These are questions of a general nature that seem to crop up
repeatedly in the AX84 forums.
Some of the answers
are clear facts, others are opinions. I've tried to carefully
differentiate between the two, but if you think I missed that
boat on a question, please send email to
the AT with "@" and DOT with "." in your email!) with the details.
- What should I build first?
- I want to build my dream amp NOW!
Is this your first guitar amplifier build? If it is, the P1 or
P1-eXtreme are HIGHLY recommended as your first build. They are
the most documented and tested projects that we have and your
chances for success are enhanced by starting there. You do want to
succeed, don't you?
Here is the cold hard truth: Unless your dream amp is very simple,
or you have a fair bit of similar experience, the likelihood that
you'll successfully build that dream amp as your first project is
very, very slim. "Building Computers", "Building Car Audio
Systems", etc... don't count because they have nothing to do with
building music electronics from scratch.
So pick the P1 or P1-eXtreme, build it, troubleshoot it, tweak it,
and enjoy it. They are great amps that you will most likely grow
to love. Then, after you have learned to crawl, consider walking.
And if you leave extra room in your chassis and plan for it in your
layout, you can always expand the amp later. But experience tells
us there's an excellent chance you'll keep that amp and build another.
- What is the First Law of Robototics? Um, I mean AX84?
Dont bozo the layout!
- I just finished my amp and it's not right...
- How should I power up my new amp for the first time?
Before asking anything else, go read Paul Ruby's startup guide
- My voltage readings are all wrong!
The schematic voltages are just what one individual observed at one specific
point in time with a specific set of components whose exact values we don't
know at whatever the wall voltage was right that minute (ambient air temperature
and airflow may impact these a tiny bit as well 8^).
If they're within 5% you should be ecstatic; within 10% is not at all unusual.
Since many power transformers for tube amps actually have a 117V primary (in
the USA) and the wall voltage is typ. 125-130V, many amps start off with high
voltages no matter what.
- My plate voltage is over spec!
- My wattage is over spec!
If the tubes are red-plating, you definitely need to re-bias and/or
(if the B+ is above spec) drop the B+. Red-plating usually means,
"Hey, you! Imminent Death of Tubes!"
But if the tubes are not red-plating, higher than spec readings
(typ. voltage, current and/or dissipation) are not necessarily
cause for alarm. In such cases, you may see somewhat reduced tube
life compared to tubes run completely within their specs. Just
how much the tube life is reduced depends on the tube brand
(design, manufacture, QC, etc), the circuit, how hard you play,
and the individual tube itself.
- What size wire do I need?
For almost everything inside any AX84 project amp to date, 22 gauge is plenty; this size wire will carry at least 7 amps in chassis wiring. Certainly this is plenty for everything besides heaters. For those, add up the heater current for the tubes you plan to use. Add in 20% to 100% for inrush current (depending on how paranoid you are). If the result is less than 7A, you're fine. Or wire some power tubes in parallel, rather than all tubes in series. See table below for other wire sizes. Larger wire sizes may not fit in your tube socket terminal holes, esp. on 7 pin and 9 pin sockets. So use the smaller of the sizes available that will carry enough current. You also care about insulation breakdown voltage (next question).
Gauge (AWG) 
Wire Gauge (SWG) 
All data from 1978 ARRL Radio Amateur's Handbook or computed from data there.
- AWG = American Wire Gauge, diameter in 1/1000 of an inch
- Amperages are for continuous duty current with insulated wire in conduits, bundles, or cables.
- Nearest British S.W.G. number, diameter in mm
- Max. wire temp. of 212F (100C), max. ambient temp. of 135F (57C)
- Estimated, not shown in original table
- Is 300 volt wire enough?
If the voltage potential between the wire and everything it
could possibly touch (or come extremely near to) is less than
300 volts, yes. But if it gets even close to that, you should
consider using wire with insulation having a higher breakdown
voltage. Otherwise a power line surge might cause problems.
- All I care about in resistors is resistance, right?
- What wattage resistor do I need?
With resistors, you generally care about the following criteria:
- Resistance : this is the obvious one.
- Tolerance : for guitar amps, 5% to 10% is fine, depending
on taste and availability. If you're really picky, go with 2% or 1%,
but expect to pay more.
- Wattage : make sure you use a wattage rating higher than
the resistor will ever see. I like to have 25% or more power in
reserve. For instance, if I calculated a resistor would dissipate
0.9 watts, I would use at least a 2W resistor, not a 1W.
- Type : some people insist you need carbon comps for a
true, vintage sound, but carbon comps drift more over time and
with heat, and are noisier (hiss, popping, etc). carbon film are
quieter but some people prefer their sound over metal resistor.
Metal film or metal oxide are quieter still. Wire-wound resistors
are mainly used when you need a 5W resistor or larger. There are
huge discussions about resistor types; feel free to search the
AX84 archives for more data.
- Voltage rating : as with wire insulation, make sure this
will handle the voltages you care about. In most resistors, you
only care about the voltage drop across the resistor, but if the
resistor is up against, or can possibly touch, another conductor
(such as a ground lug!) or another resistor, you have to worry
about the maximum potential between the resistor and the other
conductor or resistor.
- May I use AX84 or the project names on my amps?
Chris Hurley owns the AX84 trademark; the various project names belong
to a variety of people. Here's Chris's response to a question regarding
the use of the AX84 name on an amp:
``It is my intent that there are no finished AX84 amps. To be consistent,
Doberman [Chris's amp kit company -Miles] doesn't sell AX84 branded amps
either, or any AX84 logo'd items at all- no faceplates, no chassis, no
``People say "well, this amp is just for me, so whats the harm?" The harm
is that it is extremely rare that any amp stays with its original owner
``Please folks- don't use the trademarks on your amps. Make up a cool
name for your amp. You probably did something cool and unique when you
built it- put your own cool brand on the amp.''
For more information, please check out the following page:
- Where should I put my output transformer for the least hum?
- Where should I put my reverb transformer for the least hum?
- What is the headphone trick?
This is the best way to find the quietest place to put a reverb, output, driver, or any
other signal transformer to get the least hum. CAUTION: THIS INVOLVES WORKING ON THE AMP
WITH WALL VOLTAGE PRESENT. WALL VOLTAGE CAN BE FATAL.
First set up the power transformer. If it's not
installed yet, all the better, but if it is, this will still help.
If the PT is not installed, wire up the primaries to a power cord, but
do not wire up any secondaries-- tape the ends of the secondaries.
If the PT is installed, fine, just make sure there is no circuit on
any of the secondary leads (pull all tubes, and disconnect leads and
tape them as necessary).
Now set up the signal transformer (output, reverb, whatever). Hook
one set of OT secondaries (your choice, but I usually go from ground
to the highest impedance tap, don't worry about impedance matching
here!) to a set of headphones. Tape the primary leads.
Put the headphones on, plug in the power cord, and start moving the
signal transformer around. Try it all over the chassis, and also
rotate it. While you might expect the least hum with the transformers
as far from each other as possible and at right angles (and that may
be the case for you), it might come at an odd angle and/or position.
Sometimes the least hum occurs when the transformers are side by side.
When you find the quietest spot, use a permanent marker to mark where
the mounting holes should be.
If you have more than one signal transformer, repeat this for each one.
When you're finished, unplug the PT. That's it!
- What is Ohms Law?
- What value resistor do I need?
- How much voltage will this drop?
- How much current is going through this resistor?
- How much power is this using?
- How big a resistor do I need?
- How do I do the maths?
90% of what you neeed to know, math-wise, can be summed up in two
equations. Everything else derives from these.
First, Ohm's Law:
E = I * R
Basic math rules apply, so the other forms are:
|E:||voltage in volts|
|I:||current in amps|
|R:||resistance in ohms|
R = E / I
I = E / R
Let's say you want to drop 20 volts and you expect to be pulling 50mA (45mA for power tube, 5mA for preamps). You need the second form to find resistance:
R = E / I.
R = 20 / 0.05 = 400 ohms
(Remember, 50mA = 0.05A). The closest standard value is 390 ohms. Since the current drop is constant, what is the real voltage drop?
E = I * R
E = .05 * 390 = 19.5V
Close enough! So long as you know any two, you can find the third.
But what size resistor do we need? For that we need the power equation. The basic form is:
P = E * I .
In this case, that means
|P:||power in watts|
|E:||voltage in volts|
|I:||current in amps|
P = .05 * 19.5 = 0.975W
Technically you could use a 1W resistor, but that's awfully close to the edge, so I'd use a 2W resistor.
You can substitute from one equation to another, too.
Since P = E * I and E = I * R then P = (I * R) * I = I2 * R
Since P = E * I and I = E / R then P = E * (E / R) = E2 / R
That's the basics. You really should get an introductory electronics text or poke around on the web til you find something that explains it in a way that makes sense to you.
- What happens if I put resistors in series?
When you put resistors in series, you just add the resistance values.
For instance, if you connect a 100K resistor and a 220K resistor in
series, you get 320K.
R = R1 + R2
Each resistor will have the same current through it, but each
resistor's voltage drop will be proportional to its share of
Er1 = Er * (R1 / R)
Er2 = Er * (R2 / R)
- What happens if I put resistors in parallel?
This is a bit more complex. The general equation for paralleled
resistors (or impedances in general) is the reciprocal of the sum
of the reciprocal of the resistances:
R = ------------
1 1 1
-- + -- + --
R1 R2 R3
When you are just paralleling two resistances, you can simplify
R1 * R2
R = -------
R1 + R2
When the resistance values are the same (regardless of how many resistors are paralleled), then the result of N resistors in parallel is simply
R = --
Each resistor will have the same voltage across it, but each
resistor will carry current proportional to its share of
the equivalent, parallel resistance.
Ir1 = Ir * (R / R1)
Ir2 = Ir * (R / R2)
Why does it work like this?
We can use the water pressure analogy. Imagine two hoses attached to a spigot with a splitter. If the hoses are the same size, it is easy to see that the flow through each hose will be the same by intuition; they see the same water pressure. It is also intuitive to see that it is twice as easy for water to flow through two identical hoses than one. Two hoses have half the "resistance" to the flow of water.
So, for two equal resistors, it may also be intuitive to see that the resistance is half of one resistor of the same value, because you have two paths of equal value for the electricity (water pressure) to flow through. But what if the resistors are different sizes, or what if there are many resistors? That is where we need the math.
THE IMPORTANT FACT THAT IS OFTEN MISSED:
All resistors connected at a common point see the same voltage at that point.
Once you know this important fact, fomulas like R = E/I (Resistance equals Voltage divided by Current) make more sense. Unfortunately, instead of talking about how easy it is for electricity to flow, we talk about how hard it is...how much flow is "resisted". Otherwise, for parallel resistors, all connected to the same place by definition, we could just add up all the values of how easy it is for electricity to flow through each resistor. The math would be simple. But...
...we actually can and do add up how easy flow is through each resistor as one of the steps to solve the total resistance of resistors in parallel. Numerically, the reciprocol of resistance (1/R) is a measure of how easy it is for electricity to flow through that resistor, the "conductance" of the resistor. This number is called a Mho. The Mho we use it, the Mho we like it! We can just add up all these values of Mho to find out how easy it is for electricity to flow through all the resistors, and that is what we are doing when we take each resistance, invert it (i.e. devide 1 by the resistance, like this 1/R), and add up all the different 1/R results.
The problem is, we actually want to know how hard it is for electricity to flow (resistance in "Ohms"), not how easy (Mhos). Since we just used 1/R to calculate Mhos, we just need to use 1/Mhos of all the resistors to get Ohms. Problem solved.
Let's do an Example:
Requirement: Calculate total resistance (Ohms) of these three parallel resistors
- Three resistors in parallel
- R1 = 5, R2 = 10, R3 = 20
- Convert all resistor values to Mhos
- Add up all the Mhos
- Convert Mhos to Ohms (Ohms = 1/Mhos)
- Calculate Mhos (how easy electricity can flow)
1/R1 = 1/5 = 0.2 Mhos
1/R2 = 1/10 = 0.1 Mhos
1/R3 = 1/20 = 0.05 Mhos
- Add up values to get Mhos of circuit
0.2 + 0.1 +0.05 = 0.35 Mhos
- Now convert Mhos to Resistance of circuit
1/Mhos = 1/0.35 = 2.857 Ohms
- The total resistance of parallel resistors is always less than the lowest value resistor. This should make sense, because each resistor we add provides another path for the flow of electricity; it gets easier and easier for the electricity to flow (less resistance).
- For resistors of equal value, you just divide the resistor value by the number of resistors to calculate the parallel value in Ohms.
- Mhos is an old name and was replaced by the "Siemens". Note that Siemens is the spelling for both singular and plural.
- What happens if I put capacitors in parallel?
When you put capacitors in parallel, you just add the capacitance values.
For instance, if you connect a .1uF and a .2uF capacitor in
parallel, you get .3uF.
C = C1 + C2
Each capacitor will have the same voltage across it.
- What happens if I put capacitors in series?
This is a bit more complex. The general equation for series
capacitors is the reciprocal of the sum
of the reciprocal of the capacitances:
C = ------------
1 1 1
-- + -- + --
C1 C2 C3
When you are just putting two capacitors in series, you can simplify
C1 * C2
C = -------
C1 + C2
When the values are the same, then the result of paralleling N capacitances is simply
C = --
Each capacitor will carry voltage proportional to its share of
the total capacitance.
Ec1 = Ec * (C1 / C)
Ec2 = Ec * (C2 / C)
This is occasionally done to get a specific smaller value
of capacitance than one has on hand, but in general this is
done to increase the voltage rating beyond that of the available
capacitors. In this case, it's a good idea to place a resistor
in parallel with each capacitor to help equalize the voltage.
In power supplies (the main place this happens in guitar amps),
220K resistors is a good value to start with if you aren't sure
what to use. If the caps are of different voltage ratings, you
should probably use resistors of different sizes, in direct
proportion to the voltage ratings of the caps. For instance,
if you were putting a 200V cap and a 350V cap in series, you
might use 220K and 330K resistors.
- How much current (mA, amps) does my PT need to be rated for?
You need to determine this for each of the secondaries.
For the high voltage secondary, power tubes are the most
important factors. You'll have to estimate the peak
current draw per tube plate and screen and
multiple by the number of tubes. For a single 6BQ5/EL84,
the max. plate current is supposed to be 65mA, and the
screen is typically 5mA or less, but throw in a few mA
for good measure, call it 75mA. (You can get this from
the spec sheets). For two 6BQ5s in Class A, double that.
For Class AB, the average current is the same, but the
instantaneous current is higher, depending on how far
toward Class B you bias. At Class B, you'd need a
secondary capable of double the Class A current.
Now add current for each of the other tube sections.
Each half of a 12AX7 typically pulls around 1mA, but could
pull up to 3mA. If you aren't sure, use 3mA per triode.
BUT... if there is any chance you or someone
else would ever substitute in a 12AT7 or 12AU7, you'd
best figure 10mA or 20mA max per triode. That's a whole
new ball game! (You may have to hunt to find the actual
maximum, rather than typical, ratings.)
Adding these up, a basic P1 that will never use anything
but a 12AX7 is safe with an 80mA high voltage secondary
(75mA power tube + 6mA 12AX7). Any PT made should be
be able to handle 1 extra mA. But you might also want
to include a safety factor, and get one capable of a bit
Now you need to determine the main heater current.
This data is more readily available. For a 6BQ5 it's
760mA. For a 12AX7 with the heaters in parallel for
6.3V usage, it's 300mA. So a P1 needs a 6.3V heater
tap rated for at least 1.36A. But again, look at
what other tubes you might use; a 12BH7 pulls 600mA
with the heaters in parallel.
Finally, if you plan to use a 5V rectifier, make sure
the 5V winding can provide however much current your
rectifier will require. Depending on the tube and
load the load here can be anywhere from just under 500mA
to a couple of amps. If you want to go the old Bassman
or Mesa route (multiple tubes) then double or triple
the current as necessary.
- Should I build a P1 or eXtreme, or a High Octane?
Consider the following:
If you lean more to (a), consider a P1 or P1-eXtreme. If you lean more to (b) consider a Hi-Octane.
- (a) Highway to Hell (b) Hells Bells
- (a) Love Gun (b) Lick it up
- (a) Iron Man (b) Miracle Man
- (a) Smokin' in the Boys Room (Brownsville Station) (b) Smokin' in the Boys Room (Motley Crue)
- (a) Breakin' the Law (b) You've got another thing comin'
- (a) Train, Train (Blackfoot) (b) Crazy Train (Ozzy)
- (a) Dyer Maker (Led Zeppelin) (b) Dyer's Eve (Metallica)
- (a) Eighteen (Alice Cooper) (b) 18 and Life (Skid Row)
- How does speaker efficiency affect volume?
- I really NEED a 100 watt amp!
I'll give you the obligatory speech about watts VS. volume so that you don't get all caught up in the 100+ watts thing.
The way that watts translate into decibels is in powers of 10. That being the case, if you want to do twice as loud, you need to go 10x higher in wattage. The difference in volume between 100W and 50W is actually only 12% less or so. HALF the volume of 100W is actually 10 watts, and TWICE the volume of 100w is actually 1,000 Watts. And when is the last time you played a 100W amp on a setting higher then 3 on the volume without everyone screaming to turn it down!
Now, after you have digested that, we can move along to the next part.
Speaker efficiency is also key to volume. That curve is also not linear. If you take 2 otherwise identical speakers, one with a sensitivity rating of 103dB, and one with a rating of 100dB, that -3dB drop is the equivalent of sending half the power into the speakers... like moving from a 100W amp to a 50W amp, or about a 12% drop in volume.
Now, if you move from the 103dB speaker to a 97dB speaker that is otherwise identical, that -6dB drop is like moving from a 100W amp to a 25W amp. That being the case, a 25W amp played through 103dB speakers is exactly as loud as a 100W amp being played through 97dB speakers.
OK, digest that for a second and then we'll move on.
Last week, I played a 35W amp on full blast (but not clipping) through a Marshall 4x12" cabinet loaded with Celestion Vintage 30's, which have a sensitivity of 100dB. I took out a sound pressure level meter and put it 10 feet away from the speakers. The 35W amp produced a clean power level of 117dB, which is only 3dB quieter than an airplane landing.
With all that in mind, you should decide on whether you really want to go with the added expense, weight, maintenance and extra wiring that a 100w amp requires.
- What about silver-plated wire?
Silver-plated copper wire shouldn't be used in any equipment that may be exposed to humidity over 50%. The silver reacts with the copper and causes the copper to turn into rust ("red plague"). Yes, this can only happen in the presence of oxygen, but it is very easy to "nick" a wire when stripping, even with the best tools and training. I have seen lengths of silver-plated copper wire in aircraft that still had continuity but had increased in resistance to several kOhms per foot. Tinned copper does not have this problem.. Silver lowers the resistance, but I can't believe there's an audible difference.
and for the really studious types (notice by the dates that this is not a new problem):
- Anthony P L & Brown O M 1965, Red plague corrosion, Materials Protection, 4, 3, pp. 8 - 18.
- Friebel V R 1969, Corrosion problems on silver-plated copper wire, Wire and Wire Products, May 1969, pp. 39 - 43, 53.
- Peters S 1970, Review and status of red plague, Insulation/Circuits, May 1970, pp. 55 - 57.
- Peters S T & Wesling P B 1968, Corrosion of silver-plated conductors, Proc. SA M PE 13th National Symposium, pp. 395 -406.
- High Temperature Insulated Wire, Report on Copper Corrosion, NEMA Standards Publication, No. HP 2-1968 (reaffirmed by NEMA 1972).
- ESA PSS-01-720, Draft, Test Procedure to Determine the Susceptibility of Plated Copper Wire/Cable to 'Red Plague' Corrosion, 9 December 1983.
- ESA PSS-01-708, Manual Soldering of High Reliability Electrical Connections
- What do these cap codes mean?
European cap material codes:
MKP = metallized polypropylene
FKP = metal foil and polypropylene
MKT = metallized polyester foil
MKC = metallized polycarbonate foil
FKC = metal foil and polycarbonate
MKI = metallized polyphenylene sulphide
MKS = polystyrene (metallized or with foil)
- How can I tell how much power my amp is putting out?
" In order to get an accurate measurement of output power, you should use a purely resistive load... if you are just looking for a ballpark figure, a meter and a load resistor will do fine, if you have the amp set just prior to where you start to hear distortion when a speaker is connected.
Sub a load resistor equal to the speaker impedance, measure the RMS AC voltage, square it, and divide by the measured value of the load resistor. Be *sure* your meter reads in true RMS, or you will get an incorrect result. Use a 400Hz signal as the input source, because this is where most speakers have their ``flattest'', or nominal, impedance."
-Aletheian-Alex, distilled down from Randall Aiken's site
- Why don't these amps use tube rectifiers?
- Can I use a tube rectifier on my build?
In a push-pull amp, the tube rectifier will drop more voltage as the current through it increases. As you turn up the amp and dig in, the power tubes pull more current. This causes the tube rectifier to drop more voltage, causing an overall drop on the B+, changing the tone of the amp-- adding some mojo.
Most of the [AX84 amps] are single ended. SE amps pull the same amount of current no matter how loud or hard you play. There is no mojo gained. SS diodes are cheaper, and easier to use for the same result.
The mojo refered to above is commonly known as "sag", which results in a compression effect, sometimes called a "singing sustain".
For more discussion on this topic, see the thread Bobby's answer above is from on the AX84 BBS at http://ax84.com/bbs/dm2.php?id=207139.
-Miles (Harrison Ford Prefect)
- What is the value of (for example) a 4K7 resistor?
- What is the value of (for example) a 2n2 capacitor?
Many times, engineers will use the multiplier as the decimal point. It harkens back to the days of poor printing when you could easily miss the decimal point (or comma in some euro diagrams) and screw things up royally.
2.2 Megohms = 2M2
2.2 Kilohms= 2k2
2.2 ohms = 2r2
.22 ohms = 0r22
The same thing applies to capacitors. Examples:
.0022uF = 2n2
2.2pF = 2p2
-Miles (Harrison Ford Prefect)
- How do I convert between capacitor units (micro, nano, pico)?
Randall Aiken's site covers this quite well (as usual!)
-Miles (Harrison Ford Prefect)
- What type of solder should I use?
Make sure you never use solder that says "acid core". Plumbers might use that stuff to solder oxidized metals but it will corrode the $#|+ out of your electronics if used. Avoid that stuff like the plague. Here are three types that are OK to use:
"63% Tin 37% Lead" solder is the easiest stuff to use IMO since it melts cleanly at the lowest temperature. Make sure it says "Rosin Core" if it has flux added - most have it. Then if it says "no-clean flux" it's even easier. The Kester brand works for me. (By far the best -ed)
The 60/40 rosin core stuff melts at a little higher temperature, it's also OK to use but it takes a little more technique since it goes through a transition phase between flowing and setup that the 63/37 stuff does not do. IMO it's more prone to "cold solder joints" if your technique is bad. Google or wiki the term "eutectic" if you are curious about this stuff. The 63/37 stuff is a eutectic solder, all the other percentage formulations of Tin/Lead solder, like the 60/40, are not. If in doubt, go with the 63/37 stuff. (Only if you can't get 63/37 -ed)
The "2% Silver stuff" is OK to use too, IIRC it's 62% Tin 36 Lead 2% Silver. There might even be stuff with a little higher Silver content. Some folks think the 2% stuff makes a stronger solder joint than 63/37 and some even think it adds to the mojo or "sound" of the solder joint. No comment on the mojo ;-) , but it makes a good joint if you have a good iron and know what you're doing. Rat shack even has some, it's not bad stuff.
I'd go for the stuff in #1, get a small amount of it in a diameter that you hope makes it easy to feed into the solder joint and see if you like it. If you use solder that has too small of a diameter you will find that you have to feed more length in at each joint and that is a pain. Too large and you'll easily get a big blob of the stuff if you're not careful. Just like guitar string gauges, use what you like and fits your style.
If you get a big spool of solder, wind a smaller amount of in into an empty "solder wick" plastic holder thingy - the ones that are about as big as a half-dollar coin with a hole in the middle. Mount a dowel, ball point pen, or something like that in the chuck of your cordless drill, push the empty "solder-wick" spool onto the post, and then wind solder from the big spool onto the "solder wick" package until you fill it up - this make a handy solder dispenser. The outer flap holds it from unwinding but you can easily pull a few inches out at a time for a joint.
This is much better than holding a big freakin' spool of the stuff in your hand, or having to cut little pieces of it off the spool each time to use and then dropping these little unused ends around everywhere where the dog can eat them.;-)
(lead/tin -> tin/lead correction courtesy of Kristian Schlager)
- I think my tubes are too hot. I can't touch them!
The spec sheets often list a maximum bulb temperature of 180 to 200 degrees C for rectifier and power tubes. Some of the horizontal sweep types such as 6DQ6 and 6GV5 list as much as 240.
That is instant 3rd degree burn territory.
Preamp tubes on the other hand should not get much over 50 degrees C.
Note: I have seen tube data suggesting preamp tubes run at 100C or higher. Fingertip tests tell me 50C is much lower than reality. -Miles
- Is it OK to use a hifi speaker with my guitar amp?
- I tried a hifi speaker with my guitar amp and it blew up...
There is a big difference between guitar amp use and PA or HiFi use of speakers, and in some areas the advice and issues are almost exactly opposite-- but both correct in their given areas.
In HiFi and PA use you must never let the amps clip. Once you do the amp will begin to deliver significant amounts of energy in relatively high frequencies. A PA of a HiFi rig will have high frequency drivers, whose power handling capabilities are by necessity vastly lower than those of the bass drivers. Normally this is fine as the energy in normal music at these frequencies is also vastly less. But if the amp clips they will receives pretty close to full amp power straight through the crossover and their voice coils will burn out. Clipping happens easier with a lower powered amp, so counter-intuitively, their is a greater risk of blowing your tweeters with a lower powered amp than a higher powered one.
But in guitar amps and speakers this isn't the case (except for some solid state jazz rigs designed for clean only sound.) In a guitar amp we assume it will be overdriven, and that the power delivered is closer to 1.4 times the RMS clean capability. The speaker designers also assume this. However guitar speakers are full range devices, and there is no need for a separate tweeter. So the voice coil is already sized assuming the delivery of this power. Of course such a speaker has pretty rotten high frequency response-- but we neither need nor want it to reproduce such high frequencies. So the very high frequency energy just quietly heats up the voice coil-- but since it is sized to cope anyway there is no problem.
RMS is very much the accepted definition for power delivery. Indeed the entire definition is specifically based upon this idea. However there might be a small semantic issue-- since there is an implicit assumption of distortion level. Amplifiers are quoted for power delivery into a known load with a limited distortion of input waveform. Typically this is going to be as close to maximum clean power as you like. However there is nothing stopping one from measuring RMS power at full overdrive. So long as the power supply can cope you will get about 40% more power. By convention guitar amps are specified as max clean power with a sine wave input at 400Hz.
That gives the clue about HiFi amplifier specs. The spec is very stringent (which is why so many manufactures try to avoid it with PMPO and the like.) It is the maximum sustained clean output with the amp not exceeding a given temperature rise. Quoted as RMS power (which it is) these other constraints are mandated by consumer law in the US-- they are not part of the physics of the specification directly. But they do underline the issue that power specifications must be read along with the measurement constraints-- otherwise wildly differing results are found-- all of them correct.
- Do I need shielded wire in my high gain amp? I don't have any (or would rather not use it).
[You can] just use a twisted pair. You may be surprised at how close in performance a twisted pair can come to shielded wire in many applications. Not all, but audio certainly. Just treat the second conductor of the pair in the same manner as you would the shield.
Try to keep the pair tightly twisted with an even and symmetric twist.
I used twisted wire instead of sheilded wire in an amp a while back, a scratch build of a Valve Junior. Worked great, and the amp is very quiet.
I used two lengths of 20gauge, 300v, solid wire (black and green) twisted them in a drill to get a tight, uniform twist.
If you're struggling for shielded wire, have you not got an old pair of headphones or something you can steal some from? Even that cheapo stuff is better than nothing.
(RCA patch cables from stereos, etc are cheap and work great. -ed)
Build it without and see how it does!
Any wires you may need to swap for shielded later, just tack down lightly with the soldering iron (don't bend the wire much around the turrets or lugs), etc). If the end result is fine (give it a few days of playing, some at full volume, to decide), great-- go back and secure the tacked down connections. Otherwise pull them out one at a time starting from closest to the input, replacing with shielded, until you're happy with what you have.
- How do I solder?
Soldering with rosin core solder is cake. And the "let it flow" concept is simply to let the part you are soldering melt the solder, which ensures you get a good joint. To do this, you don't touch the solder to the tip of the iron when soldering, you touch it to the joint.
- Make sure you have a nice hot iron. Most folks use a 30 watt or so for everything except soldering to the chassis. I use a 230 watt squeeze trigger "gun". I never have "bad solder joints" with my gun. But you better be careful or you can melt stuff. :-)
- Tin your wire in advance if you are using stranded. Simple way is to leave a tail of solder on your spool, then touch your wire to the iron and let the wire touch the solder lead (not the iron). Take a second to wick the solder onto the wire. Component solid wires of course do not need this step.
- For the actual soldering joint, I first make a VERY good mechanical connection. In fact I probably use a bit of overkill, because I want my amp to work forever, even if I miss a joint. See this example, and note that none of the joints have been soldered in the photo: http://www.charlestonarea.com/octane/2-13-2006/AX83_HO_solder_joint.jpg]
- When you have an excellent mechanical connection, the soldering part is easy. Wipe your (hot) iron on a wet sponge (moist paper towels in a bowl also work just fine). This cleans the junk off the tip if there is any. Touch a dot of solder to the solder tip, which should turn it a nice silver color and provide some wetted area for heat flow. Then just touch it to your turret, near but not touching the joint. In a second or two you touch a dab of solder right onto your joint and let it flow to just barely fill in the gaps. With my ultra-hot iron this literally takes about one second or less. I get a GREAT joint every time.
- As soon as you have a good solder flow on there, pull the iron and solder strand and do absolutely nothing. Don't blow on the joint to cool it faster. Don't move anything. Just let it "cure" naturally.
NOTES: The iron I use is way overkill for most people, but works exceptionally well if you use it correctly. Since it is good and hot, with lots of thermal inertia behind it, I can quickly heat a very local area and get it soldered. I don't need to leave it on the joint for a while, heating up all the parts around it. However, it takes care and practice. I also use a heat shield for diodes and with shielded wire, which is just clipping a hemostat above the joint to pull heat away from the component/shield wires.
Oh! I can't remember who showed me this, but take a red marker and dot all your solder joints when you are done. This is a good way of checking each joint, and you may find you forgot to solder one.
That's the longest explanation of soldering that I could come up with. ;-)
Pliers work quite well for bending leads.
I use a temperature-controlled soldering iron with a small chisel tip, set at 700 degrees F. I also use a flux pen and apply a drop of liquid flux to every joint before soldering. I've found that this makes a big difference. I don't bother to pre-tin the turret or the wire before soldering and I have only suffered one bad solder joint on seven amp builds, and that was on a turret with a big cap and 3 or 4 other wires connected to it, where I didn't use enough solder.
- How do I install turrets?
I've found this page to be real helpful for first time turret board building. Hope it's what your're looking for:
I made my own turret insertion tool out of threaded rod and nuts. Pretty easy, and cheap.
- Does DC heater bias really help with hum?
This question arose in another discussion: how many db does raising the heater voltage by about 40 volts dc reduce the hum? These measurements (db) are from a Modified HO I am working on:
|Background (G1 up, G2 down, MV up):||-70||-52||-63||-62|
|G2 adj. for good hum, 0 v. dc||-50||-22||-42||-23|
|Same G2, 40V dc||-55||-36||-62||-52|
Yep, simple, elegant, and it works.
(These are measured hum voltages, and subject to the usual limitations of measurements. The actual improvement is probably considerably better.)
- I want adjustable bias, but the locking pots are too epensive and/or hard to get where I live.
Depending on your budget, you can use regular 25k pots.
I cut the shafts off with a dremel, so they are flush, and then cut slots in them, so I can turn them with a screw driver.
- Should I use snubber caps on my power supply diodes?
- Should I use FREDs instead of plain diodes like the 1n400X series in the power supply?
For what is close to the last word on snubbers read this: http://www.hagtech.com/pdf/snubber.pdf
Quick summary - capacitors alone are not the best answer - a series resisotor/capacitor in parallel with the diode is vastly better. But finding the right values isn't trivial and depends in part upon the power transformer's leakage inductance.
+1 on everything Francis said.
IMO, getting a snubber to REALLY work optimally is tough. If someone tries, do they have a good scope and know how to delay trigger it and know exactly where and when to look in the rectification cycle? If not, then they are guessing.
The lazy cap only can actually make things worse (its rare, but it can happen). One size does not fit all.
IMO instead of guessing with an attempted snubber design, try this
Use a Soft Recovery Ultra Fast Rectifier like the UF5408 instead of the 1N4007, and then keep that first high current pulse circuit loop small and the heck away from sensitive high impedance nodes.
UF5408s are not that expensive. Someone can get two for a little less than a dollar in small quantities at Mouser. Done. See [link] and [link]
Standard stuff. It really makes a difference. Oh, and dont bozo the layout ;-)
Why not just use FREDs and be done with it?
FWIW, it's certainly possible to build a hum and buzz free HO with the stock power supply, although probably not with the stock heater circuit.
A 50V DC heater reference and a good layout should do it. Use UF5408s and its even more golden.
But most importantly --> Dont bozo the layout!
- What is the safe bias range for my power tubes?
Bias them anywhere between the hot side limit meaning "any hotter and they get damaged" and the cold side limit meaning "wow, that sounds BAD".
Anywhere in that range is OK. There is no "correct" bias setting, keep it safe and then put it anywhere you like the sound. Biased colder, they might last longer.
(In other words, bias them so you like the sound
and make sure that bias setting is within the specs, and
you have the right bias setting for your amp. -Miles)
- What happens if I use different preamp tubes?
- What tubes can I sub for a 12AX7 in the preamp?
OK, so the question comes up a lot about what tube tou can run in the preamp of a P1 instead of a 12ax7... obviously you can run ANY tube if you want to take the time to figure out how... but i made up a chart of some common tubes just to show what it would look like if you just 'dropped them in' without changing any other component values. Obviously, this is not a good operating point at all fo some tubes... which you can see in the chart.
I am NOT advocating that you try all of these, and obviously you would have to rewire the socket to try some of them... or install an octal socket, but it is just to give a general guideline about what would work.
I plotted out what the Av (output gain) would be, the output impedance (good to know for the tone stack), the plate curent, the PSRR, the positive and negative AC swing, the voltage across the tube, and how many volts it would take at the input to push the stage into cutoff (really only possibly with EMG's or a booster pedal)
These are all dual triodes except for the triode strapped EL84... which i threw in just for fun AND to show you how similar in specification it is to a standard ECC99 (I heard that the ECC99 was based on a triode strapped EL84... looks pretty close to me-- could make a great one tube push pull output stage or phase inverter!).
It also shows how you don't neccessarily need a high mu tube to get a lot of gain... a 12at7 or 6AQ8 have less then 60% of the 12ax7's mu, but they produce over 90% of the output voltage.
- Do I have to wire the heaters and pilot lamp in one string, or can I use separate wires off the transformer?
Either way is fine. The heaters get wired in parallel in both cases. Most mfrs wire them in a single string because (a) it's easy, (b) it's cheap and (c) it minimizes the amount of wire radiating hum in the amp. Most DIYers do it that way because that's how the mfrs usually do it. 8^)
I typically run a single string because of (c) and (a), in that order. But I would have no problem splitting it somewhere and running multiple strings if I needed to for layout purposes, or if I had several big bottle tubes that required more heater current. In that case I'd run a bigger gauge string to the power tube sockets and a 22ga string to everything else. Heck, even 10 nine pin preamp tube heater sets can run off 22ga unless you have a bunch of 12AZ7s (.45A) or 12BH7s (.6A)...
I do have one tip to add that IMO will help someone verify that they have not messed up the heater wirings solder joints during the build:
Solder up the connections starting from the farthest downstream preamp tube socket and work your way back toward the PT. Make the two 100 Ohm resistor connections 2nd to last (if you use them at all), and save the final connections to the heater windings for the very last step. This way, you can easily verify during the build process that you have not caused a short between the two heater wires due to overheating the solder connections. Of course, they should do it with no tubes installed ;-)
A typical newbee problem is to use excess heat and solder-iron contact time when making solder joints. This can causes the heater wires insulation to melt through, causing the two heater wires to short out to each other. This mistake is very bad for the transformer, and it is made more likely to happen here since the builder has probably twisted the wires tightly together. Seeing melted insulation by eye on twisted pair wiring is tough since the wires are touching anyway. Finding the one bad spot after it is all wired up is even tougher since the normal resistance of a heater winding is very small. It is best if a problem can be found and corrected during the build, not after it is found to be all FUBAR at first power-up.
Make the first two heater wiring connections at the 1st preamp tube and verify both heater wires sticking out of it read as an open circuit. If they read shorted then you have hosed up the wiring at this point - rip it out and redo it. If everything is good, then run the other end of the twisted run over to the next preamp tube upstream and solder these in there as well as the twisted run that will run out of this socket and go upstream to the next tube socket - there will be two wires going into each tube socket eyelet here. Solder them up and re-measure to verify they still read open, redo if it is hosed up. I like to use pre-twisted wire Ive made with the hand-held drill trick and have the upstream run be plenty long enough so it can be trimmed to length when installed in the next step. Repeat with all sockets.
The idea is to wire-up and solder one socket connection set at a time, then verify the two heater nodes on the tube sockets read open circuit all the way back. When (or if) the two 100 Ohm resistors are added, then you will read both of them in series for about 200 ohms between the two heater nodes after they are connected. When the heater winding is finally connected up, you will read the very low resistance of the heater winding - you will have to do these last joints with some skill, but you are probably good at doing them by now ;-) .
Helpful hint: Always measure a transformer windings DC resistance BEFORE you hook it up so you know what to expect after you wire it into the circuit.
OK, you should have measured ALL the transformer windings DC resistances before a solder iron gets near them just as a good practice. Knowing what they read before you hammer it with AC will help you if you ever have to go back and re-measure them later to see if everything is still OK if there was a problem in the build.
I dont usually use those Type 47 pilot lights that suck 150mA of heater winding current, but if you did, then the above method could easily be adapted to handle it. Just make sure the pilot lamps socket wiring doesnt hose up your DC Voltage elevated heater circuit since one side of the pilot lamp mistakenly going to chassis ground will cause some issues here if youre not careful
- When trying to fix my amps belching / bad table manners / demonic possession / whatever problem, I measured the voltage on the Grid of my Phase Inverter and the measured value looks funny to me. Could this be my problem?
- Why do my grid voltages look all wonky?
No. Even on a healthy amp, you can't measure the DC grid voltages on a long-tail PI with a normal multi-meter. Your DMM's input impedance will load down the boot-strapped high-impedance input of the PI and this affects the circuit operation. Same problem happens with a regular oscope probe on that node. PI Grids are a "don't touch node".
If you really feel you need to know the grid voltage, the best you can do is measure the Cathode voltage (referenced to Ground), then measure the plate current that is flowing, then plot a proper load line on the plate characteristics chart from the tube's datasheet, then find the Vgk voltage that would give you the measured Ip with that plate load, subtract |Vgk| from the measured cathode voltage and thats a reasonable guess of where the Grid voltage is sitting with respect to ground.
Your amps problem needs to be tracked down without making this PI Grid Voltage measurement directly.
Note-- the above is generally true of grid measurements, not just PI grid measurements. -Miles
- Why do most 12ax7 PIs (phase inverters) use 82k and 100k for the plate load resistors, when Aiken's procedure to designing PIs shows that they should be 2xRa (62.5k typical for 12ax7)for a total of 125K ohms?
The second triode is driven by the cathode signal from the first triode. Because there is loss in the system you usually use a slightly larger anode resistor on the 2nd triode to increase its gain, to compensate for loss.
Firstly ra is quite variable; the data sheet often quotes 62.5k though I find it's more like 50k for the warm bias we tend to use. In any case that rule of "2x ra" is really just a loose guide. You can use anything from a few tens of k-ohms to several hundred k-ohms, depending on what you want from the triode. A 100k resistor is a nice round value that works well with the ECC83.
- How do I debug my build (or any amp)?
Here's a good link: http://www.geofex.com/ampdbug/ampdebug.htm (thanks to Mick S!)
And here's a simple way to trace where the no sound originates (sic):
With the amp belly up and on (speaker plugged in!), volume up, use a meter probe to touch each tube grid connection. Start at the power tube and work backwards. You should hear a hum each time, getting louder as you move towards the input. When you don't get a hum touching the grid terminal, you know the problem is between that stage and the next.
If you get loud hum touching the first preamp grid, then the problem is the input jack or the wiring between there and the grid.
- How does a preamp tube (valve) work?
Preamp tubes operate exactly like the power tubes do, except that they are developing a signal across a fixed resistor in the plate circuit, rather than across a transformer winding. Same amplification method, though.
As you are no doubt aware, a tube is biased on by application of a negative voltage at the grid with respect to the cathode. This is done either by directly applying a negative voltage to the grid (not seen often in a preamp, but is occasionally used and works fine if you want to do it that way), as in the fixed bias type of poweramp, or by grounding the grid (through a reference resistor) and putting resistance between the cathode and ground. This raises the cathode voltage above zero. The grid is referenced to zero, so you get the same effect > the grid is negative with respect to cathode. Tube is 'on' and will cheerfully amplify a small signal applied to the grid, which creates a current across the plate resistor.
Since Ohms Law states that pulling current through a resistor will create a voltage drop, the tube's plate now has a varying voltage, which is a larger version of the small signal we fed into the grid. Et voila! Amplification. ;-}
Distortion is created (whether in a preamp or power tube) when the signal at the grid is bigger than the ability of the tube to faithfully amplify.
For example: a hypothetical tube has a gain of 50, which means a sine wave of 1V peak to peak at the grid will be amplified into a sine wave of 50v peak-peak at the plate.
Now consider that the power supply is feeding the top of the plate resistor with a fixed voltage. Say, for example, 200V. It should now be easy to see that a signal of any greater than 4v at the grid will be more than it's possible for the tube to cleanly amplify, even if it's biased so the idle plate voltage is right in the middle of the possible range - it simply runs out of voltage.
What happens in that situation is that the tube stops tracking with the input signal for the period that the output would be above 200v or below 0v. This creates a flat top and a flat bottom on the sine wave output, which is what you hear as distortion. The bigger the signal at the grid, the more pronounced the flat spots become and the closer the output comes to resemble a square wave (a square wave is as distorted as you can make something, in guitar amp terms).
In reality, the distortion mechanism is more complicated than that, as it has to include non-linearity as well. Which is when the tube is still tracking the input signal, and not actually clipping, but isn't doing a very good job of producing an output that looks the same as the input. This too happens with big input signals, generally a little before the flat clipping starts, so the actual output is a combination of the two in most cases.
It should also now be obvious that if you bias the tube accordingly, you can arrange for the top to clip before the bottom, bottom before top, or anywhere in between. You create different distortion tones by doing this.
The mathematical description of what's actually happening is more involved than my monkey explanation, of course, but in essence that's how it works.
I suppose, for the sake of completeness, I should also mention grid limiting/clipping...
As I mentioned before, the tube works because the grid is negative with respect to the cathode. But the instantaneous grid voltage is a combination of the bias voltage AND the signal voltage. Say the bias is at -3v and a 2v signal is being applied. The grid voltage is then varying between -1v on positive signal peaks and -5v on negative ones. The plate voltage varies in sympathy with this, only a lot bigger, which is what creates the output signal.
However, if the input signal is big enough that it can drive the grid positive with respect to the cathode at any time, then you get clipping again because the tube 'switches' off for the time the grid is positive. It can only work if the grid is negative, unless special provision is made to allow the grid current somewhere to go. But that's a whole other topic. And a large and probably contentious one, so I'm not going to go into it at present. Not that important to the basic understanding of why tubes distort, anyway.
- How can I attach a link in the BBS (forums)?
What do you mean by "attach a link"?
- If you mean "have it make a link to the URL I enter", it does that automatically for you.
- If you mean "I want the URL to appear in the text" you have to trick it, typically by leaving off the http and colon, or adding a space after them or something.
- If you mean "attach a file to the post" you can't. You have to upload files to some other site (photobucket, youtube, whatever). Then just link to them (see #1).
- Miles, you moron! You missed another one!
I know. What else is new?
(Thanks to Jukka for the inspiration.)