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'How to determine Output Transfo versus Tube '
Author:Gitman (registered user: 20 posts )
Date: Thu, Dec 11th, 2008 @ 13:32 ( . )


Hi,

How can I determine the right output transformer to use
in an amplifier?

If I use a 6SN7 or 12AU7 or a 6C4 etc... at 16 ohm
for exemple?

I read the 12AX7 in SE mode needs a 25,000 ohm primary
for 8 ohm secondary (50,000/16ohm). A reverb OT.

Is there a way to calculate that.

Thank you.

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'How to determine Output Transfo versus Tube '
Author:Merlin (registered user: 4735 posts )
Date: Thu, Dec 11th, 2008 @ 18:18 ( . )

In SE, for maximum power out, the optimum impedance is 2x ra. Where ra is the internal anode (plate) resistance of the triode.
For the 12AX7 this is about 60k, so the optimum load is 120k, I don't know where you get 25k from?

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'How to determine Output Transfo versus Tube '
Author:PhilS (registered user: 1564 posts )
Date: Thu, Dec 11th, 2008 @ 21:02 ( . )

22.5K is the number I've seen for 6SN7 and 12AU7. Apparently, this is derived from testing (ears) and drawing load lines. 2x Ra would be around 15.4K for those tubes. I've also read that anything in the range of 15K to 25K works for those triodes. I'm going to see about building one self-split PP all octal (6SL7, 6SN7) pretty soon. I keep looking for a bargain OT. If I get frustrated, it will end up a SE 6V6 with 5K OT.

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'How to determine Output Transfo versus Tube '
Author:Gitman (registered user: 20 posts )
Date: Thu, Dec 11th, 2008 @ 22:08 ( . )


" I'm going to see about building one self-split PP all octal (6SL7, 6SN7) pretty soon. I keep looking for a bargain OT. If I get frustrated, it will end up a SE 6V6 with 5K OT. " PhilS

It is way easier to find 5K OT's than 12-25K...

What I find nice about low wattage 1/2W to 2W guitar amps is
the possibility to play in a room with that sound only possible
at very high volumes with higher output amps.

A 5 watter is still quite loud! You have to push it to obtain
the sustain and that tube breakin' wanted!

Keep me informed on your project... Interesting!!!!

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'How to determine Output Transfo versus Tube '
Author:Gitman (registered user: 20 posts )
Date: Thu, Dec 11th, 2008 @ 21:47 ( . )


Hi Merlin,

I read so many different opinions...

I got that number (25K/8 ohm) on a forum from
an internal description of the Blackheart BH1H.

The BH1H uses 1/2 12AX7 section as output triode and
here is exactly the notes I copied in a Word document:

( "Looks like a scratch build could use a reverb driver
transformer using a 16 ohm speaker.
Few other notes :
Wall voltage = 119V
Output at Power transformer 244V
+B to Output Transformer = 313V
V2B Plate = 307V
V2A P;ate = 274V
V1B Plate = 172V
V1A Plate = 170V
The OT primary is 50K/16ohm, on the stock Killer Ant" )

I have also read on a forum in France it was 1/10 of
the plate resistance on a specific voltage and bias.

This is the exemple given from a RCA manual:

Tube : EL84/ 6BQ5, class A,
Plate voltage: 250V
Grid no.1 : -7.3V
Plate curent : 48mA
Plate resistance: 38,000 ohms
Load resistance: 4,500 ohms
power out : 5,7W

His suggestion was to use an output tranfo of 4,500 ohms
on the primary.

I took my RCA manual to check that out. I took a triode
as an exemple: the 2A3. This time for a plate resistance of
800 ohm, the optimum load resistance is 2,500 ohm...

3 times the plate resistance. I am confused!!!!!

For the small double triodes (12AX7, AU7, AT7, AZ7, BH7)
the manual only gives specs to use them as a driver or preamp
tubes never as output tubes.

I heard a Blackheart BH1H in a guitar shop and I was very
impressed by the sound from that 1/2 watter...

I am planning to build my own version with tubes I have
in stock. 6C4, 12AU7 ( a current driver),6922, 6U8, 6SN7 etc...

Many options possible but I have to know what to use as
output tranforrmer.

I have never seen a very high impedance OT as 120K for 16 ohm
speaker.

Where can I find that?

Thank you Merlin for your answer!









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'How to determine Output Transfo versus Tube '
Author:Gitman (registered user: 20 posts )
Date: Fri, Dec 12th, 2008 @ 15:44 ( . )


I just received a message from a forum in France about
the Blackheart BH1H with pictures of the amp inside-out.

I could read directly on the transfos:

Power supply: 0 - 250V out

Output transfo: 0 - 50Kh 1W/ Audio
BYC-1625-0107 75Hz - 7Kh
16-8-4ohm - Com

So the value chosen was 50K for the primary on
the output transfo for the 12AX7 triode.

There must be a reason for that choice. Probably
ear tests as PhilS mentionned about his values for
the tubes he mentionned...

For guitar amps we don't need hifi values 20-20Kh.

Right?






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'How to determine Output Transfo versus Tube '
Author:Merlin (registered user: 4735 posts )
Date: Fri, Dec 12th, 2008 @ 17:56 ( . )

On 12/11/2008 @ 21:47, Gitman wrote :

: Hi Merlin,
:
: I read so many different opinions...
:
: : I have also read on a forum in France it was 1/10 of
: the plate resistance on a specific voltage and bias.
That's approximately true for pentodes. Triodes are different.

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'How to determine Output Transfo versus Tube '
Author:klingo (registered user: 281 posts )
Date: Fri, Dec 12th, 2008 @ 18:17 ( . )

12au7 power tube SE & PP links:
[link]
[link]
[link]

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'How to determine Output Transfo versus Tube '
Author:Gitman (registered user: 20 posts )
Date: Fri, Dec 12th, 2008 @ 21:13 ( . )


To say the truth I find a little bit obscure those
graphics since I am only a beginner in electronic.

I can build Gainclones, rebuild tube amps following
a schematic but constructing one by myself with tube specs
is another world.

Let see what I can read from it.

If I look at your link I see for the 12AU7:
Under a plate voltage of 250 V
and a cathode resistor of 1K (for cathode bias?)
Plate resistance = 13K (or impedance?)
The plate current would be 9- 10mA for 350 mW maximum under
12% distorsion for an input (signal?) of 6.6V rms

In that case I would use a 26K output transfo
following Merlin's advice (13K X 2).

Did I get it right?

First time I try to interpret and
understand one of those graphics. If I got this one right
the others are the same pattern except push-pull operation
or different voltages etc...







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'How to determine Output Transfo versus Tube '
Author:Merlin (registered user: 4735 posts )
Date: Sat, Dec 13th, 2008 @ 06:05 ( . )

On 12/12/2008 @ 21:13, Gitman wrote :

:
: : In that case I would use a 26K output transfo
: following Merlin's advice (13K X 2).
:
: Did I get it right?

You seem to have it, yes. For a class A design, 26k would give you maximum power output. If the design were PUSH-PULL, Class A, then you would need twice this: 52k anode-to-anode (A lower value would be used for class AB). Increasing the impedance will reduce % distortion, but also reduce power output.

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'How to determine Output Transfo versus Tube '
Author:klingo (registered user: 281 posts )
Date: Sat, Dec 13th, 2008 @ 06:49 ( . )

I think you're right.
first i assumed the Rp=13K SE value was the plate load impedance. it is not far from twice the ~8K internal resistance value Rp(ra?) you have here (250v/9-10mA) [link]

for PP operation graphs they give you the load impedance (Rpl-pl=30k) so my mind is confused now.
i' ve seen SE headphone amps using a 22/28K load value with each 12AU7 section.

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'How to determine Output Transfo versus Tube '
Author:Merlin (registered user: 4735 posts )
Date: Sat, Dec 13th, 2008 @ 08:28 ( . )

On 12/13/2008 @ 06:49, klingo wrote :
I think you're right.
: first i assumed the Rp=13K SE value was the plate load impedance. it is not far from twice the ~8K internal resistance value Rp(ra?) you have here (250v/9-10mA) [link]
:
: for PP operation graphs they give you the load impedance (Rpl-pl=30k) so my mind is confused now.
: i' ve seen SE headphone amps using a 22/28K load value with each 12AU7 section.
--


I was trusting your numbers to be correct.
Looking at these curves: [link]
at 250V, 10mA, I figure the internal resistance (ra or rp, whatever) to be about 8k. Therefore I would use a 2 x 8 = 16k transformer impedance for one triode. 32k for push-pull, Class A. As you can see, that's very close to the 30k quoted in the other links. For Class AB I would use a lower load.

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'How to determine Output Transfo versus Tube '
Author:Gitman (registered user: 20 posts )
Date: Sat, Dec 13th, 2008 @ 13:31 ( . )

On 12/13/2008 @ 08:28, Merlin wrote :
On 12/13/2008 @ 06:49, klingo wrote :
: I think you're right.
: : first i assumed the Rp=13K SE value was the plate load impedance. it is not far from twice the ~8K internal resistance value Rp(ra?) you have here (250v/9-10mA) [link]
: :
: : for PP operation graphs they give you the load impedance (Rpl-pl=30k) so my mind is confused now.
: : i' ve seen SE headphone amps using a 22/28K load value with each 12AU7 section.
: --
:
:

: I was trusting your numbers to be correct.
: Looking at these curves: [link]
: at 250V, 10mA, I figure the internal resistance (ra or rp, whatever) to be about 8k. Therefore I would use a 2 x 8 = 16k transformer impedance for one triode. 32k for push-pull, Class A. As you can see, that's very close to the 30k quoted in the other links. For Class AB I would use a lower load.
--



Thank you friends: Merlin, PhilS, Klingo...

I think I know how to choose the right OT with the help of
graphics.
I will be able to experiment with the tubes I have on hand.

I see you visit the site: triodeel.com to find those
nice diagrams I can't find in my RCA manual!

[link]

I will also look out for headphone amps schematics.
Very good idea mentionned earlier!

I started researching schematics. I found many using 6922's
type of tube. They end up to a 32 ohms output impedance.

The don't need OT's but naturally use a coupling capacitor
before the headphones to neutralise DC. The values are quite
high and demand high voltage caps. The're expensive!

Now the next step is to find a place that sells the needed
OT's. High impedance primaries with 8-16 ohm secondaries are
rare if we need a very specific value.

Universal units with many options would be perfect to
experiment with different tube types.

As I mentionned earlier. I read people use reverb transformers
for 25K/8ohms or 50K/16ohms values to start with. $15us. :)



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'How to determine Output Transfo versus Tube '
Author:klingo (registered user: 281 posts )
Date: Sat, Dec 13th, 2008 @ 17:32 ( . )

hunt in the old radio territory, you may find a nos 8" alnico speaker with a 5000/2.5 ohms transformer bolted on it for cheap (i paid these 5 eurobucks each+ ship.)
[link]

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'How to determine Output Transfo versus Tube '
Author:klingo (registered user: 281 posts )
Date: Sat, Dec 13th, 2008 @ 17:37 ( . )

....5000/2.5 ohms OT is a good match for 12AU7 with an 8 ohms speaker and you'll have a spare speaker for...a champ.

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'How to determine Output Transfo versus Tube '
Author:Gitman (registered user: 20 posts )
Date: Sat, Dec 13th, 2008 @ 21:18 ( . )

On 12/13/2008 @ 17:37, klingo wrote :
....5000/2.5 ohms OT is a good match for 12AU7 with an 8 ohms speaker and you'll have a spare speaker for...a champ.
--



Good idea to explore!!! Probably on Ebay!

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'Surplus dealers'
Author:Henry Ramsey (registered user: 3070 posts )
Date: Sat, Dec 13th, 2008 @ 19:32 ( . )

On 12/13/2008 @ 13:31, Gitman wrote :
: Now the next step is to find a place that sells the needed
: OT's. High impedance primaries with 8-16 ohm secondaries are
: rare if we need a very specific value.
:
: Universal units with many options would be perfect to
: experiment with different tube types.
:
: As I mentionned earlier. I read people use reverb transformers
: for 25K/8ohms or 50K/16ohms values to start with. $15us. :)
--
If you're in the US there are several surplus dealers online. This one in particular [link] has lots of OTs, one of which might fit the bill.

Here's a direct link to the OTs:
[link]

He has a key for his abbreviations at the top of
the page.

You'll need to email the guy to make sure he still has an item and that can take a few days between playing email tag, but my one and only purchase from him was a success. I got a few PTs and OTs that were more than suitable for low wattage output and one OT for something much bigger. ;-) His prices are not too bad either. A universal OT might be only a few bucks vs $15 for new. He just doesn't update his site often so you have to send him the item# for a stock check.

I hope you find something useful. ;-)

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'Surplus dealers'
Author:Gitman (registered user: 20 posts )
Date: Sat, Dec 13th, 2008 @ 21:26 ( . )

On 12/13/2008 @ 19:32, Henry Ramsey wrote :
On 12/13/2008 @ 13:31, Gitman wrote :
: : Now the next step is to find a place that sells the needed
: : OT's. High impedance primaries with 8-16 ohm secondaries are
: : rare if we need a very specific value.
: :
: : Universal units with many options would be perfect to
: : experiment with different tube types.
: :
: : As I mentionned earlier. I read people use reverb transformers
: : for 25K/8ohms or 50K/16ohms values to start with. $15us. :)
: --
:
If you're in the US there are several surplus dealers online. This one in particular [link] has lots of OTs, one of which might fit the bill.
:
: Here's a direct link to the OTs:
: [link]
:
: He has a key for his abbreviations at the top of
: the page.
:
: You'll need to email the guy to make sure he still has an item and that can take a few days between playing email tag, but my one and only purchase from him was a success. I got a few PTs and OTs that were more than suitable for low wattage output and one OT for something much bigger. ;-) His prices are not too bad either. A universal OT might be only a few bucks vs $15 for new. He just doesn't update his site often so you have to send him the item# for a stock check.
:
: I hope you find something useful. ;-)
--



I just have visited your site.

Wow!!!! Alout of material there and prices are
very good to. I'm in Canada but I mostly buy my
DIY material in USA. Better prices and service
is No.1.

Thank you!

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'Surplus dealers'
Author:Henry Ramsey (registered user: 3070 posts )
Date: Sat, Dec 13th, 2008 @ 22:30 ( . )

On 12/13/2008 @ 21:26, Gitman wrote :
:
:
: I just have visited your site.
:
: Wow!!!! Alout of material there and prices are
: very good to. I'm in Canada but I mostly buy my
: DIY material in USA. Better prices and service
: is No.1.
:
: Thank you!
--
I hope you can find a good one in there somewhere. I think I saw a few listed as PP Univ which are universal to a point (limited by the wattage). He sold me one of those back earlier this year and it currently resides in a PP 6SN7 based poweramp with the AX84 Blues preamp circuit.
The thing came with the original specs sheet so it was New-Old Stock and I think I paid between $5-8 for it. It's teeny, but it doesn't even get warm with ~1W output. ;-)

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'Surplus dealers'
Author:Dave Nix (registered user: 369 posts )
Date: Sat, Dec 13th, 2008 @ 23:49 ( . )

I've bought from this fellow and was very pleased. I got a HUGE enclosed 20 amp Variac for my bench for some silly cheap price... I think $20 plus shipping.

Thing is built like a tank and was new old stock, unused as far as I could tell.

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'Surplus dealers'
Author:Henry Ramsey (registered user: 3070 posts )
Date: Sun, Dec 14th, 2008 @ 02:23 ( . )

On 12/13/2008 @ 23:49, Dave Nix wrote :
I've bought from this fellow and was very pleased. I got a HUGE enclosed 20 amp Variac for my bench for some silly cheap price... I think $20 plus shipping.
:
: Thing is built like a tank and was new old stock, unused as far as I could tell.
--
Yeah, he's a nice guy too. When I told him what I was building (2WPP- 6SN7 poweramp at the time) he found me an OT that was a good fit. He also is quite fast with replying to inquiries about stock.

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'Surplus dealers'
Author:Gitman (registered user: 20 posts )
Date: Sun, Dec 14th, 2008 @ 01:43 ( . )

On 12/13/2008 @ 22:30, Henry Ramsey wrote :
On 12/13/2008 @ 21:26, Gitman wrote :
: :
: :
: : I just have visited your site.
: :
: : Wow!!!! Alout of material there and prices are
: : very good to. I'm in Canada but I mostly buy my
: : DIY material in USA. Better prices and service
: : is No.1.
: :
: : Thank you!
: --
:
I hope you can find a good one in there somewhere. I think I saw a few listed as PP Univ which are universal to a point (limited by the wattage). He sold me one of those back earlier this year and it currently resides in a PP 6SN7 based poweramp with the AX84 Blues preamp circuit.
: The thing came with the original specs sheet so it was New-Old Stock and I think I paid between $5-8 for it. It's teeny, but it doesn't even get warm with ~1W output. ;-)
--




PhilS wrote: I'm going to see about building one self-split PP all octal (6SL7, 6SN7) pretty soon. I keep looking for a bargain OT. If I get frustrated, it will end up a SE 6V6 with 5K OT.

Henry, your project with the 6SN7 is interesting!

As you can see PhilS might build one and I have 2 nice
NOS 6SN7GTB. You used your tube in PP configuration for 1W output.

Exactly what I want to build!

What value on your OT's primary. Can you describe us your
project? Do you like the sound?

You touched my curiosity!!!!!! ;)



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'Surplus dealers'
Author:Henry Ramsey (registered user: 3070 posts )
Date: Sun, Dec 14th, 2008 @ 02:21 ( . )

On 12/14/2008 @ 01:43, Gitman wrote :

: PhilS wrote: I'm going to see about building one self-split PP all octal (6SL7, 6SN7) pretty soon. I keep looking for a bargain OT. If I get frustrated, it will end up a SE 6V6 with 5K OT.
:
: Henry, your project with the 6SN7 is interesting!
:
: As you can see PhilS might build one and I have 2 nice
: NOS 6SN7GTB. You used your tube in PP configuration for 1W output.
:
: Exactly what I want to build!
:
: What value on your OT's primary. Can you describe us your
: project? Do you like the sound?
:
: You touched my curiosity!!!!!! ;)
:
:
:
--
Nothing special or different about what I did at all. It's all part of the AX84 projects that are on the project page. I just mixed and matched the preamp and poweramp.
The PP 6SN7 is here
[link] under the power amps section and the preamp is under the preamps section *Blues preamp*. The preamp tube I'm using vs the 7199 in that preamp is a 6AU8. The 6AW8A I had, had a faulty pentode section and the one I bought from a dealer was too microphonic at the gain needed. The 6AU8 pentode and triode sections have lower gain than the 7199, but there's plenty as far as I'm concerned.

I have to keep the volume control turned down or it gets way too loud too fast. It never gets turn above maybe '2' or so

For the output section I used the variable cathode bias option with a seperate pot for each triode section since they were mismatching their current draw and I was getting crossover distortion, IIRC.


BTW, I think I said it was 1W output with the 6SN7, but it's 2W. My mistake. sorry if I made for confusion.

The OT I bought from that dealer I linked to earlier. It's a Merit brand universal PP or SE type and it's max primary impedence into 8 ohms is 14k I think that's what I have it wired for. The OT is good to 5W I believe. I found the email where I ordered it and that's what the listing said anyway. I've lost the datasheet that came with the OT in my forest of a bedroom, sorry.

How do I like the sound? It's very nice. My low cost (read almost cheap) semi-hollow body guitar with humbuckers is LOUD! I have to keep the master volume all the way down or the walls would *shake*.
This is only 2W max output through a single ceramic 40W Celestion speaker.

Sorry, no sound clips; my playing would send you running covering your ears with horror it's so awful. ;-)

Seriously though, I don't have a good mic to record with really.

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'Surplus dealers'
Author:Gitman (registered user: 20 posts )
Date: Sun, Dec 14th, 2008 @ 14:38 ( . )


Thank you Henry for sharing your experience.

I'm always surprised how loud is a single
little watt.

Happy man playing on a Celestion speaker!

How would you describe your amp's tonality?
Bright ( fender type 6V6), warm ( Vox EL84) very warm
( Marshall KT88) or even darker (Ampeg)

Can you reach a nice crunchy sound before it gets to loud
in a small room?

I have a Vox AC30 head + Vox 2 X12. Very loud. 30W. Brought
it to +- 15W in triode mode. Can't reach nice crunchy sound
in my room. My neighbours are friends... ;)

That is the reason I will build a small amp. My friend
bought a Balckheart BH5H 3-5W. Beautiful sounding amp.
Same tonality as mine. 12ax7 + EL84. My favorite output
tube for guitar amps.

It is still very loud for a small room. He is trading it
for a BH1H 1/2 to 1W. Same kind of tonality. Warm and it
has a nice crunchy sound to. Very close to the BH5H. We
tried it yesterday in a guitar store on a Vox 12"Celestion.
The room was large. In a smaller room it will be louder.

Very nice amp. They call it the killer Ant. Now we know
why!!! Inexpensive. Around $200cad here.

I prefer building mine because I want to experiment with
many tubes, tube rectification, bias, tube voltage etc...
And learn. Aquire new skills. This forum is fantastic for
that. Nice people. Very polite with newbies like me! ;)

Really like the schematics on the site. Thousand of hours of
work and experimentation shared!

Found two versions of your amp. PP + parallel 6SN7. The
// version has lower Ra. Output impedance 6X lower. No
feedback. Purist SE version? :)
The PP must have a quite different sound and opposite
to the // conception views.

Tubes are pushed to 90% of the max dissipation point.
Excellent for nice disto easier reached. It should rock!!!

I will search on the forum for peoples opinions.


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