AX84 BBS

Amp Parts and Kits

Doberman Music Products, LLC offers a variety of parts and kits

Funny(by JeffW)

Sat, 04 Feb 2012 21:47:48 -0500

SEL DONE layout check and pics please(by automap68)

Sat, 04 Feb 2012 20:10:01 -0500

well after its of work my SEL is done I was hoping you guys could give it a quick look to see if there are any obvious issues Im about to start paul rubes start procedure here are some pics I'm using a kt88 and a heyboer O/T [link] [link] [link] [link] [link] Also here is a link to how i did my lead rhythm switch [link] I hope it is correct My switch is also done with shield wire so it kind of makes it messy. Its not as clean as i wanted it to come out but for my first build its not to to bad Thanks

Swapmeet at Houston-Greenspoint Marriot on Friday and Saturday ((by Henry Ramsey)

Sat, 04 Feb 2012 19:35:39 -0500

In case anyone else from the Houston area is interested there's going to be a swapmeet held by the Houston Vintage Radio Association next Friday 10a-4pm and Saturday 7a-11am at the Houston Marriot-North This is open to the public. Here's the flyer from their website with full details: [link] Hopefully, I'll be able to get there if the weather isn't only fit for a duck like it has been the last two days. ;-)

Octal Fatness D*mble- Wow...(by Doug H)

Sat, 04 Feb 2012 19:16:28 -0500

Years ago, David Jones posted that he modded his Octal Fatness 2nd and 3rd stage into a D*mble ODS overdrive circuit. You tweak the 2nd stage, add the interstage divider, and convert the buffer into a gain stage. I was planning on cannibalizing this amp into a SE Wreck. So iI thought while I was waiting on parts I'd give this a try. Wow, just Wow...is all I can say. This is the most nearly perfect DIY tweak I've tried yet... This little magic overdrive circuit does the trick- effortless juicy harmonics under complete control of your fingers. All it needs is a little less gain and more headroom. I will tweak the interstage divider for that. It's a drive control trimmer on the ODS- maybe I'll just use a trimmer too. But this thing is so smooth and cool sounding. I thought it would be harsh with the g12h30 but no worries, even with the PRS which is bright. Using the bright switch I can turn down the guitar for great semi clean with the preamp gain not too high. I may have to get more parts to do this other amp now...

References in RDH4(by Henry Ramsey)

Sat, 04 Feb 2012 18:26:05 -0500

Anyone know what magazine or article the reference in RDH4 chapter 15 "Cutler,S. "Audio frequency compensating circuits" Elec. 15.9 (Sept 1942) 63" actually refers to? I assume that "Elec. 15.9" is a magazine edition and 63 is a page number, but the abreviation doesn't tell me enough. Which magazine? Electronics or electrical seems obvious enough, but it's too generic to search from and gives me thousands of irrelevent hits in google. My local library may have old magazines in their closed stacks if I know for sure which one to look for. I have to be specific though or they're not going to help. Due to funding cuts the librarians are all unwilling to be helpful like they used to be. If you don't know neither do they. :-(

Turning a Heathkit EA-3 amp into a guitar amp(by Wick Man)

Sat, 04 Feb 2012 10:41:14 -0500

Hi- I'm fairly new to this site but wanted to see if anyone would be interested in making a decent guitar amp from an old but working Heathkit EA-3 amp? I believe it is 14 watts RMS and I'm willing to pay whatever. Please provide a costs estimate of time and materials if you would consider taking on this project as well as your background with amplification. Thank you very much.

Power Transformer Question(by Gregor)

Fri, 03 Feb 2012 15:38:55 -0500

***disclaimer: sorry about the xtra long post*** I'm working on plans to build a homebrew from scratch. Today I've been working on coming up with the requirements for my power transformer and I wondered if anyone would want to take a look at my figures and tell me what you think. I looked at this website for basic info [link] and then extrapolated what my circuit would need. Basically I want to build this circuit: [link] with the addition of reverb, a selectable bypass capacitor switch ala M*tchless, and a VVR. So that's how I came up with a count of 3 x 12AX7's; 1 for the PI, and 2 for the reverb circuit. Here are my figures: Rectifier 1 x GZ34 Vh 5 Ih 1.9 A Irect 160 mA 4 x EL84�s My circuit calls for 320 v on plates and screens Plates Va 300 Max Ia 92 92 mA / 300 v = 0.3067 0.3067 x 320 v = 98.13 mA 98.13 mA x 4 = 392.5 mA Screens Va 300 Max Ia 22 22 mA / 300 v = 0.0733 0.0733 x 320 v = 23.47 mA 23.47 mA x 4 = 93.87 mA 3 x 12AX7�s Va 250 Max Ia 1.2 My circuit calls for 280 v 1.2 mA / 250 v = 4.8 x 10^-3 4.8 x 10^-3 x 280 v = 1.344 mA 1.344 mA x 6 = 8.064 mA 1 x EF86 Va 250 Max Ia 3.0 My circuit calls for 280 v 3 mA / 250 v = 0.012 0.012 x 280 v = 3.36 mA Total: 160 mA + 392.5 mA + 93.87 mA + 8.064 mA + 3.36 mA = 657.8 mA Filaments 4 x EL84�s Ih 0.76 0.76 mA x 4 = 3.04 mA 3 x 12AX7�s Ih 0.3 0.3 mA x 6 = 1.8 mA 1 x EF86 Ih 0.2 mA Total: 3.04 mA + 1.8 mA + 0.2 mA = 5.04 mA Thanks!

FET preamp examples(by tedmich)

Fri, 03 Feb 2012 11:43:29 -0500

I thought I'd repost something JM Fahey put over at Ampage, pretty cool sounds from a unique circuit [QUOTE=J M Fahey;247336]Well, there�s this KMG guy from St Petersburg who has made definitely his homework, and developed a series of *IMPRESSIVE* preamps and Power amps. For one, he feeds his fets 200V DC, not puny 9V. Not that complex, he replaces each triode with a single Mosfet, to which he adds a simple buffer to avoid external loads altering the working point he so carefully chose, and another bipolar stabilizing the bias point (the real curse of Fet stages). As an example: [link] Fet version of the SloRectoTwin Be certain to hear the sound samples: [link] [link] Enjoy.[/QUOTE]

MIDI Footswitch(by Chris Hurley)

Sat, 04 Feb 2012 21:39:55 -0500

So does anyone have any need or interest in a MIDI pedal project? I've now built two MIDI pedals and would be happy to share what I've got and help others get kicked off. I'm not sure that there is enough interest to sell parts but I would consider doing so if it helped anyone out. Making a MIDI controller pedal is really very easy since the heavy lifting is done by microcontroller. Adding MIDI features to an amp really depends on how far you want to go. i think it would be very easy to add LDR switching to an amp via MIDI control. If you could work out how to use LDRs in other places, you could control those as well. I haven't done the amp side of it yet, just the pedal. Happy to share if anyone is interested.

Daisycutter'ish SE AMP(by Asdain)

Sat, 04 Feb 2012 09:02:16 -0500

Hey fellow geeks, So i have been bulding around an SE design for quite a while, went over P1Ex to SEL to Princeton'ish to trainwrecki'sh and back to a tweaked SEL (curren state) so a few days ago i started messing around in CAD and tried some alternative wiring for the DPDT switch i have in the AMP and wired it to skip the second stage, made it cooler, increased Anode loads and added and changed thinks here and there and... WAIT A MOMENT,... i have seen this somewhere,.. so i opened AX84 and checked out the daisycutter. YES! THis is where i have seen it. my preamp section got pretty close to the Daisycutter. so i made a few more changes (made it even closer) and am pretty confident that this will be the next step in messing around in my poor abused little amp. So here is the current(ly purely theoretic) schematic: [link] i would love to get some input on this from you guys here. (especially Randy if he is around;) anyone see anything missing or something i could tweak ? Cheers, Asdain

mini pc's (by Kursad K)

Sat, 04 Feb 2012 08:25:30 -0500

[link] These could make viable signal processors for home built amps, but unfortunately they come without decent audio interfaces. Nevertheless, perhaps with asio4all ( [link] ) they may still work for some purposes (inside an effect loop for example), but not for directly capturing guitar signals, obviously. Kursad

20W SE Rk w/o KT88(by DutchMarco)

Fri, 03 Feb 2012 17:09:33 -0500

I'm not planning to use a KT88 in my build, only EL34 / KT77 / 66 / 6L6 /6V6 (I actually already have those, not the KT88). So what values for Rk (r11) could I best start with? Without having to rebias the KT88 output section, by ear, or by DMM, using VR1) And how do I rebias it using a DMM? Without blowing out the glass panes by playing it at full volume (I want to use a VVR on the whole amp, pre- and power-section)

Amp with best clean headroom from AX84(by Nicolas1400)

Thu, 02 Feb 2012 22:56:36 -0500

Simply that... what's the amp from ax84 with best headroom to play clean? I mean, playing true clean sounds at high volumes without crunch. I tend to thing the SEL is, because of its 20w output, but tell me if I'm wrong. Thanks in advance. Nicol�s.

Poweramp Question(by FrooshFan81)

Thu, 02 Feb 2012 23:41:26 -0500

After much thought, and hearing the new 1-watt Marshall amps, I am gonna go with the low-power option in the power section of my High Octane build. Rather than using a 6N1P, I want to use a 12AU7. Based on what I am seeing on the data sheets, it looks like a direct swap won't be a problem. Am I right? Thanks!

Ordering extras...(by FrooshFan81)

Fri, 03 Feb 2012 21:19:58 -0500

Just wondering if anyone can tell me how much wire I need to order so that I can build one of these amps? I don't want to order wire and then have to reorder because I didn't get enough to start with and I don't want to order a huge amount either. Thanks for your help.

LED direction/polarity?(by DutchMarco)

Fri, 03 Feb 2012 12:29:37 -0500

At very long last, I'm planning to use some of the LEDs someone here sent years ago. But before I do: I couldnt find directionality markers. Do I need to koojmk closer?

Newbie question (sorry in advance)(by dam28)

Fri, 03 Feb 2012 20:01:40 -0500

Hi all, Built a little 5W tube amp recently and now got the bug for it! Thinking of building the AX84 clean preamp for a new bass amp I want to make as I have been reading it's good for that. Just got a question about the voltages on the schematic. Values like "B+3". What do these mean / is there a key to these values? Cheers! D

Output jack question(by automap68)

Fri, 03 Feb 2012 10:38:00 -0500

On the SEL schematic there some to be a difference on the output jack The schematic does not show a use of 16ohm jack the layout does show a 16 ohm jack The layout shows that R5 is connected to the 16 ohm jack and there is no resistor on the 8 ohm jack. Then on the schematic it shows that R5 is connected to the ohm jack and there is no 16 ohm jack at all. Can someone shed some light on this? Where exactly does R5 go? Thanks

It's alive!(by Bigmac)

Sat, 04 Feb 2012 19:35:23 -0500

Just fired up the amp today and it's in full working order and sounding great! Just wanted to say thanks to everyone who answered my questions over the past few weeks, it was all greatly appreciated. Cheers! Dave.

can someone simplify?(by steveoeding)

Thu, 02 Feb 2012 19:37:48 -0500

i have been researching a preamp build for a while now, the information here is fantastic, my question is...if i want to build a standalone rackmountable preamp 1U, where and how do i wire output jacks so that i can connect to my power amp via cables? and what do i do with the B+ and B-, since it's not going to have a power section? i know this is probably a dumb question, but everything on the schematic makes sense to me except this. i was going to attempt the "lead_preamp". i think i have enough soldering experience to do it. any help would be greatly appreciated

SEL features questions.(by Nicolas1400)

Thu, 02 Feb 2012 13:07:12 -0500

Hello everybody. First time here. I'm Nicolas from Buenos Aires, Argentina. Meet ax84 a month a go and I love the projects and people here. Im about to start building myself a SEL amp. I heard a lot of demos and videos of all the ax84 amps, and I decided the SEL is my choice. I have a few questions for you all: 1.Is there any commercial well-known amp that best matches the design and sound of the SEL. I mean: 2 pres stages, 1 KT88. 20W Output power. Is there any amp out there with this features that can be immediately compared to the SEL? 2. I love the high gain sound of this amp, but.... Can you also get a 100% clean sound of the SEL using its full output power? All demos I heard were a little bit crunchy... 3. About bias: Im planning to use the original KT88 output tube. Do I need to care about biasing? Can I just build the amp exactly as it is in the schematics, put the KT88 tube and expect to sound properly? Thanks in advance. Hope to hear from you.

More Gain!(by Christopher R Wise)

Sat, 04 Feb 2012 14:58:49 -0500

Whats the easiest way to get more gain on this amp. im building mine here shortly. I played a friends and the only thing i wished to change was to get a little more gain for some heavier rock. what are yalls suggestions?

Oscilloscope help(by Sunshine)

Thu, 02 Feb 2012 12:17:53 -0500

Could someone please look on eBay and post some links to a good but cheep scope. Thanks!

10x difference between PI outputs - normal?(by Henry Ramsey)

Sat, 04 Feb 2012 01:05:30 -0500

My latest project the EH185(version one w/ inductor in the tone stack) I've been gather help about for the last several weeks here has a strange issue: the PI outputs have a voltage output difference of about 10x comparared to each other. I changed the plate, cathode and paraphase circuit to match the 6SN7 tube. I put in 47k plate resistors, a 1.8k cathode resistors and 470k-6.8k-470k in the paraphase circuit. This brought the plate voltages to over 190V from the meger 123 and 125V I had before, with 5.8V at the shared cathode. The cathode voltage now is 3.91V. I'm really just trying to calibrate my eyes to the scope. I have limited time using one so I need to learn. I got a new 1x/10x probe for my scope. It's the same model as the first one. I hooked both up to the scope inputs and clipped each to the PI outputs. I made a 1khz test sine-wave oscillator and I hooked up a dummy load so I wouldn't hurt my hearing. I connected the test signal to the instrument input. Volume is set for about 1/2 or so. I set the oscillator volume to put out around 300mVAC. A tad more than my guitar puts out on a hard low-E note. Time/cm is 0.1ms One channel set to the 5v/div w/ the probe set to10x. Channel 1 signal shows a fairly clipped signal and rounds off as I back off some volume. Pretty normal I guess. Channel 2 needs me to be on the 0.5mV setting in order to see a signal of any use with that probe also set for 10x. I could also set the probe for 1x on the 5V setting. I swapped probes between channels and the issue follows the probe so it's not the scope. SO is this normal or something with a floating paraphase inverter? Do I have something wired wrong? I am going to check with another 6SN7 and see if perhaps I have a tube with one section amplifying more than the other. The almost exactly 10x difference seems not to support that but I'll try. I need to take some video of the scope outputs to illustrate this further. Now... where's my mini-tripod.

lower power supply voltage(by elbrujo)

Thu, 02 Feb 2012 01:30:25 -0500

Hello: my question out there for all you Gurus. What happens if i use a power supply which is providing me with a lower dc voltage than the one needed as per the P1 Schematic. I am getting about 206 volts DC instead of the required 266 as per the plan. This would obviously change all voltages through the circuit to lower voltages. I would think though that the amp would probably still work. Let me know your thoughts on that. P.S. I currently have a non-center tap transformer that i would like to use for this project.