AX84 BBS

Amp Parts and Kits

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

Half a push-pull OT for single ended amp?(by erer44)

Wed, 16 May 2012 13:34:54 -0400

Hi, I have push-pull output transformers out the wazoo from old organs and test equipment. Would one work on a P1x or Champ if I only wire up half of the primary? Thanks!

Clean preamp sparkle/grit stage(by bagghist)

Wed, 16 May 2012 09:04:34 -0400

This is about tube V6A (the amplifying half of the 12AU7, right before the CF). As I'm contemplating building this, the cathode resistor's value (90K) seems way too high. Is this because it's in parallel with R104 and VR104 (sparkle/grit)? If so, then I guess the cathode sees something more like 299-5005R, depending on the sparkle/grit pot? Also, any guesstimates about what the cathode voltage should be, e.g., with a B4 of 300? Cheers, James

Blackface Clone Without Vibrato(by bpursley)

Wed, 16 May 2012 09:07:01 -0400

Hi, I'd like to get a wiring diagram of a blackface style amp with reverb, one channel only, without vibrato. Any ideas of how to best go about getting this? Thank you

Interesting noise issue(by bancika)

Wed, 16 May 2012 04:48:45 -0400

I've never considered my mark IIc+ build to be very noisy, considering the amounts of gain but an interesting thing happened when I tried to slightly push it with an overdrive pedal. The noise level actually goes down when the pedal is on. I'd guess it has to do with the lower impedance of the OD compared to the guitar, but is there anything I can do to make it that quiet without the pedal? tnx

cathode cap switch(by ColinD)

Wed, 16 May 2012 06:56:44 -0400

OK I have a cathode biased power section, and I'm using a SPDT to switch between two different values for the shared power tube cathode cap. I'm switching the tube side of the capacitors right now. Will I see different results by switching the ground side of these caps? I'm wondering because the very first time I flip the switch the tone changes, but after that the switch seems to have no effect.

6JT8 or 6LY8 in Valve Jr(by Max Blast)

Wed, 16 May 2012 13:34:26 -0400

I have some 6JT8 and 6LY8 tubes laying about that I intended to use for a low powered PP biuld someday. Now someone has given me a Valve Jr. v2 and of course I want it to be lower volume but more distorted, at least to AC/DC crunch level, than just the normal tweaks may provide. I have this idea and am looking for some input as to whether or not to even pursue it. What I have in mind is to substitute the 6JT8 in place of the el84 and take advantage of the extra triode to supply more drive to the pentode section. Is it possible for me to use the signal path currently going directly to the el84 from the existing 12ax7 and run it thru the triode of the 6JT8 and then drive the pentode for about 2 watts max single ended? I am not afraid of buying new OT if neccessary. I currently have the 16ohm cab that came with the VJ as well as other 12" 8ohm spkrs available if needed. I know just about enough to be dangerous here and feel no pain when others point that out to me so feel free to criticize my inexperience. thanks, Max Blast

Consequences of unbalanced grid leak resistors for LTPI(by Greg Robinson)

Tue, 15 May 2012 09:41:27 -0400

Hi, I'm just wondering if anyone knows how to calculate the input impedance of an AC-coupled cathode biased long tail pair phase inverter that has different grid leak values on the two inputs. Would it simplify to something approximating Rg1||Rg2? Thanks.

Ohm selector switch(by RichT)

Mon, 14 May 2012 10:59:39 -0400

Anyone have a suggestion for an Ohm selector switch other than the marshall one for 20 bucks? I am designing for a circuit board, I was looking at [link] Is this one feasible for a 100 watt tube amp?

RIP(by steve25)

Tue, 15 May 2012 04:10:22 -0400

I just heard of Jim Marshalls passing.

WTB: Hammond 270DX(by DaveF)

Mon, 14 May 2012 03:39:10 -0400

Wanted - used/NOS Hammond 270DX power transformer (or similar). I need about 500 to 550 volts CT @ at least 90mA along with 5V and 6.3V taps. Please email me at foda01@epix.net Thanks, Dave

October Stage/Club PT selection(by Dan McCormack)

Mon, 14 May 2012 02:58:53 -0400

Hi guys. I'm Dan, and I've been idle for a few years. Back in 2010, I started designing a project, a modified October Stage with added tube-driven spring reverb based on the California Dreamer. I got as far as ordering all the parts, including $250 worth of custom-wound Heyboer iron as specified in the October�Stage docs, and a custom made enclosure from a forum member (Brian S), big enough to house my relatively large design. Despite my plans to support the full 50w of output power, the plan was always to run 6V6's most of the time, for about 20w of output power. At the time, I'd only ever run my first build, a High Octane with EL34, through a 1x12 cab, and it wasn't nearly loud enough to compete with a drumset. Since that was about 8-10w, I figured the 20 or so I'd get from my October with 6V6's would usually be loud enough but might still have trouble in some settings, hence the desire for the option to run the KT77s. In the intervening two years, I've run my EL34 HO through a 4x12 cab and realized how much louder it gets with that configuration. That setup alone is almost but not quite loud enough for my needs -- I think 20w will be plenty. So now that I've returned to my project, I'm questioning my design choices. I know I can still run 6V6's with the bigger transformers, but I'm realizing there are costs. Aside from the very significant weight of all this iron, I realized that this big 300-0-300, 250mA PT is going to have pretty much zero sag with the 6V6's, and while I haven't actually heard what sag sounds like, it sounds like something I'd like my amp to be able to do, and I made sure to stick with a tube rectifier in my design for that specific reason. So I'm wondering if I should just order the transformers specified for the October Club (or at least the PT) and build an amp that can only handle up to 6V6-range tubes, but do it better and lighter. Or should I just build it with this fancy, expensive iron even if I will probably only rarely plug KT77's into it? Thanks for any input you guys can provide. Dan

WTB: Hammond 370DX or similar(by DaveF)

Mon, 14 May 2012 03:38:00 -0400

Wanted - used/NOS Hammond 370DX power transformer (or similar). I need about 500 to 550 volts CT @ at least 90mA along with 5V and 6.3V taps. Please email me at foda01@epix.net Thanks, Dave

Paul's Amp Raffle in Support of the Leukemia and Lymphoma Societ(by Paul Fawcett)

Mon, 14 May 2012 11:08:19 -0400

Hi All, As the old thread [link] is getting rather crowded, I thought I would start a new thread to document the actual construction phase of the raffle amplifier. For those of you that may have missed it: On 3 June, I will be running the San Diego Marathon as a Team-in-Training (TnT) fundraiser for the Leukemia and Lymphoma Society. The LLS is a wonderful charity that funds research and patient support in the battle against blood cancer. My fundraiser page is here: [link] I have been a participant and fundraiser with TnT since 2007 -- so this year I thought I would do something a little bit different to help out my fundraising efforts. In short, I am designing and hand-building a unique guitar amplifier that will be raffled off following the race and sent to the lucky winner. Every $25 donated on my fundraising page [link] counts as one ticket in the raffle. The amplifier itself, I'm hoping, will be something really special. First of all, AX84 stalwart Ben Sykora has very generously agreed to donate a beautiful Cherry wood head cabinet that he is hand-constructing. Take a look at Ben's work here: [link] Also, our own Brian Schmittling, owner of Dirty Dawg amp chassis [link] , has donated one of awesome aluminum enclosures. For those of you that don't know, Brian is the go-to guy around here if you are in need of a high quality amp chassis at a very fair price. Finally, the amp will have a custom faceplate donated by MasonAtom of TMI guitar amps [link] On my side, the amp features: 2x6V6 power section 3x12AX7 preamp Three-band tone stack (James style B&T, Framus style mid) Channel switching - clean and Dumble-style OD Tremolo Built-in reverb (LM386 driver, LND 150 initial recovery) Footswtiching for trem, verb and OD DC heaters for preamp tubes But enough chat. Building has begun. Please visit my blog to see the current schematic and layout. I'm going to try and document this build so that you can follow along as it comes together during the next few weeks. I've just posted some pictures of the construction of the mainboard: [link] Thank you so much for taking the time to read through all of this, and I hope you'll consider making your tax-deductible contribution to the LLS and getting in on the raffle! As you can see from my donations page, we've already collectively raised over $2200 for cancer research. But my goal is to reach $3000 this fundraising season. So I would really appreciate it if you would share this link on your facebook page, or where ever really! Help me make this go viral, and let's kick cancer's butt!

Repairing a damaged head - advice?(by Harris Snyder)

Tue, 15 May 2012 21:07:45 -0400

Hello, I recently acquired a laney GH100TI, used. I noticed that the output tubes were really hot all the time. Now obviously I should have checked the bias, but, well, I didn't. The second day of playing it, I turned it up a bit and it blew up :P Well okay it didn't blow up, but something did, cause the tone is really quiet and thin sounding now. Also, it makes weird crackling/whirring sounds, even when no guitar is plugged in. I checked the bias on the tubes, and sure enough, those EL34s were biased practically at max dissipation. Dunno whats up with that, but anyways... Obviously I'd like to fix it, and I have experience working on tube amps (I built a marshall 2555 silver jubilee clone), so I know about the safety and whatnot. I don't really know where to start with this issue, however.. Could anyone with more experience speculate? does this sound like an OT issue? It doesn't seem to be the tubes, cause I put another set of EL34s in and the result was the same (though the bias was down to about 18 watts). Thanks guys

Duck Dunn, RIP(by Paul Fawcett)

Wed, 16 May 2012 02:56:06 -0400

You'll be sorely missed: [link]

Ideas for a 3-5w push-pull distortion (near metal) build(by Turd Ferguson)

Wed, 16 May 2012 13:07:21 -0400

Been in a custody battle for the past two years, where the only clear winner is the lawyers. It's nearly wrapped up and I have focus, time, and parts to do a build. Want to build an amp with 2 or 3 preamp stages and push pull output stage around 3 to 5 watts. I have all the parts needed except PT and output tubes... seeking recommendations.

2 or 4 el84 power amp build(by Nomad100)

Tue, 15 May 2012 10:33:23 -0400

I've been wanting to build a power amp for a long time with a toroidal power transformer, and an options for switching to SE mode. I've not found any kits for this kind of build, and the 20W PP core power amp uses 6v6, where should i start? What changed need to be made from 6v6's to el84's?

FX loop: need assistance(by JHoff)

Mon, 14 May 2012 21:15:16 -0400

I could use some input about using Merlin's effects loop. I have a 20W PP amp with the simple preamp, and they work fine together. I built the effects loop schematic by Merlin and now I get no signal. Here's the schematic: [link] I subbed in a couple of different parts because that's what I had laying around: There's a 1.1 Meg resistor in place of the 1 Meg; a 39k in place of a 33k, and a 25k pot in place of the 22k pot. I don't think there's a big enough difference in values to be important--am I wrong there? I have a hunch about what's wrong, but I'm not sure. With my super sleuthing skills, cheap DVM and $30 eBay oscilloscope, I noticed a couple things. I checked the values of the components. 2 of the resistors were off: Both the 1.1M and the 470k were measuring less than 100k. I don't think I misread the color bands. Maybe I've got 2 bad resistors, but it just seems unlikely. Then I noticed that I could trace the signal fine on the clean preamp up until C100. It just stops on the far end of the cap. My guess is that I've simply put the loop in the wrong spot. The wiring looks good to me. Is my placement of the loop filtering out the signal? Also, on Merlin's website, it looks like he designed the loop with a B+ of about 200 volts. B+4 on the 20 Watt Core Amp is around 340V, IIRC. Do my component values need to be adjusted accordingly? Thanks for taking a look and for any suggestions! Joel

e85: the worst idea ever(by profes)

Wed, 16 May 2012 13:04:52 -0400

[link] it is 24% less efficient (for highway) and only 12% cheaper 3.35 vs 3.80

October Voicing(by Bill Durham)

Tue, 15 May 2012 22:32:17 -0400

I have been rehearsing with some guys in the ATL area for a gig we are playing July 7. Its the 2nd time I have used my October in a band situation, the first time being a couple of years ago. We have been rehearsing once a month for the last couple, and one thing that I have finally figgered out is the amp is voiced for a 4x12 cabinet and really doesn't have the full balls and low end fullness that it would generate using a single 12, open back combo cabinet. doh!! In the context of the rehearsal room, it sounds fine and cuts through when I solo with the help of a TS9, but the overall tone is really bright. I have been using a fleece blanket wrapping around the cab to knock down some of the volume and brightness for the small room, but still.. its really bright. Don't get me wrong, not complaining at all.. just an observation that if I really want to get the balls of a Marshall at the lower level.. I gotta use the 4x12 when I play the gig.

My New Fuzz Box(by FredB)

Fri, 11 May 2012 13:27:33 -0400

Here's a couple of sound clips from my latest creation. The first clip is miked, the second is recorded direct from the effect into the digital recorder. This is the only effect I have ever seen that records better direct. [link] [link] A video will be forthcoming when I get the thing in an enclosure.

Radio as Guitar Amp(by Vimau)

Sat, 12 May 2012 08:28:07 -0400

Hello ax84, I've been asked to try my hand at turning a working Tube radio into a guitar amplifier and I'm hoping you're willing to offer some advice. safety first: I'm reasonably new to tubes but not so green that I'm about to go poking fingers _anywhere_ without a lot of thought, a belief that I have some idea about the properties of the pokeable thing, and a little prodding with a multimeter. I've not yet seen the radio, but I do have a schematic ( [link] ) and this is what I make of it: Jacking a guitar into the v4 would be nice and easy, don't even need to BYO coupling capacitor, but it's not likely a guitar will produce nearly enough voltage to drive it. V3 looks more promising, though I'm unfamiliar with the tube configuration. I'd be tempted to try injecting a signal at pin 5, or the plate of v2 (removed). I'm not sure what L6 and L7 achieve though. Am I on the right track? Vim.

Bent string won't reach correct pitch(by Henry Ramsey)

Thu, 10 May 2012 15:31:39 -0400

Anyone know why a bent string would have trouble reaching the correct pitch? I'm learning Johnny B Goode (I can't play it for $hit!) but I'm trying... In the opening riff of JBG the 3rd string is bent from 9 to 10 and plucked whilst alternatly plucking the 2nd string at the 6th fret. The two notes should be the same, but the bent string is flat even when I bend it much farther than it should go. I have a simliar issue with the solo of She's A Woman. Two strings get plucked at the same time then one is bent so both are the same pitch. The bent string is sounding flat. In this case it's a whole step and the note sound like 1/2 step. I just changed to a slightly bigger gauge string from the ultra lights I've always used since these are nickel flatwounds. I think they 12s or 13s with a wound 3rd string. Is this perhaps the problem. I do know how to set the intonation, but my changes don't seem to be affecting this problem. The neck *seems* straight. I may be checking it wrong. I put a straight-edge on the middle and there's a gap that's not more than the thickness of a sheet of paper along most of the fret board. This is a 7 yr old SHB Oscar-Schmidt(by Washburn) Delta King which was modified for lefty use. The guy I took it to actually swapped the direction of the bridge by removing it and doweling the holes then drilling new ones. It's worked well until I changed strings actually. I just really like the nickel flat wound strings. I used to use Pyramids on my Ric. Any ideas? Do I just need to go back to the lighter strings?

Newbie Firefly Build(by Craig Kenfield)

Wed, 09 May 2012 23:08:27 -0400

Hi, I'm a newb here. I am currently working on a 5F1 and while researching that I came across info on the firefly. While ordering parts for the Champ clone I added caps, resistors, and a few misc things for a firefly too. I'll be building it on turret board but haven't gotten that far yet, I'm still collecting parts. This is the layout I'm working from (I'm pretty sure I got it off this forum): [link] I already have a couple questions... Would it be stupid to use a Champ-style on/off/volume switch on the firefly, with the on/off controlling the standby? I've also thought about simply eliminating the standby switch and using it for master power just like a Champ. I haven't got a standby switch on either my Champion 600 or my VT15 so I'm not sure why I really need one for the use this is expected to see. On the layout, the bottom run connects the grounds of various components. I'm new to this, but shouldn't that be grounded to the chassis too? I'm going off the layout for the Champ which has a couple leads leading to ground from that part of the amp. I do have a couple books on tube amps but a recent storm has knocked us into a state of disarray... I'm not really sure where the books ended up. They're probably with the first speaker I ordered for the Champ. Can't find that either. I ended up ordering a new one... which led to ordering the firefly parts... which led to me joining here... Thanks in advance for the help I'm sure I'll be needing on this thing. Hopefully my questions won't be TOO basic for people! ;-)

Help me troubleshoot my noise symptoms(by RickTown)

Tue, 15 May 2012 14:05:44 -0400

Hey all, I'm new here and just finished my first build. It's a HO and it sounds awesome!! I've done a few of the suggested fixes here to eliminate some problems but this one has got me scratching my head. First I noticed interference through the guitar cable so I put a 22k Ohm on the input and fixed that. Next I had heater hum so I tried an artificial center tap then went to a humdinger with an extra 100R to the power cathode and was really able to tune it out. Now I'm going after this buzz that seems to be affected by external things that I wouldn't expect would help. With the input grounded, it goes away. with a dummy jack plugged in it appears with a guitar cable it's worse until i hang the other end of the guitar cable right near the chassis and then it quiets signifigantly. Plugging in a guitar doesn't seem to change it until i get too close then i get a bad hum. when I touch the chassis it gets really quiet On the scope I see 120Hz on the OT primary but not on the secondary. I've tried playing around with adding to the reservior cap to adding another filter cap but it seems that this buzz is still the same. I'm not sure how to best employ a snubber or even if it will help. It almost seems like I've got a ground loop on the chassis allowing interference in but the only places that have ground is the power cord and and the input jack where the buss bar is also grounded. Both transformors cases are grounded to the chassis also. Any help would be greatly appreciated. Rick