AX84 BBS

Amp Parts and Kits

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

Interesting noise issue(by bancika)

Wed, 16 May 2012 15:05:56 -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

Ocilliscope Safety Video(by FredB)

Wed, 16 May 2012 14:55:58 -0400

More then once over the years I have read posts of people using scope probes like a battery operated multimeter, where they connect the ground clip of the scope probe to a point in an amp that is above ground potential. Here is a good video that explains in detail why that should be avoided in most cases and also how that can be done safely in certain cases. [link]

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

Wed, 16 May 2012 14:29:01 -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

Clean preamp sparkle/grit stage(by bagghist)

Wed, 16 May 2012 13:56:20 -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

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!

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.

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

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

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.

Duck Dunn, RIP(by Paul Fawcett)

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

You'll be sorely missed: [link]

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.

Commanchero tone stack replacement(by sambo69)

Tue, 15 May 2012 21:59:48 -0400

I'm experimenting with my Commanchero. It's completely stock and I want to change the way it sounds. I read Merlin's preamp book( about a million times) and found the james tone stack interesting. I've redrawn part of the schematic and would like you ( anyone) to sanity check it. [link] All comments are welcome ! Regards Sam

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

First post...Bunch of questions(by principia)

Tue, 15 May 2012 19:40:17 -0400

I recently picked up an old tube amp for 20 bucks at the flea market...It did not have a power transformer so I hooked up an triad n68x iso tranny to it and switched to 3 prong. It does not seem to need to be rectified as it has a 50L6 which I believe should be a full rectifier? To be safe I tried rectifying with a 8A/400V Full-Wave Bridge Rectifier and got no output, then tried it without it and it worked (the transformer did not get hot). The amp has 3 tubes a 50l6gt, 50y6gt, and a 12sl7. It has an input and output that are 1/4". It also has 3 pots one that's obviously volume, another that seems to do nothing, and a third thats not on the face but between the 50l6 and the 12sl7 on the top that seems to kill the volume at high and low settings, allows sound in the center, and makes a ton of noise if adjusted when the amp is on. I can post a picture of the layout and pics of the amp tomorrow. Now for the questions Any idea what this amp was originally for?...What size fuse should be added before the iso? I tried 1/2 amp and it blows immediately, just work my way up?... Also anyone have any idea what these pots are actually controlling? Also I cant seem to overdrive these tubes, the sound is pretty clear at any level, any ideas how to remedy this other than with a boost pedal? Finally is there any good schematic to add some tone pots to the amp? I would like to convert it to a more legitimate guitar amp as the clean sound pretty good and it seems like a fun project. Just pretty new to working on and building amps.

reverb tank modeling with LTSPICE(by dludwig)

Tue, 15 May 2012 15:40:59 -0400

Hi all. I've added a one tube reverb circuit to my SEL build using a 12AX7 triode and it sounds ok, but a little bit lacking. I'm thinking redoing it using an ECL82. So, to hopefully cut down on all of the soldering/desoldering and guesswork, I decided to model the amp and reverb circuit with LTSPICE. So far so good, but I'm down to modeling the tank. I have a rough transformer model for the tank as you can see here: [link] (you'll have to click a few times to get to the SELreverb jpg file), but would like to have something more refined. I'm not so much concerned with modeling the reverb as much as the overall attenuation through the circuit. Anyone have a good tank model or know where I can get more info? Thanks for any help. Dave

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

Belton digital reverb in to a new amp(by AleksiS)

Tue, 15 May 2012 13:40:17 -0400

Hi guys, quick question: I'm planning Marshall-ish amp and though about trying the belton Digital Reverb module. Consider the preamp as a JCM800 with a LTPI. I would take reverb input signal from the cathode follower (before the tonestack) and the feed the output from reverb module to the other grid of the LTPI? Any thoughts on why this wouldn't work? Im presuming the difference between the dry signal (which goes through the TS) and the reverbed signal will not cause problems? I'm also guessing I can make bias the LTPI halves differently so that the smaller signal from the reverb would amplified more? BR and thanks in advance, Aleksi

strat sound(by Kursad K)

Tue, 15 May 2012 12:16:12 -0400

soo... my fender highway-1 strat sounds a bit boring and somewhat dark compared to my ibanez made 335 clone. I wonder what may be the cause. - too heavy strings? (probably they are 11 now but I dont recall since I dotn play this guitar very often) - blocked tremelo arm (I've set it up one way, it touches the guitars body on the other side) - overwound pickups (kinman woodstocks) - or.. maybe it's just that that's the way they are? What do you think? Thanks, Kursad

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?

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.

RIP(by steve25)

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

I just heard of Jim Marshalls passing.

Lead Rhythm Squeal(by Matthew Kennedy)

Mon, 14 May 2012 23:24:40 -0400

I started up my HO last night and everything worked great except when I switched to rhythm it would squeal like a stuck pig. I thought that I may have switched lug 2 and 3 on the switch but I did not. Now I'm a total noob so I fear it is probably something really stupid. I mounted the switch on the front of the amp, and to do that I have very long leads coming from the tubes to the switch. And I thought I'll try to keep them as short as possible so they are running under the board and while I was at it I twisted all three together. Its the Doberman kit so you can visualize the layout.

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

50 Watt P-P(by boblove)

Mon, 14 May 2012 13:44:04 -0400

Has anyone here built the 50 watt P-P from the core section? I would like to know how it turned out and which preamp it was coupled with. Any audio samples would be great too. Thanks in advance.

Feature films which show valves / tubes:(by Merlin)

Mon, 14 May 2012 13:31:00 -0400

For some trivia I thought it would be fun to collect a list of feature films in which valves / tubes make an appearance! Can you add to the list? Here are the films I know of so far: Coneheads (1993) (As I recall they make a device to communicate with their home planet, and the "final component" is an octal) City of Ember (2008) (Features a time-locked suitcase. The timer is made from nixie tubes. Of course?!?) Ordinary People (1980) (I *think* there is a valve radio in the shrink's office). The conversation (1974) (There is a valve hifi amp in the main character's apartment. It nearly gets smashed at the end!) Infernal Affairs (2002) (A high-end valve hifi amp plays a crucial role in the bad guy's undoing!)