AX84 BBS

Amp Parts and Kits

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

Micro Amps? (ie. Champ)(by DrHugenstein)

Sat, 31 Jul 2010 06:00:27 -0400

Hi, I am having to travel lots for work. I realized I can fit a tele with the neck off and a small amp with 6 to 8" speaker in my luggage. I was looking into building a Micro Champ but wondered if there are any other small tube amp PTP builds that are approx 2-5 watts output and very small. If not then if anyone has a build of materials list for the Micro Champ, please let me know.

EF86 High gain?(by TrevorGoodchild)

Sat, 31 Jul 2010 03:13:51 -0400

I've heard some really great blues and classic rock level amps that have been made using an EF86 in the preamp. I'm wondering if there is any precedent for a high gain EF86(or other pentode) amp. On the level of a SLO, Recto, Engl, or similar. I've been looking around for ideas, and I'm wondering about a serial pair of EF86s, since I've only seen parallel ones in previous designs. It seems like you could get a lot of different sounds out of it, using some inventive singnal routing. I've been thinking of a signal chain something like: Input ->12ax7(a1) ->EF86 ->EF86 ->ToneStack ->CF[12ax7(b1)] ->PowerAmp Any ideas?

Fixed Bias Problems(by Bonfire)

Sat, 31 Jul 2010 03:02:40 -0400

Hey guys, I've switched to fixed bias in a build of mine and I can't get a negative voltage from this circuit...in fact, the DMM pretty much shows no potential at all (similar to when the probes are not touching anything, i.e. some millivolts which is unchanged when removing the probes from the test points)... [link] Any ideas what might be wrong? I've switched both caps and the diode to rule out busted parts, the resistors all test fine, all connections seem solid and show zero resistance.

Is this worth the effort?(by 3rdof3)

Sat, 31 Jul 2010 01:39:11 -0400

I've got a design that I'm looking to get a bit more gain/tonal change out of and was wondering if this circuit would be worth the effort. The bootstrapping would be switchable. Thanks! [link]

12AU7's out the wazoo. And some xformers. etc.(by yecrom)

Sat, 31 Jul 2010 00:42:51 -0400

I've picked up a bunch of 12AU7's from various organs, and other sources. RCA Clear-top grey-plate. $7 each -- 20 tubes (at least) RCA Blackplate. $7 each -- 10 tubes Sylvania/raytheon/GE mostly greyplate. $5 each. -- 17 tubes -- 12BH7 -- various brands -- $5 each -- 4 tubes Transformers: 10k:8 PP OT from hammond S-6 chord organ. Z-mount. 15-20 watts. $15 PT from hammond chord organ (285-0-285 HT, 6.3VAC, 5VAC, 25VAC windings), It's really huge, but I'm not sure of the current rating on the HT primary. It supplied HT for 2x6V6, 18x12AU7, 3x12BH7, 2x6BA6, 6C4, 6BJ6. I'm guessing about 200mA, but I'm not positive. $30 (still in the little chassis + rectifier tube socket) 2 x 1100:4 UL-SE OT from Zenith console. Power tubes were 7695's. Low-voltage (120VDC) but lots-o'current. 50VAC filament. $10 each and I'll throw in the tubes also. :) Quad (or 2 pair) of Conn-branded Sylvania 7868's $25 per pair. Pulled from a working organ, but I can't test them in my tester (yet). -- Soundmasterg has right-of-first-refusal in these. I was trying to get him some earlier, but that fell through. Conn Stereo Organ amp Chassis + transformers. PT + pair of 7868 OT's (I think they're Thordarson 7200:4,8 for the 7868's above). Again, from working organ. $50. (Or $90 for both pair of 7868's and the chassis) Hammond AO-43 chassis and trannies: $40. Good for high-voltage 18-watt or October-type builds. Lower-48 only. Shipping not included. If all you're buying is tubes, it'll be $5 shipping regardless of how many. Chassis and transformers will be actual shipping cost. All tubes (except 7868's and 7956's) tested w/ a B&K 500 mutual conductance tester. e-mail me w/ any questions. mcorey at xmission (point) com Matt

Convertible Phase Inverter(by Zoe N Iain)

Fri, 30 Jul 2010 21:10:00 -0400

Just in case anyone is interested in how you do this... [link] With the switch in the current position, it's a standard LTPI. Flip the switch and it's a floating paraphase with about double the gain. Both are fairly well balanced when running clean, but the paraphase will create more even harmonics when overdriven as more imbalance creeps in. There are ways round that, but it kinda defeats the object of using a paraphase in the first place. Hope someone finds it of use.

Tolex ?(by mac dilard)

Fri, 30 Jul 2010 20:06:37 -0400

How do you do the corners on a flat board. Say a 1/2"x 4" X 18"? I have tried every way I can think of and still can't get it right.

07/07 - 6AQ5 - 7 pin project - provisional preamp(by MasonAtom)

Fri, 30 Jul 2010 15:28:54 -0400

Here is a provisional preamp that I drew up. [link] This runs into a 5964 long tail pair with presence control, feeding two 6AQ5s in push-pull. I'll post poweramp and power supply when I have the time to draw them up. The biasing is pretty 'vanilla' at this point. Basic idea is to have a switchable boost channel that brings a 6AV6 in and out of the circuit. The boost channel has a mid cut. The 5654 pentode is biased for moderate gain... I've used much higher plate loads on this before but never liked the sound, plus I'm always a little worried about microphonics. I've used Merlin's triode/pentode morph control on the screen - may require a little tweaking to get this right. The output from the pentode has a rotary switch to control the low end roll-off. The cathode bias cap on the pentode is actually a lot lower than you commonly see, but I find the 5654 to be really full-bodied and I think it gets a little blatty if you go larger. Any comments would be much appreciated, particularly if you see any egregious errors. I'm more of a builder/tweaker than a designer at this point in my amp building career. Thanks for those who have shown interest. I'm not sure exactly when I'll have a chance to build this, but I WILL build it at some point and I'll post pictures along the way. Mason ps - July 7 seems like a right fine day to post this, no? The Lucky 7 seven pin project on 07/07.... I'm heading out to the post office in a few minutes to send Ben a set of tubes for this project.

UL wiring(by stringbend)

Fri, 30 Jul 2010 15:25:12 -0400

What would happen if the UL taps were cris-crossed relative to the primaries?

50w PP - first attempt at PTP(by GuiteLectrique)

Fri, 30 Jul 2010 15:13:46 -0400

Hi everybody! Sorry it's been a while, I have been playing instead of building :) Now I'm back at it in my first attempt at point to point. I would be grateful if some of you guys could check my layout and tell me if I did something wrong. The amp is a lead preamp - 50w pp with a single knob tone control. I know the speaker output section looks odd but I've got my reasons for that. By the way, the control knobs will be on TOP of the chassis (a little bit over the circuit). Thanks!

Tube mismatch by half?(by JeffW)

Fri, 30 Jul 2010 14:57:26 -0400

So I have a PP PA with KT77 tubes in it that has one valve drawing 20ma and the other 40ma. At first I thought it was my bias meter so I soldered in two 1ohm resistors and still the same. Switched tubes and it is the actual tube causing the mismatch. Is this a bad valve or just some super poor QC from JJ. I realize that there are differences but 20ma? Anyway, if you have any advice or insight please pass it along. Thanks

conjunctive filter - primary vs secondary(by Miles O'Neal)

Fri, 30 Jul 2010 13:15:21 -0400

It seems conjunctive filters usually get put on the primary side. But it seems they should work on the secondary as well. Since putting them on the primary side requires higher voltage components, why is this the norm?

Ready to fire up my first HO!(by AlterEgo)

Fri, 30 Jul 2010 12:40:15 -0400

All, I finish my first HO build (yeah), and I was going through the Paul Ruby's step to safely power up my new amp. I'm wondering if the intent of R1 is to be a bleeder resistor? I'll take picture before power up just in case on self destruction :) and post them later. I'm using schematic V08.05.07 and just realized that a new schematic was uploaded. I'm sure someone on the thread caught that, but the R5 on the schematic which is as it was from the previous version (connected on the OT Yel wire) but it is incorrectly represented on both Axial and radial layout (connected on the OT White wire) on page 7 and 8. just FYI :) Cheers,

HOT PT - Overfiltering?(by Alex G)

Fri, 30 Jul 2010 12:27:11 -0400

I've used Magnetic Components 100-watt Marshall style power transformer in both 100-watt and 50-watt builds. I noticed in both cases that the PT becomes really hot. After playing full bore for about a half hour or so, I can't put my hand on the transformer for more than 5 seconds (which I've read is a very general rule for determining whether a PT is running 'too' hot). The OT stays cool, however. Being paranoid, I thought it wouldn't hurt to write the company, especially because I'd read other reports of these particular transformers running hot, however not everyone had this experience. This was the response I received: "Yes, these are built to withstand high continuous running temperatures without any problems. All of the magnetic wire and layer insulation in this unit is rated for these high temperatures. It is normal for these particular units to run hot. Please rest assured that these should perform well for you." I thought this was an adequate response, especially since I'd read on a HIFI forum that certain transformers run hot with no problems because of the materials used. Still a little paranoid, my question is about using too much filtering for the first filter cap. I used 100uf in both the 100 and 50 watt amps I built. That particular transformer is intended to be used with only 50uf of filtering, I presume, like the original 100-watt design, so I'm wondering if the PT can tolerate double the filtering? Here's a link to the datasheet: [link] And FWIW, I'm not using the Center-tap, and I'm not sure if that affects anything concerning current draw. Thanks!

LTP plate resistor bypass caps(by AleksiS)

Fri, 30 Jul 2010 10:48:28 -0400

Hi everyone, just something I started wondering about with my last build: Suppose you have a regular LTP as your PI. Now adding a small cap between the PI plates kills the treble (as the out-of-phase signals cancel out each other). But what if you have that cap in place (let say 100pF) but still add a bypass capacitor to only one of the PI's plate resistors (lets say 500pF)? Now what happens? The 500pF kills some treble and creates an imbalance on the PI. So does it actually lessen the effect of the 100pF cap between the plates as the signals are unequal in amplitude? Just something I can't seem to visualize in my head. Thanks in advance :) BR, Aleksi

Amp for double bass(by roystonoboogie)

Fri, 30 Jul 2010 10:45:38 -0400

Hi. I've been reading a lot of the threads here about bass amps, but they all seem to be about bass guitar. I am considering a valve amp for my double bass, and I am not quite sure where to start. What I want is the sound I get when I plug into an SVT with an 8x10" cabinet, turned down to about 2 on the volume scale. No overdrive, no massive assault of decibels to rattle the windows; just a warm and faithful reproduction of the tone of the instrument, but at a modest useable volume. I don't need enormous, thunderous bass either: I want to hear the wood and the strings. I also don't want to carry an 8x10" around with me - I'm already carrying a double bass, after all. I will happily use a 2x10" or a 1x15", but I don't want to go any bigger than that if I can avoid it. I don't mind the extra weight of a valve head over a solid state head if it will give me that warm tone I am looking for. From what I have read, I doubt I would need more than 100W if that, but I am keen to keep the sound 'clean' right up to near its peak. I usually play bars and clubs - 100 to 200 people max - but I am playing alongside a bluesman with a Fender Deluxe Tweed and a Les Paul, so I need to be audible with that going on. The drummer is OK, he adjusts to whatever volume level we are playing at (violence is so persuasive, don't you find?). So - what do you think? Is there a variant of the AX84 project that springs to mind? I know the sort of sound I want, but I cannot find a tube amp in the shops that will deliver it. Which leads to my question: what sort of hardware do I need to use to get that sound? Thanks, Royston.

Bit of detective work(by Wattie)

Fri, 30 Jul 2010 10:15:44 -0400

Just received a donated amplifier from an acquaintance who is moving country, and this is excess baggage. His only statement was "I think there might be something wrong with it". This is a well known P1 type kit build, that I helped him put together. I will hesitate to mention which one because there is absolutely no blame on the kit or how it was built. What is wrong with it is that it hums, it smells, and the two resistors directly in series with the EL84 screen look like toast. My other half made me put it in the garden - it smells so bad. I summise that a bad tube went in, but there won't be any more information from my "pal". Could be he left it on for a few weeks with no speaker, I don't know. We are talking about R3 and R7 if it were a standard P1 - these are obvious replacements. My questions are, if the forum can help: What components should I be double checking and replacing before going through the start up procedure? Apart from the toasted resistors. What might have caused this unfortunate event? A crappy tube?

power supply for all 12ax/au7 amp(by mynyka)

Fri, 30 Jul 2010 08:16:40 -0400

So I recently built an amp based loosely on a 2203 but using a pair of 12au7s for the power amp (3x 12ax7 + 2x 12au7). It works well but if I want to run all the plates at 250v or so, should I take 1 tap off the last filter to feed them all or filter and feed each one separately? The reason I ask is, I have it wired up the first way at 220v but if I raise the voltage to 250 I get bad hum when I turn up the gain. Reasonably quiet at 220v though...Seems like I need more filtering..... but 5 caps....????

"Unecessary Quotes"(by Alex G)

Fri, 30 Jul 2010 07:21:39 -0400

Thought you "guys" would "like" this. There's definitely "a lot" of this that goes on in "amp message boards!" (i.e. "my posts"). [link]

PT & Toggle(by Adddakid)

Fri, 30 Jul 2010 06:23:14 -0400

Hello - Newbie Noob Here!I built my first last year from an HAO 43 Hammond Organ Chassis (2xEL 84'2). Sounded very nice until I made some faceplates for it. I had a longtime tech helping me along the way (he is now unavailable). My Problem: I had to unsolder the fuse holder, the on/off toggle and the standby. After sitting for a month a few id tags I wrote on are unreadable and 2 fell off. For the life of me I can't remember how the wiring went. I have a standard 2amp fuseholder, a 6 tab radio shack DPDT on/off toggle and a 3 tab standbay on/off (no ground switch) it is a 3 prong ac supply. The standby I have figured out, except a jumper that runs to on/off toggle. The 2 black leads from the transformer and the ac wh/bla are giving me fits (have the green grounded to a star). Thought I finally had it last night and "boom" blew another fuse! I have been running everything to the fuseholder so it blows that and doesn't fry my amp. All the schemos I see are for a (ground switch in the mix, which I don't have or want). Any help would be greatly appreciated. PS - this amp worked freaking fine until I put the faceplates on - the {6} tag toggle is what's throwing me for a loop!!!!

Elevating heater voltage without CT?(by Dennon)

Thu, 29 Jul 2010 23:05:04 -0400

Hey guys, The power transformer I got (hammond 272 dx) doesn't have center tapped heater wires...is there anyway to re-create the clever trick of raising the reference voltage to around 50 V, as recomended, to reduce hum? If not, is there any other way to bring it down (other then good lead dress, of course!)? Just need to get PS hooked up and everything installed, and then should be ready to rock! Thanks, Dennon

Help me choose a PT(by Henry Ramsey)

Thu, 29 Jul 2010 16:07:00 -0400

I want to use a stand-up type (not through-chassis)PT due to tight space. I'd like to buy a hammond, but I found a used parts dealer with lots of PTs to choose from and about 1/3 the cost. He doesn't have many stand up PTs. These are listed as possibles. Can someone help me narrow these down? 1003-118 POWER STANCOR PC-8402 240-0-240V @55MA, 5V @2A, 6.3VCT @2A, FIG 1, 2" LEADS, USD 3 2 1/2 3 0/0 2 1/2 11.75 1340-717 POWER ZENITH 95-306 345-0-345V @100MA, 5V 4A, 6.3CT 4.8A, CHS 1202, FIG 1, USD 1 4 0/0 5 1/4 4 0/0 24.00 1347-169 POWER THORDRSN T-44917 370-0-370V @100MA, 5V @3A, 6.3VCT @3A,FIG1,SHORT PRI LEADS 1 3 3/4 4 7/8 3 7/8 14.25 1365-093 POWER2 UNKNOWN B-20162 375-0-375V @90MA, 7.5VCT @6A, 6.3V @2.5A, FIG 1, USED..... 1 3 1/8 3 7/8 3 3/4 16.00 1424-242 POWER STANCOR PC-8406 325-0-325V @40MA, 5V @2A, 6.3VCT @2A, FIG1, 2 1/2"LEADS, U 2 2 5/8 3 1/8 2 5/8 8.75 1522-319 POWER UNK 63P028-5 235-0-235V 60MA, 6.3VCT 1.7A, BOTH CTs CASE GND, FIG 1,USD 1 2 1/2 3 1/8 2 1/2 9.50 4529-269 POWER SEARS 1011045953 310-0-310V @90MA, 5V 2A, 6.3V 2.1A, MODEL 101.672,FIG1,USD 1 2 7/8 3 5/8 3 1/2 14.00 4530-053 POWER BELL B20236-EW1 380-0-380V @100MA, 5V 3A, 6.3V 3A, MODEL 2200P, FIG1, USED 1 3 3/4 4 5/8 4 0/0 17.00 4538-168 POWER AMPEX 3953 360-0-360V @85MA, 5V 2A, 6.3VCT 2A, 23VCT 40MA, FIG1, RUST 3 3 1/8 3 3/4 3 1/4 13.00 4544-014 POWER PHILCO 32-7180N 320-0-320V @55MA, 5V 2A, 6.3V 1.5A, MODEL 80/84, FIG 1,USD 1 2 1/2 3 1/2 2 7/8 14.00 4546-006 POWER ADMIRAL 80B60-1 240-0-240V @60MA, 5V @2A, 6.3VCT @1.2A, FIG 1, USED....... 1 2 1/2 3 0/0 2 7/8 12.00 4546-023 POWER WEBCOR 689001 340-0-340V @60MA, 6.3VCT @2A, BOTH CT'S TO FRAME, FIG1,USD 1 2 5/8 3 1/8 2 7/8 10.00 4550-006 POWER UNKNOWN 375-0-375V @100MA, 5V @3A, 6.3V @2.5A, SURFACE RUST, FIG 1 1 3 3/8 4 0/0 3 7/8 14.00 4561-034 POWER STANCOR P-4073 350-0-350V @50MA, 5V @2A, 6.3VCT @2.6A, FIG 1, USED....... 1 2 7/8 3 3/8 2 7/8 15.75 4585-004 POWER UNKNOWN P-2063 330-0-330V @95MA, 6.3VCT 2A, 6.3VCT @8A(8V NO LOAD),FIG1,U 1 3 1/2 4 1/4 3 3/4 15.00 4586-020 POWER UNKNOWN RK-804 360-0-360V @70MA, 5V @2A, 6.3VCT @2A, FIG 1, USED......... 1 2 7/8 3 3/8 3 3/8 13.75 4588-318 POWER PHILCO 32-8055N 262-0-262V @52MA, 6.3V @2.3A, FIG 1, USED................. 1 2 5/8 3 0/0 2 3/8 11.50 BTW, the dealer is Play things of the Past: [link] I've bought from him before and have not had any problems.

This dead HRD(by nonzero)

Thu, 29 Jul 2010 15:45:49 -0400

I've got iron from a dead (dropped and shattered) HRD, so I would naturally like to use it to build something. The PT delivers 440VDC and about 65mA rectified. The OT is already set up for 6L6 PP with both 4R and 8R outs. I would like to make a 6L6 PP power section. I've got a Philips 5879 I'd like to use for the input (seems like it'd have just enough gain). I'd probably use a 12AX7 for my LTP PI. I'm not too interested in a FMV tone stack, but I've got some current to spare (just more than enough for another 12AX7) so I'd like to do something interesting. Any ideas?

a few questions on the theory.(by monongahela monster)

Thu, 29 Jul 2010 15:35:22 -0400

I am researching to gear up for my first build here and i have a couple elementary questions. okay so since the cathode is connected to ground i assume that that is where the current is coming from. and i am assuming it is ac.( or is it?) i was also under the assumption that current in a tube only flows out of the plate. so why have a cap to block ac signal in the cathode circuit? it seems like either ac is coming out of the tube from the grid via the cathode ( which doesn't sound right.) or it is blocking ac coming in, ( which leads me to believe it must be dc but i see no rectifier anywhere near here.) what the hell's going on? also, the theory page for the p1ex has two preamp tubes. I was under the impression that it had one preamp tube ( and that having two would make it a sel)also some of the resistors are labeled differently on some schematics rather than others, this doesn't worry me any but i would like someone to let me know if it should....and lastly , is there some other way to look previously asked questions up rather than endlessly scrolling through potential answers in marginally related threads? whew. I know that was a lot but any help would be appreciated. thanks.

5751's, I'm a believer(by brawler)

Thu, 29 Jul 2010 09:57:49 -0400

I have always thought that 12AX7's to be cold sounding. I have built several amps and most of them have used tubes other than 12AX7's in the preamp because of that. I finally picked up a few 5751's to try. Just for a quick test I put one in the V1 position of a Fender Blues Jr. Wow, I see why these have such a big following. The 5751 I tried has a nice mid range and tight lows. No sign of that brittleness of the 12AX7 at all. When I pushed the tube with some overdrive pedals, it had a real grit I just loved. Grit without harsh high frequency harmonics. Very nice. I'm going to have to make an amp based on these. I just loved what I have heard so far.