AX84 BBS Threads with new messages as of Tue, May 26th, 2015 @ 03:55:17 Amp Parts and KitsDoberman Music Products, LLC offers a variety of parts and kits Merlin's Ultra High Gain (by Denny Gracey) Components just arrived for converting my L2L (Brian’s) into Merlin’s UHG preamp. After building the L2L, I decided to use the chassis & power amp to build the UHG, basically to expand my education on high gain topologies. Also want to build it point-to-point. First, I pulled apart the L2L pre-amp, moved the preamp tubes just behind the front panel (and 4” apart per Merlin’s book), and rebuilt the L2L preamp using P2P. Wow – what a difference in noise! No hum and only modest hiss with everything dimed. There are only 4 wires in the circuit, including the input lead which is only 2" and not shielded, and only the longest (4” between stage 2-3) is shielded. Totally diggin’ P2P for high gain designs. Looking for anyone who’s built Merlin’s UHG to share ideas, etc. I plan to install pots for voltage dividers and cat resistors to tweak values for fun. Mon, 25 May 2015 20:35:58 -0400 Merlin's B9A Board(by Denny Gracey) Merlin's "Designing Pre-Amps" book just arrived today - totally stoked. I've decided to convert my L2L into a "pre-amp lab" - basically building a small power supply board for all the B+'s, a small tag board for the PA circuit - leaving rest of the chassis to build out various pre-amps and experiment around. His B9A boards look ideal for this project. Anyone had any experience with these? Mon, 25 May 2015 19:59:10 -0400 Introduction + what to build ?!(by Ryan Boeur) Hi all, This is my first post on ax84. I'm up in Vancouver BC Canada, play "professionally" with Fish & Bird ( and Jenny Ritter ( I suppose that's my main occupation. I also build pedals, and just about anything else I can think of.. Good with my Hakko. I'm here because I would like to build myself an amp to solve a logistical situation I find myself in. I am occasionally touring back to back in the UK and North America, and I would like to use the same amp wherever I go, and not lug step down power transformers around with me ( I have not found a rental solution that does the trick for my needs.) I used to play a silverface Champ, now I play a brownface Princeton, so I am used to low watt amps (play in bands with lots of acoustic instruments/mics on stage). I suppose I could change my power transformer in the Princeton to dual primary, but I would like a lighter weight amp for fly in touring. I'd like 2 to 10 watts of power, 115/230v input, very light weight (luggage fees!), push/pull output. It will either be a head, or a combo with 1 10" speaker. Hoping to use a Hammond 1590BB or similar size/weight chassis. I think the ECC99 may be a good power tube for my application, but am open to suggestions on all of this, that's why I am here! Any recommendations on what to build would be super, this will be my first tube amp build, tho not my first time doing mains/HV wiring. Mon, 25 May 2015 19:27:45 -0400 HO shuts down with a full volume hum(by Chris Linkous) Hi everyone, This is my first build. HO has octal mod and external bias adjustment. The amp plays fine with very little noise until about fifteen minutes and then it switches to a full volume hum. After cool down it will play again and repeat. Preamp tubes make no difference when pulled. All noise stops when power tube pulled. When switched to standby the hum drops considerably in volume but is still loud. I've checked all the ground points against the input chassis ground and they all check out. Have 255V at plates and run 40ma on el84 or 46ma with 6v6. Have triple checked (resoldered) the OT and PT connect points as well as filter cap joints and I don't SEE any bad solder joints. This shut down will happen if I'm playing with either tube installed, wide open or low volume. I've timed it at eleven minutes twice in a row with no input and all knobs zeroed. Always have a speaker connected and have experienced this with different speaker loads on different outputs. Also checked resistor values, all line AC connections, filament connections and standby connections and everything looks right. I am obviously missing something important! Any suggestions would be GREATLY appreciated at this point as I've long since outrun my knowledge level! Thanks in advance, Chris Mon, 25 May 2015 17:58:46 -0400 JCM 800 voltages(by mategenomics) Hi guys! I am building JCM 800 100w clone. Schematic is wired and I proceeded to check the voltages against table. (Note : I inserted only preamp tubes as I measured voltages). Heaters are all good. For bias I have 44V tap on my mains tranny. Behind BIAS diode I get -69V (I omitted the 27k feed resistor as the voltage is already quite low). I get -37.3 to -43.4V on pins 5 on power tubes. Quite narrow range but I think I can manage that by swapping higher resistance BIAS pot (I have 50k, so maybe 100k would do that?). However I get some weird measurements on various pins compared to the table as following : V1 : 1-270 2-0 3-3 6-247 7-0 8-210 V2 : 1-199 2-0 3-2 6-352 7-199 8-210 V3 : 1-373 2-165 3-329 6-368 7-165 8-330 V4 : 1-0 (on all power tubes) 3-515 4-510 5- -43(for all power tubes) 6-510 8-0 (on all power tubes) V5 : 3-512 4-0 6-0 V6 : 3-514 4-0 6-0 V7 : 3-511 4-0 6-0 I double checked the wiring and possible shorts but nothing unusual. Still I think voltages on V2/V3 are too high for preamp tubes and there is the missing reading on pins 4 and 6 ond V5/V6/V7. Do you have a clue about what is wrong? Thanks in advance :)Mon, 25 May 2015 09:07:17 -0400 Adding a gain stage(by Sirppi) So yesterday I finished building a combination of the EA MV120 preamp stage and the KT88 SE 20W power amp. I'd like to add the unused V1a in the circuit to get more gain out of it, since I have no phase inverter to drive into clipping that the original EA has. Would this be as simple as calculating the right values for the anode and the cathode and putting it in somewhere, or do I need to do major modifications to the whole amp? Here's the schematic I made: <a href="" target="newwin1432626917">[link]</a> Mon, 25 May 2015 07:34:28 -0400 Jon Fleig's son of F2-B preamp(by Chris84) Hi guys, I am Chris from France and I am brand new on AX84. I would be very interested in the building of the Son Of Alembic F2-B preamp, especially Jon Fleig's model with the White Cathode Follower. Has anyone here built this preamp ? I've uploaded the schematic PDF file but there are some wirings I don't understand so, I need a little help. Thank you very much for your help. Chris. PS : I play a Spector EURO 5LX bass with a TECH21 SANSAMP BDDI direct in a QSC RMX850 power amp, driving a 4X10 Ampeg SVT410HE cab. Mon, 25 May 2015 07:14:32 -0400 Celestion G12C(by James Marchant) Hi folks, I've got a couple of G12C prototypes if anyone wants them. In case you don't know, the C was the immediate predecessor to the G12M greenback. The C was produced for a few months before the final spec of the M was settled on. They are a little softer than the M and slightly lower efficiency. They sound a but more broken in. If you are interested, drop me a line on ax84 (at) jmarchant (dot) co (dot) uk Sending from UK so more interest to uk dudes. Will take £40 for them plus postage. As they are protos, they don't have labels, but they do have the greenback cover.Sun, 24 May 2015 14:54:36 -0400 Custom chassis(by Gruszie) Just wondering if anyone would be interested in purchasing custom sized chassis? I am considering buying a box and pan brake to make chassis for my builds and was just wondering if any one out there would be interested. Is seems that all the custom metal shops charge a fortune for custom sized chassis and thought it would be cheaper in the long run to just buy a box brake and possibly sell a few as well. I figure price could be around $30-40 depending on size. If there are a handful of interested buyers out there I just might purchase a box Brake and start making my ownSun, 24 May 2015 13:39:14 -0400 BAD MONKEY!!!(by RickTown) Well, spider monkeys and parts bins don't mix. We have self-closing hinges on most of the doors in the house including my shop door but the spring in one of the two self-closers broke a while back which I've been ignoring. Lucy found the shop door unlatched and decided to explore. Luckily, she only had a few seconds before she was caught. <a href=" " target="newwin1432626917">[link]</a> <a href=" " target="newwin1432626917">[link]</a> Looks like I've got some sorting to do. Here's a picture of the culprit. <a href=" " target="newwin1432626917">[link]</a> She dragged that guitar down the stairs to bring it in the living room and play with it. It's the only one I don't keep locked up. She loves to sit at the piano and "tink" as well.Sun, 24 May 2015 07:14:19 -0400 Another Tube Reverb Question (by Alva Goldbook) Well, not so much a question, as I'd like the opinion of the board. So I'm working on a reverb circuit. I'd like to get the kind of reverb that's as rich as found on blackface amps. These circuits are typically driven by parallel 12AT7 triodes with a 12ax7 triode on the recovery side. The amp I'm working on already has 4 pre-amp tubes without the verb. (3 pre-amps, plus 1 for a tube driven FX loop) I'd hate to end up with 6 pre-amp tubes and just leave one half of a 12ax7 unused. To avoid this, I'm kicking around two ideas. A) Use a parallel pair of 12AT7s to drive the reverb, and use the recovery triode from the FX loop for the verb return. Or B) Use a 12DW7, and drive the reverb pan with the single 12AU7 side of the 12DW7 and use the other half of the 12DW7 (a 12AX7 triode) as the recovery. Each approach has it's significant down sides. Approach A would mean that a normal reverb control would also effect the FX return levels, while Approach B would provide the reverb pan with less current than a pair of 12AT7s would provide. Sat, 23 May 2015 12:16:47 -0400 Feature films which show valves / tubes:(by Merlin) 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!) Sat, 23 May 2015 11:05:55 -0400 Tube FX loop on HO(by sajyho) Hi guys, It's been a while since I added Mark L's FX loop to my HO: <a href=" " target="newwin1432626917">[link]</a> It sounds amazing with my Rebote delay, but when I use it with my FV-1 based digital multi-effect it sometimes overload the effect at high gains and the overload LED on the effect keeps blinking. My question is; What is the best way to attenuate the signal before the send jack? Can a simple knob do the job without any significant tone loss? I totally a noob in impedance concept so please help me through this one... Thanks SajadFri, 22 May 2015 22:22:07 -0400 bias output tube(by jmpj46) I was thinking about building the uber sel looking at the schematic it has a bias control on a single ended kt88. I thought you only needed a bias control on 2 are more output tubes looking at all other single ended amps none have a bias control just the uber sel is there something different about the uber sel output. thanks Fri, 22 May 2015 10:34:10 -0400 Class AB2. If the Grids want current, they can have it.(by CChurchill) There are many ways to tackle the issues of blocking/crossover distortion in Push Pull output stages, particularly in cathode bias configuration. I like the zener assisted circuit Paul Ruby expanded on for 18 watt amps. R.G. Keen has a tempting suggestion to couple the grids to a source follower. Cool circuit and not difficult to set up with mosfets, but thats cheating. Then is good old fashioned math, by figuring out which capacitor/grid leak resistor you can get away with without sacrificing bass response and loading down gain. I recently read something where Randall Aiken developed a bipolar clipper to balance the voltage on the cathodes, and now I have to know what that is... But I'm really falling in love with the idea of driving the output tube grids with a direct coupled cathode follower. This means I have to redesign the power supply or modify it for a negative supply and learn what ever I can about designing this output stage. I was thinking of possibly using cathode followers with BJT constant current sinks. But I know there are those of you out there who know what I need to know about this.Fri, 22 May 2015 09:12:26 -0400 Modded my Simple Preamp with additional gain stage -too much bas(by Darren Porter) So, when I built my Simple20, I did it with an eye towards expansion by adding two additional 9-pin sockets and running the heater wires to them. So far, I have used one of the sockets for a one-tube reverb. This week, I wired-up one triode of the remaining socket as an additional 12AX7 gain stage between the second stage and the MV. I used Ra=100k and Rk=680R and unbypassed. Basically the final stage of an October/JTM. My ultimate plan, after I get it tweaked to taste, is to make it foot-switchable. WOW! This thing sounds freaking amazing. I think I am getting VERY close to my goal of not having to rely on pedals for overdrive. I do only have the one tone stack that needs to cover both clean/crunch and overdrive modes. But, there is just WAY too much bass and I have to turn the bass knob all the way down to tame it - obviously I don't want to run my clean-ish mode with no bass. So, what do you think the best way to attenuate that bass is? Should I just put in a fixed high-pass filter, should I add a bass tone control? If I add the bass knob, would I need to drive it with a CF or would just the one knob not have a huge impact. I'd like to get away without using a knob if I can though. A switch for "more cut" and "less cut" might be good for going between my strat-ish and les paul-ish guitars. Or can I get away with just a different coupler value (currently 22n)? This circuit would of course be inside the switched area so it would only be active when this gain stage is active. Thanks for any insight!Thu, 21 May 2015 12:02:14 -0400 A transformerless reverb driver(by John Polstra) Recently I've been experimenting with a tube-based reverb driver circuit that doesn't use a reverb transformer. It's designed to deliver a constant signal current (not voltage) to the reverb tank regardless of the frequency. That's what Accutronics recommends in their technical documentation. The circuit is working out very well, so I wrote a short article about it. I thought some of you might be interested. Here's the link: <a href=" " target="newwin1432626917">[link]</a> There's a sound clip at the end of the article. John Thu, 21 May 2015 11:32:24 -0400 5F6A Bassman Mods(by robrob) I have just completed a 5F6A Bassman mod web page. Most of the mods have accompanying layout diagrams. The mod page can be found here: <a href=" " target="newwin1432626917">[link]</a> Thu, 21 May 2015 08:22:43 -0400 Tube Reverb and Phase. (by Alva Goldbook) I'm working on an amp with a classic Blackface style reverb circuit, and I'm trying to figure out where to place the reverb return back into the preamp circuit without having phasing issues. The reverb driver tube will invert the preamp signal, and the recovery triode will flip the phase again to a non-inverted signal, but the thought occurs to me that I'm not sure of the reverb driver transformer will invert the signal, or the reverb pan itself might invert the signal. I can imagine that reverb pan transducers may in fact do this, but I wasn't sure. Does anyone have any clarification on this?Thu, 21 May 2015 05:43:02 -0400 Finally built my Crazy El84 6V6 Parallel, Problems.(by lkrasner) Alright, I finally built this thing. SEL like preamp, cleaned up a bit, switch on the first stage to give a clean and lead setting, FX loop, simplified LOW HIGH tone stack, parallel 6V6 and EL84 output with independent volume control. Overall, working well, but there are some issues. The FX loop isn't working. the recovery triode is dropping to 111 volts on the plate, and it causes the whole amp to be distorted with low output. It's a bit better if I put a 12ax7 in to draw less current, but still bad. Works fine if I completely bypass the FX stage. Any Ideas? Here's the latest schematic and layout. note that the layout is not perfect in the jack wiring and some of the other wiring, but the electronics are all correct. Schematic: <a href=" " target="newwin1432626917">[link]</a> Layout: <a href="" target="newwin1432626917">[link]</a> Mon, 18 May 2015 15:22:46 -0400 weird tone artifact at very low volume levels(by vicrattlehead) So I think I’ve got my build just about perfected but I have one lingering issue. At low volumes (low on the MV; maybe set at 1-1.5 if it were numbered), and only when being pushed with a pedal (a Digitech RP500 if it matters), I have a kind of ‘choppy’ crappy sounding artifact in my distorted tone. The best I can describe it as, is like that really bad power amp distortion that comes from pushing a solid state amp too far. It is most noticeable when the guitar is switched to the neck pickup (the warmer, bass-ier tone sets it off more). It’s not the dominant sound that comes out though, it’s more like if my tubey goodness was being mixed 70/30 with some SS crap. I’ve tried adjusting pre-gain, tone controls, and every conceivable adjustment on the pedal but the sound is always there to some degree. What strikes me as odd though is that if I turn up the master volume past 2 or so (maybe 20-25% vol) it clears up and sounds awesome. It’s only at very low levels that I get this artifact. The problem is, even at 2-3 on the MV this thing is pretty loud and is really pushing what I’d consider ‘bedroom levels’. Anybody have any thoughts on this? I’m kind of at a loss. I thought maybe it might be some kind of blocking distortion but increasing the volume actually makes it sound better and adjusting the preamp Vol or the pedal has very little effect. Mon, 18 May 2015 04:06:02 -0400 a quick question regarding an equation, and variables.(by proton45) Hello... Here is the equation.. Plate voltage^2 / tube dissipation. This is used to roughly determine the output tubes impedance load, when figuring out the best OPT primary impedance. Now my question...would this calculation be "more" accurate if you used your "actual" bias watt(?). the calculation so (generalised) rough, that it matters little? I realize that there are certain realities of the fluctuating loads...that maybe the "max" dissipation is used? Thanks in advance... I realize that some numbers could be close enough...and then there is the "sound' and preference...but I would still like to know. Thnx CSun, 17 May 2015 14:47:53 -0400 P1-eXtreme vs Marshall preamp(by Henrik Lidbjrk) I'm trying to design a simplistic Marshall-esque amp based on the P1 eXtreme, keeping the SE octal tube power amp, but perhaps doing something different with the preamp. I'll probably simplify it to 1 volume and no other controls and let the power amp provide the distortion. I'm wondering if I can make the whole amp sound more Marshall (JTM45/plexiish) by changing the preamp to an 18 watt style preamp? What are the essential practical differences between the eXtreme preamp with two stages in series and the 18 watt preamp with two stages in parallel (correct me if I got that wrong)? Minimalist 18 watt: <a href=" " target="newwin1432626917">[link]</a> P1 eXtreme: <a href=" " target="newwin1432626917">[link]</a> Sun, 17 May 2015 14:11:45 -0400 Definitions, Equations, and Reference Data for Tube Amp Design(by CChurchill) I compiled a PDF as a primer to use when I'm reading technical papers and some of the older Tube circuit design books. It contains reference material defining some of the nomenclature (symbols, etc.) commonly found in vacuum tube circuit design equations. I consolidated some of the pages from "Reference Data for Radio Engineers, 2nd Edition" that are more specific to tube amp design. I ran it through an OCR (text recognition software) so the text can be searched for terms, or you can use bookmarks as a guide for quick access to certain topics. I downloaded it from, so before I post it, I want to ask Pete Millett's permision. He did the hard work of scanning volumes of work(and I respect him), so if He's cool with it, a link will follow.Sat, 16 May 2015 17:26:45 -0400 stand alone power amp question(by Darren Porter) If I was to build a stand alone power amp is there any reason why I should NOT put an effects loop and reverb in it so I would not have to then add those to every preamp I build to go with it? I can't think of any reason why it wouldn't work. I'm thinking that would be a good way to keep costs down.Sat, 16 May 2015 10:33:34 -0400