AX84 BBS Threads with new messages as of Mon, Mar 30th, 2015 @ 18:29:11 Amp Parts and KitsDoberman Music Products, LLC offers a variety of parts and kits Opinions wanted: Low wattage amp vs attenuator(by Martin Schrder) This is my first ever post here so please forgive and instruct me if I break any etiquette rules. Also - this might not be the most well-structured of posts because I am confused and wish to ask a million questions but also keep it short and give people a chance to read and answer. In short I consider building an amp and would like some initial advice. Because I also consider building an attenuator in stead. I keep going back and forth over the decision. Last year I built a JTM45+ as seen on It is a sort of JTM45/1987 cross-over and it just begs to be cranked. The volume knobs could be labeled "more" and "better" - but it is 40-50W and obviously somewhat loud when at its best. I play it in my living room (I am not a gigging musician) and yes - I did put a master volume on it but you probably all know how it is: It sounds nicest at full blast. Tone-wise I tend to love the sound of a les paul through either a superlead a JTM or a 1987 so I try to move a little closer to this goal without the monstrous volume. I have been considering an October Studio because it seems to be a low-wattage version of the type of amps I like and I find the soundclips available awesome. However I worry that it might still be too loud when cranked and would need some sort of attenuation. (I know the loudness of any amp is in part dependent on the speakers used. I play through an '97 JCM900 50W combo so my only speaker at this point is a single slightly old celestion g12t-75.) So I am thinking that perhaps a better choice for me at this point is to build an attenuator and I am considering either an airbrake-style diy project or the PoS kit at Both are passive/non-speakermotor types as far I understand. Based on all this - what are your thoughts and advice? Attenuate the JTM? Build an October Studio? Other options? I know "both" may be an obvious answer so if that is your reply please include some why's and wherefore's. Not least because the "both" answer opens up a lot of other options: If I have an attenuator an additional small amp might not be a marshall type at all, but perhaps one of the other designs here. Cheers, MartinMon, 30 Mar 2015 17:31:24 -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.Mon, 30 Mar 2015 16:43:07 -0400 Unfinished P1-eXtreme: 11 years!(by TripperRB) I've taken my time with my P1 11 years! I was cleaning up the garage this weekend and saw my unfinished project and thought it was about time to finish it up. The transformers and circuit board is complete and installed and a lot of wiring already performed, so all that's left is just to finish up the wiring. But I have a couple of questions. This project is obviously based on an early schematic (see link below). Is there any reason I should abandon this build in favor of a new schematic? Any inherent flaws in this design that warrants starting over? Since most of this project is done, I'd rather not start over. I designed the layout (link below) based on the schematic, but I added two jewel lights: one for power and one for standby. Going over my notes from 11 years ago, I was instructed by neatpete that the filament wiring was wrong. Here's what he said I should do. filament wire1 ---> one side of pilot light ---> pin 2 octal socket ---> pin 9 9pin socket filament wire 2 ---> other side of pilot light ---> pin 7 octal socket ---> pins 4&5 9pin socket However, I think he was basing that off of only one light, not two. I'm hoping someone can take a look at my layout and let me know if this is the proper way to wire up both lights. If not, I can just remove the standby light altogether and follow neatpete's advice, but I'd like to have both lights if possible. Schematic: <a href=" " target="newwin1427754551">[link]</a> Layout: <a href=" " target="newwin1427754551">[link]</a> Parts: <a href=" " target="newwin1427754551">[link]</a> Thanks for your help! Mon, 30 Mar 2015 14:25:16 -0400 How to organize your parts collection?(by David Staepelaere) I use bins or those plastic divided boxes to hold those small parts that are relatively few, odd shaped, or recognize quickly by sight (jacks, switches, lamps, etc), but I haven't figured a good way to organize low wattage (<5W) resistors and coupling capacitors in a way that it easy to find what I need. The best I've come up with is keeping them in zip lock baggies -- each different part in a separate baggie along with a slip of paper (or mouser label) identifying the type/value/rating/etc. Then I store the whole mess in a cardboard box (well resistors in one box, caps in another). If I want say a 470R 1/2W resistor I sort through the whole stack looking to see if I have one. It's easy (parts from Mouser, etc already come in baggies), its flexible and works OK but it seems like there's gotta be something better. Anyone got a better way?Mon, 30 Mar 2015 11:35:43 -0400 Diode Rectification and Power transformer questions(by Ty Penrod) I'm interested in building a 5f1 based amp, it'll be my first plunge into tube circuitry. I've done a healthy bit of research and have a basic understanding of the circuitry, but I'm getting confused at a couple points. 1, the power transformer I intend to use does not have a 5v output lead, therefore I can't use tube rectification, that's fine as I would like to use diode rectification. with that being said, the output and preamp tubes I'm using are a single 6AQ5-A and 12AX7-A respectively, what diodes should I use and should I do a full wave or bridge rectifier? 2, with using diode rectification in a 5F1 circuit, what do I run to the output and preamp tubes where the 5v would have normally come from? do I use the output lead of the diode rectifier? or am I missing something completely? from what I'm seeing, I should wire the high output leads from the power transformer into the diode rectifier, and if I use a bridge, ground the negative side, and run the positive side to the output and preamp tubes, does that sound about right? if anyone has a easy to read schematic for a 5f1 with diode rectification, and no 5v lead. that would help tremendously. I'm sure the answer is staring me in the face, but for some reason, I just seem to be missing it and I'm getting caught at this point. thanks in advance.Mon, 30 Mar 2015 04:59:45 -0400 debugging a P1(by mikerl) I purchased a P1 from Chris and just finished the build recently. I have correct voltages at all points on the schematic. The amp sounds good but I have an issue. The volume pot appears to be at full volume (whether I'm getting 100% or not I can't tell) when the knob is at 10 o'clock. When I max the volume knob to the right I get no sound. And, when the knob is fully to the left I get no sound as well (as it should be). So....I assume I have something in the signal chain wired to ground somewhere or the potentiometer is bad or internally grounded at some point??? (Or, not wired properly) I've checked the layout over and over and I think everything is correctly wired. I don't see anything that looks suspicious. Has anyone encountered anything like this? Any suggestions on where to start? I think the volume pot is wired correctly as well as I've checked the volume, TMB, gain, wiring a number of times. Any help or guidance would be appreciated. Thank you!Sun, 29 Mar 2015 21:23:36 -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 ownSat, 28 Mar 2015 15:49:56 -0400 Would like feedback on clean preamp idea(by Darren Porter) If anyone would be so kind to take the time and look over my idea and give some feedback, I would really appreciate it. I'd like to hear what others think before I take the time to actually build it. Thanks! <a href=" " target="newwin1427754551">[link]</a> Sat, 28 Mar 2015 12:43:38 -0400 First Draft Layout for HO build (PCB)(by Ben Sykora) PCB Artist library files are here: <a href=" " target="newwin1427754551">[link]</a> PCB Artist schematic and layout files are here: <a href=" " target="newwin1427754551">[link]</a> This layout is for a 4" x 8.5" board. The idea is to fit into a Hammond 13.5"x5"x2" chassis, which I haven't confirmed yet as far as fitup/interference issues. Orientation of the board is tubes down, components up. Heater wires are via external twisted pair. I'd consider the board layout to be 80% complete. Seeking constructive comments, especially from those interested in building one.Sat, 28 Mar 2015 09:52:15 -0400 Resistor and signal cap footprints for HO build(by Ben Sykora) For resistors, I plan to use a footprint large enough for a standard 2W metal film. The 1/2 carbon films will fit in there no problem and its only a minor upsize. Pretty straight. The most popular tube amp caps seem to be Orange drop 225P, 715P, 716P and Mallory 150. There are a number of other candidates on digikey/mouser, for example Epcos/Kemet have the boxy PCB style and Panasonic/Nichicon have something similar to orange drops. Is there an interest in accommodating those? At this point, I'm leaning toward the orange drops due to their popularity.Sat, 28 Mar 2015 09:21:27 -0400 Udating Ampeg Gemini II... how far can I go to keep vintage valu(by CChurchill) Firstly, I need to convert the power cord to a 3-prong earth grounded cord. I was going to just run a 3-prong through the same opening as the previous power cord. Since this modification is pretty much standard at this point, I got me thinking (as I was cursing under my breath trying to get the clamping/grommet/cord holder/thing into the chassis)... I would rather install an IEC outlet into the chassis. They are much more convenient in my opinion and I install them in all my modern amp builds. Will this hurt a vintage value on the amp any more than a 3-prong chord which isn't original anyway? Also, looking at the schematic, I hate the way the power switching, standby switching, fusing, and polarity is designed and really don't even want to sell it to someone the way it is right now. Would altering this be detrimental to maintaining its value? Also, I would like to make a solder connection to the chassis for the earth bond going forward. What do I need for proper equipment to make sure this is done correctly, and meets all codes and standards. Do I need a special solder when using aluminum chassis? Sat, 28 Mar 2015 07:50:51 -0400 Treble bleed capacitor.(by garry parker) Is it worth using a more expensive capacitor for a treble bleed tone control? The cap is just dumping treble down to earth, but does the quality of the cap affect the remaining signal which carries on to the next stage? Would the same question apply to a guitar tone control as well? Cheers Garry Fri, 27 Mar 2015 18:14:27 -0400 Silly Jack Question(by turtle441) Seems like a simple question, but does it matter if you use switched vs non-switching jacks for the output speaker jacks? I've got some extra cliff-style switching jacks sitting around, didn't want to have to order some more. Obviously, I won't be using the switching function, but it should work... right?Fri, 27 Mar 2015 16:25:09 -0400 Lowering B+ and Zener diodes(by basicaudio) I'm looking in the mouser catalog for zeners to lower about 100 volts. I see a 24 volt 5 watt zener for 50 cents. Is this the type I want? (using 4 between the PT centertap and ground) page 303 Part# 610-CZ5360B I think I may be wrong because I've heard of them being expensive... Where else are they in the catalog if i'm wrong? John Fri, 27 Mar 2015 08:50:14 -0400 Bridge rectifier with zener B+ lowering and bias(by AneirinE) I'm building an amp with an EL84 poweramp. The PT doesn't have a CT on the high voltage secondary so I am using a bridge rectifier. The PT doesn't have a bias tap so I thought I could try the AC coupling technique in a Marshall Artist 3203: <a href=" " target="newwin1427754551">[link]</a> After rectification I'm getting too high a B+ (about 400v), so I thought I could lower this with some zeners by about 45v (3 x 15v/5W zeners in series). So - I have some questions: 1) To bring the bias within range for a pair of EL84s, do I just reduce R1? If so, how much to? I was going to experiment with 15k or so but that is a total guess. I was also going to replace R2 with a 15k resistor in series with a pot to ground so it is adjustable. 2) How can I do this with zeners to reduce B+ as well (can I even do it?) and what kind of effect does this have on the bias voltage etc...? Thanks! Fri, 27 Mar 2015 00:54:48 -0400 Tonestacks. (by Alva Goldbook) I've been pondering tone stacks lately, and it rather bugs me that so many amps have used the Fender/Marshall tone stack. Because, in all honesty, it's a rather lousy circuit. The performance of the mids is just awful. In recording studio gear, no one would tolerate an EQ circuit that performed this poorly. I'm not sure of the origin of this circuit, but I once read something about how Leo Fender found it in an old manual from the 1920's or 1930's that was originally designed for telephone use. I think for new amp designs, this circuit ought to be put to bed. In a lot of tube driven studio gear, passive EQs tend to be far more sophisticated. Which would be a bit too cumbersome and expensive to place in a guitar amp. But with the exception of Ampeg SVTs, I have never once seen a single inductor used in a tone stack. Is there any reason for this? Perhaps inductors were expensive years ago, but these days you can get a shielded axial inductor that's about the same size as a resistor and cheaper than what most people pay for capacitors in tube guitar amps. So I was thinking that we should have a thread here for tonestack circuits other than the standard Fender/Marshall tone stack. I was reading through this the other day. <a href=" " target="newwin1427754551">[link]</a> And so I pieced this tone stack together based upon it. It's sort of similar to a James type circuit, and I'm sure it could be improved upon. <a href="" target="newwin1427754551">[link]</a> Thu, 26 Mar 2015 17:56:40 -0400 "Raffler" DC drive (mod) schematic(by proton45) Hello all, and thanks (in advance) for any help that you can give me... I have used the "search feature", but I dont seem to be able to find a working link, that will take me to the "Raffler mod schematic". It seems, that the only LINK I can find, no longer leads to a working web page. I get a dead "Earthlink" page. I have read the mod description, in the "The High Octane Modification Guide", but I would still find a schematic very helpful... Could someone please post a copy of the "Raffler DC drive mod" integrated into a schematic...thank you very much!!!Wed, 25 Mar 2015 10:55:23 -0400 Converting Roberts Tape Deck to P1(by vonholt) so last summer I scored a Roberts 90c tape machine and the accompanying 901 amp. After messing with the 901 amp and studying the schematics I thought this could be a good conversion to a P1. But this is my first tube build so I have some questions. Here is the schematics and part list of the 901. <a href=" " target="newwin1427754551">[link]</a> My main question is can I keep the power supply circuit as is? I am planning on replacing the filter caps due to age but I would take the 190 volt supply and put it on the plate of the 12ax7. And I don't think sending 260 volts will make a big difference vs 254 or will it? And what about the voltage for the OT? Should that remain at 270 or will that fluctuate too. I am trying to keep this as simple as possible for my first build. Wed, 25 Mar 2015 09:41:18 -0400 Testing for short in tubes(by Darren Porter) I do not have access to a tube-tester. Is there another way to find out if a tube has an internal short other than plugging it into an amp and waiting for something to burn-up? I am trying to fix this Bassman amp for a friend. The 470R screen resistor fried on one of the 6L6 output tubes and the tube was arcing blue sparks inside. I am guessing the tube is bad... very bad.Wed, 25 Mar 2015 07:31:39 -0400 Source Gain & Boost pots?(by Bugsi) I have three Rev.8 boards and just ordered parts from tubesandmore and Mouser today to build them. The PC mount 250K boost and 1 Mohm gain pots were no longer available: Mouser 313-1510F-250K and 313-1410F-1M. Respectively, Taiwan Alpha parts # RV16AF-20-15S1-A250K and RV16AF-20-15S1-A1M. Since these are very specific sizes with 5mm pin spacing 3.8mm back from the mounting surface, 12.5mm from board to shaft axis, 6mm dia. shaft and 8.382mm shaft length; which allow them to mount on the firefly PC board and fit the drill guide, I'd like to find these exact items or source something as similar as possible. Anyone have a line on a source for these or suitable alternative pots? I'd really like to have the pin mounts for directly mounting to the circuit board if possible.Wed, 25 Mar 2015 02:07:30 -0400 Gretsch Pro Bass 6170(by JIM T MULE) Hello, I'm working on bringing a late 1960's Gretsch Pro Bass 6170 back to life for a friend. I have seen some discussions out there about this amp (Valco-made, very similar to a Supro Thunderbolt) but not many, and I'm hoping someone out there has some experience with this circuit. I have rebuilt the power supply with new discrete filter caps and power resistors (along with a new Classic Tone OT) according to a Sean Weatherford schematic for a Harmony H420 which is identical to his Supro S6420 Thunderbolt schematic as far as I can tell. The original cap can values don't match those schematics so I split the difference on the values (higher capacitance and voltage ratings) but everything else seems to be in line with the schematic. The issue I'm having is that the single bias supply resistor for V3 & V4 is overheating. All of the voltages are in order except the phase inverter voltage was high the last time I took readings. I can't figure out why the resistor's burning up when the voltages are so close to spec. I have tried grounding it in a couple of different places to no avail. Any ideas or general feedback about this circuit is appreciated! Thanks, Tom Tue, 24 Mar 2015 17:37:41 -0400 Reticon SAD1024 BBD-chip... what to do with it?(by EeroNiemela) I have an old DOD Flanger 640 that I have not had any use for. It sucks tone really bad in bypass mode, is in poor condition in general (covered in sticky duct tape goo...) I opened it yesterday to see which BBD it uses, and it has the Reticon SAD1024, which I know are pretty rare and expensive these days. I don't really feel like restoring the pedal, I thought about maybe converting it to true bypass and changing the electrolytics etc... but and I'm not a big fan of flanging sounds in general and this unit in particular. But the SAD1024 seems too cool of a chip to leave in the pedal. So my question is, if anyone is aware of any cool analog delay circuits I could build around it? Chorus would be an option too, but I find I use delays more often. Thanks!Tue, 24 Mar 2015 12:45:54 -0400 Preamp signal voltage swings and switching(by Kostic) Hi, I have a reasonably good experience building pedals using solid state stuff, but recently decided to try myself in the tube territory. I can see there is a good bunch of fairly experienced people here, so hopefully somebody can shed the light on my question. I can generally understand tube based amp stages, load lines, biasing etc. However what I still can't quite understand is what kind of voltage swings are really happening in different stages of a typical tube preamp in practice? The context of this question is channel switching. I have seen schematics of preamps such as Soldano x88r, and also Framus Cobra amp. I tried simulating in LTSpice using different models of 12AX7 but I believe I get quite unrealistic results (even though the models do take account of grid current overdrive), e.g. around 200V swing (i.e. roughly 100V amplitude) after second stage of the x88r lead channel. In Soldano vactrols are used for switching, but even they (VTL5C1) are spec'ed at 100V max (obviously they only will see in the "off" state). In the Cobra they use H11F3 opto-fets, which are only spec'ed at +/-15V (30Vp-p), which leads me to believe that even at the last stages of the preamps the amplitudes are reasonably low. So, as I haven't got my hands "dirty" yet to try it out (still trying to understand as much as I can about all of this), can anybody share their practical experience? Thanks KosticTue, 24 Mar 2015 11:11:23 -0400 Need Some Sizzle With My Steak!(by vicrattlehead) So I finished my amp build (based off the HO) and I’m kind of ‘dialing it in’ to taste. So far where I’ve varied from the original preamp schematic is: -removed the 2nd preamp vol (my chassis is a little tight and didn’t have room for another vol.) -Moved the .001uF cap (C13) on the 220K divider to between stg 2 and 3. -Used a 120K carbon comp plate resistor on V4b and 91k on V4a and V3b. I’m not convinced they did much for adding ‘mojo’ but I figured ‘what the heck, why not try em’. -removed the cathode bypass cap from stg 3. (cath. resistor value has not been changed-yet). -changed the cathode cap on V4b to a 2.2uF (along with an 1820R resistor adjust bias w/ the 120K plate resistor). Aside from some resistor value tweaking to work with my non-standard power section it’s basically a HO. I’ve played with a few tubes (I don’t have many on hand) and what I noticed is that I actually prefer the sound with a Chinese tube in V1 (V4 on the schematic; the first tube in the preamp is just always V1 in my head for some reason). With a JJ in V1 it sounded good but it was just too ‘smooth’, for lack of a better word. There was tons of distortion on tap but it just lacked a little in ‘sizzle’ or bite. I’m actually running a Chinese 12AX7 in V1, a JJ 12AX7 in V2 and a Chinese EL84 for power. With a JJ in the power spot again it was just a little too ‘smooth’. I like the ‘bite’ I get with the Chinese tubes but they just lack ‘depth’ so I’d like to get that ‘bite’ without having to use them. How do I get just a little high frequency ‘sizzle’ (not looking for a ton, just a touch to add some ‘bite) without resorting to these cheap tubes? Turning up the treble pot really just adds some creamy highs but not really any more ‘bite’. I don’t want to get too much into chasing a sound through tubes though, as that can get expensive. I’d rather build a little more bite into the amp; unless anyone has an affordable tube suggestion. I’m kind of a NOOB at this (OK, not ‘kind of’; I’m REALLY a NOOB) and not sure where to start. It’s kind of funny, my other amp (a Jet city combo) has been an exercise in getting rid of sizzle and trying to smooth out the sound a little and this one is the exact opposite! Tue, 24 Mar 2015 10:39:56 -0400 What is making this buzz?(by DaveW) I have just built a 3 valve (ECC88) headphone amp, it sounds fantastic with no background hum noise or buzzing, until I turn down the volume pot. If I change the source volume no noise but if I wind down the in-built volume control it buzzes louder and then goes to zero again as the pot grounds the signal. Any ideas? Link to circuit below: <a href=" " target="newwin1427754551">[link]</a> Thanks for looking. DaveMon, 23 Mar 2015 07:13:03 -0400