AX84 BBS New Threads as of Fri, Mar 27th, 2015 @ 23:58:39 http://ax84.com Amp Parts and KitsDoberman Music Products, LLC offers a variety of parts and kitshttp://shop.dobermanamps.com 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, 27 Mar 2015 21:17:42 -0400 http://ax84.com/bbs/dm.php?id=489897 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="http://www.1darren1.com/2015/03/idea-for-clean-preamp.html " target="newwin1427515119">[link]</a> Fri, 27 Mar 2015 18:57:39 -0400 http://ax84.com/bbs/dm.php?id=489895 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 http://ax84.com/bbs/dm.php?id=489891 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="http://www.drtube.com/schematics/marshall/3203pwr.gif " target="newwin1427515119">[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 http://ax84.com/bbs/dm.php?id=489884 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="http://home.online.no/~jaeioluf/sound/eqpassiv.htm " target="newwin1427515119">[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="http://i.imgur.com/BOYmeLd.png" target="newwin1427515119">[link]</a> Thu, 26 Mar 2015 17:56:40 -0400 http://ax84.com/bbs/dm.php?id=489866 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? Fri, 27 Mar 2015 20:40:00 -0400 http://ax84.com/bbs/dm.php?id=489859 "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 http://ax84.com/bbs/dm.php?id=489857 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="http://imgur.com/a/J8iPN " target="newwin1427515119">[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 http://ax84.com/bbs/dm.php?id=489852 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 ownWed, 25 Mar 2015 07:20:46 -0400 http://ax84.com/bbs/dm.php?id=489839 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 http://ax84.com/bbs/dm.php?id=489838 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 http://ax84.com/bbs/dm.php?id=489829 Master volume control(by Denny Allen) I am building an amplifier for a cigar box guitar. I am using the simplest circuit possible, a 12ax7 preamp with a Marshall single knob tone control and volume control between the 12ax7 sections. The 12ax7 will drive a SE output stage with a 6005/6AQ5. My question is this. Would it be worth while to put a pot in the grid circuit of the 6005 as a master volume control? I want to crank open the volume control so as to overload the second stage 12ax7 and get some distortion and at the same time control the output to the speaker with the master volume control. I do not know if a single 12ax7 has enough gain to do this. Second question: I have a 16 ohm 2 watt pot which I could put between the output transformer and the speaker to use as a master volume control and get the gain of all three stages for distortion. Would this work? I will only have about 180 volts on the plate, so I will expect about 2 watts outputSun, 22 Mar 2015 22:45:14 -0400 http://ax84.com/bbs/dm.php?id=489822 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 http://ax84.com/bbs/dm.php?id=489821 Can you guys explain this one to me?(by Alva Goldbook) I've seen this in a few schematics here and there, mostly on a few more modern British amps. There's a standard triode gain stage, but there's a cap (typically between 50pf and 100pf) tied between the plate and the cathode. I don't recall ever reading about this in Merlin's book or any old tube manual. I figured it had something to do with Miller capacitance of the triode. Or maybe something to do with stabilizing the current in the cathode. <a href="http://i.imgur.com/aVDgxEy.png" target="newwin1427515119">[link]</a> Sun, 22 Mar 2015 15:14:38 -0400 http://ax84.com/bbs/dm.php?id=489811 Flame Proof vs. Flame Retardant(by KenInVic) Can one assume these two descriptions are synonymous when shopping for resistors?Fri, 20 Mar 2015 15:37:20 -0400 http://ax84.com/bbs/dm.php?id=489795 B+ current? needed for preamp(by Moose) Hi all, I am building a 2 channel, relay switched preamp, with jt 10K transformer output for balanced line out - solid state guy here trying to get up to speed on valves and need to know how much current it generally takes to drive each stage of these preamps if i am building a 2 channel preamp, lead 2 lite and simple preamp channel, with a cathode follower 12bh7 driving a JT-10K61-1m transformer for a balanced line out, what B+ current would i need on my power supply? i have a Triad R-23B sitting in a box, which is 250 volts at 22ma. - the trans has enough current for heaters, but i can't figure out if it has enough current on the B+ side for preamp power, no output tubes i'll pick up a R22 if i need to, but i am trying to keep this small, portable and pedal like - its driving a class D amp, after a line in/out effects unit 3 - total 12ax7,s 1 - 12BH7 6 preamp stages plus the cathode follower thanks, MSat, 21 Mar 2015 05:12:58 -0400 http://ax84.com/bbs/dm.php?id=489783 Two Switch Jacks(by GeoMan) I'm not sure how to ask this question correctly, so bear with me. Is there such thing as a switched jack with both a normally open switch and a normally closed switch? I have vision of a combo amp with a 16R load, and a jack to plug in a 16R external. If those are parallel I now have an 8R load with the external plugged in and need a different tap off my OT. I'd like to be able to affect this change simply by plugging a cabinet into the external jack. I can't see a way to do this using only one jack unless it has two switches - NO for the 8R tap, and NC for the 16R tap. I can't find anything that seems like what I'm looking from the normal suppliers, but I'm not sure I'd recognize it if I did. Have I meandered off the road somewhere? Thanks -PLFri, 20 Mar 2015 13:13:45 -0400 http://ax84.com/bbs/dm.php?id=489782 Laying Out the Ground Bus(by KenInVic) I'm new to amp building and have just acquired a P1 kit from Doberman Music. A number of items come to mind regarding attaching and laying out the ground bus on the eyelet board Chris so kindly provided. Here are my questions: Do I lay the ground bus out first or sequentially as I add components? or after adding components? What is the proper method for tagging the ground bus to the eyelet board? How do I tie a component to the ground bus and the eyelet at the same time? Instructionals or links to show-and-tells greatly appreciated.Fri, 20 Mar 2015 09:41:42 -0400 http://ax84.com/bbs/dm.php?id=489774 Inline fuse holder Voltage rating(by Leroy from Oz) Hello All What do you guy use for your inline fuse holders? I can only seem to find 12v auto ones and I'm concerned about the insulation withstanding 400v. Are they safe to use? Leroy Wed, 18 Mar 2015 04:34:56 -0400 http://ax84.com/bbs/dm.php?id=489758 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 http://ax84.com/bbs/dm.php?id=489742 Panel Meters(by lkrasner) I've seen this discussed a few times before, but haven't gotten a good answer. I want to add a panel meter or adjusting the bias without any kind of meter (let's assume cathode bias for now). Should I put the classic 1 Ohm resistor with a millivolt meter across it? or should I put 0-100 ma current meter in series and leave out the 1 Ohm resistor? If I want to test parallel tubes, what's the best way to add a switch to let me use just 1 meter? Along the same lines, should I always leave the meter in the circuit, or should I switch it on and off? If this is not a terrible idea, does anyone know where I can buy small analog meters for a reasonable price?Tue, 17 Mar 2015 16:53:43 -0400 http://ax84.com/bbs/dm.php?id=489728 double SE EL84 PT?(by sixstringer) I have been using the recommended Hammond 269EX with 1 EL84 power tube. The B+ current draw for a single EL-84 is 46.1 mA but with two EL84s it's 85.9 mA which exceeds the 269EX current limits. (although it seems to work just fine, don't ask me how I know this) :). I have not been able to find a suitable replacement with specs similar to the Hammond 269EX with a current capability of 100mA or more. Do you have a recommendation for a suitable PT for this application? Thanks for your help.Thu, 19 Mar 2015 21:40:29 -0400 http://ax84.com/bbs/dm.php?id=489725 October with Antek PT question...(by Darren Porter) I'm having a hard time making sense of the datasheet for this transformer for some reason. If I wanted to do an October Stage with SS rectifier, would this transformer work? <a href="http://www.antekinc.com/as-1t300-100va-300v-transformer/ " target="newwin1427515119">[link]</a> How would I wire it? That thing is only $34.50!!!Wed, 18 Mar 2015 14:26:01 -0400 http://ax84.com/bbs/dm.php?id=489710 Basic blackface Tremolo question(by Alva Goldbook) I've been looking at the tremolo circuit on Fender blackface amps, which uses a neon octo-coupler. Does anyone know if such a device only works on one side of an AC waveform, like using a diode shunt to ground clipper circuit only effects one side of the wave form, or does the neon octo-coupler work on both sides of an AC waveform? Tue, 17 Mar 2015 07:03:18 -0400 http://ax84.com/bbs/dm.php?id=489708 Problems (I think) with 5E3 B+ voltages(by GuiteLectrique) Hi everybody, I posted this on TDPRI without results so I thought I'd have better chances over here :) I recently noticed the output volume on my 5E3 clone was significantly lower than before. I just checked the B+ voltages and they give me 380VDC at first cap (plates), 269VDC at second (screen) and 193VDC at third. Am I wrong to assume that screen voltage should be around 325DC with a plate voltage of 380DC? Both filter resistors read their appropriate values (4.7K and 22K). The only changes I made are the addition of a VVR with an additional filter cap after rectifier (measuring 385VDC at B+) and I changed the stock first capacitor to 30uF (so filter caps are 16uf (rectifier) - 30uf (B+1) - 16uf (B+2) - 16uf (B+3)). I tried two sets of power tubes with the same results. What could cause the B+ to drop so significantly after the first resistor? I guess I could always lower the first resistor value but I wonder if the problem doesn't lie somewhere else. Thanks for your help, GuiTue, 17 Mar 2015 13:37:23 -0400 http://ax84.com/bbs/dm.php?id=489699