AX84 BBS Threads with new messages as of Mon, Dec 22nd, 2014 @ 04:05:11 Amp Parts and KitsDoberman Music Products, LLC offers a variety of parts and kits JTM45 Split Cathode Switch(by Bluto) Hey Guys, I installed this little mod on a '67 JTM45 circuit that I built [IMG] <a href="[/IMG] " target="newwin1419239112">[link]</a> it works fine but I get a loud popping noise when I throw the switch in either direction. What would be the best way to quiet this down without compromising the splitting of the cathodes? I'd appreciate any input... Thanks BlutoMon, 22 Dec 2014 03:08:31 -0500 Valve brands(by Thin Freddy) Hi, Is there a voted preferred order of current EL84 valve manufacturers? What sounds best etc. Ie 1st, 2nd , 3rd, 4th etc I am using a Ruby EL84, how does it stack up?Mon, 22 Dec 2014 01:48:10 -0500 Distortion(by Thin Freddy) Hi, Does most of the overall overdrive sound come from the 1st or 2nd valve? Sun, 21 Dec 2014 14:12:29 -0500 EL34 Suggestions(by wolf papa) It's a two part question - What are some of the favorite homebrew amps, using the EL34/6CA7 (probably SE)? Run at a lesser B+ could be a plus. Secondly, What are the good, great, terrible ones to choose from? Manufacturer and retail name would be especially helpful. ThanksSun, 21 Dec 2014 13:52:20 -0500 pots in ltspice(by Dan Moos) What's the best way to do pots in ltspice? I know there is a thing somewhere on the Yahoo site, but I can't seem to find it. My dream would be a sub circuit that is just two resistors as a voltage divider, where I could specify the value of the pot and the percent of rotation, and it automatically sets the values of the two resistors. For a linear pot, I feel that would be easy, but I am not savvy enough on creating custom components in ltspice to know if it is even a capability. Worst come to worst, I know I can enter the resistor values by hand, but brings me to my next question. Logarithmic pots. I still want to use percent of rotation as my parameter, so how do I do the math to figure out the resistor values if I know the poet's value, and percent of rotation. I realize if I figure out one of the resistors, I can subtract to figure out the other. Any thoughts?Sun, 21 Dec 2014 13:35:35 -0500 pt idle voltage(by Nortube) Hi, I am a new member to this forum. First of all, I want to say thank you for all the effort the AX84 guys have put into this site and thank you for sharing your knowledge with the world. I'm gonna built two basic p1 heads, and I'm winding my own power transformer for them. I just pulled some trannies out of an old tv and a broken hammond organ. I'm keeping the primary windings and just winding up new secondarys. (I'm just building two because I got two OTs laying around. And it might be cool for stereo delay use) Can anybody tell me what the idle voltage (unloaded) on the 269ex / 369ex PTs? I'm guessing they're about 400 and 7VSun, 21 Dec 2014 09:13:25 -0500 Spitfire build troubleshooting (by dakota) I am building an AO43 conversion into a Spitfire Clone. I am using this layout: <a href=" " target="newwin1419239112">[link]</a> I've used donor OT, PT and tubes from the Hammond AO43. I have no sound. No hum, no anything. This is my first amp build and I'm at a loss! I feel like I'm missing something obvious. All voltage measurements match almost exactly the notes on the above schematic. could anyone help me out?Sun, 21 Dec 2014 04:36:57 -0500 Cheap EL84 Aternative, Anyone?(by Douche Baggins) Just in case anyone needs a hardy Russian tube that sounds nice, durable and an EL84 alternative here's a site that sells them cheap: <a href=" " target="newwin1419239112">[link]</a> Here is Chris's notes for using one in a P1: <a href="" target="newwin1419239112">[link]</a> Sun, 21 Dec 2014 03:16:47 -0500 help me plan new project(by Dan Moos) I am about to start a new design, and though I'd see if you guys had some input. Here is what I am after. I am very intrigued by Trainwreck style amps that are really sensitive to volume control on the guitar for distortion levels. I have glanced at some schematics, and am curious about a few things. Is negative feedback a big part of the recipe for this kind of amp? Also, I am having difficulty deciding on a wattage target.I want a decent mix of preamp and power amp breakup at club show levels, but when it cleans up it still needs to be heard. My most recent design is in the 25 watt range, and loaded with a fairly efficient Eminence Gov'ner, it gets plenty loud cleans, but I have to rely mostly on preamp distortion to keep volume reasonable. I envision a 6v6 or el84 based push pull amp. So far I've only worked with 6l6s, so any pros and cons to help me choose between those lower wattage tubes would be valued. Tonewise, I'm going for acdc type distortion, so think higher rock and roll gain, but not much more. I'll depend on pedals for my high gain stuff. The cleans just need to be decent. Tweed Fender would be optimum, but my usual gigs lean more towards classic rock type dirt, so I don't want to sacrifice that to get better cleans. My previous builds have been getting pretty complicated. FX loops, relay switching, extra gain stages, etc. I want to go the other direction with this one, including no master volume if possible. I'm even thinking one knob tone control. (The conjunctive filter I see in some Orange amps looks intriguing). Any ideas?Sat, 20 Dec 2014 09:33:02 -0500 HIgh voltage supply with mosfet regulation(by DanGu) Hi. I've got two power transformers rated 720VCT @ 260mA which I'd like to use for a bench high voltage supply to use for amp testing. I'm bored to build always the power supply searching for random transformers here and there or adapting them back to back or similar. So this has to be regulated with HT mosfet, and I'd like to select the voltage with a pot from 0V to about 480V. For the circuit I'd like the following approach <a href=" " target="newwin1419239112">[link]</a> Only needs to change the mosfet for higher voltage and sense resistor for current limiting The other thing I would change is the pot. It would have about 450/500V across it, and I think it's a bit dangerous if it fails. How can I regulate the voltage in a safer way? Do you know of any good 600V mosfet? Sat, 20 Dec 2014 02:58:36 -0500 No Boost problem(by broth) OK, I've built probably a half dozen Firefly's. After a long absence from building I put one together for a friend and I fired it up tonight and the clean channel sounds great. When I flip on the boost I get more volume but no distortion even with both pots at full. The amp is super quiet like my others but no distortion. I must have checked and rechecked a dozen times my connections. Anybody? Maybe just need to walk away from it for a bit. Thanks all.Fri, 19 Dec 2014 17:40:17 -0500 OT placement/wiring (by Denny Gracey) The only residual issue from the SEL build was OT hum, so decided to pull the board and do the headphone trick. Ironically the original placement (right next to PT per the standard board layout) turned out to be the worst, but unfortunately the quiet spot is directly over the pre-amp circuit. Looking for advice to minimize hum induced by secondaries. Couple initial questions: * The OT is equally quiet with 180deg rotation, so I can drop the secondaries thru the hole over gain 1 circuit, or over the tone stack. Suggestions? * I found on the forum recommendation to twist the secondaries but not clear on how to do that since I've got 4 wires (4/8/16 plus ground). Ideas, links, advice appreciated!Fri, 19 Dec 2014 04:03:14 -0500 Soft-start (standby) switch -- before or after rectifier?(by Henrik Lidbjrk) I'm about to solder a "soft start" switch, as Merlin has outlined on his Valve Wizard site ( <a href=" " target="newwin1419239112">[link]</a> This is in place of a standby switch and has (in my case) a 68 k resistor across the switch. But I'm not sure if I should put this switch before or after the rectifier? In Merlin's example, he's putting the switch after the rectifier, but on the same page he writes "A switch will lead a happier life in an AC circuit rather than a DC circuit [...] putting the switch before the rectifier is to be preferred." Maybe the AC/DC factor doesn't matter with the soft start switch design? My build is a 6v6 Plexi by Mark Huss: <a href="" target="newwin1419239112">[link]</a> Fri, 19 Dec 2014 03:34:51 -0500 California Dreamer build?(by Darren Porter) So, I have the summer off and I am thinking I'd like to do a California Dreamer. I mean, why not? It's summer and I just happen to live in California after all. I have a few questions... I've been doing some research and have found quite a few variations on this. There are obviously the plans here in the Archived Projects section, there is a "version 3.0" schematic here that I like: <a href=" " target="newwin1419239112">[link]</a> ...and I have also read that it is basically just an AX84 Core Simple preamp paired with a Core 20W PP plus-or-minus a few features and cap/res values. I have compared the projects and this does actually seem to be the case. If I was after that classic Blackface sound would I be happy with this setup (the core simple/20W PP)? I was wanting to do a core 20W PP someday anyways. What do you think of using a 1760H as the OT? I know you get what you pay for but it is about $70 cheaper than the 1650F. One final question... if I wanted to add an EF86 "Vox" channel and some channel switching relays, the 270FX spec'ed in the 20WPP should handle that load just fine shouldn't it? Or do you think there would be no point in doing that? I really like the EF86 channel on the DC30 clone I built. Thu, 18 Dec 2014 10:17:26 -0500 Wanted: fuses 400+ V(by Henrik Lidbjrk) I want a fuse for the B+ line (after rectifier) in my 6V6 Plexi build, but can only find fuses rated 250V. I need circa 400 V. Maybe 500 V is the next step after 250? Preferably 250-500 mA or thereabout. The usual small size glass fuse for mounting in a clamp style fuse holder inside the chassis.Thu, 18 Dec 2014 05:13:16 -0500 crackling sound when amp is warm(by JaapK) Guys, can you help me on this? Iím trying to repair a Fender hotrod deville 2 x 12 combo. It works and sounds great at first but after ten minutes of playing the sound crackles when you strum. Specially on loud chords and low strings, low notes. First I took the signal out of the pre amp out and put than into another amp. Result: no crackling sound. So the preamp section is oke. Then I changed te PI tube: no difference. Then I installed new power tubes: no difference. Then I cleaned the power tube sockets: no difference. No I donít know anymore. Itís that kind of failure that we you play higher notes or higher chords and not so loud it sounds oke. But when you play lower strings the crackling is there. Has to do something with the amp getting warmer cause it takes around 10 minutes before it starts. And I think it has to do with the power amp. Any suggestions? Thanks! Jaap Thu, 18 Dec 2014 03:19:13 -0500 Debugging a PI(by foulowl) Hi folks, I'm trying to build a p1 into a Peavey Mace chassis, using the power and output transformers. Here's my schematic: Preamp: <a href=" " target="newwin1419239112">[link]</a> I know the preamp is missing a few tone shaping components, but I don't care about that right now. I only care that the output of the preamp is an undistorted sine wave, which it is. Also note that I have tried several different preamp designs, include the full correct hi octane, my own designs, etc. Power amp: <a href=" " target="newwin1419239112">[link]</a> Power supply: <a href=" " target="newwin1419239112">[link]</a> (Bias supply is omitted from the schematic, but it is not relevant here AFAIK.) I get really harsh trebley clipping no matter what I do. I actually bought a scope just to try and debug this problem. I believe I have things narrowed down to the PI. Amp is being driving at the input with a roughly 9V peak-to-peak sine wave. Gain is set low. Power tubes are pulled for these measurements. Please note that I'm using a 100x probe. Here's the scope at point "F" on the schematic. (After the volume pot and before the cap leading into the PI) <a href=" " target="newwin1419239112">[link]</a> Decent looking sine wave. Here's the scope at point "G" on the schematic. (The inverted output of the PI) <a href=" " target="newwin1419239112">[link]</a> Here's the scope at point "H" on the schematic. (The non-inverted output of the PI) <a href=" " target="newwin1419239112">[link]</a> It's ugly as hell. I'm 99% sure that this section is the cause of the harsh clipping. I have checked my wiring over and over and over, no errors found. I have tried swapping out the 12AX7 being used for the PI. I am now at a loss. What do I do now? Thanks!!Wed, 17 Dec 2014 14:49:01 -0500 Anyone ever wind their own transformers?(by Darren Porter) Any good guides on how to do this? I know the Internet is full of stuff, but how does someone who's never done it before discern what guide is a good one... so hoping someone here can point me to a good source of info for this. I'd like to try it and see how it comes out. The transformer is really the only part of an amp build that we COULD do ourselves but don't. I know it may cost more, I know it WILL take longer. This is not driven by cost/time savings, etc. It's driven by a desire for more of the amp to be DIY and because I think I might enjoy the challenge. I see that Edcor sells everything I'd need to wind my own.Wed, 17 Dec 2014 14:33:31 -0500 .25 (1/4) watt Project(by Jazz5dad) Request a little help here, may have been addressed somewhere but donít see where. I want to design and build a .25 watt tube amp with volume and master only without sacrificing tone. I already built the Gilmore Jr and the tone is fantastic. Iím thinking about using design of the Gilmore Jr as a base to modify but not educated enough yet to make those changes without some guidance. Also, if there is a prof tested design on this, or any other forum, already in existence with similar design, I would like to take a look and study. Your help would be greatly appreciated. Thanks Ken Wed, 17 Dec 2014 14:32:02 -0500 The secret to a hum-free stand-alone preamp is...(by Darren Porter) .... what??? I've built two now and I'm totally frustrated because they both hum like crazy. I've done other complete builds (one with a layout of my own making) that are hum-free so I _think_ I have a good understanding of a good grounding layout. I do NOT want to go the route of lifting the third prong on the cable either. I want a real, safe solution that also does not require buying a $45 Hum-X or building a clone of one into my circuit (unless that's the only/best solution.) I have other "commercial" preamps that don't hum. How do they do it? My last attempt was Merlin's mid-gain example from his preamp book as a preamp only feeding my Simple Preamp/20W Push-Pull and it buzzes like a hive full of baritone bees. I think I like how it sounds but I can't tell for sure with all the buzzzzzzzzz..... Tue, 16 Dec 2014 18:37:00 -0500 curve tracers... again!(by DanGu) Hi. I've recently build that merlins curve tracer posted in his site, wich uses a scope to reproduce the plot. This works great so I actually started to think on an improved version of it, always using the external X deflection of the scope to trace the plot, to test even bigger pentodes or tetrodes like kt88 6550 etc So, for anode supply I decided to use a center tapped transformer and place a small resistor there to ground for current sensing, and a voltage divider from anode to ground for the voltage axis. Aiming for about 500V scale, how much current should the transformer be able to supply for bigger tubes?? I'm aiming for a maximum screen voltage of 400Vdc Second point: for grid bias I'd like to generate a max of -80VDC , 8traces at max due to 50Hz flikering. The 4040 binary to analog conversion with resistor ladder is great but cant reach -80V. Is there a way to amplify the bias steps to desired voltage? maybe morfets or opamps? this is the darkest point for me to solve Thanks Tue, 16 Dec 2014 14:22:03 -0500 FF Boost hum advice needed(by Ben H) Hello, First poster here, long time lurker! I have just completed my first valve amp and decided upon a firefly amp. Sound great on clean but, there is considerable hum on boost. At present I am using 12AX7 for the pre amps and 12AT7 for the output, sounds great. However when I engage boost there is considerable hum. Without boost it is quiet. I have shielded the cable from the switch to the board, tried shielding the signal path through the boost section, but signal just vanished, so back to un screened cable. There is good separation between input voltage and B+ I have moved the valves around and got rid of oscillation that was there to begin with, but can't seem to find out the cause of this hum. I have rotated the output transformer which did not make a difference. I have also swapped valves, but to no avail. I have gut shots, but do I need to host them and will then post a link to them Thanks for any suggestions! BTue, 16 Dec 2014 11:15:43 -0500 Where to solder the 50V bias wire?(by Henrik Lidbjrk) I'm building a Mark Huss 6V6 Plexi, and in his schematic there is no separate bias wire from the OT, so he connects one of the HT secondaries to the bias circuit through a 220k&#937; 3W resistor. My OT has a dedicated 50V bias wire and I thought it would be neat to use it. So, do I simply connect this bias wire directly to the diode in the bias circuit and skip the 220k&#937; resistor?Tue, 16 Dec 2014 03:07:37 -0500 Smoke!(by winghamwill) Have an SEL which hasn't worked for a little while now, but I started looking at it again, before I was getting a much quieter sound than I should have, now I am getting smoke, and on inspection i can see that the 5k5w resistor is getting red hot... Any ideas what this would be happening? A possible sign of a dud resistor? Cheers WillMon, 15 Dec 2014 03:04:53 -0500 FMV tonestack "sweep" idea.(by Greg Robinson) Hi everyone, I got to thinking about sweepable mids on a FMV tonestack recently, I often get requests for this and have always done the "shift" type control, adding a pot in series with the slope resistor, as is quite popular. It works, but I've always wanted something that behaves a little differently, isn't quite as lossy, and works a little more intuitively and controllably with commonly available pots (I never liked the "backwards" behaviour of an audio pot for the shift control, and using a linear lost all fine control). So I just did a quick simulation of an idea, I'm sure it's not original, but I don't believe I've seen it anywhere else, so I thought I'd share. I haven't actually tried it yet, so it's worth as much as you've paid for it right now ;) It simulates as working equally well plate driven, although in this example I've used typical cathode follower source impedance, and Marshall style component values. If you were to try this with Fender style values, you'd probably want to up the "sweep" pot to a 250k or thereabouts. Let me know if you've seen this idea before (and where) and what you think, and especially if you've tried it before. I'll give it a try as soon as I get a chance. <a href="" target="newwin1419239112">[link]</a> Sun, 14 Dec 2014 23:53:08 -0500