AX84 BBS Threads with new messages as of Tue, Sep 16th, 2014 @ 12:47:06 Amp Parts and KitsDoberman Music Products, LLC offers a variety of parts and kits Biasing for a KT88(by Asa Patterson) I decided to install the variable cathode bias mod for the P1Ex so I could use a KT88 for the output tube. I'm getting 410v on the plate and the PaMax for the KT88 seems to be 42W. (The tube bias calculator on the site uses 35W, dunno why). Plugging those values into the formula given on the Variable Cathode Bias Mod Page: (42 * .9)/410 I get .092mA. The problem arises when I actually go to bias the tube. Turning the bias pot, the max and minimum values I get when measuring across the test points are .037v and .02v respectively. If the max at .037v represents 37mA, that seems way too cold doesn't it? Any help would be greatly appreciated, thank you!Tue, 16 Sep 2014 10:41:23 -0400 fixed to cathode switch in PR clone(by cameron nicklaus) I'm about to get started on this one and I thought I'd put the switch in, I'm wondering if there's a way to keep the bias trem working in cathode bias mode. any thoughts? seems to me that the cathode bias will have current through it, so just connecting the oscillator circuit won't work. But maybe disconnecting the fixed bias voltage, then grounding the osc circuit but leaving it connected to the grid might do it? I've built a few amps but am a relative novice. Tue, 16 Sep 2014 10:05:18 -0400 he Pink Floyd Tribute Show (2011)(by Kursad K) <a href=" " target="newwin1410886026">[link]</a> I don't know who these guys are, but seems to be really well played. KursadTue, 16 Sep 2014 08:21:40 -0400 P1 eXtreme transformer question(by aeronaut) Greetings, Has anyone played around with other transformers for the P1 eXtreme? Thinking of doing it for a first build, and it looks like one can get an Edcor CXPP30-MS-5K for about the same price as the Hammond 125ESE output transformer. The Edcor is rated at 30W, 20 Hz to 20 kHz, and has a 5k primary and 4, 8, and 16 ohm outputs. Seems like a lot more iron for the money, and rated to handle the bigger tubes the P1 eXtreme can take (perhaps a little shy for the KT88, but much closer than the Hammond's 15 W). Might be more than one needs, but for the same price, seems like a no brainer. Any concerns or potential problems? Or is the 30 Watt push pull rating the same as a SE 15 Watt because in push pull it only conducts part of the time? Even so, the better frequency response is still an upgrade, albeit perhaps not a useful one. Can't link it, but go to, menu bar class X, item push-pull. Seems to be way better value than the hammond tranny. Regards, Martin Tue, 16 Sep 2014 07:09:47 -0400 I NEED an NOS 12ax7(by GeoMan) So here's my situation. My buddy gave me a really old Supro 8 to look at and see if I could fix up. Since he was waiting to come up with the spare change for the parts I had it in a closet in my spare bedroom. One night during a thunderstorm my dog freaked out, ran into the closet, knocked over the amp, stepped in it, and broke the pre (12ax7) and power (6v6) tubes. Now obviously i have to replace these tubes and i feel like they need to be vintage since they were vintage. I have a supply of old 6v6s that I recovered from a radio station, but no 12ax7s. Before I run off and pay internet price for one, I was wondering if anyone here might have one they'd be willing to swap for some NOS 6v6s. If so, reply and I'll get you an email address so we can work something out. Thanks PatrickTue, 16 Sep 2014 02:18:18 -0400 power transformer for reverb(by Narciso Fiore) Hi guys, very pleased to be here once again!!0About two yeras ago you helped me very kindly with my first building: an hi octane amp (with octal tube and variable bias mods). I'm very proud with the amp but now I'd like to add a reverb tank. Actually I'd like build a new HO with such a feature. I've found a simple mod that picks up the audio signal from OT (low impedance) to feed the reverb tank and requires a 12ax7 tube to recovery the reverbered signal. The mod can be found at, it has been realized and a couple of demos is available. Now, I've a question on the adding the third 12ax7 tube to the amp: taking into account (theoretically, with octal tube mod) 300+300+300+1500 mA as current consumption at the heaters and 1.2+1.2+1.2+100 mA for the plates consumption, I think I need a power tranformer that can supply,say, 3A for heater secondary and 125mA at power rail secondary. if I'm not wrong.... In this case I think the hammond 369ex (universal primary) is no more sufficient. Any idea on a good substitute (I think I'll be able to find only hammonds)? ....or I'm wrong? thanks in advance ps: sorry for my poor englishMon, 15 Sep 2014 22:20:04 -0400 Phase Inverter Buzzed(by Dire Wolf) I am getting a loud buzzing from my PI even with the preamp tubes out. I have used a wooden pencil to poke and prod and found as I tap on the grid connection wire for one side of the PI I can hear the tapping in the output. The preamp works and can be heard however this buzzing is dominate. With a lightbulb limiter the sound is still present, not as loud. I have built the circuit before and had no issues. The connection in question is pin 7 of the PI. Any thoughts. Here is the schematic. <a href="" target="newwin1410886026">[link]</a> Mon, 15 Sep 2014 17:44:14 -0400 Any blues preamp clips?(by David Staepelaere) Are there any good clips floating about for the blues preamp? (All I've found so far are mentions of intentions to post and offers to host.)Mon, 15 Sep 2014 08:16:34 -0400 Ugly distortion on power amp/clean setting(by foulowl) Hi folks, I've been pulling my hair out on this one. Nothing fancy, just a p1 preamp into a Marshall/Fender long tail PI, into a 6x6L6 push/pull Marshall style power amp. Getting really ugly distortion at high volume. It sounds like a fuzz pedal. I have rebuilt the preamp a total of three times, looks like that's not the issue. I pulled the cathode caps off the preamp too to try lowering the distortion, no luck. I have checked the power supply, power amp, and PI a number of times but haven't found anything. I have swapped the power and preamp tubes as well. Finally, I have also tried pulling all but one pair of the power tubes. I feel like maybe something is wired incorrectly in the PI or power section and I've just missed it. Not sure what I can measure for more clues. I do have a scope. Thanks!! Mon, 15 Sep 2014 04:51:31 -0400 bad 120Hz hum on 18-watt build(by Darren Porter) OK I finished my 18-watt build last night and I can tell I am _really_ going to like this amp BUT I have a really bad 120Hz hum. I don't have a scope but I think it's 120Hz based on comparing it by ear to a generated 120Hz signal. The level of the hum is affected by the volume, tone and VVR control. With the volume all the way down, I get no guitar signal but I still do get a slight hum. It is VERY loud with the VVR and volume cranked. Things I've tried to fix it (none made it better or worse)... - I elevated the heaters' center-tap to the cathode of the power tubes. (I make this connection directly off of the pins 3, and not between the Rk/Ck and ground, correct?) - I ran shielded wires to the input jack/V1. - I replaced V1 with a known good 12AX7. - "Chopsticking" around inside. - I fixed an "iffy" looking solder joint at one of the heater connections. What is the 6V lantern battery trick? If I run my heaters off of it instead of the AC supply and it goes away, does that mean my heater wiring is bad and I need to re-do it? Hoping I can try some easy things before I start figuring out where to post some gut-shots... It's a Lite IIb build, FWIW.Sun, 14 Sep 2014 20:34:09 -0400 Blackstar Gem or Not?(by Douche Baggins) Purchased this amp a few weeks ago and linked a noodling/demo: <a href=" " target="newwin1410886026">[link]</a> . Like to get opinions about it's tones. Sun, 14 Sep 2014 19:12:10 -0400 Quetion about octal option(by lentzell) Hello all, this is my first post. I have a background in electronics and audio and could possibly do the math myself, but I'd like to make a quick question: Why the cathode resistor for the EL84 alone is 130 ohms, and when both sockets are connected in parallel with the option implemented, turns out to be 220 ohms?. This is actually good for a 6V6 with roughly the anode voltage given, I believe, but since the two sockets share the cathode circuitry, doesn't it change significantly the bias point for the EL84? The two tubes, of course, are never to be used at the same time. Thanks in advance.Sun, 14 Sep 2014 11:48:29 -0400 wrecking a blues Jr. lol.(by Zambo) Ok so i am wrecking my blues Jr. Its already done. I just have to start chasing the squeals , hums and other crazy oscilations, and boy are there a lot of them this time. I am missing the .1uf 630v capacitor the goes between the primary leads right before the full wave bridge recto. Would that be causing a lot of hum? damage? I also added a presence knob and it just starts sounding fizzy from 12 oclock down to zero. wide open its ok. My layout is awful but i was working with the stock chassis. Any ideas on the capacitor on the primary or why my presence knob would sound so bad? Thanks , GregFri, 12 Sep 2014 23:49:10 -0400 Thrift Store Find(by boblove) Was in the salvation army store yesterday and stumbled across a hammond m3.... No price on it, no reverb unit on it either. Looked like the lower photo in <a href="! " target="newwin1410886026">[link]</a> I couldn't stop thinking about it. The back was off and could see the tube recto and a pair of 6v6's. I don't know if it worked but was really intrigued to the point of dreaming about it... Worth picking up? For what price? Like I need another project....and I don't have the room either...I guess thats what my storage unit is for huh.Fri, 12 Sep 2014 05:33:28 -0400 Proposed Solution to Impedence mismatch(by boblove) I'm looking for some feed back on an idea I have for using an extension cabinet. I'm currently using a single 12 in 16 ohm speaker in a combo amp and I'm looking to us an extension cabinet. My current problem is that the extension cabinet really needs to be 16 ohms as well but I've only 8 ohm cabinets right now. What about using an intermediate box to increase the load resistance like <a href=" " target="newwin1410886026">[link]</a> as a temporary solution until i fabricate or purchase a 16 ohm cabinet. Its hard to see in the link but thats supposed to be an 8 ohm 50 watt resistor in the box. 1. Will it work without damaging my OT ? 2. Will it sound okayThu, 11 Sep 2014 09:24:20 -0400 yellen-on-fed(by Kursad K) The forum rules clearly state that any political discussion is not welcomed, but, maybe we could consider this to be technical subject (as technical as economics could be, I have no idea). In short I wonder what does this mean: <a href=" " target="newwin1410886026">[link]</a> and what has been happening in your side of the world during the last 7 years or so, because apparently nowadays it affects almost everybody on the earth. Thanks for any comments and remember to keep it technical..:) KursadThu, 11 Sep 2014 08:04:09 -0400 radial or axial(by clints) Any opinions on board layouts for axial vs radial? I have some radial 47uf 450's from a previous build i could reuse. Wed, 10 Sep 2014 18:47:22 -0400 bunch of tubes(by J Wiggins) I have a bunch of tubes that my father in law gave me, thinking of trying to piece an amp together with surplus tubes. Tubes are: 7ag7 7a6 7f7 7y4 12sj7 35l6gt 50l6gt 12sa7 victoreen corotron gv5a-2500 Any ideas?Wed, 10 Sep 2014 13:11:24 -0400 rectifier voltages question(by Darren Porter) Hey all. My current build is an 18-watter with VVR and I have wired the PT up but I am not sure if the voltages I am getting are correct... Everything looks good except for the heater voltages on my rectifier socket. With NO rectifier plugged-in, I am getting 9.1 to 9.4 volts across the pins for the heaters. (FWIW, the rectifier will be an EZ81, so it's pins 4 & 5.) Will this drop to normal once the rectifier is plugged-in? I don't want to risk blowing-up the tube with 50% more than it's rated heater voltage! Again, there is NO load to the socket at all... NO rectifier plugged in and I have NOT wired-up the post-rectifier filter network or the VVR. Thanks! Wed, 10 Sep 2014 03:27:35 -0400 New Amp - pics(by Stevo) Hey all, just wanted to share pics of the new beast. This is my most ambitious build so far - complete with two tone cabinet covering and hand printed faceplates and tag strip construction inside! The preamp is an Uber lead - stock so far - while the power amp is a 50 watt push-pull with 6CA7s fed by a cathodyne PI (lifted from Merlin's article here: <a href=" " target="newwin1410886026">[link]</a> ). The album is here: <a href=" " target="newwin1410886026">[link]</a> When I get around to recording some clips, I'll revisit the thread to post them. In the meantime, however, I'd just like to say this is amp is already my favorite. The preamp gets nice and heavy, but remains tight and focussed. Backing off on the gain really cleans the amp up, though, which is pretty cool. If I were to build this amp again, I'd probably add an Orange style FAC to introduce some more bass at lower gain settings, but I'm not really using this amp for low gain stuff anyway. The power supply is rather more conventional than the UberSEL - no choke, no capacitance multiplier, 100uF reservoir cap with 47uF caps for the rest. Kudos go out to Alex for the preamp design and Merlin for making his knowledge and schematics freely available.Tue, 09 Sep 2014 08:20:58 -0400 Can the PT Take It?(by turtle441) Question regarding selecting an appropriate PT for a project. I'm working on a homebrew 2-channel amp, current plan will require 4 12AX7's, plus 2 EL84's running push-pull and a tube rectifier. I've built a couple of small kits before, but this is my first stab at putting together the pieces to design my own. Big question is, is there any formula/calculation I can use to figure out whether a particular PT would be adequate for the amp? I'm looking at a Heyboer 18W set that I think would work. Looking at the # of amps drawn by the heaters, it should be able to handle the load with a 10% safety margin. But, I haven't been able to find any calculations to figure out if the high voltage secondary's up to the load. Do I need to worry about this?Tue, 09 Sep 2014 04:31:01 -0400 Passing the time(by Ken Graves) On my job I have to stay alert but there's not much activity for me. So, when I don't have a project going I need to keep my hands busy. So I bought some mild steel from Home Depot, some files and wet or dry and went to work. Since this is mild steel and can't hold an edge long I call it a letter opener. <a href=" " target="newwin1410886026">[link]</a> <a href=" " target="newwin1410886026">[link]</a> When my daughter in law saw it she asked if I could make a letter opener for her so . . . <a href=" " target="newwin1410886026">[link]</a> <a href=" " target="newwin1410886026">[link]</a> My next couple of tries will be with carbon steel to be hardened so they will be sure enough real Daggers. One will have a Rosewood handle, the next will use Bocote. Mon, 08 Sep 2014 14:23:53 -0400 New Vishay MKP 20uF & 30uF 700V low profile film caps(by CChurchill) for sale: Brand new Vishay metalized polypropylene film capacitors in the following values: 20uF/700v <a href=" " target="newwin1410886026">[link]</a> <a href=" " target="newwin1410886026">[link]</a> 30uF/700v <a href=" " target="newwin1410886026">[link]</a> <a href=" " target="newwin1410886026">[link]</a> High Voltage rating, low ESR, and compact size make it a good option for DC filtering in amps where board and chassis real estate is a consideration. Plus, they look cool. Here's the deal with the 4 leads... These caps with 4 leads have about half the ESR (Equivolent Series Resistance) of the same capacitors with 2 leads. The leads at each end share continuity with the other(at the same end). In other words, (in theory)you could attach one lead at each end into an appropriate circuit and have a functioning capacitor. That is probably the most importand distiction and if you want anymore info, datasheets are available. I hope that clears up any confusion someone may have who isn't familiar with this style. Mon, 08 Sep 2014 14:16:45 -0400 Merlin's new curve tracer design(by Ardencaple) As usual, a concise and interesting project description from Merlin: <a href=" " target="newwin1410886026">[link]</a> There are some other interesting curve tracers out there. Merlin links to the u-Tracer, which is well regarded (a friend of mine has one, and rates it) [ <a href="] " target="newwin1410886026">[link]</a> There is also this idea, which uses a PC sound card to capture the I/V values, and also to generate the grid levels: <a href=" " target="newwin1410886026">[link]</a> Merlin uses a Supertex LR8 high voltage regulator in his design, and notes that they are almost impossible to source in the UK. I bought a couple from Tube-Town with my last order. They are in Germany, but I have found them to be very good on delivery, and not too expensive. Mon, 08 Sep 2014 08:02:29 -0400 Clone LP Goldtop Project(by RickTown) So I went to eBay and spent $99 plus $65 shipping from China for a Les Paul P90 kit because at this price, why not... <a href=" " target="newwin1410886026">[link]</a> It's got a flame veneer over a plain maple top. Even though the flame looks nice, it's got planing chips out of it and a few small dings. I'm not bothered by the condition of the top because I've intended to do a goldtop since before it arrived. The neck looks and feels nice. I still need to measure it and compare it to my more comfortable guitars to see if I'm going to spend time contouring it or not. The black P90 covers might have to be swapped for cream covers and the black speed knobs might get swapped for amber witch hat knobs. The tuners are pretty authentic looking. Hopefully they're good. Same goes for the TOM style bridge. I do plan on checking the neck angle before glueing. I read somewhere that a 5 degree slope is typical on an LP. I've ordered goldtop gold aerosol and nitro lacquer from Reranch and hopefully will receive it this week. I've been mentally listing the best sequence of build stages and was hoping for some input. Step 1 Fill dings and chips. Sand and seal body and neck. Step 2 File fret edges. Step 3 Prime and sand top and headstock. Strip the bindings. Step 4 Set the neck. Step 5 Drill for bridge pins. Step 6 Paint top. Strip the bindings Step 7 Paint headstock. Step 8 Apply decals. Step 9 Lacquer. Step 10 Buff and polish. Step 11 Oil fretboard. Step 12 Hardware and setup. Although it might be easier to paint before setting the neck, I can't drill for the bridge until the neck is set and I don't want to drill after I paint. Or do I? Filing the frets might be easier on an assembled guitar except for the tops of the upper frets. Opinions would be welcome. This will be a slow process because of limited free time as well as wanting to do a quality job. I might even like how it looks, plays, and sounds when I'm done. So tell me what to expect and what I'm forgetting. I've never done a set neck and never worked with nitro lacquer. Mon, 08 Sep 2014 07:53:51 -0400