AX84 BBS New Threads as of Sat, Aug 29th, 2015 @ 01:55:22 Amp Parts and KitsDoberman Music Products, LLC offers a variety of parts and kits KT88 screen question(by Denny Gracey) In pursuit of hum and noise reduction for the UHG build (using the stock SEL power supply), I built the rectifier/reservoir circuit on it's only little board, mounted in far corner of chassis, but left the original reservoir cap in place to add an extra filter. My question is which HT should I now feed the OT/KT88 plate for 'ideal' plate/screen condition? B+1 is 370V, B+2 is 350V (currently feeding OT/plate) and B+3 (to screen grid stopper) is 330V. Any advice or links to threads on the topic appreciated. Fri, 28 Aug 2015 23:05:50 -0400 Anybody with 6-8 inch guitar speaker cabs (sealed ported)(by sburck) Hello, So I have a dilemma - my ax84 1x12 is way too loud for a bedroom/apartment amp regardless of wattage. It simply moves too much air. Lower power amps (like my ax84 HO) won't push nearly the level of bass through my 1x12 (or 4x12), but the mids and highs are still more than loud enough to piss off everybody. I love how my ax84 1x12 is tuned to sound bigger and tighter than an open back configuration would. Would it be possible to do the same with a much smaller speaker, so as to create a cab that can sound... really good compared to tinny open back small speaker combos? I personally don't know anything about designing speaker enclosures, however I do have a background in math/engineering/computer science, so I can probably understand technical details/theory and make use of software tools if needed. I really have no idea how complicated designing one of these would be, and if it would be quiet enough and possess the desired frequency response (similar to a 4x12 or sealed 2x12)... Here are a couple speakers I have been looking at. <a href=" " target="newwin1440827723">[link]</a> <a href="" target="newwin1440827723">[link]</a> Fri, 28 Aug 2015 06:46:28 -0400 SEL PT options help.(by billpol66) Hi folks , Was wondering if this CT PT will work for an SEL , I'm just not sure about the 6.3@ 2A part. If not is tbere a list of usable sets other then the Hammonds . I am not looking for output power but would like the any octal option eventually. Thanks !Fri, 28 Aug 2015 07:18:05 -0400 LND150 implementation(by boblove) I'm looking to use a transistor based gain stage for a clean boost prior to a tube based gain stage and while I've been on this forum for several years and have successfully built several amps, I'm still kind of a noob. I looking to add an LND150 gain stage and I'm always baffled by the data sheets. I've seen several examples here and else where but I was wondering if any here has an example of (resistor values) and biasing..... I know this is probably alot easier than im making it out to be.Fri, 28 Aug 2015 05:36:24 -0400 Short "How Tubes Work" Video(by robrob) I edited and annotated the original 1943 film "Electronics at Work" to create a 3.5 minute video that has pretty good animations which show how electrons flow through vacuum tubes to amplify signals and rectify DC: <a href=" " target="newwin1440827723">[link]</a> Wed, 26 Aug 2015 14:20:20 -0400 UberSEL PS - is it a regulated PS?(by Denny Gracey) I've been reading up on regulated power supplies, mostly in pursuit of minimizing noise in a my UHG build and of course expanding learning. I've found references on the forum to regulated PS schematics, but the posts are older and links to schematics in those posts are no longer active. The UberSEL PS looks interesting - definitely different than the SEL PS I'm currently using. Is it a regulated PS and/or will it generate less high freq noise, both radiated and ripple? I'd also be interested in studying other PS schematics for single ended high gain designs, so any links much appreciated. DennyWed, 26 Aug 2015 14:32:59 -0400 a Queen covert from the good old days(by Kursad K) <a href=" " target="newwin1440827723">[link]</a> circa 1994, a mostly unknown all-female cover band from Istanbul, and 2 years later it was discovered by large music producers and started sounding like this <a href=" " target="newwin1440827723">[link]</a> and the singer became a major star in her country. That's what they call 'investment' I guess, like buying the stocks of a small company and watching them grow exponentially in just 2 years. Kursad Wed, 26 Aug 2015 05:09:42 -0400 High Gain 12AU7(by Octal57) Hello,could someone look at my rather crude schematic and give me some hints on achieving Mesa/Soldano style aggression but still able to dial back for bluesy stuff? Just to say i always use a simple clean booster in front of my amps so that has to be taken in to consideration and the poweramp will be a fixed bias SE. <a href=" " target="newwin1440827723">[link]</a> ThanksThu, 27 Aug 2015 06:15:45 -0400 Is there a way to drive or excite a small plasma globe using aud(by CChurchill) I'm really just investigating an idea to use a small plasma ball in my head cab as an indicator lamp, using audio signals for visual effect. You might be wondering why I would go through all the trouble of having to integrate all the needed circuitry, the added cost(etc.), all for a glorified "on" jewel lamp. Well, I think it could look cool and I like learning new things. Now, I'm not proposing that I drop a commercially available globe in the middle of my amp chassis and call it a day. That would look ridiculous, cheap, and gaudy. I was thinking about something small, subtle, and distinct. Having said that, I admit, I don't have any background in high frequency/high voltage circuits or flyback power supply designs, so on and so forth. Plus, I have some concerns that this may look cool, but would invite major problems in a guitar amp that really make it unrealistic. Namely, very noisy circuitry, high voltage arcing, and massive interference caused by the electric fields generated near sensitive high gain, high impedance signals. I bring it up on this forum because, well, it never disappoints- The knowledge pool is deep and I'm calling on you hardcore science enthusiasts, who never let a perfectly good high voltage supply go to waste and wind your own Tesla coils, or build Rife beam ray tubes. This idea was influenced was probably influenced by two things. The first was experimenting with LED biasing in the output stage of my amplifier and finding it interesting to see real time, fairly drastic, visual indications of how the output tubes transitioned from small signal level to full drive while actually playing in real time and not relying on a signal generator under test. Nothing ground breaking there, just interesting to "see" it that way. The second, is a local start-up company from here in the Boston area, called oneTesla. They developed a line of DIY "musical" Tesla coil kits. Their coils are controlled by midi commands sent from files on an SD card or a midi keyboard. The midi files feed an interrupter which receives an audio frequency command and feeds the resonant pulses which generate the sparks at the audio frequency waveform peaks (I would imagine analogous to a square wave peak?). I love the idea of it and it is a cool experience to see a tesla coil spark playing the Mario Bros. theme, however, the actual sound generated is almost unlistenable to me. Because of my concerns about using traditional plasma globe designs which I've listed about, I did some searching and came across something cool and different. This is a project where a guy turned a nixie tube into a plasma ball. see here: <a href=" " target="newwin1440827723">[link]</a> and here: <a href=" " target="newwin1440827723">[link]</a> This is small, doable, and creative. Is there any way of exiting the plasma using audio signals as a apposed to touching the surface?Fri, 21 Aug 2015 15:17:32 -0400 Opinions on post PI MV.(by Alva Goldbook) I wanted to get everyone's opinion on PPIMVs. Do you like them? Hate them? I've never used an amp before that had one and I was wondering if they sounded better and were more User friendly than standard MVs. Are they problematic in amps with a fixed bias and NFB?Fri, 21 Aug 2015 16:56:36 -0400 Bias Question(by SX4Amps) I have a 20W PP power amp with a Lead Preamp. I have been experimenting with different sets of 6V6GT tubes and noticed that some of them that I would like to bias a little hotter (not more than 70%) but I run out of room turning the bias pot. I am using the 50k pot which the schematic calls for. Would I need to lower higher the value of R9 or lower or higher the value of R10 room get more room on the pot? Wed, 19 Aug 2015 10:03:47 -0400 Homemade Amp(by EPFahey) I found a homemade amp. Looks like the design idea was Twin Reverb, blackface era. 4-5881s. I recapped the pwr supply and changed out the screen and control grid resistors. Same values. It distorts at low volumes, and all volumes. I researched biasing, and found that the plate dissipation on the power tubes was around 9.00 watts. The recommended PD is 16.1 watts [70% of 23 watts, from tube data sheet]. It is biased cold; and I cannot seem to raise it. I am reading -30V on pin 5, the control grid. How to lower grid bias or raise plate dissipation? Ther is a bias pot, but the range of negative voltages on it will not go lower than about -36V. I drew a diagram, if anybody wants to look at it. The date codes on the Xformers are 1959.Mon, 17 Aug 2015 11:50:39 -0400 Software amps(by Jamie Anderson) I haven't been building and playing for a while, but built many diy amps here over the years. Just recently I decided to carve out the time to play a little guitar again and have been playing through an app on the ipad, so my ear is completely unfamiliar with real tube sound after such a long layoff. Just an opinion from guys here who build real amps and use software amps as well, does the Bias app sound realistic? My ears have reverted to "tin" and I like playing through the emulation and purposely haven't pulled out my P1, Hi Octane, champs, or any of my other DIY stuff just in the name of simplicity and I want to play and fear that if I get out a real amp again I will be reaching for the soldering iron. Also, I like how I can go in on Bias and "build" an amp in the app, choose tubes, preamp stages, transformers, rectifiers, speakers, etc. Most of my creations in there are mediocre, but some of the stuff others have created sound pretty close to some of the original amps. There is a champ simulation that nails the clean of what I remember my silverface champ sounding like, and the tweed stuff sounds bassy/farty, but in a good way and modelling usually fails pretty bad here. I haven't played around with any high end modelling software/hardware, just various modelling pedals and some windows software. Not trolling, just wondering if my ears have gone completely. Maybe I should pull out the 18 watt or 5E3 builds and forget about emulation. Fri, 28 Aug 2015 11:11:53 -0400 How linear are tube rectifiers?(by Alva Goldbook) I realize this topic has been covered before, but I was thinking about taking some new approaches to it. The idea is to accurately simulate a tube rectifier with solid state parts. Typically this is done with a lowish wirewound resistor after the solid state rectifier. But resistors are linear, and tube rectifiers are not. As I understand it, lower frequencies require more current to drive speakers, so when you play a lower note on a guitar, the tube rectifier has a harder time providing the necessary current, and you get a compression effect on the lower notes of the guitar. Would it be possible to split the B+ supply into two paths, in which you'd add an extra filter cap that would need to be charged, but only when the current demands are higher, thus more closely mimicking a tube rectifier? It would require, I think, that the cap would need to drain when the current demands were lower. Or am I simply over thinking this, and the same thing could more or less be accomplished just using lowish filter caps?Tue, 18 Aug 2015 00:05:47 -0400 Isolation transformer safety question(by David Staepelaere) Hi all, How useful/important is an isolation transformer for working on tube amp gear? That is, how much real safety benefit does it give? How many people use them? Up until know when I've needed to work on a powered-up amp, I simply plugged it into a mains plug and was careful. Pay attention, one hand in pocket, all that. Recently I've been watching a bunch of youtube videos on the subject of how important mains isolation is for safety, but I think you still really need to keep your wits about you even with an isolation transformer. Here's what I've gathered so far... First if one buys an isolation transformer unit (rather than building it from scratch) he would need to modify it so that "isolated" output is no longer earth referenced. This would require (a) defeating any internally connection between the secondary neutral and earth ground, and (b) defeating the safety ground at the isolated output plugs. Without (b) the moment I plug my amp in, it would become earth referenced again by virtue of the fact that my chassis has connections to both safety ground and signal ground. Second, even after I get the isolation transformer unit set up to allow the amp to "float", the moment I hook up a scope, it becomes earth referenced again by virtue of the scope's ground connection. So in the end it doesn't seem like the isolation transformer unit really buys all that much, at least while you are using a scope to debug your amp. Is this true, or am I overlooking something important? Thanks DavidSun, 16 Aug 2015 01:27:23 -0400 Output transformer impedance.(by GeorgeG) I am doing an SEL build. As I understand it, the impedance seen by the output tube as presented by the output transformer is a function of the load impedance of the transformer output. Is that correct? What is the range of impedance that my octal tubes can work with? I currently have 16 ohm speakers so the white tap on my 125ESE would reflect 5K back to the tube but it looks like that should be more like 2.5K. Second question, what is the purpose of the 220R/5W resistor from the white tap to gnd?Sun, 16 Aug 2015 05:30:51 -0400 Push pull transformer with parallel resistor across primary(by Darren Porter) I am looking at a design (it's from an in-print book so I can't/won't post schematic) but basically it is a push-pull output amp using a 12AT7 as the power tube. Bridging across the output transformer primaries/tube anodes (that is, in parallel) is a 33k/1W resistor. I believe from other sources that the transformer is rated at 150kR primary/8R secondary which is not an easy transformer to find. Does this effectively drop the primary of the OT down to 27kR? (via parallel resistors formula) This would make WAY more sense as an Raa value for a 12AT7 going push-pull than 150k does. Does it affect the output resistance value at all doing this? I would want it to be 8R still. This has me flummoxed...Sat, 15 Aug 2015 09:10:09 -0400 Cheap, small Function Generator?(by BlackGlass) Hi all, Been a long while since I've been here (although I surf through and keep up on discussions and attempting to build my knowledge as regularly as I can!)... I finally finished my first amp design and build (a small 5w converted from an old 50s radio) a while back, with a ton of help from the awesome Stephen Keller (thanks Stephen!), and when time permits I'll make a few recordings and share some images and clips. For now, whilst I'm pretty chuffed with that, I remain - obviously - pretty inexperienced. I'm hoping to start a new amp project later in the year, but I've just been asked by an acquaintance if I'd be able to fix up an old Selmer amp he has, which has had one channel fail, followed by the other. I'm quite keen to try and restore the old beast (not least as it will help me scratch my electronics itch without costing a fortune!), and I'm lucky enough to have access to an Oscilloscope, but I don't have a Function Generator. Now, here's the rub!... Obviously, an old, cheap analogue bench beastie would be the best bet on a budget, but space is a serious concern (read: my better half has made it extremely clear than more electronics-related paraphernalia cluttering up the house is not going to be welcome!), and so I'm trying to find something cheap and small that will do the trick, just for the minute, at least. All I'm planning on using it for is testing amps (diagnosis in this case, and ideally also for design/analysis on my next build), and that being the case would a cheap Chinese ebay-special like this one be any good: <a href=" " target="newwin1440827723">[link]</a> Or is that simply worthless? I'm kind of guessing the latter, but perhaps given the lowly requirements of my use case I could get away with it? Will I have any issues with analysis in terms of it not actually injecting the amplitude it's telling me it is due to lacking drive capability and impedance issues or anything like that? I know (and firmly agree with!) the old adage of getting what you pay for, but I guess I'm just hoping that in this case I don't need any more than what I'm paying for! Any thoughts?? Thanks as ever all!... Dom.Sat, 15 Aug 2015 09:59:37 -0400 How safe is cloth covered push back wire?(by Alva Goldbook) Ever since I first used cloth covered wire, I've loved the stuff. I've always found stripping plastic covered wire to be quite tedious, and I've always loved the look of cloth covered wire. I've put the stuff in a few guitars, but I've always been afraid to put the stuff in an amp. I figured the stuff could catch on fire. But I've noticed that there's plenty of old Fender amps out there that still have their original cloth covered wire, and they don't seem to catch on fire. Is the stuff really safe to use in an amp with all those high voltages?Sat, 22 Aug 2015 18:19:23 -0400 Tone and OT (by VanHanlon) Hi folks, I built a Firefly a while back, but I'm unhappy with the overall tone. It could really use a bit more top end. How would one go about adding some more sparkle to the sound? Also, when I put it back together recently (had it partially disassembled for mods that never happened), I mistakenly reversed the brown and red primary wires coming from the OT (color blind). The result was an amp that had a real heavy sag after I hit a chord, but the distortion was brutal - in a good way. What did I do? Can it be safely replicated? Thanks, JonathanThu, 27 Aug 2015 07:19:23 -0400 1x12 speaker cab(by epaoletta) I've finished my first build. It's a AX84 SEL. Works excelent in a chinise speaker... I want to make a speaker cab for a Eminence Wizard. I was about to start with the 1x12 speaker cab that AX84 design. But I read in the Wizard specifications that it has not data with a vented cab. Has someone tried that configuration? What are your recomendations? Thanks in advance. Excuse my poor english. Thu, 20 Aug 2015 05:53:58 -0400 Rivera(by Longview) Has anyone out there had much experience of Rivera amps? I am having an absolute pig of a day trying to diagnose a fault on the Negative side of the stabilised heaters, any help would be appreciated!Mon, 10 Aug 2015 12:25:37 -0400 b+ Too High! EL84 Red Plating(by Hank Cohen) I am building a homebrew ~18W reverb amp with an 2x EL84 push-pull power amp. It is a heavily modified derivitive of the Hoffman Stout design. One of the modifications was to replace the tube rectifier with a full wave diode rectifier. (In retrospect it seems this might have been an unwise choice) The transformer is center tapped so it's not a bridge. The power transformer is a MOJO762EX rated at 350VDC. With 100V AC in I'm seeing 488V before and 484V after the choke. Perhaps it should come as no surprise but one of the EL84's is red plating. I don't know yet if it is the socket or the tube but let's assume that it's the tube since only one tube is red plating. Obviously there's too much B+ on the plates and too much at G2 too. I'm wondering what to do about it. As I see it I have several options in order of decreasing desirability: 1. Try a different primary tap. I am using the 100V tap (Japan) but the 120V tap might give me a more suitable B+. This is obviously the most desirable alternative because it only requires flipping a switch. However if that doesn't do it I am considering: 2. Replace the EL84's with higher voltage tubes EL84M's from Sovtek or 7189's or 7189-A's. The 7189-A's are rated at 440V so they might work out of the box but if not it won't require a lot of sag resistance (and therefore dissipated power) to bring it into range. I'm a little leery of the Sovtek EL84M because I can't find a manufacturer's data sheet in English (or any other roman alphabet language). EL84M's are cheap but 7189-A's are expensive so I'd appreciate opinions on the merits and demerits of these tubes. I am constrained to Noval 9 pin tubes because I'm using Merlin's B9A boards and they don't take 8 pin sockets. 3. Drag down the B+ voltage with resistors or zener diodes. I'd appreciate hearing the pro's and con's of this approach. I expect that zeners will deliver a consistent voltage no matter what the current and might be favored for that although that might be a con for a guitar amp where you might want to see some power supply sag under load. My big concern here is that dropping over 100V with either resistors or zeners will generate a huge amount of heat. 4. Change the power supply to a tube rectifier. This would be a challenge to find space for the socket and also because I have co-opted the 5V secondary to drive possible future logic or relay functions. 5. Buy another power transformer with lower primary voltage. Of course this is the most expensive alternative so it is at the end of my list. Any comments, recommendations or suggestions would be greatly appreciated. Hank Sat, 15 Aug 2015 01:02:46 -0400 Modding AmpMaker donor to HO / L2L - advice needed(by tristanc) I'm in need of some advice for modding my amp to HO or L2L spec. Given the lack of AX84 kits readily available, the information here may be useful for others. The starting point is an AmpMaker N5X, very similar to a P1-eX, but 5W with VVR and being able to use both EL84s and octal valves. This is a great amp as stock, perfect for living room playing. But I found myself needing to use an EQ pedal and boost in front to get the level of OD/distortion I wanted. Ideally, I'd have this in the amp and be able to dial in cleaner sounds with the guitar controls - my dream is to have just amp, cable and guitar. I play blues, classic rock, 90s rock, indie etc. Nothing too extreme - anything else I can use pedals. Of the AX84 designs, both the HO and L2L seem to fit the bill. So the challenge is to modify the N5X to have the additional gain stages of these designs. I've had a go at grafting the circuit designs of the HO and L2L on to the N5X. The two files show the new or modified areas shaded in pink. L2L: <a href=" " target="newwin1440827723">[link]</a> this involved inserting the 3rd stage between the existing 2nd stage and tone stack, then the 4th stage after the tone stack. HO: <a href=" " target="newwin1440827723">[link]</a> the 3rd stage and CF are inserted between the 2nd stage and tone stack. NOTE - I've copied the wiring ciruit for the 1st gain control from the L2L schematic, R51 connecting lug 3 to just after C54. There are a few component changes that would need to be considered (R_ks and pot values). I've the following questions intially: - How do I supply power to the new / existing stages? Can I run a connection from 'E'? Or do I add a resistor & cap (R60 & C53) and make 'D'? The former is easier! - Are the bright / fat selector and tone stack bypass more or less redundant? - Given the voltages indicated, would there be additional bias changes needing to be made to get a similar sound to the 'stock' L2L / HO? - Is the change in HO wiring described above a good idea? I like the idea of keeping the volume similar whilst OD increases or decreases (again, perfect for living room playing). Thanks for any advice!Thu, 27 Aug 2015 05:55:12 -0400 Looking for how to measure power supply ripple on oscilloscope(by Darren Porter) Can anyone give me (or point me to a resource online) on how to see and measure power supply ripple using my DSO? I don't want to just go poking around looking for it because I think there are concerns with blowing it up if your grounding reference is not correct? I haven't found much in my searches. Thanks!Mon, 10 Aug 2015 02:48:32 -0400