AX84 BBS New Threads as of Thu, Jan 29th, 2015 @ 14:53:07 Amp Parts and KitsDoberman Music Products, LLC offers a variety of parts and kits Are Siemens capacitors any good?(by Henrik Lidbjrk) I have access to an old surplus supply of components, which includes many old Siemens capacitors, exactly like these: <a href=" " target="newwin1422561188">[link]</a> They're probably from the 1960s-1980s some time. Anyone know if they are known to sound good / work well in tube amps? I've used two or three and those worked fine. Just wondering if they're considered good compared to for example Mallorys and Orange drops.Thu, 29 Jan 2015 12:26:35 -0500 Which one of these layouts do you prefer?(by Henrik Lidbjrk) Barely have I finished my 6V6 Plexi and I'm experimenting with a new build. The plan this time is to build a physically very small single ended amp based on the P1 eXtreme, with only one or two knobs, a small or no tone stack, octal output tube and about 15W output power. I've drawn a few ideas for an external layout here: <a href=" " target="newwin1422561188">[link]</a> Which one ya'll think is best? I'm leaning towards skipping the cap can and using a rectifier tube. The chassis is 178x127 mm (Hammond 1444-12).Thu, 29 Jan 2015 09:12:22 -0500 Helpful Hint - typo in PSUD 'rectifiers.txt'(by Rob Ott) I downloaded the extended file for 'rectifiers.txt' on Duncan's site and was modeling using 6AX5GT and getting an incorrect result. Looking at the file, I found Ipkr = 375A (not mA). So, if you are using an oddball or uncommon rectifier, check for typos before modeling. To correct: - open the .txt file (in wordpad or similar) - find the value to change and edit it - save the file using the same name and to the same directory where you keep PSUD.Thu, 29 Jan 2015 08:27:44 -0500 Triple single ended, palmer drei(by RoyGuitar) Hello, I've recently build the hi-octane with some mods and an extra clean channel. For that amp I also used a single-ended el34 power amp. Probably some of you have heard about the Palmer Drei. This amp has three knobs which allow you to dial in certain output tubes. Does anyone have a basic explanation. I'm not looking for a schematic or something. Thankyou very much! RoyThu, 29 Jan 2015 08:28:44 -0500 DC heaters?(by Alva Goldbook) I read something about this years ago. The idea was to rectify your heaters so that your preamp tubes ran on DC instead of AC. Wouldn't you have to use Pin 5 only and run it at 12.6v? Would you still be able to use a reference voltage in such an amp to protect the cathodes on cathode followers? Is it possible to get a useable DC heater voltage from an ordinary 6.3v tap?Thu, 29 Jan 2015 09:53:27 -0500 WIMA Capacitors - These will be fun to turret board!(by scr4mbles) When I was buying the parts for my upcoming build, I went with WIMA film capacitors because they seemed to be a pretty good brand and they weren't too expensive. Stuck with them for consistency. After receiving them, I realised that the leads on the things are tiny! Is there anything I can do that would be less time consuming than soldering longer leads onto the legs? I could just try to work around it, but I'd end up with the tiniest turret board ever seen! <a href=" " target="newwin1422561188">[link]</a> Serious gaffe on my part. The joys of the first build! Just glad I went with these screw blocks to see how everything would fit together first. If I'd bought everything all at once I'd be sorely disappointed!Thu, 29 Jan 2015 08:58:32 -0500 Max capacitance on a 6AX5GT???(by Rob Ott) Tube sheet says 10uF. The original amp (a 50's PA) had 2 x 10uF in parallel. Not a lot of info here or on the web re this topic. Modeling on Duncan PSUD says ~30uF gives a decently clean supply. Questions: 1. Can I go to 30uF? 2. What are the benefits of doing so or is it a waste (since early Fenders used 16uF)?Thu, 29 Jan 2015 07:30:23 -0500 Converting 6V6 plexi to EL34 or KT66?(by Henrik Lidbjrk) I love the sound of my newly built 6V6 Plexi (essentially an October Club Birthday model), but I'm tempted by idea to actually try EL34s or KT66s in it, mostly to see if it gets even more Marshall crunch. Now I'm basically wondering if this is a reasonable idea at all. Firstly, I know the PT doesn't have enough heater current for EL34s. 3.9A total with three 12AX7s and two EL34s is too much for this tranny which only delivers 3A. Is it a big no no to actually try for a short while? Secondly, would the B+ voltage have to be kicked up a lot in order for EL34s or KT66s to work in place of the 6V6s? The PT secondary is 330-0-330 (diode instead of GZ34 would give about 10V extra). Is KT66s more realistic than EL34s? (The Tung-Sol KT66 supposedly only draws 0.9A heater current so that shouldn't be an issue). In the end, what sound difference would EL34s and KT66s make, respectively, in place of 6V6s?Wed, 28 Jan 2015 01:26:14 -0500 Balanced line out(by quayhog) I have a Hilgen B2501 bass amp that I'd like to put a balanced line out circuit into. Factory schematic here: <a href=" " target="newwin1422561188">[link]</a> There Is plenty of room on the chassis to accommodate a transformer, jack and tube if necessary. I'd like suggestions on circuits. I have a lot of line transformers from Altec, Dukane and many others. I would like to put a a transformer coupled balanced line out for use in the studio. I have enough surplus current to wire in a small tube if necessary. What primary and secondary impedances should I expect to use? Any favorite circuits Jonathan Horne Quayhog53@gmail.comWed, 28 Jan 2015 04:42:17 -0500 No Bass amps?(by Len Ovens) After going through many of the threads in this area, it seems that the general feeling is tube for guitar and solid state for bass. In truth I run my bass clean all the time right now... because it does not sound good pushed in my SS amp (Traynor TB50). I have not been happy with 15 inch speakers, prefering 10s for sound. I have run the DI into a subwoofer when the TB50 is not enough... but I have found even though the crowd could hear fine, I could not. The sound went right out between my legs so far as I could tell. So far I have run my rig with more high end than I like because I can hear myself better. The DI from the amp goes to the PA where the it can be modeled to whatever the tech feels fits and balances with everything else. 50 watts has been enough for anything I have done but loud guitars. Drums have not been an issue. I have a clean tube mic pre that I use for recording. One of the things I like about it is that it uses the tubes for peek limmiting. I can push it a lot harder than my ss pre while still sounding clean. I am thinking something like this for a bass pre. This would allow the use of a smaller power amp. Or to put it another way it would allow one to get the most out for whatever power amp they had. So a tube pre, not for a tube sound, or maybe it is. Just not a distorted sound. does this make sense?Thu, 29 Jan 2015 01:35:09 -0500 Amp hums when I don't touch it but it doesn't know the tune.(by GreenCanoe) I built an Ampeg B15n (1968 version, Fixed bias)into an 18 Marshall chassis. I'm having a problem in that when I do not touch the strings or the chassis the amp has a buzz. None of the other amps I've built have this problem. I'm assuming the capacitance of my body to the shield of the chassis or strings kills the buzz. What is the best way to implement a fix to this problem?Wed, 28 Jan 2015 03:53:25 -0500 Marshall 50 watt build(by blindbrett) I am planning to build a Marshall jcm 800 with kt66 power tubes and was hoping to get some advice regarding specs needed for the power and output transformers. I don't want to risk using a power transformer that can't provide the juice necessary for a pair of kt66 tubes. Any help will be much appreciated.Tue, 27 Jan 2015 15:45:51 -0500 Electrodynamic to permanent magnet speaker conversion?(by Alanh) Can someone tell me how to do an Electrodynamic to permanent magnet speaker conversion? I have a 1947 Harmony model 200 and the ED speaker is shot. I would like to convert it over to permanent. My schematic does not give any value for the field coil. Thanks, AlanMon, 26 Jan 2015 14:25:05 -0500 Yey!(by dsmith) Yey! ax84 is back.Mon, 26 Jan 2015 15:21:07 -0500 2W PP Poweramp Bias Pot/BOM(by Jazz5dad) I'm in the process of selecting parts for subject and not sure what all the requirements are for the BIAS Pot. Also, is there a BOM for the 2w PP version? I notices all the other power amps have BOMs except the 2 W PP.Mon, 26 Jan 2015 09:50:15 -0500 Blues peramp motorboating(by Celeste hall) I gvE my laste bulespre fronted combo away to A newphew AS thanks for helping he so much in these times when I have to be supervised 24/7, tolds him play them all and you can have the one you lile the best, figured I could just build anothwer for myself, so I did on a little SE EF84 chassis I have lying arounnd, did not have room for thr tone stask, so I replaced it with the single knob Moonlight. Wire it up straight from the scematics, connect a speaker and fire her up, quickly turn it off when it instently starts motorboating. Well I guess I can't expect my first build not fully recovered from HSE to fo pwerfect. SO go back over my wiring, thinking the most common cauase od motorboating is preamptube grid that is not correctly ground references , and sure enough the triode aside of the 7199 has no 'grid leak resistor, and whwt's this, no coupling cap to the pentode's anode. go bck to the schenatics, and it seems I built it just as draWN. Now, I have several of these build, and this is new to me. Lots ofs things are new to me because so much osf my memories were lost to HSEMon, 26 Jan 2015 20:43:40 -0500 Power transformer suitability(by DaveW) I am about to build a 4-4-0 but would like to add single 12AX7 valve reverb and 12AX7 bias tremolo options. I am trying to build this out of what I have hanging around to keep costs down, as money is tight. Could anyone help with a couple of issues I have with this concerning voltage and current draw. 1. Before the idea of adding trem and reverb I was going to use Danbury transformer which I have, but now I will exceed the heater current so I would like to use another transformer that fits the heater requirements (It has two heater winding, one would easily supply the pre, reverb and trem the other the output stage) but is 300-0-300 not 250-0-250. I could fit a zener or Mosfet in series with the centre tap to drop the B+ a bit. Is this practical? I have a 100ma 10H 150R choke I am considering adding that also should drop the voltage a bit. 2. I got the Electro Voice A.192 transformer I would like to use off eBay some time ago. It is unused and has a typed tag attached which rates it at 60ma HT and 3A heaters (I think this might be conservative if the size and quality is to go by) the recommended Hammond 269Jx is rated at 69ma but using Ohms law the drop over R2 (1K) is 24v which is a current draw of 24ma, this is the quiescent current of course and I will be adding a couple of 12AX7s. Does anyone know what sort of current is drawn when it is in use? Thanks for taking time to read and any help you can offer. DaveWed, 28 Jan 2015 01:32:16 -0500 Signal generator sound(by tdmur) Powered up my new build night before last and everything seemed to go great.All voltages are were I believe they should be and fairly low hiss and hum.I set up my signal generator so I could play with the controls and found the sound of the generator coming from both the speaker and inside the amp.PORed the amp using a load resistor and the sound is still there.Pulled all preamp tube and placed the signal directly on the power tube grid and it's still there.The amp is a modified 5E7 bandmaster front end with a single ended power section. Any thoughts or advise would be greatly appreciated. Much thanks,TomWed, 28 Jan 2015 06:30:58 -0500 2 x 12AX7 and a 6AV6 - how to use?(by Rob Ott) I have recently acquired a 2 x 6L6 cathode biased @~350V B+ amp using 2 x 12AX7 and 1 x 6AV6. I want a 'Fendery-y' sound for this one. My firt thought is a 5F6 Bassman pre going into a 5E3 power amp (except using the 6L6s - maybe go to 6V6, not sure yet). I like the Cathoodyne cause it's simple and I am not terribly versed on LTPIs. That is, Common Cathode (5F6) -> Volume (5F6) -> Common Cathode (5F6)-> Cathode Follower (5F6) -> Tone stack (5F6) -> Driver (5E3) -> Volume (new add) -> Cathodyne (5E3) - > Power Tubes (Cathode biased - either 6V6 or 6L6). Assuming I use one 12AX7 for the 5E3 driver/PI, that leaves 1 12A_7 and a 6AV6 for the pre. How would you folks divie up the other 12AX7 and 6AV6 for the pre? Obviously I cannot get 1/2 of a 12AY7 in there (unless there are 12AY7 equivs for 6AV6 - are there)? Or, forgetting my idea altogether, what are yours? Mon, 26 Jan 2015 11:18:06 -0500 Help characterising a vintage power transformer(by askem87) Please could anyone help me out? I bought an old power transformer on eBay years ago. It is labelled: JD Electronics (B'ham) Ltd. 8-39 RT.261 I can't find anything about them online (it seems the company stopped trading in the 60s). Other than a page on a 60s Vox AC-4 which uses an RT.290 I've measured the unloaded (open-circuit) voltages on the secondaries and they're 238V RMS and 7.22V RMS. My question is: how do I know what the voltages will be when it's loaded? I need to know this so I can design the rest of the circuit. Is it just a case of putting a resistor across the output? I'm assuming the 7.22V is for the 6.3V valve heaters, so should I use that ratio to determine the loaded voltage? Any help that anyone could offer would be greatly appreciated!Wed, 21 Jan 2015 21:46:16 -0500 Single Tube Reverb(by DaveW) Hi, I have just started to build a 4-4-0 but would really like reverb. I found the following link to a cool looking single tube reverb circuit which I am sure I could squeeze in to my build. <a href=" " target="newwin1422561188">[link]</a> I have uploaded the core schematic here if you have trouble reading the above <a href=" " target="newwin1422561188">[link]</a> Am I correct that the easiest way to add this is to put a 100k between the v4b anode (junction with R29) and C11 then add the reverb across this? Do you think that the 100k make a noticeable difference to the circuit? All advice gratefully received.Wed, 21 Jan 2015 10:11:41 -0500 Alternate 2 channel option SEL (by Denny Gracey) After playing around with the standard Lead/Rhythm option, I decided to work on a foot-switchable, clean channel option with separate volume and tone control, and eliminate having to deal with the high DC voltages in the standard option. Also the standard rhythm option is still a bit crunchy with G1 at 2:00 where I like it for lead, and was going for something cleaner w/o having to back off on guitar volume knob. Got it going this weekend and it works great! Thought Id post in case anyone might be interested although its likely been done by others on here in some archived thread. For about $50 in parts, heres the summary: Disconnected the input lead into G2 (i.e. the output lead from C11) and connected to common on the relay. One output from relay connects to G2 input, the other to single tone control Used a 500K concentric pot to control bass (lead channel) and single tone control for clean channel. Im still playing around with different tone control configurations but current fave is the Marshall 18watt; has nice full sweep and it runs in parallel with main signal vs series so it seems to retain more low and mid when adding high end. Tone control then feeds a second 1meg MV. I looked everywhere and couldnt find concentric 1meg pots so I used a mini pot for space considerations, mounted next to the standard MV, along with the panel switch for manual channel switching The second relay simply switches outputs from the 2 MVs common is the lead into R8 I used DPDT low signal relays and power supply from Hoffman. Since I was trying different options, I ordered two relays/boards so I had flexibility to mount them separately to avoid long leads. The power supply can drive multiple relays. In my case, I ended up mounting one near the bass/tone pot and the other near the MVs; nice and clean with short leads. Any standard marshall-type LED footswitch works just fine; no modifications needed for the LED to work. Note that the clean channel is very clean even with G1 on around 2:00, which is what I was after. Takes pedals really well too. I'll work on some clips if anyone interested. Mon, 26 Jan 2015 19:04:21 -0500 Amp Layout Program(by turtle441) Kind of a simple question, but are there any "secret" programs out there that folks use for drawing up layouts for amps? I've found a couple of great freeware programs for generating and tweaking a schematic. But, as close as my amp is to a couple on Ceriatone, it's got some differences that are going to require shifting things around a bit (not to mention a different chassis shape). I want to generate an exact layout before I start putting everything in "for good" as I can see myself getting painted into a corner otherwise. If everyone's just using a graphics program like Inkscape or Photoshop, I guess I'll just have to start making alot of rectangles.Tue, 27 Jan 2015 19:38:09 -0500 Cathodyne grid R placement ala Merlin(by Rob Ott) I assume the grid resistor Merlin mentions to tame cathodyne goes at the location in blue in the schem below. Can I put it at the location in red? If so, what are pros/cons and why? Any guidance on min/max value starting values or doesn't it matter? <a href=" " target="newwin1422561188">[link]</a> Also, if I wanted to DC raise heaters could I attach heater CT to the cathodyne cathode (noted in brown)? If so, why do most layouts I have seen usually use the power tubes cathode? Tue, 20 Jan 2015 03:12:27 -0500 Clean Preamp Chassis Layout?(by SX4Amps) Does anyone have a clean pre amp chassis layout? For some reason it's missing on the site. Wed, 21 Jan 2015 17:47:57 -0500