AX84 BBS Threads with new messages as of Wed, Jan 28th, 2015 @ 07:23:14 Amp Parts and KitsDoberman Music Products, LLC offers a variety of parts and kits 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 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="newwin1422447794">[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? Wed, 28 Jan 2015 05:55:08 -0500 advice on building and designing amps for a living(by CChurchill) I fell into this rabbit hole of amp, guitar, and effects building about five or six years ago. To make a long story short, building tube amps quickly became as much of an passion as playing guitar. I've been reading everything I can get my hands on regarding the subject. I want so badly to build and design amps for a living, and the thought of doing anything else blows(quite frankly). It can be frustrating learning on my own, but there aren't many job postings for Tube amp apprentices. I'm looking for any advice I can get. Wed, 28 Jan 2015 05:40:45 -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)?Wed, 28 Jan 2015 05:23:15 -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="newwin1422447794">[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 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 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 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 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?Wed, 28 Jan 2015 01:21:15 -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 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 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 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 Yey!(by dsmith) Yey! ax84 is back.Mon, 26 Jan 2015 15:21:07 -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 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 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 Rebuilding Hot Rod Deville and turning into Blackface(by Alva Goldbook) So after some consideration, I think I'm going to tear out the guts of my Hot Rod Deville and turn it into a handwired blackface type amp with a few mods. The idea is to use as many of the original parts as possible (because I'm cheap). <a href=" " target="newwin1422447794">[link]</a> <a href=" " target="newwin1422447794">[link]</a> Just like a Marshall 2203 has a high and low sensitivity jack, which just bypasses the first gain stage, I'm going to do something similar, where the 2nd jack will be the blackface circuit, while the 1st jack will just add a gain stage in front of the BF circuit. A master volume will be added for more flexibility, and I'm going to put a tube driven tremolo and reverb circuit in there. I think I'm going to go more of the British route for the tremolo circuit because of cheaper and simpler circuitry, and use a 1 tube reverb circuit. This is less than ideal, but judging from the chassis, I only have room to put one more pre-amp tube in there, so that would mean I'm limited to 1 pre-amp tube for the entire reverb circuit. I'm wondering if I should use a 12AT7, a 12AX7, or perhaps a 12DW7 for the reverb. I pulled out the reverb tank today to see what model it was. I'm hoping I can use it, but I don't have high hopes since it's built to be driven by op-amps. <a href=" " target="newwin1422447794">[link]</a> Correct me if I'm wrong, but I believe this has a 600ohm input and a 2250ohm output. I have room to put a reverb driver transformer on this chassis, but I'm intrigued by the idea of doing a reverb circuit without a transformer. Not sure how this would be done, or even if it's a good idea, but I noticed that a 12ax7 as a cathode follower has an output impedance of around 600ohm. I noticed this little oddity on the springs though. Has anyone ever seen this before? It looks like at some point the springs broke or replaced with shorter springs connected with looks a lot like the ball end of a guitar string. Color me confused. <a href=" " target="newwin1422447794">[link]</a> <a href=" " target="newwin1422447794">[link]</a> <a href=" " target="newwin1422447794">[link]</a> I also had this mysterious little piece fall out of the amp. I spent a few minutes trying to figure out what it was or where it went. I laughed when I finally figured it out. See if you can guess this one. <a href="" target="newwin1422447794">[link]</a> Thu, 22 Jan 2015 09:30:52 -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 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 P1 adjustable bias setting(by Nortube) so I finished the P1 amp yeasterday. Fired it up step by step and it works perfectly. Had some problems, as I apparently had installed rectifier diodes with 50V breakdown voltage. Blew a few fuses before I found the error. But all is good now. And it sounds amazing. First of all. Thank you all for making and contributing to this wonderful source of knowledge which AX84 is. I made it with the adjustable bias option, but I dont think I understand how to use it properly. Could someone wiser then me explain how to do it, step by step in a practical manner with example values of the measurements and the calculation of the mV value which is to be set with the trim pot? Wed, 21 Jan 2015 10:13:46 -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="newwin1422447794">[link]</a> I have uploaded the core schematic here if you have trouble reading the above <a href=" " target="newwin1422447794">[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 How to use tube conversion factors?(by sajyho) Hi guys, I'm trying to build a SE amp based on Tweed Champ but with a 6AK6 instead of 6V6. I'm using MAX1771 SMPS as the power supply and my B+ voltage is 170V; so I need a set of grid curves for around 150-160V screen voltage but the datasheet only has the curves for 180V screen voltage: <a href=" " target="newwin1422447794">[link]</a> I draw a load line for 180V screen voltage. Using Hammond 125E 11.2K primary taps and if I want to hit the knee of 0V grid curve I draw the purple line: <a href=" " target="newwin1422447794">[link]</a> As you can see this line's way over the max anode dissipation, and also I want the screen voltage to be around 155, so I lowered my bias point to 170V plate voltage (the blue line). The writings below the curves is my calculations for the purple line, and I want my purple line to be in place of the blue line; according to RDH4 all the voltages will change in the a proportion, and currents will change in order, so I did some calculations: Anode voltage C factor: 170/200= 0.85 So; other voltages would be: Vsc = 180 x 0.85= 153V , Vg1 = -8 x 0.85 = -6.8 Current C factor= (Voltage C factor) ^3/2 = 0.85 ^1.5 = 0.78 So; Ia= 18 x 0.78 = 14mA , Isc = 3 x 0.78 = 2.35mA Can you please take a look at my calculations? Is it this simple; or am I missing something? Thanks Sajad Wed, 21 Jan 2015 05:11:36 -0500 rhythm/lead switch question(by Denny Gracey) Installed the rhythm/lead switch in my SEL - seems to work as expected with one nuance- when in 'clean' mode (bypass gain stage 3), the gain 2 knob must be at zero otherwise the signal attenuates as i turn up gain 2. From the schematic it would seem that gain 2 knob is completely bypassed. Is this normal or do I have something wired wrong? Here's the way I wired it: * pole 1 to V4 pin 1 * pole 2 to V3 pin 2 * pole 3 to V3 pin 6 comments anyone? Wed, 21 Jan 2015 03:54:17 -0500 New reverb kit from AES(by Darren Porter) Have you guys seen this new stand-alone reverb kit from AES? <a href=" " target="newwin1422447794">[link]</a> It's only $275!!! It should work either in front of the amp (like the Fender 6G15) or in an effects loop if that's where you prefer your reverb to be, don't you think? Also I think that since it's all PTP wiring it would make a great test platform for a push-pull reverb unit or even a low-wattage amplifier. There is a TON of room in that chassis. Wow... I'm really considering doing this. At that price, you get something that should sound really nice if you build it to spec or a GREAT tinkering platform for your own projects! Tue, 20 Jan 2015 23:53:49 -0500