AX84 BBS Recent Posts as of Sat, Dec 20th, 2014 @ 07:29:53 http://ax84.com Amp Parts and KitsDoberman Music Products, LLC offers a variety of parts and kitshttp://shop.dobermanamps.com 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 7VSat, 20 Dec 2014 07:07:01 -0500 http://ax84.com/bbs/dm.php?id=487948 RE: HIgh voltage supply with mosfet regulation(by DanGu) aaaahhh ok! What comes in my mind would be a simple LDR! Here the concept: the led of the LDR is powered from the output making hits resistor low on resistance. The LDR resistor would drive on some bjt or something else connected from transformers CT to ground. So, if the output is shorted the led would power off, resistence will rise to HiZ triggering off the device(bjt relay or else) placed between CT and ground. The main problem to solve for this would be at start up, when there is no output voltage yet and CT is held Hiz. It should be bypassed at startup while circuit charges the caps.Sat, 20 Dec 2014 02:58:36 -0500 http://ax84.com/bbs/dm.php?id=487947 RE: HIgh voltage supply with mosfet regulation(by Merlin) <div class="quoteblock" >On 12/20/2014 @ 01:29, DanGu wrote : This leads me to a bit of confusion. The power dissipation on the mosfet is greater when its output voltage is very low with respect to the input voltage. This beacause it has a greater voltage across it. Am I correct? Or am I confusing with bjts? </div> It's the same for BJTs or any device. If you short the output you'll have virtually all the voltage left across the pass device, and the current will be maximum too. Therefore P = IV means the pass device is going to sweat! As well as NTCs you can get analog temperature sensors. They look like a TO-92 transistor, but deliver a voltage proportional to temperature- you can even get then as free samples.Sat, 20 Dec 2014 01:40:14 -0500 http://ax84.com/bbs/dm.php?id=487946 RE: HIgh voltage supply with mosfet regulation(by DanGu) <div class="quoteblock" >On 12/19/2014 @ 14:28, Merlin wrote : It's not likely to handle 250mA with 500V across it, <i>permanently</i> (125W)! -- </div> This leads me to a bit of confusion. The power dissipation on the mosfet is greater when its output voltage is very low with respect to the input voltage. This beacause it has a greater voltage across it. Am I correct? Or am I confusing with bjts?Sat, 20 Dec 2014 01:29:04 -0500 http://ax84.com/bbs/dm.php?id=487945 RE: HIgh voltage supply with mosfet regulation(by DanGu) <div class="quoteblock" >On 12/19/2014 @ 14:28, Merlin wrote : : (One solution would be to mount a temperature sensor on the heatsink, which would shut down the supply once the MOSFET got too hot) -- </div> That's what I thought I too. An NTC which switches on/off a bjt which drives a relay. This should disconnect the power line from the transformer primary. And an always running cooling fan onto the dissipators. Sat, 20 Dec 2014 01:17:30 -0500 http://ax84.com/bbs/dm.php?id=487944 RE: HIgh voltage supply with mosfet regulation(by sajyho) I like what you're doing here, keep it up;)Fri, 19 Dec 2014 22:47:01 -0500 http://ax84.com/bbs/dm.php?id=487943 RE: No Boost problem(by broth) Found it. C1 was a 10u instead of a 1u. Its cranking good now. Fri, 19 Dec 2014 17:40:17 -0500 http://ax84.com/bbs/dm.php?id=487942 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 http://ax84.com/bbs/dm.php?id=487941 RE: HIgh voltage supply with mosfet regulation(by Merlin) <div class="quoteblock" > I've found this <a href="http://www.neurochrome.com/audio/?page_id=454 " target="newwin1419078596">[link]</a> : Could this be a good starting point? </div> Yes that's the sort of thing I was thinking of. However, the trick will be to make it short-circuit proof. The regulator you linked to, I think, is mainly meant to be put into an amp circuit, not a multi-purpose bench power supply. It's not likely to handle 250mA with 500V across it, <i>permanently</i> (125W)! If you want high voltage, high current, and robustness, simultaneously, this needs thought. (One solution would be to mount a temperature sensor on the heatsink, which would shut down the supply once the MOSFET got too hot)Fri, 19 Dec 2014 14:28:07 -0500 http://ax84.com/bbs/dm.php?id=487940 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 http://ax84.com/bbs/dm.php?id=487939 RE: Soft-start (standby) switch -- before or after rectifier?(by Henrik Lidbjrk) Thanks! Now I get it. Since I have a couple of spare diodes and two free and usable lugs on the switch, I'll solder the diode in. Great idea I would never have come up with myself! :)Fri, 19 Dec 2014 03:34:51 -0500 http://ax84.com/bbs/dm.php?id=487938 RE: HIgh voltage supply with mosfet regulation(by DanGu) <div class="quoteblock" >On 12/18/2014 @ 13:00, Merlin wrote : Hmm, you've just given me an idea for a Maida regulator that would use the LM317s built-in thermal shutdown function to protect both itself and the main pass device by automatically reducing the current limit as the output voltage is reduced... I'll get back to you... -- </div> I've found this <a href="http://www.neurochrome.com/audio/?page_id=454 " target="newwin1419078596">[link]</a> Could this be a good starting point? By makeing R9 variable through a selector switch, with R4 I could chose the voltage needed. R3 can be reduced to allow 500mA max to flow, and 2 or 3 n-mosfet could be paralleled to help power dissipation. Could this be possible? I bought a few STW13NK60Z (600V 150W N-mosfet) yesterday to experiment withFri, 19 Dec 2014 03:13:26 -0500 http://ax84.com/bbs/dm.php?id=487937 RE: HIgh voltage supply with mosfet regulation(by DanGu) <div class="quoteblock" >On 12/18/2014 @ 12:08, Merlin wrote : : Dissipation is going to be the killer in a supply like this. If this is for bench use, you're bound to need it to be totally short-circuit proof, not just current limited. This probably means a relay or SSR to crowbar the device out of action in the event of a prolongued short. -- </div> Yes. And a lot of power would be wasted as heath with this design. Maybe the solution would be to give up on semplicity and design a proper high voltage switching circuit with those transformers. What's a maida regulator by the way? My designing skills lacks in regulated and switching power supplys, so a bit of help with this project would be appreciated. Fri, 19 Dec 2014 00:13:05 -0500 http://ax84.com/bbs/dm.php?id=487936 RE: HIgh voltage supply with mosfet regulation(by Merlin) Hmm, you've just given me an idea for a Maida regulator that would use the LM317s built-in thermal shutdown function to protect both itself and the main pass device by automatically reducing the current limit as the output voltage is reduced... I'll get back to you...Thu, 18 Dec 2014 13:00:45 -0500 http://ax84.com/bbs/dm.php?id=487935 RE: HIgh voltage supply with mosfet regulation(by Merlin) <div class="quoteblock" >On 12/18/2014 @ 10:42, DanGu wrote : <div class="quoteblock" >On 12/17/2014 @ 13:09, Merlin wrote : If you're happy with simple 'variable voltage' rather than 'proper regulation' </div> I'd prefer a proper regulation mantaining a simple approach. </div> Bare in mind that the circuit you linked to in the first post does not regulate; it's just variable voltage.Thu, 18 Dec 2014 12:21:04 -0500 http://ax84.com/bbs/dm.php?id=487934 RE: HIgh voltage supply with mosfet regulation(by Merlin) A further option would be to use a pot but have a switch to select a high voltage option (240-480V) or low voltage option (0-240V), so the pot needn't have more than 240V across it. The switch would make various other changes to minimise dissipation, such as selecting a different tap on the transformer, if your transformer allows (e.g. if you're in the US you could switch the primary to wiring to the 230V configuration, which would halve the secondary voltage). Dissipation is going to be the killer in a supply like this. If this is for bench use, you're bound to need it to be totally short-circuit proof, not just current limited. This probably means a relay or SSR to crowbar the device out of action in the event of a prolongued short.Thu, 18 Dec 2014 12:08:44 -0500 http://ax84.com/bbs/dm.php?id=487933 RE: HIgh voltage supply with mosfet regulation(by DanGu) <div class="quoteblock" >On 12/17/2014 @ 13:09, Merlin wrote : If you're happy with simple 'variable voltage' rather than 'proper regulation' then you can run a 1M pot off the rectified AC, before the resevoir cap. A 1M pot will then dissipate less than 0.1W, and you can use one with a plastic shaft for perfect safety. -- </div> I'd prefer a proper regulation manteining a simple approach. So I think I'll go with a 12step selector to chose a voltage from 200 to 480/500V (20/25V each step)which is good to setup a tube output section. Going lower with B+ would be useless other than a preamp, and the fet would dissipate too much in those nasty conditionsThu, 18 Dec 2014 10:42:59 -0500 http://ax84.com/bbs/dm.php?id=487932 RE: California Dreamer build?(by Darren Porter) Wow your's came out sounding great! I love mine. I might use it tomorrow night for my first gig with my new blues band. I actually went with the Simple Pre/20W PP core combination, which is almost identical to the C.D. I also added Merlin's Scale Control to the LTPI just to try it out and I like how it tames the volume and adds a subtle overdrive at the same time. With it I can dime everything and get a really nice hard-rock-ish drive at a decent volume. Thu, 18 Dec 2014 10:17:26 -0500 http://ax84.com/bbs/dm.php?id=487931 RE: Wanted: fuses 400+ V(by Paul Fawcett) Merlin's just accustomed to not having nice things, being in old Blighty and all. But so far as I'm aware all of the manufacturers with HT fuses seem to just use standard 250V parts. Interestingly the amp user manuals I've looked at seem to dodge the question by just quoting an amperage rating alone.Thu, 18 Dec 2014 05:13:16 -0500 http://ax84.com/bbs/dm.php?id=487930 RE: crackling sound when amp is warm(by steve25) yes, time for new sockets!Thu, 18 Dec 2014 03:19:13 -0500 http://ax84.com/bbs/dm.php?id=487929 RE: Soft-start (standby) switch -- before or after rectifier?(by Merlin) <div class="quoteblock" >then add a fifth diode after the switch? I don't understand what the extra diode does, since the current has already been rectified (though not filtered)?</div> The extra diode stops the DC voltage on the first cap from getting 'back' onto the switch. The switch then sits at a point in the circuit where there is rectified pulses, but not smooth DC. As far as the switch is concerned, rectified AC is the same as ordinary AC; it reduces arcing across the contacts. <div class="quoteblock" > : GZ34 -> soft-start switch -> diode -> caps etc.? </div> Yes, exactly.Wed, 17 Dec 2014 23:06:43 -0500 http://ax84.com/bbs/dm.php?id=487928 RE: California Dreamer build?(by Ken Graves) Yes, it's finished. <a href="http://i269.photobucket.com/albums/jj55/sepulchre10/Harlow-Amp_zps9a9e0114.jpg " target="newwin1419078596">[link]</a> It sounds Great! Far better than I expected. A little taste: <a href="http://www.soundclick.com/player/single_player.cfm?songid=12899388&q=hi " target="newwin1419078596">[link]</a> (No tonal boxes, just a little echo.) It's Very versatile and can go from chimey clean to hard rock crunch - not high gain metal but not too far from it. And it can be dialed in to be pretty touch sensitive, going from fairly clean to driven crunch with the guitar's volume knob. The tone circuit makes it even more versatile. It's my take on a Carman Ghia style tone rig; one knob that controls both highs and lows. It tended to be very scooped which is cool but maybe not what I wanted all the time so I modded it, adding another control on the back of the amp that varies how much effect the tone knob has and thus can add mids how the degree desired. Also, if the Drive control is not maxxed out the Platinum / Blonde switch adds a gleaming sheen to the tone. One last note - this amp is Far louder than I anticipated. My wife can hardly believe so much sound comes out of such a tiny box. From my measurements with a 1K 1 volt sine wave it produces about 5 watts clean. Crank it up and it puts out 8 or 9 watts. These figures are actually lower than I expected but boy, does it ever Speak! I don't think it would keep up with a drum set (well, maybe a really tamely played set) but then I've only played it through a single 12 Celestion Greenback. With more speakers it might be fine for a small venue. In the house it will definitely ring your ears, but with its PPIMV it can still get that nice tone at low levels. So yeah, I'm very pleased with how it came out. Hope yours is all you're looking for too.Wed, 17 Dec 2014 20:27:30 -0500 http://ax84.com/bbs/dm.php?id=487927 RE: Stuck(by Stephen Keller) When all else looks good, start looking for the unusual.... Perhaps you have a bad/corroded connection somewhere in the mix that's throwing the phase inverter or power amp out of whack. Is this a new build with new parts or did you reuse sockets and passive components? Some years ago we ran into an issue with a Hagstrom 614 rebuild. There was a ton of discussion and suggestions. Eventually, the problem turned out to be a high-impedance socket connection in one half of the power amp that was introducing some pretty evil harmonic distortion. The builder (Lars Kohkoinen) sent me all the relevant images and I posted a write up after all the dust settled. You can read it here: <a href="http://www.thermionic.info/misc/Lars/Hagstrom_614_Debug.html " target="newwin1419078596">[link]</a> I'm not saying you have exactly the same issue, but it may be of a similar nature: some high-impedance junction in or around the PI circuit could be giving grief. StphWed, 17 Dec 2014 14:49:01 -0500 http://ax84.com/bbs/dm.php?id=487926 RE: Anyone ever wind their own transformers?(by Darren Porter) <div class="quoteblock" >On 12/17/2014 @ 14:14, Lord of the Livingroom wrote : Found two more references that I found useful; : : Audio Transformer Design Manual - Robert G. Wolpert. Looks like it's an internal design manual from the 70's/80's. Found the pdf online somewhere. : : Output Transformer Design and Winding - R.G. Keen. It's pretty good. Looks like a work in progress that didn't quite get finished. Still worth a read. -- </div> Thanks for the suggestions. I've read the Keen writeup. Yeah it kind of leaves you hanging there in the middle. I'll definitely look for the other one.Wed, 17 Dec 2014 14:33:31 -0500 http://ax84.com/bbs/dm.php?id=487925 RE: .25 (1/4) watt Project(by Darren Porter) <div class="quoteblock" >On 12/17/2014 @ 13:07, Jazz5dad wrote : : That being said, I'm thinking of the Simple Preamp (without the tone stack)and coupling with the 2W PP Power amp (without Presence). Bbelieve I can redraw the preamp without the tone stack but not sure how to eliminate the presence. Some help would be appreciated.. -- </div> I built a Simple preamp with the 20W Push Pull poweramp, with a couple tweaks, and I like it A LOT. It is a fantastic sounding amp. A lot of the Fender-ish tone comes from the 6V6 power tubes though. It would be interesting to hear how the Simple sounds with the 12AU7 triodes! Personally, I think EVERYONE should build the Simple/Fender basic circuit at least once in their lifetime. I can see wanting to eliminate the tone stack from the preamp, but take my word for it, you want the Presence knob in the power amp. When you start to push the gain and the master volume the highs will get VERY pronounced. The Presence knob lets you tame this down. By VERY pronounced, I mean painful to listen to. This will be VERY different sounding from the FireFly and your Gilmore Jr. It's not a very overdrive-oriented design but it will work very well with any effect pedal distortions you may have. Wed, 17 Dec 2014 14:32:02 -0500 http://ax84.com/bbs/dm.php?id=487924