AX84 BBS Recent Posts as of Wed, Apr 23rd, 2014 @ 06:40:08 http://ax84.com Amp Parts and KitsDoberman Music Products, LLC offers a variety of parts and kitshttp://shop.dobermanamps.com RE: JTM45 clone startup problem(by ormen) And this was more severe this time with the preamp tubes disconnected. Like I said it was more subtle one time but this time the tube flashed up a lot.Wed, 23 Apr 2014 05:12:41 -0400 http://ax84.com/bbs/dm.php?id=484269 RE: JTM45 clone startup problem(by ormen) Okay I tried this out without preamp tubes in and now I saw much more clearly what actually happend. When I flipped the standby on, BOTH tubes flashed upp with a blue light and the fuse blows. When the amp is in standby there a fairly loud hum and when the standby is switchen on it rushes up loud at the same time as the tube flashed blue and fuse blows. There's no DC present when in standby.Wed, 23 Apr 2014 05:10:53 -0400 http://ax84.com/bbs/dm.php?id=484268 RE: JTM45 clone startup problem(by DanGu) Seems strange. while in stand-by bias is ok and no dc is present. have you tryed to power on with the power tubes only? I suspect it could be the coupling cap between v3 and v4. if so v4 grid would go positive when standby is swithed. to test this remove all preamp tubes and switch on. if you ear satan come out again, the cap could be the problem, and at last you have excluded all preamp related issues. a scope would help greatly to debug this outWed, 23 Apr 2014 04:19:24 -0400 http://ax84.com/bbs/dm.php?id=484267 RE: JTM45 clone startup problem(by ormen) I just realized I used a fast acting fuse this time instead of a slow. Maybe thats why it was more subtle this time.Wed, 23 Apr 2014 04:01:22 -0400 http://ax84.com/bbs/dm.php?id=484266 RE: JTM45 clone startup problem(by ormen) Yes -49 VDC on both grids directly on the socket. I tried to change the phase of the OT as suggested, the fuse still blew, however it was more subtle now, the tube didn't go bananas as before, I tried to capture it with a video camera this time but it didn't happen now, it's too bright here now anyway so the video won't serve any purpose unfortunately. I don't know why it was more subtle this time, it might be because of something else? I've noticed that the filament of V4 doesn't light up as much as on the V5, but this could be becasue of the tube? Heater voltage are the same for both sockets with the tubes in. I might have been a bit unclear from the beginning. When the tube started to light up "like hell" as I said I forgot to mention that something made a nasty sound also. I could take pictures of the amp but to be honest, I can asure that everything is hooked up as it's supposed to be and I've checked for shorts with the DMM several times. Everything seems to be in order. Why doesn't the 470R resistors drop any voltage? I've checked every resistor and capacitor in the amp and they all read fine on the DMM. I've also checked the electrolytic caps with an ESR-meter. Everything seems fine. Wed, 23 Apr 2014 02:58:27 -0400 http://ax84.com/bbs/dm.php?id=484265 RE: JTM45 clone startup problem(by DanGu) I don't think positive feedback would cause red plating only to one tube, but never used an amp in these conditions for to long too. At last try to remove the feedback for the testing and see what appendsTue, 22 Apr 2014 21:37:57 -0400 http://ax84.com/bbs/dm.php?id=484264 RE: Single Ended Negative Feedback(by radu christian) thank you very much! Pots are installed and working great. I dialed in a certain amount of resistance and the sound changed considerably. Listening as we speak. Tue, 22 Apr 2014 18:31:45 -0400 http://ax84.com/bbs/dm.php?id=484263 RE: October Bright Volume Noise(by John Bgron) No noise on any of the filter caps. The noise definitely starts at the plates of V6.Tue, 22 Apr 2014 18:14:52 -0400 http://ax84.com/bbs/dm.php?id=484262 RE: Adding Tremolo to P1-Ex(by chris jeffers) <div class="quoteblock" >On 04/22/2014 @ 16:24, JKCavitt wrote : Okay. Found it: <a href="http://www.rru.com/~meo/Guitar/Amps/Kalamazoo/R12/Images/KalReverb12-b.jpeg " target="newwin1398249613">[link]</a> : : You can see the power tubes grid leaks both connect to the trem intensity pot. You can do the same with your SE amp. -- </div> thanks! that's exactly where i thought it should go. figures i'd ask someone that had already done one similar. Tue, 22 Apr 2014 17:56:09 -0400 http://ax84.com/bbs/dm.php?id=484261 RE: October Bright Volume Noise(by RickTown) I doubt it. Bad switches and sockets would typically crackle if they're still working. A constant buzz is usually a cap. If you have any extra filter caps, it might be worth swapping the last filter cap. Use your scope to see if the noise is present on the B+5 power node. Tue, 22 Apr 2014 17:06:32 -0400 http://ax84.com/bbs/dm.php?id=484260 RE: Adding Tremolo to P1-Ex(by JKCavitt) Okay. Found it: <a href="http://www.rru.com/~meo/Guitar/Amps/Kalamazoo/R12/Images/KalReverb12-b.jpeg " target="newwin1398249613">[link]</a> You can see the power tubes grid leaks both connect to the trem intensity pot. You can do the same with your SE amp.Tue, 22 Apr 2014 16:24:40 -0400 http://ax84.com/bbs/dm.php?id=484259 RE: Adding Tremolo to P1-Ex(by JKCavitt) See the power tube's 220K grid leak going to ground? You could try and have that run to the intensity pot of the tremolo circuit. The tremolo I built was based off a Kalamazoo amp - can't remember which one, but I noted it on the schematic. Miles had a copy of that schematic on his website. I'll see if I can find a link for you.Tue, 22 Apr 2014 16:18:26 -0400 http://ax84.com/bbs/dm.php?id=484258 RE: October Bright Volume Noise(by John Bgron) Yep I tried swapping those caps. I was checking continuity to ground in different parts of the circuit and got some intermittent beeps testing the Vintage/Modern switch and the pins of V6 it's connected to. The switch does connect to ground via some components but I can't get continuity testing the actual components, just at the switch and tube. Would it be worth swapping out my tube socket and/or switch?Tue, 22 Apr 2014 16:06:04 -0400 http://ax84.com/bbs/dm.php?id=484257 RE: October Bright Volume Noise(by RickTown) I was wondering why none of it made sense. Have you swapped the cathode bypass caps yet? Other than that I'm out of ideas. Tue, 22 Apr 2014 15:46:46 -0400 http://ax84.com/bbs/dm.php?id=484256 RE: October Bright Volume Noise(by John Bgron) Sorry Rick, that's not my picture. I just linked it because I have my input jack wired the same way. My amp is identical to the schematic.Tue, 22 Apr 2014 15:37:17 -0400 http://ax84.com/bbs/dm.php?id=484255 RE: Single Ended Negative Feedback(by Stevo) <div class="quoteblock" >On 04/22/2014 @ 14:13, radu christian wrote : Hello everyone, this is my 1st post on this forum. I have a SE 4watt/channel 6v6 (and 12au7) based amplifier that uses around 6db of negative feedback (wire from the speaker to input tube cathode through 5k of resistance). The amp is now sold in a different version that has adjustable feedback (from 0 to 6db infinitely) and I of course intend to install that in my amp too, curious as I am to hear it play without feedback. I have a pair of 100k alpha pots, but I don't know their power rating. It is probably either1/4 or 1/2 watt. I ordered 2watt bourne pots, just to make sure. Can you please help me figure out the power dissipation of the pot I can use as a rheostat between speaker lug and 5k resistor? : Thank you in advance. : PS: should I be aware of any dangers of using the amp without any feedback? it drives 96db 10 inch audio nirvana fullrange speakers. : : Radu Christian -- </div> Well, Power = V^2/R, so the voltage with an 8 ohm load and a full 4 watts will be 5.66v. This is the voltage that will appear across your feedback network. If we just take the 100k and ignore the rest, we get Power = 5.66^2/100000 which works out to 0.32 mW if my math is correct. Even a 1/4W pot should be fine there.Tue, 22 Apr 2014 15:33:54 -0400 http://ax84.com/bbs/dm.php?id=484254 RE: JTM45 clone startup problem(by Stevo) Your sound issue sounds like positive feedback. The negative feedback connection from the speaker out is actually positive instead of negative due to the output transformer phase. The fix is to swap the plate connections from your OT to your power tubes - they're usually brown and blue, so connect the blue lead to the socket where the brown lead is currently, and the brown to where the blue is. Not sure about the red plating, but I suppose if you have the amp running in that condition for long enough it might do that. I've never left an amp that was exhibiting this behaviour run for very long, though.Tue, 22 Apr 2014 15:14:31 -0400 http://ax84.com/bbs/dm.php?id=484253 RE: JTM45 clone startup problem(by boblove) Do you have any photo's of the board and sockets? Tue, 22 Apr 2014 14:26:53 -0400 http://ax84.com/bbs/dm.php?id=484252 RE: Single Ended Negative Feedback(by radu christian) Hello everyone, this is my 1st post on this forum. I have a SE 4watt/channel 6v6 (and 12au7) based amplifier that uses around 6db of negative feedback (wire from the speaker to input tube cathode through 5k of resistance). The amp is now sold in a different version that has adjustable feedback (from 0 to 6db infinitely) and I of course intend to install that in my amp too, curious as I am to hear it play without feedback. I have a pair of 100k alpha pots, but I don't know their power rating. It is probably either1/4 or 1/2 watt. I ordered 2watt bourne pots, just to make sure. Can you please help me figure out the power dissipation of the pot I can use as a rheostat between speaker lug and 5k resistor? Thank you in advance. PS: should I be aware of any dangers of using the amp without any feedback? it drives 96db 10 inch audio nirvana fullrange speakers. Radu Christian Tue, 22 Apr 2014 14:13:20 -0400 http://ax84.com/bbs/dm.php?id=484251 RE: JTM45 clone startup problem(by DanGu) both tubes have -49V directly on the socket? Is there a sense resistor on the Cathodes to check the current draw?Tue, 22 Apr 2014 12:06:51 -0400 http://ax84.com/bbs/dm.php?id=484250 RE: JTM45 clone startup problem(by ormen) It's definitely at the most negative I can get at the socket, which is around -49 VDC as I wrote. According to the voltage chart in the document, that should be appropriate.Tue, 22 Apr 2014 12:00:52 -0400 http://ax84.com/bbs/dm.php?id=484249 RE: JTM45 clone startup problem(by DanGu) Or more probable V4 bias pot is turned the wrong way at startup (on the most positive position). If so turn it the opposite side, check grids and start up againTue, 22 Apr 2014 11:50:38 -0400 http://ax84.com/bbs/dm.php?id=484248 RE: JTM45 clone startup problem(by DanGu) <div class="quoteblock" >On 04/22/2014 @ 09:56, ormen wrote : : So here is what happen when I plugged the power tubes in and as soon as I flipped the standby on a loud hum comes from the speaker and V4's tube lights up as hell and the HT fuse blows. It's like it gets invaded with a big current? As far as I've been able to see it looks like it's only on V4 and I've tried switching the tubes and it seems to be on that particular tube socket. It Looks like that I'm not saying this as absolute truth just that it really looks like that, it happens fast so it's not so easy to see with the amp upside down. : </div> The problem obviously is V4 bias. Check directly on the power socket grid if bias voltage is present, while the amp is in stand-by. I bet my ass it is not connected or at 0V potential. Check both socketsTue, 22 Apr 2014 11:47:44 -0400 http://ax84.com/bbs/dm.php?id=484247 RE: ECC88 / E88CC linearity(by DanGu) Thanks to both! I'll do a bit of more testing. Anyway I managed to get 0.15/0.16% at about 13/14W and I'm quite happy with this with a SE. Only think that I don't like much with my design is the bypass polycap in the SRPP driver. I'll post project if anyone is interested Tue, 22 Apr 2014 11:25:57 -0400 http://ax84.com/bbs/dm.php?id=484246 RE: October Bright Volume Noise(by RickTown) The switch that is in your picture looks like it switches a bypass cap in and out of the circuit. I also see a transistor of sorts coming off what looks like one if your plates. I would strongly suggest that you troubleshoot the amp stock before adding these features. Getting the amp running stock will allow everyone to help you more easily by following the schematic. Once it's working properly, then it's time to add features one at a time so you can troubleshoot each option more easily against a stock amp's baseline noise level and operation. Tue, 22 Apr 2014 11:24:35 -0400 http://ax84.com/bbs/dm.php?id=484245