A Self Draining Amp?

[TheDirewolf]

Ok guys. I have been through this amp 50 times over with a fine toothed comb and I can't for the life of me figure this out. I noticed the other night, in frustration, when I struck a chord on the guitar and turned the power switch off without hitting the standby that the sound sharply faded out in about two seconds before going silent. Under normal circumstances this is supposed to be a gradual fade. Correct? So I pulled the chassis and checked my filter cap voltages only to find they were all drained to about 12mv. I flipped the amp back on and powered down again properly and remeasured and the caps were down to about 45vdc and draining swiftly just by meter probing. All grounds have been done and redone 3 times. During one of those redo's all voltages checked out and caps maintained voltage. My excessively high B+ was even down to normal range. 454vdc as opposed to the usual 480+ that I've been getting. I shut the amp down for the evening as it was late and decided to call it a night. Came back to it the following day only to find it had drained over night and my high B+ number was back. It's done this scenario twice now. I even installed a brand new PT to replace the other brand new PT and still having the same results. Could it be a bad OT? My mains and PI and screens are reformed ARS from Valvestorm. The preamps are F&T. Any suggestions or advice is GREATLY appreciated.

[neikeel]

This is a really hard one.
Without going through the amp checking the layout and wiring slowly and methodically it is hard to say, I suspect a B+ wiring mistake, a poor connection (hence the variability in the voltages) but I am not experienced enough to pick out where over the net.
I did not think it was your PT faulty and I do not think that kit is your OT. Obviously you did not add a 220k bleeder to your filter cans (a more correct value than the 56k Marshall used on the 100 watters!)

[TheDirewolf]

Thanks for the reply, Neil. I'll go back through and redo all of the B+ connections. Could this be the culprit for my high B+ readings? I've been having issues with V4 slightly red plating no matter which tube is in that socket when the PPIMV is bypassed and I use a HotPlate to knock the volume back down. I'll probably attach all the outboard wires on top of the board and wrapped around the turrets instead of underneath. Like I should have to begin with. One would think that after using a soldering iron for almost 10 years now that I'd have this figured out. As Albert King once sang, "If it wasn't for bad luck, I'd have no luck at all."

[danman]

I checked my amp for you Direwolf and mine also slowly drains down after powering down. It normally takes between five to ten minutes for B+ to read under 25volts. If yours is dropping rapidly and you do not have a bleeder installed then it could point to an issue.

[TheDirewolf]

Thanks for checking that out danman. Mine is already pretty well drained after initial shut off. Down to around 50-60vdc. I can put a meter on it and it falls faster than the numbers on a gas pump goes up. I'm in the process of swapping my B+ rail topside. I'll keep you posted. Thanks guys!

[TheDirewolf]

I spent most of my weekend basically rebuilding the amp. I swapped all of my off board wires topside instead of underneath. I even made sure I had a full wrap of the post with every wire. I replaced my preamp and screens and PI filter caps as well. I also built a dim bulb tester and the amp passed with glowing reviews. The verdict? I still have high B+ numbers and V4 still red plates slightly when LarMar is bypassed. The caps are also still drained within 2-3 minutes after shut off. I threw the variac on it and I had to drop the voltages down to 112 to get my B+ at 450. Heaters are sitting pretty at 3.15vac. They're normally 3.45vac. I have new preamp sockets, fuse sockets, IEC socket, LarMar kit, and bridge rectifier on the way. If this doesn't do it I'm going to ditch all of the CT iron and start over in that department. I should've saved my money and put it towards a MetroPlex. Must. Have. One.

[danman]

Is your reading of 3.15v at the heaters with all tubes installed in the amp? Any signs of redplating when you have the voltage dialed down to 112vac?

[TheDirewolf]

Sorry for leaving out some info. Heater voltages are with all tubes installed. With the variac set to 112vac the red plating does not occur. However, it took dropping it to 112vac to get it to stop. Any higher and it still does it, only slightly less intense than at full wall voltage.

[Lbrown3743]

When you say you are seeing "red plating" , what exactly are you seeing? Are you talking about the red or orange glow you see from the heaters? Reading through this post I cant figure out exatly what is wrong with the amp.

[TheDirewolf]

Thanks for checking out my post Lbrown and sorry for any confusion. When I say "red plating" I'm referring to the anode in the power tube over drawing current and turning red. It's not a full on melt down and it only occurs when signal is applied when LarMar is bypassed. Once I stop playing the tube calms down and the amp idles perfectly. The red plate comyment was a bit off topic but was in reference to my other post in 1987 reissue issues. Thanks again for posting and any and all suggestions are appreciated. I'll get new pics of the wire swap.

[danman]

If the 112vac gives you 3.15vac at the heaters and you are not seeing any redplating then you should be good to go. As I mentioned in the other thread, there was a post several years ago concerning this same issue. It was discussing the fact that in certain Marshalls with high plate voltage you can run into the problem of redplating no matter where the idle dissipation of the power tubes are set. If I remember correctly, it only occurred in certain amps and was caused by low impedance in the OT. I remember Randall Aiken stating that the only way to cure the issue was to raise the OT impedance to avoid over dissipation at high power levels. It was a long discussion and Aiken has wrote about it in his "tech pages" also. Could be that the OT in your amp just happened to be wound with a slightly lower impedance and that is what is causing the issue possibly.

[Lbrown3743]

So does the amp work?

[TheDirewolf]

Yes it does. I'm just chasing down some gremlins. I removed the LarMar completely last night and did the Larry Grounding scheme. LarMar removal was partly to see if there was something wrong in that circuit that was inducing the red plating. I fired the amp up at full voltage and dimed it and there was still some red plating but it was not as intense as before. And this was at 488vdc B+. I have some new stuff coming this week just to rule out faulty parts. I'm even considering a new OT. If this doesn't lower my B+ I'm going to admit defeat and send it to someone. I've enjoyed the build part of it and I definitely enjoy learning how it all works but the gremlins have wore me down.

[neikeel]

Yes it does. I'm just chasing down some gremlins. I removed the LarMar completely last night and did the Larry Grounding scheme. LarMar removal was partly to see if there was something wrong in that circuit that was inducing the red plating. I fired the amp up at full voltage and dimed it and there was still some red plating but it was not as intense as before. And this was at 488vac B+. I have some new stuff coming this week just to rule out faulty parts. I'm even considering a new OT. If this doesn't lower my B+ I'm going to admit defeat and send it to someone. I've enjoyed the build part of it and I definitely enjoy learning how it all works but the gremlins have wore me down.

Presonally I think the Classic Tone PTs deliver too high a B+.
In my experimenting a B+ of 420-430v is perfect in a stock Marshall 50w, lower than 400v it can lack headroom and higher can be too bright and too much voltage. 480-490 on a 4 potter is fine

[TheDirewolf]

Thanks for the encouragement, Neil. I guess my motivation for exorcising the gremlins from my amp is the fact that I've had it at "proper" voltages twice now. Each time it's changed for the higher voltages all I did was turn it off and back on again. I did swap a dropping resistor for the effects loop but that shouldn't have changed my B+ at all. I've even changed the PT twice. I'm really tempted to ditch the iron all together in favor of something else. It just doesn't sit right with me knowing that it's not 100%. I can't have that doubt lingering at a gig.

On a side note, I really do appreciate everyones input and effort in helping me figure this thing out. You guys are top notch.