Redplating

[Smokebreak]

I've been playin my 50w plexi 1987 I built a few months ago for the past few days and yesterday rewired it so bright channel is stock and normal channel now cascades V1a to V1b. One wire or RR mod I guess. Sounds good(not as much more gain as I thought cascading would provide but that's another story) and the amp has been trouble free for a while. I can't crank it in my apt so I just put in larmarPPIMV to really be able to hear the difference in the newly channel vs stock. Well, as soon as i fired it up, one EL34 (shugs) redplated and blew HT fuse. I replaced fused and watched tubes again on a few more powerups and red plating continued. All my wirings check out, bias V is fine. I swapped the EL34s and redplating stopped. Problem didn't follow tube OR socket. I played for about 10 minutes and amp sounds totally badass with both both gains cranked and master down low. What the heck is going on? Any ideas?
FWIW this amp blew a brand new 6CA7 a few months ago in the same socket the one that redplated tonite was in. I chalked that up to a bad tube, as I put the shuguangs in right after that and it's been fine ever since, till tonite .That may just be a coincidence.

[JazzGuitarGimp]

I'm betting against coincidence. I'd start by looking at the wiring to the grid of that socket. Could be a cold solder joint on the grid pin. Or perhaps the socket pin is loose and needs to be retightened a bit.

[Reeltarded]

Totally agree with that which hath been sayethed.

[JazzGuitarGimp]

Thanketh youeth thyne support, Sir Miles....

[Smokebreak]

I'm just gonna replace the dang socket tomorrow with a belton and be done with it. The one in there now is one of those ceramic ones that I don't like, for no good reason, really .Those things just seem flimsy.
Thanks fellas!

[pdf64]

Oscillation, which is likely to be affected by control settings, is another possibility.
The likelihood of this is increased by the cascaded gain mod and re-arranged (probably non-optimal) control grid circuit wiring on the power tubes, due to the larmar master.
When the socket has been replaced, power it up with all controls zeroed, then turn master and tones up full, then, gradually, the other volumes; closely monitor for redplating, weird noises.
Pete

[martin manning]

Totally agree with that which hath been sayethed.

Forsooth! The Lar-Mar installation involves connections to the bias supply as well as bringing in the possibility of an ultrasonic oscillation. Before replacing the socket, I'd go over the solder joints carefully and examine the lead dress- are the Lar-Mar leads twisted or shielded? If shielded, are the shields connected to the bias supply?

[Structo]

I agree, you might have made a wiring mistake on the LarMar.

Since it is in a critical part of the circuit, any mistakes could easily cause what happened.

[Smokebreak]

This was the second amp I built, so there are likely issues everywhere.
Before I get in there, let me explain what I did, so I don't make the same possible mistake when I rewire. Working off this: [img1250]https://taweber.powweb.com/store/6m45p_schem.jpg[/img]

Cascade: Disconnected R34 from mixer network where it meets R33. Attached it to junction of R3,R4. Done. Now bright channel is stock, and normal channel is cascade

PPIMV : took out 220Ks. Ran bias feed from where they once met to outer lugs 3 of dual250k pot. Ran twisted wires from .1s to lugs 1 . Ran non twisted non shielded wire from wipers to 1.5k grids toppers, mounted directly on sockets. I will shield these wires in a minute. 2.2M across lugs 3 and wipers .
So far so good?
One thing that is bugging me is the 1.5k stoppers. I have them right on the socket pins, then shrink wrapped to leads, but they are just flying. What's a better solution for mechanical stability? Term strip?

[Structo]

It sounds like it might be right but a picture is worth......

[Smokebreak]

Yep that's the one I was referencing. Well everything seems to be in working order. Thanks for the tips guys. Im able to turn everything to 10 without getting any oscillations in either channel. Originally after I finished and fired up it squeeled a bit at the very top of the settings, but I replaced jj v1 with a sovtek that is my go-to when an amp oscillates and that fixed it. It turs out I think my original problem with the redplating was the HV screen wire's topcoat insulation was a little melted into the tube pin, so it wasn't getting a great connection. Now I get to see what these 6CA7s sound like !

[Structo]

Yep, tubes can make a lot of difference, either good or bad.

It's frustrating to power up a new amp with new tubes and have problems.

Maybe the best way would be to borrow some known good tubes from another amp if possible and use those to verify proper operation.

[Roe]

v4 will often redplate in 1987, esp with higher voltages and low NFB (100k etc). if v4 redplates, but not v5, try decreasing the 1m on v3a to 390k ala park 1210 (if you use 100k nfb, try 470-560k if you use 47k nfb). also, increasing the tail resistor to 22k can help

[Smokebreak]

Very interesting. It was actually V5 that did it but I'll keep that in mind as I'm not quite finished with this one yet... after rehearsing with this amp last night, I am NOT a big fan of the larmar in this amp. Clean tones are ok, shred tones are good, but everything inbetween just sounds kinda lifeless. The whole Marshall mojo just gets lost with the master dialed back. I do like the cascade mod though.

[Smokebreak]

I take that back about the Larmar. Turns out the sovtek I put in V1 that squelched oscillation just sounds awful, and hums. I will now shutup and tube swap.