MM PT in AC15RI, EL84's redplating.

[petemoore]

I've got a RI Vox factory document with some broken schemata on it, kinda big, and the tubes don't show connections I can read, then what I can't read goes to C or something, I just want to reduce the current through the plates.
Plate voltages are ~358v.
What I see looks like it might be:
Grids connected through 1k5's to 220k's which fork into a 1meg which goes to ground/ does that sound like a wiring topology for an ac15 output grid bias?
Grid to ground resistance measures 150k.
.........................1k5-------GRID
......................220K
GND--1MEG--<
......................220k
.........................1k5------GRID
[Ignore decimal points]
Would reducing the 1meg to say .5meg be a good approach?
[That's what I just did and it's still redplating].
I took it back out.
Any other approach welcome...
I have increased the shared cathode reisisor, it started as 130 ohms, now 150ohms. There was slight distortion that has since ceased rearing it's ugly head, output tubes replaced then, perhaps increasing to 180ohm would reduce plate current sufficiently?
Was easy to open and work inside the amp [~6 screws], hard to see the tubes though until I figure out just slide the amp 1/2 way out and they're right there.
'Mild' redplating [test to visible RP, then shutoff].
The MM power transformer starts with higher secondary voltages, and the big improvement sounds great.
Removing the board...ughhh
Tacking two resistors across the 220k's [dunno, awful tight betwixed the two].
The only thing left I see to do now is further increase the shared el84 cathode R value, perhaps increase screen resistors?
Reducing a resistance here is sometimes shoddy but possible [if theres room to do so.
Increasing resistances would require cutting and pinning the left in lead [so it doesn't fall out the bottom of the PCB, or top heating the pad to pull and install the new resistor [not a good idea to solder a distant to trace blindly.
I haven't surveyed for a possible zener injection at recto/cap.
Screens are right on the tube socket...that'd reduce the plate voltage, what would be a ballpark > of value to put in there?
Tried 200ohm per el84 screen, still RPing.

[Enzo]

Did we try different power tubes?

[petemoore]

Rather than delete below, I'll put this above.
Upped the shared cathode to 144u + 136u = 280ohms, which is 10ohms more than what I saw in an old VOX ac15 schematic.
That cured the redplating.
I think 220ohms or 230ohms would do it, at least for now the amp is working and I have a handle on the bias.
Jammed loud for 40min. no sign of RP.
I'll go back and clean it up, and the cement in probably ~250ohms there, and put the screen resistors back to stock values.
I wanted to try dropping grid resistor, but I think it might be easier to build one than mod that.
Yes,
I typed 'design a tube amplifier' in google, and figured the cathode R, made large enough would cure the redplating problem. it does.
Below is description of problem/attempted fixes for anyone concerned.
The new ones didn't glow for long, and were working fine until too hot.
The other new ones have barely red plated [enough to see the red plate and power down].
two Very slightly redplated new tube matched pairs JJ's [unboxed this morning, eh.
Biased fine when original tranny was in there.
I've eliminated the possibility of bad tubes..I think, brand new ones aren't biasing..
I think the MM is maybe designed for an old AC15, and in the new RI voltages were like a lot lower, so I'm starting off with too high input AC to rectifier, If I can't get the bias under control with resistors...
Each plate resistance increased from 100 to 200ohm.
Cathode resistor [shared cathode best I could draw up], is now 150 was 130ohms.