little teisco amp red plating

[rexindigo]

Hi , ive got an very old Teisco amp on the bench, the model seems to be "Teisco 10". its a EL84SE poweramp driven by an 12ax7 and the second 12ax7 does tremolo and reverb.
i attach a schematic ive found on the net.
The 6x4 tube rectifier was broken and the former owner hat tried to fix this with a bridge rectifier soldered wrongly onto the socket of the tube rectifier. the OT was dead.
after replacing the OT and adding a full wave rectifier ( 2 x 1N4007) on the socket it works fine for about 5 minutes, then the sound starts to get distorted and low. El84 is red plating.
voltage reading on the cathode are about 15v-20v. that seems to be high?
New power supply filter caps are on the way (the 40µF cap is already replaced
I am about to replace the cathode resistor (130 ohm) bypass cap (10µF)
what else could i do the stop red plating? any reply is much appreciated!

[Jon Snell]

I would replace the capacitor circled in red.

[mozz]

Can't you put a 6x4 back in there, it will drop your hv.

[rexindigo]

6x4 is on the way with the filter caps, should arrive today, then i will also replace the cap on the grid like jon suggested

[Helmholtz]

Cathode voltage should be around 7V.

To see if the grid coupling cap is leaky, disconnect one ot its ends and see if the EL84 cathode voltage goes down.

The schematic shows a 7189A, which has higher voltage and dissipation limits than an EL84.

What are plate and screen voltages?

https://frank.pocnet.net/sheets/106/7/7189.pdf

[rexindigo]

Replaced the grip coupling cap (with a big WIMA 2000volt block...) and the filter caps, put in the 6x4 and now its fine! the reverb works, the tremolo works, cathode voltage is7.7 volts,plate voltage is 272 volts, screen voltage is 283 volts.
i still have the original 7189, try it as soon as the EL84 has cooled down. it sounds astonishingly beautiful, clean and bright with my 1978 Tele, just love it!.