Bad red plating problem!

[DRH1958]

[QUOTE=Dave H;387037]Removing the PI tube will make any leakage worse (higher voltage). The voltage readings in post #6 are far too high. I think one of the coupling capacitors must be leaking. Remove the PI tube and both power tubes and measure the voltage on the EL84 grids. It should be no more than a few mV.[/QUOTE

Agreed that the grid voltage is too high( the one with .75v and 1.25v). The cathode voltage is right(or close anyway) except it should go up as the amp is turned up. It probably isn't because as one tube goes to redplate, the other goes to cutoff, thus the net change is only .1v. 1. Get plate and screen voltages. 2. Check for coupling cap leak.

[Silvio55]

I will check for leaking PI caps, it seems to be the most probable cause, I'll check it as soon as I get new caps, I don't have .01 uf caps right now, have some .022, maybe I'll put those and test.

[DRH1958]

There will be very little difference in tone with those .022s in there. If you want a touch more bass, you might want to leave them in and save yourself some work.

[Silvio55]

I just put those .022 and tested, i checked the grid voltages without the preamp and PI tubes and they are much lower now, the higher voltage i measured was 0.25V on one grid with the vol full up (the other goes to almost 0V), with the volume full down both grids are at 0.02V.

[pdf64]

Have you verified the grid leaks, ie that both el84 control grids have got a good path to ground?

[Silvio55]

Yes pdf64, I checked them and they're fine, all grounds seem to be fine to.

[Silvio55]

I did a quick test and the problem seem to be gone now, no more red plating like before, it was just for a couple of minutes, but I want to make sure by playing with the amp several minutes while checking the tube at all volume levels, can't do it today