Musicman HD-130 Reverb / Bias Problem

[plexi50]

I have (2) EL34 power tubes linked together as a pair off one of the coupling caps that want to red plate.

Yet a bias voltage of -42 is present on all tubes and does not fluxuate.

I can not really make out for certain the bias trimmer voltage number on the schematic.

My max voltage at point (Y)  is -43vdc

The left pair of power tubes is not effected. This amp has a 12AX7 PI driver

[plexi50]

I just backed down the bias trimmer pot and i have -31.35vdc.
A nice blue glow and no apparent red plating at -31.35

I could swear i am seeing -45 volts at point (Y) on the schematic
There was a faint bias hum with -43 on the grid of the EL34 tubes

Do i even have an issue now?

Hi power -31.35
Low Power -21.30

Sorry for any confusion. I'm Confused today.

[DummyLoad]

check c46 and c47


--pete

[DummyLoad]

check c46 and c47


--pete

do you see -46V at point C and +46V at point A on recto board?


--pete

[plexi50]

check c46 and c47


--pete

do you see -46V at point C and +46V at point A on recto board?

I have -44.5V on both point C and +44.5V at point A on recto board

It started to slowly red plate @ -31.35. I raised the voltage until the red plate went away @ -37VDC

C47 & C46 are not saturated or look bad. I suppose changing them wouldn't hurt either. They are 37 years old.


--pete

[plexi50]

I put new caps in C46 & C47. I still have -44.5. No more red plating after raising grid voltage to -37.5

But why was only that one tube trying to red plate. They are brand new tubes. Not that, that means a lot these days.

[Willabe]

Looks like the output tube sockets have a few different components on them?

[plexi50]

Musicman amps are built a little differently. They have 1kv ceramic caps and a few diodes on the tube sockets. 700VDC plate voltage.
I have had it on for over 2 hours and no red plating has taken place.  Sounds good as well. They are good amps.

[DummyLoad]

I put new caps in C46 & C47. I still have -44.5. No more red plating after raising grid voltage to -37.5

But why was only that one tube trying to red plate. They are brand new tubes. Not that, that means a lot these days.

that's probably it. hate that scheme to get +/-16V. notice the PCB burn around the zeners. it's not like Vregs weren't available in those days...     


--pete

[plexi50]

Thanks for the help!

[DummyLoad]

YW, though you had it covered already. 


--pete

[plexi50]

The new old stimulus for amps/