ashdown peacemaker 60 bias adjust

[Mars Amp Repair]

This is the same amp is the same in the last thread...just different issue. This amp has cathode as well as fixed bias. The cathode has common resistors to ground as well as about a negative 6volts. the resultant bias is still at about 60ma per tube. about 60v on the common cathodes & neg 6 on the control grids.
There isn't enough range on the bias supply (which is -12v right out of the diode) to adjust this bias down to a resonable level. I have a feeling it has been this way for a looonng time judging by the darkened pcb below the cathode resistors. (BTW, the resistors in the other log for this amp I said Screen resistors...they were the Cathode resistors...I am relaying from my other tech).
any ideas..any experience? thanx, glen

[Enzo]

Not familiar with the model, but geez, 60v on the cathodes and -6 on the grids? That is 66 volts of bias, and its not enough?

Is the bias supply clean.

What is the B+ and are all the screen resistors now OK?

Have you checked bias tube by tube, or are you using the common cathode resistor and dividing? If so, try one tube at a time and see if one was running a lot hotter than the others.

COme to think of it, just what do you mean by common cathode resistors? COmmon to me means they share one resistor. You have referred to cathode resistors, as in more than one. What's the deal?

[Mars Amp Repair]

Yeah sorry, was in a rush.
The 2 output tubes share the cathode resistor. 2ea EL34's.

Subsequently, I received a schematic from one of the forum guys for a similar model. It looks like originally on this model, they were just going to use cathode bias & reference the grids to ground though a couple 100K resistors (or for leakage reasons).

They apparently realized ater the fact that different tubes biased differently & the cathode bias would not be adequate.

They klooged on to the regulated -12v supply for the opamps & brought it over to the output tubes through a couple of 100K resistors for neg bias on the grids. They left the resistors to ground from the grids in place.

I'm not certain but will check to see if I can remove or vary the value of those now divider resistors to get the bias up to a level that will bias the tubes in a acceptable range. I'll probably just inject a neg voltage to see if -12v will even be enough.

I'll also e-mail the factory & see if they have any words of wisdom for their klooge job.....BTW, plate voltage is ...about 495V.

glen

[Mars Amp Repair]

update from Ashdown on bias

This from Guy Morel @ Ashdown;

From: Guy Morel
Sent: 25 May 2007 10:47
To: glen@marsamprepair.com
Cc: Martin Burrows
Subject: RE: Ashdown 60


Hi Glen,

The Peacemaker 60 is actually an Auto Biassed amp. The biasing is
developped by R47 and R61 in the cathode Circuit of the EL34's.
Initially, they were two 390R resistors in parallel on the board.
These were uprated and moved off the board and bolted to the chassis ( 2
x 100R in Series ).
The actual Grid bias on the valves should be around -30volts. The
-12volt tap was to ensure the valves did not overcook themselves while
the Auto biasing was establishing itself.

Regards,
Guy