On Thu, 12 Jul 2007 16:58:51 +0100, "Keith G"
wrote:


"Don Pearce" wrote in message
...
On Thu, 12 Jul 2007 15:44:49 +0100, "Keith G"
wrote:


OK - powered up with dummy loads.

Grid to Ground:

Good Amp - a *continually* changing 0.04 to 0.06 VDC

Bad Amp - steady 202 VDC (???)

Are we on the money so soon?

If you swap the output valves between amps, does the fault go with
them?


No - all or any of the valves/rectifiers can be freely swapped to and
fro with no adverse effect, I'm told. Even the meters were swapped (at
my suggestion) and the bad amp stayed bad and the good amp continues to
work OK. (I will check this for myself a little later.)


That grid voltage should be pretty much tied to ground. This one
uses an interstage transformer, yes?


No. This is the circuit the amp is *supposed* to be based on:

http://www.apah69.dsl.pipex.com/show/300Bcircuit.gif


OK, so the grid of the output valve is grounded through a pair of
resistors in series - total 101k. The bad amp is 40 odd k, so what
that says to me is that the coupling cap between the valves has gone
short. That means that the hundred and one k is now in parallel with
all those resistors in the power supply - a quick look at values says
they come out to about what you measured. So you need a new 0.33uF
from the anode of the 6SN7 to the grid of the 300B.

Turning it off in the meantime would be a really good idea.

d
--
Pearce Consulting
http://www.pearce.uk.com