"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


Check the resistance of the
secondaries, and make sure it hasn't gone open circuit. That grid
voltage should only move with signal.


Hmm....

What bothers me is that on my 2A3 amp there is only a single wire to
each of the heater pins (from the filament trannies, naturally) - on
this amp with the meters, there's a whole bunch of crap connected to
them and I'm lost to see how any adjustment can work which is ultimately
connected to the cathode/heater ???

These are the 300B connections:

http://www.apah69.dsl.pipex.com/show/GoodAmp.JPG

http://www.apah69.dsl.pipex.com/show/BadAmp.JPG