On 2007-07-12, Don Pearce wrote:
On Thu, 12 Jul 2007 16:58:51 +0100, "Keith G"
wrote:
... 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.

Seconded. The anode current in the bad amp looks like it may be
*significantly* more than the 100 mA maximum (if I read correctly
a 300B datasheet I found online).

154V / 825 Ohms = 187 mA (if the cathode resistors are actually as
in the circuit diagram at the link).

--
John Phillips