"Nick Gorham" wrote in message
. uk...
Iain Churches wrote:

I have a fixed bias PP amp on my bench that has 10mm
skeleton pots for bias adjustment.

The owner who imported it directly, told me that it
worked only at very low power when it arrived, so
thinking it had been damaged in transit, he took it
to a local TV repair chap. He soon found the problem,
there was, and never had been, no connection between
the bottom end of the OPT secondary and the ground
terminal:-(

?

Do you mean the secondary wasn't connected to the LS terminal, or it just
wasn't grounded? the former should result in no output at all, and the
latter should make no difference to the output.

As I inderstand it, the latter. The output was apparently very low.

Hmm, I just fixed one that had a valve rectifier that was designed to
drive into a max of 4uf, and it was using 100uf.

Yes that's another common design fault. The reservoir cap is often
220µF.

Oddly, the rectifier had failed. The amp was only earthed via its phono
sockets (the IEC earth was unconnected) and it oscillated at 60Khz when
running.

So low (or no) stability margin. The oscilaltion can be cured with a
Zobel network. But one should really find the cause:-)



Apart from that, it was a cracker :-)

Aren't they all? :-)

(other than it didn't sound very
nice).

I can'd descide if the appearance of these things is a good or
bad thing. It shows that there is clearly an interest in thermionic
audio, buit the Chinese experience may not be a positive one for
so many hopeful people.

Iain