Ian Iveson wrote:

"tomrp" wrote

It gives me a place to start.

or maybe not...

if it is a dynaco III clone, then it is a combination of fixed and
cathode bias, with the two cathodes sharing the same, unbypassed
bias resistor.

A classic way to ensure bad results !


Adjustment is through an additional "fixed" bias
supply to the grids.

In that case, if one valve switches off, the voltage across the
cathode resistor drops, so the other valve will pass more current.
In this respect my previous argument still applies.

However, if the adjustment is to the fixed bias part of the circuit
like the dynaco, you may really need to trace the circuit, because
the dynaco lacks a DC balance control as far as I can see
http://www.triodeel.com/dynamk3.gif

It may possibly be a dry joint or poor socket connection or a short
to ground between the bias circuit and one grid. That valve may then
pass enough current for the common cathode resistor to turn the
other valve off.

Conversely, it could be a dry joint or poor socket connection to one
cathode as before, which would tip the balance the opposite way.

Could be lots of other things too.

If you can check whether the pots are associated with the cathodes
or the grids it would be a help. Much better if you could trace it
out, or tell us how it differs from the dynaco. Grids are more
likely I think. Could conceivably be both...

The point I am labouring is that, if the valves are balanced like a
see-saw across a shared cathode resistor, then your symptom can be
caused in four opposite ways: either side can be pushed or pulled,
and the result appears the same. Combine this with an intermittent
fault and things get tricky. Without a circuit you could have an
intractable problem.

Agreed.


Graham