wrote:


NOTE: All other voltages are correct and clean.
When these caps are new, the amp is hum-free.
As they age, a pronounced *60* cycle hum begins
that appears to be volume dependent - that is,
the hum goes down as volume increases.



** The electros are leaking badly as they heat up - cos they are being over-voltaged. Even 10mA ( about 1W ) would get them damn hot.

So the negative bias voltage applied to the output tube grids has a high level of 60Hz ripple. If the 6550s were perfectly matched, this would not matter - but they aren't so some of this voltage get amplified.

I don't buy your story of the 1N4004 diode being reversed, 6550s draw over 500mA each with positive grid bias and the amp would have quickly blown up if that were the case.


.... Phil