Just wanted to post the solution to the amp buzz even though that was not my main question...
The 100hz buzz was solved by replacing the tube with a matched set and then using the "tube matching" pot on the back of the amp to remove the last bit of buzz.
I did not have to replace any filter cap when doing this... it's dead silent now.
Thanks to all of you for your input!
Yeah, about that.
In the old days, filtering a high voltage high current supply was difficult, as it required large values of capacitance & seriously big filter chokes, all of which cost money.
Luckily, designers devised a "cunning plan".
It was easy enough to filter DC supplies for the low level/high gain, (but low current draw) stages, so they filtered the supply in two stages, with fairly poor filtering for the output stages, & good, (but needing smaller & cheaper RC components) filtering for the lower current stages.
With the output stages, they took advantage of the inbuilt common mode rejection of push pull stages to cancel the 100/120Hz ripple down to a very low level.
The inherent matching of "off the shelf" tubes in those days meant that once the balance pot was set, the rejection stayed in spec for years.
Eventually, one tube would lose emission before the other one, & 100/120Hz hum would become audible.
At my old work, we had a number of these type of amplifiers mounted along the wall of the transmitter hall in large, efficient speaker boxes, so when one of the EL34s died, the 100Hz from that amp was
very audible!
We would swap a new amp in, take the faulty one into the workshop, chuck in a new pair of EL34s, tweak the balance, & that was usually that!
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