Over Capacitance

[jaster55]

I've been rebuilding a Gibson GA-77. It appears to be 100% original. I replaced all of the electrolytics and some questionable bypass caps. Reflowed all solder joints. I had to replace one volume pot. All of the other pots, jacks and sockets cleaned up well. I replaced all the glass and found the bias to be a little high so I raised the value of the bias resistor which brought it down some. The amp sounds fantastic except for an annoying hum which I traced to the first filter cap. The Schematic calls for a 20mfd 450v cap. If I increase that to 50mfd it gets a little quieter. If I increase it to 100mfd, It goes away and the amp sounds good enough to record with.
Is increasing the capacitance by that much going to cause a problem somewhere else.

[sluckey]

If I increase it to 100mfd, It goes away and the amp sounds good enough to record with.
Is increasing the capacitance by that much going to cause a problem somewhere else.

Possibly. Probably if there is a tube rectifier involved. You risk damaging the recto tube which in turn can damage the power transformer.

I see about 5 different ga77 schematics in Hoffman's library. Take a look and post a link to the schematic that matches your amp.

[jaster55]

I have the GA-77 1954
https://el34world.com/charts/Schematics/files/Gibson/Gibson_ga77_1954.pdf

[PRR]

With the C-L-C filter, on a push-pull pentode, hum should be very-very low and buzz even less.

Are you sure both 6L6 are drawing equal current? A real unbalance would throw most of the supply ripple to the output.

Could the choke be part-shorted? (Does hum get worse when you wire around it?)

Why did you "traced to the first filter cap"? Upping the second cap does not change the hum?

Have you tried another rectifier? If one of the two diodes has quit working the hum will be bad. (I don't see how that is possible, but the tube and the amp are very old.... bad socket contact?)

[Williamblake]

This looks like cathode bias with an additional 24k connection to HT i dont understand.

[sluckey]

Check the AC voltage on pins 4 and 6 of the rectifier. Are the voltages equal?

[pdf64]

This looks like cathode bias with an additional 24k connection to HT i dont understand.

It’s a mixed bias arrangement, a combination of self bias from cathode current and fixed bias from the potential divider, which stabilises screen grid and cathode voltage (and hence the control grid’s bias with regard to the cathode), thereby making things less dependent of valve variance.
And allowing a colder operating point than would be conducive with good fidelity with self / cathode bias alone.

[PRR]

This looks like cathode bias with an additional 24k connection....

Voltage divider for G2, along with semi-cathode-bias. The main point, I think, was to allow higher plate voltage without exceeding the 270V limit on G2 on the older 6L6 types. A simple G2-dropper's drop would vary about 3:1 from silent to loud.

[jaster55]

Not sure what this means. Would any of this cause the amp to hum using capacitors called for in schematic, yet reduce hum significantly going from 20mfd to 100mfd.

[jaster55]

Both 6L6s are drawing the same current... 72ma. I thought that was high. Increasing cathode resistor from 150Ω to 200 brought it down some, did not affect hum, and amp did not sound as good, so I changed it back. No red-plating and amp sounds incredible, except for hum which is now gone, by installing a 70mfd in parallel with the existing 20mfd. I ran it for over 2 hours last night. Transformer gets hot but not too hot to touch...

I checked as per last nights recommendations...
... Swapped choke with known good one...  No Difference
... Upping second cap instead of first reduces hum but upping first cap eliminates hum
... Tried different rectifier tubes...  No Difference
... AC voltage to rectifier pins is equal