Sunn 100S preamp noise

[glebert]

Working on a 1967-ish Sunn 100S amp. Getting lots of noise (white and some hum) in the first two gain stages (both using the first tube). Some noise with volume all the way down, but gets really bad when volume pot turned up. Used a big cap to AC short the tube plates to ground. Shorting out the first gain stage plate (V1 pin 6) reduced the noise a great deal but not completely. Shorting out the second stage (V1 pin 1) kills the noise completely. Moving the additional cap above the plate resistors to node D actually made the noise worse. Tried adding a hum balance resistor to the V1 heater but that didn't do anything. Adding 82uF cap in parallel with the 250uF cathode cap at pin 8 of V1 reduced the noise some, I didn't have a bigger cap on hand to try in that spot.

Any suggestions on likely culprits? The wiring on this is meticulous original PTP for this part of the amp, I don't want to start hacking parts in and out guessing.

TIA.
Greg

[Helmholtz]

Scope pics of the noise would help. Might be heater buzz. If so a humdinger often helps.

Did you try a different V1?

[glebert]

Scope pics of the noise would help. Might be heater buzz. If so a humdinger often helps.

Did you try a different V1?

Forgot to mention that I did try a new V1, no improvement.

[glebert]

Returned to this amp and working on reducing the hum at least. I measured ESR on the multisection (30u, 20u, 20u, 20u) can cap. For three of the sections I get about 1 ohm and one (at node B) is measuring about 17 ohms. It would be nice to tack a cap in parallel to that one to see if that helps the problem. Do I have to worry about the capacitace seen by the tube rectifier? Spec on GZ34 is 60uF, but I've never understood if that was the sum of all the caps or just the first one or just the ones before the resistors.

[Jon Snell]

If your hum is twice the mains frequency; 100 or 120HZ, then it is worth replacing the smoothing capacitor. ESR is of little consequence with valve amplifiers, unlike SS amplifiers.
If the hum is not 100 or 120HZ then the smoothing is fine.
Do Not exceed 60uF before the choke. 40uF to be on the safe side for the life of your GZ34. No need for more than 20uF at this point though.
White noise is usually caused by carbon composition resistors breaking down, pins 1 & 6 of first pre amp or the valve itself and sometimes a coupling capacitor.
I would remove the first preamp vale to check for noise level before anything else.

[Helmholtz]

I measured ESR on the multisection (30u, 20u, 20u, 20u) can cap. For three of the sections I get about 1 ohm and one (at node B) is measuring about 17 ohms. It would be nice to tack a cap in parallel to that one to see if that helps the problem. Do I have to worry about the capacitace seen by the tube rectifier? Spec on GZ34 is 60uF, but I've never understood if that was the sum of all the caps or just the first one or just the ones before the resistors.

As Jon Snell said, only capacitance before the choke has a limit of 60µ. So no problem paralleling another 22µ cap at node B.

An ESR of 1R is fine, but 17R is definitely too high.
High ESR not only means increased ripple but also poor decoupling, which can cause oscillation and related noise.
Also increased ESR indicates that the section has dried out and most likely also lost some capacitance.

BTW, a GZ34 has a current limit of 250mA, which will be exceeded above an output of about 60W.

If you have a scope, please post a pic of the noise at the speaker output.

The amp should have a shorting input jack. Does the switching contact work?