Help to get rid of hum please.

[catalin gramada]

Hello. Have a box with tens of nos Russian 6n9s, 6sl7 equivalent. I tried to make a simple preamp but get a lot of hum. I didn't identified a loop in the ground. The ac heaters wiring is proper done. In order I tried to shield the tube with a steel cage but no noise difference. Elevating the heaters to 80v makes no difference. Supplied the heaters from DC source makes the hum a continuous highest pitch tone. I.m off ...the circuit is as simple as the sketch show. I tried a new production tung sol 6sl7 and is far more quiet but have same comportment. The tubes I tested are absolutely new and measure well from different lots from '66 to '75. Is clear I did something wrong but I don't get it...Any help please ? Thanks.

[mozz]

Bypass the first cathode.

[catalin gramada]

Bypass the first cathode.

+1. Thanks. It did a bunch of difference. It still hummy but just with volume pot wide open otherwise I may consider the noise in normal level. Still....this was not the ideea. I tested before the circuit with 6n2p which is noval 6n9s equivalent and have no this kind of hum. ...May I consider this tube is not usable in actual configuration ? Otherwise modern Russian tung sol 6sl7 is more quiet by far...

[Helmholtz]

Post a scope pic of the hum.

[catalin gramada]

Post a scope pic of the hum.

This is the output of first stage after 10n cap please.

[catalin gramada]

50 cycle hum seems heaters modulation. As I said elevated voltage with ca.80v positive not did any difference. I tied at the end of rail a bridge and an 1000uF over to DC supply the tube heaters and get same amount of hum at high frequency... the picture looks weird...why it modulate only half of cycle...? 50mV of noise is a lot of noise...

[Helmholtz]

Obviously the fundamental is 50Hz, but looks unusual.

Is this with standard AC heater wiring?
How does the AC ripple on the supply rail look?

[catalin gramada]

Obviously the fundamental is 50Hz, but looks unusual.

Is this with standard AC heater wiring?
How does the AC ripple on the supply rail look?

I cannot figure any ripple in supply rail. Is very small.
this is the supply node:


And this is v1 plate



Standard AC heater wiring .

[Helmholtz]

I guess you mean the input triode.

What do you see at the plate when you ground the grid?

BTW, I don't see a grid leak resistor.

[catalin gramada]

All test is with grid grounded. Why grid leak do a difference if the grid is strapped to ground ?

[Helmholtz]

Directly connect the grid to the grounded end of the cathode resistor using a short wire.

[catalin gramada]

I guess you mean the input triode.

What do you see at the plate when you ground the grid?

BTW, I don't see a grid leak resistor.

Yes input triode. The grid is strapped to the ground.

[catalin gramada]

Directly connect the grid to the grounded end of the cathode resistor using a short wire.

Is exactly as You said. I just removed also the wire to input jack...makes no difference as time is strapped to cathode.

[Helmholtz]

Is the cathode bypass cap large enough, say 100µ+?

[catalin gramada]

The AC heaters voltage looks fine