Noise problem

[Helmholtz]

As long as DCV at the input end of the cap is constant, the voltage at the output end should be zero when referenced to ground by a resistor.
As said, an electrolytic cap might not be suitable here.

[catalin gramada]

This I.m afraid, The dcv in cathode is not consistent. The x axis on my scope is swinging random up and down at the output of coupling cap, sometimes slow, sometimes fast...with a max+/- 0.4v dc amplitude sometimes, sometimes less...This high 0.4v amplitude is over a 2M load in parallel with my scope input of course. Fast time constant may help to suppress the transients...who knows...?

[catalin gramada]

..

[Enzo]

I'd bet your B+ is also bouncing like that.

[catalin gramada]

I'd bet your B+ is also bouncing like that.

You mean Green Energy ? Hell, I.ll check late, thx. The Russian power was better and cheaper by half.
I thought is a circuit related problem...

[Helmholtz]

I'd bet your B+ is also bouncing like that.

I think it would require rather large B+ fluctuation to cause this effect.
Reason is that a CF uses heavy local NFB, which makes its output a constant voltage source with good suppression of B+ variations.
Bottom line: A CF has excellent self-stabilizing properties.

A while ago I saw similar DC level fluctuations with a LTPI. Reason was the tube (which tested perfectly good!).

Apart from that, I don't think such slow fluctuations should be audible.

[Enzo]

I have watched B+ jumping around from mains variation for years. Seemed like it might apply here. Just the bouncy part, not the HF hash.

[Helmholtz]

I have watched B+ jumping around from mains variation for years. Seemed like it might apply here. Just the bouncy part, not the HF hash.

Sure, B+ varies with mains variation, typically by a few volts per minute.
I'd expect a CF to suppress B+ variations by roughly the intrinsic gain of the tube, meaning a factor of 100 with a 12AX7.

[Enzo]

I find mains bouncing at similar rates to the scope display. Whenever a large motor starts or other surge demand, the mains will pulse up and down. Many times a novice scope user will put the scope on AC coupling to ;look at a small amount of ripple and wonder why the B+ "was so unstable". The display is only moving a few millivolts. A one volt kink in the mains typically results in a 4 or 5 volt blip on the B+.

[catalin gramada]

The tube used was a 12bh7 biased for 8 mA around. The fluctuations comes to 0.4v sometimes, sometimes less than 10 mV and is completly random, meant have not any timing consistence.

[Helmholtz]

I don't exclude the mains voltage influence.
But to me the cathode voltage fluctuations look a bit too fast (as the filter caps will slow down B+ variations) and too large at maybe +/- 0.1V

I might be wrong. though.

[Enzo]

I just offer it as a thought. A quick look with a scope takes less time than the discussion.

[catalin gramada]

Yes, B+ on its supply point bouncing as same as Enzo said.

[Helmholtz]

Yes, B+ on its supply point bouncing as same as Enzo said.

How much, how fast?
Did you use 2 scope channels to verify simultaneity?

Always eager to learn.

How does the effect relate to your noise problem?

[catalin gramada]

I will try to record the plate and cathode simultaneously .
I really don't know, I started to do a noise diagnose and I get that to fix. Really don't know much the problems are related., if there are any.