High Voltage Output Cap dilemma

[JFace]

I've made a few tube preamplifiers for MI use, and I've had some difficulty mitigating the issues of using an electro vs. a film cap on the audio output of a high voltage tube.

A 1u or 2.2u film cap rated for 250V is quite large. I have found that the 60 cycle hum that it picks up is about 12dB higher than an electro. Perhaps layout is an issue here, but I fail to be able to shield it.

An electro is cheap and seemingly more resilient to stray e-fields compared to the film caps. The issue I have with electros is the higher the DC it is blocking, the higher the leakage and therefore significant DC on the output (10s of mV), even when using as low of a value shunt resistor as possible on the output. The DC value can be problematic for some external equipment, such as having pops when switching the device in/out.

Do you have any recommendations for mitigating these issues?

Thanks in advance.

 

[rayma]

the higher the DC it is blocking, the higher the leakage and therefore significant DC on the output.

There are low leakage electrolytic caps, maybe not in the value/voltage you want.
Some circuits use two series output capacitors, with the nfb taken from their junction.
That will give less output leakage, since there is less voltage across the second cap.
Layout must be a factor with your film caps picking up hum, so post some photos.
The impedance level at the output node will also be a factor in pickup.

 

[DF96]

Yes, two caps in series is one solution.

Alternatively, run the valves from a bipolar supply and arrange DC conditions so the output node is near 0V anyway. Then one cap will be enough.