Cathode follower: biasing DC heaters to safe Cathode-to-heater difference. How?

[Kingston]

I'm working on a tube circuit where I need cathode follower. Cathode-to-heater negative voltage difference can't exceed certain safe value, like 200V, same with positive difference. That would mean I should reference all the heaters of my amp to somewhere around 75V so that cathode and plate followers still work within their safety limits. I've done this several times with AC heaters, but this time I need DC and I'm not sure how to bias it.

How do I safely bias the DC heater supply? Can I just use around 75VDC drained from B+ acting as ground reference for the DC heater circuit? This reference drains very little current, right? And how to calculate that? Should I regulate this reference?

Thanks,
Mike

 

[pstamler]

A voltage divider will do fine. Set it up to pull 1mA from the supply, and connect the junction of the two resistors to the - side of the first filter capacitor on the filament supply, which of course has no other ground connection. The only current you're draining is the 1mA from the divider -- none flows through the filament circuits.

Peace,
Paul

 

[Kingston]

Thanks!

I knew I could do a cheap "drain", but got caught in the safety net. I thought I should make sure the reference won't follow the very high B+ of initial start up. I hope those 5-10 seconds of start up chaos are a non-issue.

 

[abbey road d enfer]

In fact, most of the designs using this scheme have a capacitor  to ground, so at start-up the reference ramps slightly more slowly than B+.

 

[Kingston]

In fact, most of the designs using this scheme have a capacitor  to ground, so at start-up the reference ramps slightly more slowly than B+.

like this? (ignore the rest of it, just look at the divider)

I landed on that divider using this: http://www.sengpielaudio.com/calculator-voltagedivider.htm

But I don't seem to understand what "load" is for that 1mA. Nothing is drained through the reference when it acts as heater ground, or is there?

 

[abbey road d enfer]

Yes, that type of divider, although I wouldn't use such a large cap, takes about a minute to ramp up, plenty of time for cathode stripping.
As you mentioned, there is no current drain in the heaters.

 

[zebra50]

The Sony C37 mic - a cathode follower - sprang to mind. Thought you might like a look.

 

[Kingston]

That's a good one, thanks.

I wonder why they put 20uF cap to ground from that "positive side" 100ohm resistor. Looking from the bias voltage point of view it's just another RC filter, but why not do it with the bottom 100ohm as well, while you're at it?

 

[truzz]

Sorry for the noobie question,
I was wondering:
in a balanced circuit like the Sta-Level, if you want to use a DC supply for the heaters,
is there the need to bias? if yes, to what voltage?

 

[abbey road d enfer]

When changing AC heaters for DC, all the rest should be left unchanged. If the AC heaters were biased, you bias the DC ones the same.

 

[emrr]

No need in a stalevel.  Cathodes are all relatively close to ground, already near heater reference. 

 

[analag]

If you can vary the floating point you can tweak the voltage for best noise level. Therefore use a pot to find the spot then hard wire it with resistors.

 

[truzz]

No need in a stalevel.  Cathodes are all relatively close to ground, already near heater reference. 

Thank you for the replies.
Ref. Sta-Level:
so using dc heater supply, should I tie the negative side to gnd? Can it pollute the audio gnd?
Isn't the biasing useful to isolate heater neg. from the audio gnd?

I was thinking of using an LM338 to regulate the heater supply.

 

[emrr]

DC needs one side or other tied to ground, and I've never seen it pollute anything. 

I've also never heard a Stalevel that needed help with hum on the traditional AC heaters.