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Maybe someone in the group can help me out with this:

I have a direct-coupled front end. The input stage is direct-coupled to a long-tailed pair phase-splitter and driver (typical Mullard style). I need to keep the plate voltage of the input stage at 75 VDC. It's important because I don't have a lot of leeway to drive the 300Bs. Finding perfectly matched tubes for the input (6J5GT) is pretty hard.

Is there a good way to do this? I could:

1) Vary the B+ to that stage with a pot across the decoupling resistor in the power supply. Then I've got a pot sitting at 400VDC.

2) Vary the cathode resistance with a pot across the cathode resistor. But this could introduce noise, and I'm changing the internal impedance, albeit very slightly.

3) Use a 75-volt regulator tube across the stage to fix the plate voltage--I think?

Any ideas, folks? I'd appreciate it!

Is it B+ of 75V for the input stage, or Plate voltage of 75V?
If it's a resistive load on that stage, the question is, which side of the plate load resistor do you need the 75V to be...?

If it's plate voltage you need to be 75V, then the 75V shunt regulator is too low, since it will be supplying the B+, right?

The plate voltage needs to be 75 volts. Can you usae an OA3 from plate to ground to fix the plate voltage? The safest and easiest way woukld probably be to adjust the cathode resistance. The input stage isn';t swinging a lot of volts...

Where are you taking the output from? In a typical anode follower, if you fix the plate voltage with a gas tube, you'll get no output...

Roscoe

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I can explain it to you, but I can’t understand it for you.

Well, that's what I thought.

Perhaps the topology mentioned in this forum post might work for you.

http://www.audioasylum.com/forums/tubediy/messages/24/242313.html

ray

Treed! You beat me to it! Regardless of plate load (CCS, choke, resistor), if you use a current sourced and shunt regulated power supply, you can adjust B+ to get the plate voltage you like.

I might suggest the Camille shunt reg like Charlie uses in phono stages, as opposed to a gas tube. That gives you complete control over B+.

For instance, if you need 5mA @ 75V on the plate, set your CCS (could be a 10M45S) for that 5mA. Then select a plate resistor that meets your design needs from the plate curves. If that is, say 20k, then you'll be dropping 100V across it. Set your shunt regulator for 175 volts.

Your raw B+ needs to supply a minimum of 20-30 volts drop across the CCS under the most adverse conditions to maintain regulation.

Grover, you had the right idea, just the wrong side of the plate resistor.

Having the adjustable shunt reg means that you can compensate for different tubes, adjusting the SR to get 75 volts at the plate.

Stuart

A simpler way would be to use a 90v gas tube, choke load with a choke that has the proper DCR to drop you to 75v at the plate. The DCR of the choke will be low enough that the plate voltage won't change much due to variations in tube characteristics...

Roscoe

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I can explain it to you, but I can’t understand it for you.

Okay, interesting CCS idea, I'll have to look at that.


Roscoe, how exactly would the choke loaded regulator tube work? How would that be hooked up?

The regulator works in the the normal manner. The choke is the first stage plate load.

B+ -> dropping resistor -> gas tube to gnd -> choke -> 1st stage plate. Dropping resistor sized to drop B+ to 90 (or 85 depebding on the gas tube) with the plate current in the driver plus 15-20mA through the gas tube.

Roscoe

_________________
I can explain it to you, but I can’t understand it for you.