Bias supply idea

[astouffer]

Is there anything wrong with doing something like this for a grid bias supply? In the real world I would probably include something like a comparator looking at each tubes cathode current to cut out the b+ in case of failure.

[DigitalJunkie]

I don't think so.
I've seen similar arrangements in older amps,they just used a resistor to "lift" the ground,since they didn't have zeners.

[Geek]

Bias supplies typically require only a few mA, hence why you see them with halfwave rectification instead of fullwave as shown.

Cheers!

[Colt45]

looks fairly standard to me

[Ty_Bower]

I think that method is described in detail here:
http://www.aikenamps.com/BackBiasing.html

Scroll down to the middle of the page to see a similar schematic.

[andyjevans]

What's the problem in case of failure?

also, can this be used for a -15v supply for a CCS?

andy

[sajti]

It could be OK. The only problem is, that the regulation of the bias voltage can results overdissipation if the HT of the output tubes is not regulated too.

Sajti

[astouffer]

Thanks everyone. Ty_Bower I'll have to give that idea a try. andyjevans: If the bias supply fails the output tubes will conduct fully and glow red. I guess you could use this for a CCS supply if you only need a few milliamps.

[andyjevans]

I had a little read of the very good web page referred to. It seems to me that for a CCS you don't have a problem. And for a start it's on the input tube. But if the HT goes up and down, the CCS could have a -15v voltage regulator on it anyway. So it should be stable. No?

[kevinkr]

It will work, and I have done similar things in the past, but I wouldn't do it this way now for the following reasons:

• Lots of power dissipation in that resistor depending on load current.
  
• Zener goes open and you end up with more than -50V across that cap.
  
• Not a good idea to regulate grid bias in an amplifier with unregulated plate or screen supplies. To some extent properly designed amplifiers with unregulated plate/screen supplies and unregulated bias supplies will compensate for significant variations in mains supply voltage, regulate one without the other and this is no longer the case.
  

I'd use a small 12V - 15V transformer with a bridge rectifier and CRC filter and the pots instead - more reliable, less heat and mostly addresses last concern listed above.