Aiken's improved back-biasing

[hylaphone]

Hey folks, I am beginning a spartan BF Fender style build with 2x 6L6, 12at7 pi, 12ax7 pre, etc. Single channel...
The donor PT is 400-0-400, with no bias tap... I just stumbled upon Aiken's page on back-biasing, looks like a nice solution here as I'd like to shed ~50v B+ and make a -bias source..

What is Back-Biasing?

The circuits on this page are for tubes requiring less bias voltage i.e. EL84..
Do you see any problem using a honkin stud mount zener to get around -50v for the 6L6?
And lastly, does one of Aiken's circuits look more suitable for this situation?

Just looking for the most elegant solution.. (other than springing for a new PT)
Thanks!

[Tom Phillips]

...Do you see any problem using a honkin stud mount zener to get around -50v for the 6L6?
And lastly, does one of Aiken's circuits look more suitable for this situation? Just looking for the most elegant solution.. (other than springing for a new PT)
Thanks!

The big stud mounted zener will work if it is attached to a spot that will allow sufficient heat dissipation. I.e not on a chassis location already hot from adjacent tubes.
However, I think the best solution would be to use the zener for B+ reduction and then derive the bias from the B+ line as done in many Marshall amps. I say this because, as pointed out in the Aiken article, the bias will then track the B+ better as the line voltage varies. This also will allow you to independently set the final B+ and bias supply voltages to the optimum values. Keep in mind that you need to start with more than a -50 V bias supply to insure that you will have sufficient range adjustment for 6L6s at the output of your bias adjustment circuit.
Cheers,
Tom

[hylaphone]

Sound advice Tom, thanks!

[Shadrock2]

This method of bias generation perfectly solves an issue i was having with to much B+. Except I have not been able to source any stud mounted ("big honkin") zeners with more than a 6.0V rating. Do you have a source for 10W plus zeners with 30 - 50V ratings?

[Jazz P Bass]

Zener Diodes | Mouser

[Mick Bailey]

I use a Mosfet dropper rather than a high power Zener (or string of them) to reduce the B+. Easy to construct and suitable power Mosfets are inexpensive and commonplace. Then use a capacitor in a reactance-coupled bias supply if there's no bias winding.

[Shadrock2]

I've gotten parts from Mouser all the time. If you conduct a search from the exact link you gave Jazz you will find no results for power zeners above 6 volts. I like zeners and I suppose I could string 3 5 watt 12V zeners in series.

I'm not real comfortable with MOSFET's but I understand Bipolars fairly well ... except I always get confused when trying to use PNP's for negative regulation and also getting the polarity correct between transistor and tube schematics.

I was thinking of something like this circuit.
What would a MOSFET circuit look like?

[trobbins]

Not quite. The simple method is a zener from gate-drain, and a resistor from gate-source.

RG Keen did a write up:
http://www.geofex.com/Article_Folder...osfetfolly.htm

A high voltage one I've used:
http://dalmura.com.au/projects/Selme...Schematic1.gif

[Mick Bailey]

Take a look at R.G.'s Mosfet Follies and scroll down to the B+ reducer;

http://www.geofex.com/Article_Folder...osfetfolly.htm

Very similar to the BJT 'amplified Zener'.