Adjustable Bias in 5E3 circuit

[KingCameron]

Hello Guys,
I am looking for a "best known method" of installing adjustable bias in a 5E3 circuit. I have looked at Ted Weber's Mod (see attached), but it requires an additional CT wire I don't have on my Mercury Mag transformers.(380-0-380)

What is the best known method for installing adjustable bias?

Thanks,
KingCameron

[dwhutchens]

Find the 6g3 brown deluxe diagram and use that circuit, just leave off the tremelo circuit starting at the intensity pot and the .o5 cap.

http://ampwares.com/schematics/deluxe_6g3.pdf

David

[MWJB]

Do you mean adjustable fixed bias, or adjustable cathode bias?

[Bruce / Mission Amps]

Use the 6G3 schematic but eliminate the vibrato circuit.
Join the new negative supply voltage directly to the 220K grid load resistors of the 6V6s but don't forget to lift them from ground, remove the cathode biasing from the 6V6 and ground the two 6V6 cathodes.
The easiest way to make it adjustable is to use a 25K trim pot (wired as a variable resistor) in series with a grounded 12K-15K resistor.
The 12K-15K resistor is the one you would change to set the bias voltage range you'll need.
I've never used the 380v PT from MM and you might find your power tubes are still too cold with the pot set to minimum resistance and actually need a smaller, say 4k7-6k8 resistor and the 25K trim pot or you might find the power tubes are too hot so you need a 15K resistor with the 25K trim pot... etc.
Use a 1/2 watt, 100K to 220K resistor at your high voltage AC tap to help limit current through the bias supply (in case of a dead short somewhere in the bias rail), a simple 1N4007 diode and at least a 47uF@100v cap for the filter.

[stokes]

With a similar PT,I used a 220k from one of the high voltage taps before the rectifier and a fixed 4.7k to ground and a 50k trim pot,like Bruce described.Cant say for sure if the 50k pot was necessary,but it was what I had handy,and it is fine.

[KingCameron]

Will try / what wattage?

I will try as you have described.
Ted Weber recomended using a 39K 5W resistor off of the AC. Is it nessesary to use the high watt resistor?

[dwhutchens]

By "off the AC" I assume you mean the PT secondary and not the mains voltage. I used all 1/2 watt resistors in mine. The bias circuit isn't going to draw much current. Looking at the 6g3 circuit I replaced the 22k resistor with a 15k resistor and a 10k linear pot in series to make the bias adjustable.
David

[KingCameron]

Thanks!

You guys rock!
I'll get this right

[hasserl]

If you want to make it switchable so you can go back and forth between fixed/cathode bias: http://www.geocities.com/rlhassebroc...?1067011161549