6v6 Grid Bias Circuit.

[wozt]

I am using a 290 - 0 - 290 transformer and it doesn't have a 50v tap on it. So taking a feed off one secondary i will go dropping resistor, diode, capacitor to ground, resisistor, capacitor to ground, resistor, potentiometer to ground. Bias taken from the junction of the two resistors. Is that order correct? Can I just use ONE resistor and a potentiometer? Can I determine a value for the dropping resisistor?

[Chuck H]

You want the extra resistor as part of the Pi filter (two caps with a resistor between) which reduces ripple. You seem to have the circuit correct, but I hate arranging circuits in my head via text description. Too many ways for that to go wrong. Any confusion can be solved by just copying a known working circuit. Traynor did a lot of amps with a full wave rectifier, no bias tap and fixed bias so you might check out some old Bass Master schematics. Just use a pot for the dropping resistor in the final divider and it's adjustable. FWIW I prefer to keep the pot range limited so that it's impossible to dial in a dangerous bias condition. This would just be the use of another resistor between the pot and 0V. You need to figure for the voltage division to get the actual voltage range you'll need.

[TomCarlos]

Hi Warren...

I am attaching a diagram for the bias control from a Crate Blue Voodoo. I no longer have that amp and not sure what kind of voltage the Orange Windings put out. But at least you can see how they wired up the circuit.

I want to build the same thing for another amp that I have. As a project, I want to convert a Cathode Bias to Fixed Bias Amp. So I will try this circuit (assuming I can find a good place to tap as my incoming source).

Tom

[wozt]

Thanks both. Does the dropping resistor before the rectifier count in a voltage divider string from secondary to ground, so 400v gets dived across all 4 resistors?

[Chuck H]

Thanks both. Does the dropping resistor before the rectifier count in a voltage divider string from secondary to ground, so 400v gets dived across all 4 resistors?

Yes.

The illustrated supply includes the resistor in series with the adjust pot I mentioned above and is just as straight forward as it gets for a two cap adjustable bias supply with a safety margin. There are simpler bias circuits, but they'll be lacking somewhere by comparison.

Thanks Tom

[TomCarlos]

I just checked my Fender Twin 65 Reissue.... Something similar ... looks like the second cap is on the output.

I also have a schematic for a Deville Hot Rod Deluxe... looks "more different" ... two diodes, a 51K in parallel with the series resistor and trim pot.

Anyhows... something to study and try out. (I too will be doing this soon!)

Tom