6v6 PP 20w Bias Voltage Range

[chunkitup]

I am building a PP 6v6 power amp and with bias voltage running off of the HV sec on the PT. I started with a 220k to drop the voltage and I have -31v maximum bias from my trimmer. What range should I be looking for? I dont want to put the tubes in and not have enough negative bias voltage. I cant find in any of my tube books the way to determine proper range of neg voltage. The plate voltage will be in the 390-400v range.

[Jazz P Bass]

-31 volts bias sounds a tad low.
I would shoot for -36 to -38 Volts
My reasoning: Fender Deluxe value is -37 volts.

[Tage]

For some reason I feel like you usualy want 10-15% of the screen voltage. But that might just be me confusing memories.

I say better to have more than you think you'll need.

[Leo_Gnardo]

If you're looking for 20W "at clip" you'll have to up the B+ and the bias both. If you don't mind 10-20% distortion at your power figure, your planned B+ will be about right. I'd increase the range of bias adjustment just to be on the safe side. You can always dial it down. Sure would be a bummer to red-plate on your initial test run.

[Gingertube]

I built a 4 x 6V6 parallel push pull into a 3400 Ohms Raa, 50W Marshall Output Tranny. It had full power scaling and switchable fixed or cathode bias (in cathode bias each output pair had a common unbypassed 470 Ohm)

Here are my set to work notes:

Set to Work of the Power Amp.

Recall that I am using Hammond 370FX Power Tranny, 4 x 6V6G (ST Shape) Output Tubes and a 50W Marshall Output Transformer(Raa = 3K4).

Measured B+ unloaded (no tubes) => +378 Volts
Measured Raw Bias (no tubes) => -77 Volts

Set Power Scale Control to 10
Set Compression (SAG) Control to 0
Set Fixed Bias
Set pentode Mode

A quick calc for 70% of 12 Watts Anode dissipation gives 22mA per tube.

Fit the 4 x 6V6G Output Tubes and selected 12AX7 PI tube (selected for matched triode sections). Actually the best I had was a CV4004 so used that.

Set the 4 bias contrls for 20mA per tube as a "start point".
Remeasure (loaded) B+, now at +349 Volts
Redo the calc above and get 24mA per tube.
Allow to warm up for 10 minutes and then set bias pots for 24mA per tube.

Checks:
B+ = 345 Volts
Bias Current/Voltages
24.0mA @ -27.1V
24.0mA @ -26.9V
24.0mA @ -26.6V
24.0mA @ -26.7V

Switched to Triode Mode - each tube idle current drops about 0.3 to 0.4 mA
Switch back to Pentode Mode

Set Power Scale Control for B+ = 250V
Checks:
B+ = 250V
Bias Current/Voltages
16.5mA @ -19.5V
16.9mA @ -19.3V
16.6mA @ -19.1V
16.6mA @ -19.1V

Set Power Scale Control for B+ =150V
Checks
B+ = 150V
Bias Current and Voltages
9.8mA @ -11.3V
10.4mA @ -11.2V
9.7mA @ -11.1V
10.0mA @ -11.1V

Set Power Scale Control back to 10 => B+ back to +345 Volts

Wind Compression Control from 0 to 10 => B+ drops to +105V

Set Compression back to 0 and switch to Cathode Bias.

Checks:
B+ = 343V
Bias Currents / Voltages
Pair#1 Cathode Volts = +23.9V
25.9mA , 25.3mA tube currents
Pair #2 Cathode Volts = + 23.7V
25.2mA, 25.4mA tube currents

Cheers,
Ian