adjustable bias 4 peavey bravo.

[tedmich]

I put in a 10k resistor with a 25k pot but I wasn´t able to bring the bias voltage down below -4.5V. I ended up putting the 18k resistor I bought for a possible dual bias in row with the former and got the bias voltage down to -4.3 and -3.5 Volts with a plate Voltage of 385.

Thats an extreme value based on the -18V stock value, how does it sound? Too cold and blocking distortion can roll in pretty bad, too hot and the tubes won't last.

I might put a 25K pot in series with a 15k resistor and use this to replace R60 (33k ) to give accessible values from about -12.7V to -19.2V, then measure actual current.

As an example the old Traynor Bassmate YBA-2 had a B+ 395V and used a fixed bias at -15.5V

Good luck!

[Helmholtz]

I agree that a bias of - 4.3V looks wrong. Such a low negative value should result in excessive plate currents. If not, I suspect a problem with screen voltage (or bad tubes).

Did you measure grid bias at the junction of R47/R49? It should not be measured directly at the grids.

Do you get -27V at the negative end of C113?

Pease measure screen voltages at the tube sockets.

[vintagekiki]

Question
Is FAN connected to pins J38 / J39.

[Helmholtz]

The schematic is misleading
The fan is a parallel load to the negative supply.
If it's disconnected, negative output can only increase, i.e. get more negative.

[vintagekiki]

PV Bravo has 2 groundings. In order to be different on the attachment, they are marked with different colors.

Question.
Is there a solid connection between the "red" and "blue" ground.
If there is any resistance between the "red" and "blue" ground, a "floating" ground appears, which can bring to unstable operation.

[tedmich]

My Bravo schematic shows 4 different grounds (maybe more

and the one on the far left is designed to elevate the ground 47 ohms for the heaters, helping minimize noise, no instability should result and almost all Peaveys of this era (and later) do this. And its always a FP or flame proof resistor, one of the few spec'd as an extra measure of safety.

[Grrrg]

My voltages for the bias might be wrong. Since I didn't measure the bias voltage but the voltage on the OT (shunt method). I cannot remember, it's been a while. I recall setting the bias while playing and I stopped when it sounded good and wouldn't change significantly when set hotter.

[Grrrg]

The voltages I mentioned were the ones at the OT. Not the bias voltage.

[g1]

The voltages I mentioned were the ones at the OT. Not the bias voltage.

That makes more sense.

got the bias voltage down to -4.3 and -3.5 Volts with a plate Voltage of 385.

one half measures 150 Ohms, the other 200 Ohms.

So you had approx. 21mA for one tube and 23mA for the other. At 385 plate volts, that works out to around 8W per tube. About the same as maximum suggested by Helmholtz in post #28.

[vintagekiki]

http://web.archive.org/web/20120623045707/http://www.diyguitarist.com/GuitarAmps/PowerTubeBias.htm
Common Power Tube Bias Charts

6L6/5881 Bias Chart
6V6 Bias Chart
EL34 Bias Chart
EL84/6BQ5 Bias Chart
KT66 Bias Chart
KT88/6550 Bias Chart

[tedmich]

that EL84 chart doesn't even go to Traynor (or Bravo) range; their B+ is 395-410VDC! (another reason to not bias them too hot IMO)
30w is claimed from fresh tubes but I would not aim for that...

[Enzo]

The stock Bravo is not a tube killer. And the only reason anyone wants to make bias adjustable is so they can make it run hotter.

[tedmich]

http://web.archive.org/web/20120623045707/http://www.diyguitarist.com/GuitarAmps/PowerTubeBias.htm
Common Power Tube Bias Charts

6L6/5881 Bias Chart
6V6 Bias Chart
EL34 Bias Chart
EL84/6BQ5 Bias Chart
KT66 Bias Chart
KT88/6550 Bias Chart

EL84 chart had typo and values ended at 360V so I extended data.