Can I jumper the bias range resistor?

[Baguette]

Hello,

I'm workin on a 5F6A Bassman and I can get the bias voltage from -40V to -30V approx, the B+ being 440V.
Even when set for the coldest bias, my 6L6 run at 35 / 40mA which is already quite hot.

A 100ohm bias range resistor is better than the stock 1kohm value... but still, I can only choose between "hot" and "freakin' burnin'".

Would it be safe to simply jumper the bias range resistor, so the PT bias tap would be directly connected to the diode?

Thanks very much.

[FYL]

IMO, 470R is the minimum safe bias range resistor when using a Fender-style dedicated tap. Jumpering it could lead to a dead winding if something goes very wrong in the bias circuit.

Original voltages were app. -48V bias, 430V plate.

BTW, what kind of 5F6A? Fender or clone? Original or reissue? Std or third party board? What kind of iron?

[sluckey]

There is no "stock 1kohm value" bias range resistor in that amp. You must be using a modified bias circuit.

Measure the neg dc voltage directly on the anode of the diode. Do you have more than the 30-40v range you get from the pot? If so, increase the value of the resistor connected from the pot to ground to increase the bias voltage. I would not use less than 470Ω for the range resistor.

If you still can't get the voltage range you want, consider tapping the ac input from one of the HT leads rather than from a bias tap. You'll need approx a 100K range resistor. Look at the Princeton Reverb schematic for an example.

EDIT... I just realized you're probably using the Hoffman layout? If so, decrease that 15K resistor down to about 1K. Then adjust up/down to get the voltage range you want.

[jjasilli]

Your bias circuit does not seem to match the 5F6A schematic, which has a voltage divisder of 15K / 56K off a dedicated tap.  Please provide your schematic.

[Baguette]

EDIT... I just realized you're probably using the Hoffman layout? If so, decrease that 15K resistor down to about 1K. Then adjust up/down to get the voltage range you want.

Sluckey you're spot on. That's a Hoffman type layout I'm using.
I realised the first filter cap in the bias line does impact the voltage. Increasing it from 10uF to 50uF made the voltage more negative (colder bias). And I did drop the 15k resistor (between the two caps) to 10k.
Now I can adjust the bias in the proper range (30 to 50mA per 6L6).

Thanks a lot for your answers, all you guys make this forum an awesome place.