Need to increase Negative bias voltage

[pbman1953]

I need more negative voltage on a Fender tube amp. At the moment I have a 10k pot with a 15k resistor to ground. I'm reading -57 volts. The pot is already pinned counterclockwise. I need more.


What values need changing?


Thanks!

[shooter]

-57 is pretty low, with tubes OUT, what is the "range" from full CW to full CCW??

[pbman1953]

With tubes out-


-60 - pot input and at full counter clockwise


CW- as low as - 5




Can the negative bias feed resistor be tweaked to get more?

[Merlin]

What amp is it, exactly? If we see the schematic we can give a definitive answer.

[pbman1953]

Fender Studio Bass amp

[Merlin]

Did you install your own bias trimpot then? Sounds like you need to increase the 15k resistor, but a diagram of your setup would help!

[pbman1953]

Yes, 10k with at 15k grounding resistor

[pbman1953]

Couldn't I leave the pot and resistor alone and adjust the 220 ohm resistor coming from the feed?

[JPK]

What 220ohm resistor? People keep asking for a schematic and then you posted only a small part of it. As Merlin suggested the correct way is to increase the 15k resistor. It's very common to have to adjust this resistor value to get proper negative bias range.

[pbman1953]

What 220ohm resistor? People keep asking for a schematic and then you posted only a small part of it. As Merlin suggested the correct way is to increase the 15k resistor. It's very common to have to adjust this resistor value to get proper negative bias range.

I sent in more items through the phone but I see they they didn't make it.


Any, I change the 220 ohm coming from the feed and got -63 volts which was enough to cool down the tube set. I was at -57 to start. Now the pot has little swing.


Thanks for looking an helping!