Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: pbman1953 on July 30, 2025, 09:57:37 am

Title: Need to increase Negative bias voltage
Post by: pbman1953 on July 30, 2025, 09:57:37 am

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!

Title: Re: Need to increase Negative bias voltage
Post by: shooter on July 30, 2025, 10:45:31 am

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

Title: Re: Need to increase Negative bias voltage
Post by: pbman1953 on July 30, 2025, 10:50:28 am

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?

Title: Re: Need to increase Negative bias voltage
Post by: Merlin on July 30, 2025, 10:59:53 am

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

Title: Re: Need to increase Negative bias voltage
Post by: pbman1953 on July 30, 2025, 11:01:33 am

Fender Studio Bass amp

Title: Re: Need to increase Negative bias voltage
Post by: Merlin on July 30, 2025, 11:13:29 am

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!

Title: Re: Need to increase Negative bias voltage
Post by: pbman1953 on July 30, 2025, 11:17:35 am

Yes, 10k with at 15k grounding resistor

Title: Re: Need to increase Negative bias voltage
Post by: pbman1953 on July 30, 2025, 11:39:28 am

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

Title: Re: Need to increase Negative bias voltage
Post by: JPK on July 30, 2025, 11:50:31 am

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.

Title: Re: Need to increase Negative bias voltage
Post by: pbman1953 on July 30, 2025, 02:05:07 pm

Quote from: JPK on July 30, 2025, 11:50:31 am

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!