59 Bassman RI added bias pot uses R31 and R41

[Analoggear]

Years ago a tech added a bias pot to my 1990 Bassman RI and it doesn't look to be the typical set up of replacing R41 with a pot. It worked as far as sounding fine but I didn't check the bias. He also mounted screen grid resistors on to the tube sockets and only one looks a bit burnt and has decreased in ohms down to 440 as opposed to, is it 470? I can't figure out what he did for the bias pot and I want to make sure how he did it is OK.

He removed R41 and did not put anything back. The new pot is attached to the top of R31 and maybe R30 which are the 220K power tube grid leak resistors. From there the pot grounds out to the chassis near the speaker outa. R30 might not be attached and I can't tell because everything is covered in RTV. It almost looks like the master volume Mod on the robrobinette.com site?

I'm mostly worried about the missing R41. The amp is from 1990. I order capacitors since I haven't replaced those yet and probably should have done that already. Maybe that caused some damage to the 470ohm resistor. The amp has always been quiet and really reliable so I left it alone.

Thanks,
Steve

 

[Paul G.]

The layout you described (a bit hard to be sure without pictures) should work fine. The bias pot is replacing R41, taking some of the bias voltage to ground. Is there a resistor mounted to the pot to set a minimum value? You really need to know where the bias is set. First thing I'd do is measure the negative bias voltage at the grids. Fender specifies -54.5, you should be in that range.

As far as the burned screen resistor, I'd replace it, and then make sure the bias is set to a reasonable value.

 

[Analoggear]

Thanks for the feedback. Glad to hear R41 isn't needed. I'll replace the 470ohm on the tube socket. I will check bias once I get the new capacitors in place.
Thanks again for the help,

 

[Pete Farrington]

Some people fit bias trimmers that can adjust the bias voltage all the way to 0V. From the description, I wonder if that’s the case here? It’s a bad idea / poor practice, in my view. A bias voltage range +/-25% of nominal should be sufficient, ie a range of about -40 to -66V.