Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Jacobjohn on July 15, 2020, 12:21:26 pm

Title: Bassman Reissue w/ Hoffman 5f6a rebuild power tubes
Post by: Jacobjohn on July 15, 2020, 12:21:26 pm: Hello everyone. New here. I recently purchased a 90’s bassman reissue (not Ltd version) that has a complete ptp rebuild done by Hoffman in 95. It was changed to 5f6a specs but with an added master volume and higher gain “Marshall” sounding channel.

I need to replace the power tubes. If I replace the tubes with a matched pair of the same kind of tube (6L6GC) is there a biasing thing I have to do? Also, how would I do that? There’s no mark on the board that I can see that says anything about adjusting or testing bias.

Also any recommendations for tubes?

Thanks everyone!
Title: Re: Bassman Reissue w/ Hoffman 5f6a rebuild power tubes
Post by: shooter on July 15, 2020, 12:43:16 pm: with NO power tubes installed;
put your black lead on chassis
put your red lead on one of the power tubes pin5 and measure the negative VDC
assuming you have -xx, probe other pin 5, should be close to same.
now adjust bias pot to one end, record, adjust to other end record, set back to ORIGNINAL setting and post values
Title: Re: Bassman Reissue w/ Hoffman 5f6a rebuild power tubes
Post by: sluckey on July 15, 2020, 12:43:27 pm: read...

http://www.duncanamps.com/technical/lvbias.html
Title: Re: Bassman Reissue w/ Hoffman 5f6a rebuild power tubes
Post by: Jacobjohn on July 15, 2020, 01:10:05 pm: Ok so my question is:
Do I need to bias if I replace power tubes in this amp with same kind of power tubes?
If so, where is the bias pot adjustment thing?
Title: Re: Bassman Reissue w/ Hoffman 5f6a rebuild power tubes
Post by: sluckey on July 15, 2020, 01:14:30 pm: Yes. The bias pot is that white thing next to the two blue caps on the board. There are also two 1 ohm resistors on the board. You may want to let someone else do this for you.
Title: Re: Bassman Reissue w/ Hoffman 5f6a rebuild power tubes
Post by: Jacobjohn on July 15, 2020, 04:27:08 pm: Are the 1 ohm resistors something that needs to be messed with as well when biasing new power tubes?
Title: Re: Bassman Reissue w/ Hoffman 5f6a rebuild power tubes
Post by: shooter on July 15, 2020, 04:55:18 pm: :laugh:
only if you want to know if the tubes will melt or freeze
Title: Re: Bassman Reissue w/ Hoffman 5f6a rebuild power tubes
Post by: sluckey on July 15, 2020, 05:54:05 pm: Quote from: Jacobjohn on July 15, 2020, 04:27:08 pm
Are the 1 ohm resistors something that needs to be messed with as well when biasing new power tubes?
Did you read and understand the link I posted above? You may want to let someone else do this for you.
Title: Re: Bassman Reissue w/ Hoffman 5f6a rebuild power tubes
Post by: drew on July 15, 2020, 06:51:31 pm: Pay particular attention to the "Safety Advisory" section of that link, which is about 2/3 of the way through the page.
Title: Re: Bassman Reissue w/ Hoffman 5f6a rebuild power tubes
Post by: punkykatt on July 16, 2020, 01:18:05 pm: Jacobjohn, do you live near Sarasota Florida?
Title: Re: Bassman Reissue w/ Hoffman 5f6a rebuild power tubes
Post by: Jacobjohn on July 16, 2020, 01:29:34 pm: Quote from: punkykatt on July 16, 2020, 01:18:05 pm
Jacobjohn, do you live near Sarasota Florida?

No I’m in Seattle