Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Meteorman on February 16, 2026, 11:55:43 am

Title: 5F6A bassman voltages - something off ?
Post by: Meteorman on February 16, 2026, 11:55:43 am

Just completed a build and start-up procedure (as per RobRob's startup guide) for the 5F6A Bassman.
On initial power-up with all tubes in, some squealy pops happened, and I shut it down immediately, and swapped the OT leads.
Started up again, and no noises.
Took voltages on all tube pins - see below (omitted the VAC heater readings, they were all normal).
Compared these with "normal" Bassman voltages provided on RobRob's page, and I'm not sure what's going on with pins 5 & 6 on V4 and V5, the 6L6 power tubes.
Are these 2 pins supposed to be the same negative voltage, from the bias circuit ?  Or is pin 6 supposed to mirror pin 4 voltages ?
And if the latter, were those pops I heard likely bad news ?
The good news is it seems to sounds OK when playing a guitar through it, although I haven't tweaked the bias yet.
Just wanted to confirm voltages before I got into biasing.
thanks for any insights.


(https://i.imgur.com/e9OJ7ao.jpeg)

Title: Re: 5F6A bassman voltages - something off ?
Post by: tubeswell on February 16, 2026, 12:27:01 pm

Quote from: Meteorman on February 16, 2026, 11:55:43 am

… some squealy pops happened, and I shut it down immediately, and swapped the OT leads.
not sure what's going on with pins 5 & 6 on V4 and V5, the 6L6 power tubes.
Are these 2 pins supposed to be the same negative voltage, from the bias circuit ?  Or is pin 6 supposed to mirror pin 4 voltages ?
And if the latter, were those pops I heard likely bad news ?

The pops and squeals could be anything with a new build. Check that none of your solder joints are cold/bad. A good wriggle with needle nose pliers is likely to show where solder joints are cold (cold joints may look superficially okay but they cause pops when current arcs across the joint)
The squealing could be NFB wired the wrong way around. Common mistake that will tend to show up as you dial up the gain.


Assuming you have grid stopper resistors wired between pins 5 and 6 (no pics of the way you laid it out) then seeing the same negative bias voltage on each end of the grid stopper is normal.

Title: Re: 5F6A bassman voltages - something off ?
Post by: Meteorman on February 17, 2026, 12:22:07 pm

thanks !....  I do have the grid stoppers between pins 5 and 6.
I fired it up and it sounds great
(https://i.imgur.com/JJnuMzT.jpeg)