Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: pbman1953 on January 13, 2026, 11:56:57 am

Title: 1977 Ampeg B15n- pin 4 screen voltage is well over the stated spec
Post by: pbman1953 on January 13, 2026, 11:56:57 am

My amp seems to have lost some power and this is very recent.


It's had its caps replaced and its been fine. Last night it wasn't the same amp.


A friend told me to check the screen voltage which should be 445 but it's well over at 550


What's to check first?

Title: Re: 1977 Ampeg B15n- pin 4 screen voltage is well over the stated spec
Post by: SEL49 on January 13, 2026, 12:11:47 pm

Replace 6L6s first.

Measure voltages on both 6L6s pins 3, 4, 5, and 8. What have you?

Is the PT original?

Title: Re: 1977 Ampeg B15n- pin 4 screen voltage is well over the stated spec
Post by: pbman1953 on January 13, 2026, 12:27:01 pm

I've tried a few sets of tubes.

One side is 519 volts and the other is about 509-  using A Bias Rite meter


PT is original

pins-
3-548
4-542
5- -051


-