Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: LooseChange on December 17, 2011, 09:20:04 am

Title: Ampeg B-25b Standby POP - Can't seem to fix.
Post by: LooseChange on December 17, 2011, 09:20:04 am

Got this B-25b on the bench. Changed the standby switch thinking that was the problem but still pops.
The standby lifts the center tap of the HV winding.

Any thoughts on a fix?

Schematic:
http://www.el34world.com/charts/Schematics/files/ampeg/Ampeg_B25B.pdf (http://www.el34world.com/charts/Schematics/files/ampeg/Ampeg_B25B.pdf)

Thanks!

Title: Re: Ampeg B-25b Standby POP - Can't seem to fix.
Post by: HotBluePlates on December 17, 2011, 09:31:18 am

Due to the huge d.c. shift that results from essentially "turning off" the PT high voltage secondary, I have a hard time thinking of a case where this would not cause a pop.

Perhaps a small-valued high voltage cap would help; I'm thinking something in the 0.01uF range.

Title: Re: Ampeg B-25b Standby POP - Can't seem to fix.
Post by: Tiny_Daddy on December 17, 2011, 09:48:01 am

That standby switch looks like bad news to me. The bias supply is going to spike way negative when the switch opens. I would re-wire the switch to the B+.

Title: Re: Ampeg B-25b Standby POP - Can't seem to fix.
Post by: ernie_jr on December 17, 2011, 04:47:55 pm

I built a b25 WITH 6550'S  Put the standby on the B+ like fender and sunn.  No issues with pops
Ernie