Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: fredouille on July 24, 2025, 01:04:03 pm

Title: bass 400 +
Post by: fredouille on July 24, 2025, 01:04:03 pm

Hi anybody with experience on this beast ?

http://bee.mif.pg.gda.pl/ciasteczkowypotwor/%23pro_audio/Mesa%20Boogie/Mesa_Boogie_Bass_400+_Schematics.pdf

working on mine, I found out that no zener 15v on the B+... I guess originaly like this

I don't see why the B+ should need a drop, bassmans bassed on 6L6 don't have as well

any idea ? I can add still.

Title: Re: bass 400 +
Post by: AlNewman on July 26, 2025, 07:49:49 pm

I think what it would do, is if you had a tube fail, and one of the screens were shorted, it would limit the current to protect the rest of the circuit.  Where normally you would open up the fuse, or a screen resistor, or a dropping resistor, or in worst case, fry a transformer. 
Maybe it isn't a bad idea?  And maybe if you have an earlier version of the amp, and they added the zener as a change up, it might be a good idea to install it.  As you mentioned, most other amps don't have one.

Title: Re: bass 400 +
Post by: jansch on July 27, 2025, 04:51:31 pm

If you decide to have screen grids 15V lower than B+ and more stable than with a dropping resistor (sometimes a requirement with older beam tetrodes), this is the simplest way. The lower g2 voltage helps reduce the g2 power. Unless it interferes with obtaining full output, I see it as OK.

Title: Re: bass 400 +
Post by: tubeswell on July 27, 2025, 06:08:17 pm

If you decide to put a zener in to drop the screen supply voltage, it will need to have sufficient power rating for 12 x 5881 screens...