Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: plexi50 on December 04, 2024, 08:45:21 pm

Title: THD V Front Bassman Style Amplifier / Sorta!
Post by: plexi50 on December 04, 2024, 08:45:21 pm

Anyone ever see one of these THD V Front 410 amps? Again another one with no schematic for biasing. I feel safe around -48vdc - -52vdc but would like some input here. Amp hums at these values but silent and loud @ -60vdc. I think -60 is a way bit high but God only knows what THD had them set at. No info except for calling THD.
I don't believe they would give me their info as the amp isn't even in their catalog anymore.This Preamp board is dated 4/3/1993

Title: Re: THD V Front Bassman Style Amplifier / Sorta!
Post by: Latole on December 05, 2024, 03:52:58 am

No need schematic for biasing a amp.
Grill negative voltage is not useful to bias. -48 to -52 may be right or not

Know power tubes maximum plate dissipation ( watts )

1- Read power tubes plate voltage
2- Read  "         " cathode current  ( ma ).

An exemple ;  with 25 watts tube
                     plate voltage 400 vdc
                     Cathode current 25 ma
400 X 0.025 = 10 watts

Adjust bias ( grill negative voltage ) to reach 60 % of 25 watts =  15 watts