PSU design; what does it do????

[Bernardduur]

Hello all!

Right now I am repairing an old MACCS M100 amp which is in fact an upgraded Klemt BS40. Mainly the ELCO's are old and rusty and the tubes need replacing.........

Then I watched the PSU design (power trafo was clearly replaced) and I couldn't get what was happening in this design........

Little schematic of what happens in the first stage of the PSU

What happens here and why is this added.

Edit: When I sim the circuit it acts as a voltage halver; when I measure the plate voltage on the powertubes (EL34) I get around 800V........ this is a voltage halver???? Can EL34 run safely on these voltages? Do I need to take extra precautions?

[Albert Kreuzer]

Hi Bernard,

this is a full wave bridge with a center tap, very common in the 70s for class B tube amps. Output 1 goes to the plates, output 2 to the screens.
R1/R2 seem to be placed a little odd though - are you sure they are wired that way?
And yes, EL34s were designed for 800V max in class B operation. The screens get 400V, bias them no hotter than 19-20mA per tube.
Also keep in mind that most European equipment back then was designed for 220V operation, not 230, which makes all the secondary voltages 5% higher than intended.

Cheers,
Albert

[Bernardduur]

Thanks man! I am currently also repairing my old Bass King from the files on your website!

The resistors are placed in that way and I measured them to be sure.

[Albert Kreuzer]

Thanks man! I am currently also repairing my old Bass King from the files on your website!

Let me know how it turns out.

The resistors are placed in that way and I measured them to be sure.

Well, ok then - they will do no harm.

Cheers,
Albert