"Iain M Churches" wrote in message ...
Hi RATS,

I have a 30W push pull amp, with a faulty mains
transformer. It has a SS full wave bridge rectifier.

The transformer secondary is marked 345V, and
according to the schematic the DC rail is 425V at 400mA

I would like to replace this transformer with a centre
tap unit, so that I can use a tube rectifier.

I know that tube rectifiers are not so efficient as
SS bridges.

Question: How much voltage does the tube rectifier drop,
and how much extra should I allow on the transformer
secondary to compensate for this?

Thanks in advance

Iain

By what you say about the voltages, it is a condenser input filter. I
would say a 375-0-375 volt transformer will be right. Consult a manual
and look at the graph to see what voltage you need from the
transformer for the output voltage you need. If it was a reactor
input, the secondary would be 550-0-550 volts. A pair of 5AR4s would
be the best rectifier. These have lower heater consumption than the
filament types and better voltage regulation from mimimum to maximum
current. It also delays the B+ so there is no cathode stripping.
Cathode stripping is when there is B+ on a valve before the cathode is
hot thus causing a much shorter life and cathode to grid shorts.