300B PSE clone

[Stephen C]

I am looking for parts to build a pair of WAD 300B PSE amplifiers.
I would welcome any advice or first-hand comments on these amplifiers.
I am looking for a source of suitable single-ended output transformers in the UK.
I have a couple of questions concerning the heater supply for the 300B valves

1. The original design has separate 5V windings for the two output valves. Should these be wired in-phase for minimising hum, or out-of-phase?
2. What is the reasoning behind having hum bucking only on one of the 300B heaters, as these are supplied from separate transformer secondary windings?

[John Caswell]

Hi Stephen,
You may have minor problems here as I am not aware of the anode impedance of the optx. There is a company on Poole that I think will supply you with a whole set but be seated when you ask the price!
As far as wiring the heaters, there is some evidence that in phase wiring does help with hum reduction. I could measure a difference on my amps but it was small.
Hum bucking is only need on one valve to essentially match to the other valve.
You could of course go the whole hog and get your power tx. with, say, 8V heater windings then rectify and regulate it to 5V DC.

John

[Stephen C]

John, Thank you for answering my heater questions.

Hi-Fi World supplement 53 shows the output transformers have an input impedance of 1k5, and an output of 8R. I do have some old (unused) Sowter transformers, from a 811a/SV572 project that stalled. I may have to dig these out and see if the impedance ratio is suitable.