Monoblock valve amp restoration (and modification?) project - Advice needed!

[El Sid]

Would counter-windings make the mains transformer run cooler? The primary winding is on the inside, the 340-0-340V in the middle and the 6.3V on the outside. I bet counter-winding the primaries on the outside would introduce interesting but unwanted coupling effects ...

Cooler - wouldn't there be lower current in the windings and lower flux in the core?

As for the coupling - I don't know enough about transformers to be able to comment.

 

[Tubekwk]

Regarding the input voltage, you stated it was between 235 and 239VAC, one possible solution is to fit an extra 230VAC to 12VAC 1.6A transformer wiring the primary across the mains and wiring the 12VAC such that it subtracts 12V from the supply to the amplifier, the voltage the amplifier has on its primary will vary from 235-12=223V to 239-12=227VAC. Some toroidal transformers are quite small and may fit under the chassis.

 

[Groundloops]

Regarding the input voltage, you stated it was between 235 and 239VAC, one possible solution is to fit an extra 230VAC to 12VAC 1.6A transformer wiring the primary across the mains and wiring the 12VAC such that it subtracts 12V from the supply to the amplifier, the voltage the amplifier has on its primary will vary from 235-12=223V to 239-12=227VAC. Some toroidal transformers are quite small and may fit under the chassis.

Like that?

 

[Groundloops]

Like that?

Somehow the picture disappered. Here it is:

 

[Tubekwk]

The image you posted did not seem to work for me. I did a quick simulation in LTspice to show how a buck or boost transformer works, be carful of the phasing of the buck/boost transformer as it can add 12V or subtract 12V.

The *.asc file will open and run in LTspice, the jpg is just a quick look.

 

[Groundloops]

The image you posted did not seem to work for me. I did a quick simulation in LTspice to show how a buck or boost transformer works, be carful of the phasing of the buck/boost transformer as it can add 12V or subtract 12V.

The *.asc file will open and run in LTspice, the jpg is just a quick look.

Wow, that is helpful! Thank you so much!!!
I'll try it and then report here.

 

[Groundloops]

The image you posted did not seem to work for me. I did a quick simulation in LTspice to show how a buck or boost transformer works, be carful of the phasing of the buck/boost transformer as it can add 12V or subtract 12V.

The *.asc file will open and run in LTspice, the jpg is just a quick look.

IT WORKS! 🙂 Took a while until I got a pair of nice and cheap surplus 12V 2A toroidal transformers. Today's mains ac voltage is 239V, and wiring up the transformers as you show gives 226V on the amplifier's primaries.

Thank you very, very much!