Psu 2

[Chris]

It's been some time now since building the psu, I've made some changes to the caps, checking all the test points everything seems spot on.

Just a few questions to help my understanding of things, Test point 7 shows 50.3v dc, on the board it has 6.3v? and Test point 5. 44v dc.

pin 1 and 4 are for the heater suppy, does the - and + leave 6.3 volts? (50.3 - 44)

Cheers Chris..

[NickG]

Yes, the heater sees 6.3v but with respect to the valve cathode its raised up by 44v so the cathode follower on the phono-II doesn't see too great a heater-cathode voltage difference.

[Chris]

Thanks Nick.

Chris.....

[julianr]

Yes, the heater sees 6.3v but with respect to the valve cathode its raised up by 44v so the cathode follower on the phono-II doesn't see too great a heater-cathode voltage difference.

Can you explain why this would make a difference?

Julian

[Global]

If a valve cathode is sitting at (say) 100v and the heater is referenced to ground then there is a 100v PD between the heater and the cathode. This may be enough to break down the heater/cathode insulation or, even if that does not happen, the heater and the cathode can become a sort of 'diode' resulting in noise in the circuit.

Raising the absolute voltage of the heater will keep the PD away from the breakdown limit and reduce the chances of noise generation.

Mark

[julianr]

thanks for that, may have to do some more thinking!!

Julian