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RE: Kit1 Anode Voltage

Posted by Frank E (A) on October 5, 2007 at 13:10:17

In Reply to: RE: Kit1 Anode Voltage posted by RhythMick on October 5, 2007 at 02:51:37:

"but actually you think it should be 435V anyway?"

No, I said the B+ should be 435 VDC, on the load side of the filter choke.

The mains TX should be putting out 425 Vac/rms either side of centertap, i.e. to rectifier tube.

(sorry for calling the mains TX a "power TX", I hope that didn't throw you off; power TX is such a North American term)

It looks like you found the problem for sure - 230V tap instead of 240. The higher voltage now is probably due to your mains voltage being a little higher than nominal. It's common for that to happen depending on how close you are to the utility HV distribution station, how many customers on your feed, time of day loading, etc. The utility runs the voltage high enough such that after line drop the customer farthest away is still in spec.

"Brian suggested using a dropper resistor on the output of the rectifier"

That is a good solution. Make sure the resistor is after the choke but ahead of the connection to the 220uF/450V cap. That way the resistor will filter out more of the ripple voltage and also the capacitor won't be subject to long-term overvoltage.

I've used R/C (resistor/capacitor) networks for filtering and voltage drop on other amplifiers to good effect. Depending on the models and parts chosen, the sound can be much better than with L/C filters. I've found the Mills Non-inductive Wirewound Resistors to sound very nice. You could use two 12W in series or parallel to get your 20W rating. (adjust resistance values appropriately)

After your voltage is optimized, you could also alter your cathode bias resistor values slightly to optimize the 300B operating point. One thing for sure, those WE 300Bs are worth taking care off.

Cheers

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