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RE: Kit1 Anode Voltage

Posted by RhythMick (A) on October 5, 2007 at 02:51:37

In Reply to: RE: Kit1 Anode Voltage posted by Frank E on October 4, 2007 at 09:10:56:

Thanks Frank. Yes I ended up checking all the way back to the mains transformer, which was putting out 467V. The diagram shows 425V. I checked the manual and it had been wired for 230V rather than 240V. I changed it and it's putting out 435V now, still higher than the diagram says, but actually you think it should be 435V anyway ?

So, with the transformer now closer to where it should be and with the 900R cathode resistors back in place, I'm seeing the following ...

- measured voltage coming out of the rectifier = 488V (this includes the 1.4x smoothing effect of the caps, so I think the rectifier is actually putting out 349V)
- measured voltage drop across the cathode resistor = 74V, so I have 83mA through the valve - still too high in my view
- measured anode voltage is 457 - MUCH better, though still above spec.

Brian suggested using a dropper resistor on the output of the rectifier, say 200R. I think that will drop the measured voltage from the rectifier to around 446 but I'll have to put it in and see what happens. It will dissipate 4.5W so I'll go with 20W for safety. Once I can see the effect of that dropper I'll calculate my way through the circuit but I expect to have to change R5 and R7 to make sure that the 5687 and 6SN7 valves get the correct voltage also.

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