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RE: Well......

Posted by maxhifi (A) on April 30, 2016 at 00:57:21

In Reply to: Well...... posted by vinnie2 on April 29, 2016 at 14:21:40:

I am confused by a few things here

1. There is no cathode in any of these tubes. I assume plate to cathode means plate to one of the filament pins, or to the centre tap of the hum balance pot.

2. If you are getting only 33V plate to filament, the issue is with bias, the tube is conducting too much because there isn't enough bias voltage to throttle it down. Remember a triode tube is more or less a variable reaistor which is controlled by grid voltage... More grid voltage gives a higher resistance and less gives a lower one... This varying resistance is used to control power, hence amplification.

Play with the value of the self bias resistors (centre tap of him pot to ground) to set the plate voltage, not with the plate load reaistor. For example, to get a higher plate to filament voltage you would increase the value of the self bias resistor. . The plate to filament voltage should be about half the B+ voltage for the first two stages, and for the output stage should be almost full B+

Make sure to measure all the resistors a second time, maybe one or two are off in value and throwing the design out of whack.

Edits: 04/30/16

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