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I traded for a pair of MKIII'S , that have a missing power transformer , I have two nice power transformers , the right size that are 390-0-390 , at 200 ma , 6.3 at 6 amps and 5.2 at 3 amps , but no bias winding .I have a pair of Poseidon driver boards that I plan on using , and was wondering , how much power out put it would loose with cathode bias , or should I just use a small transformer , like the one picture with 6.3 volts feed into the seconday , to get about 55volts AC for the bias circuit .


The third choice is to use a choke input for the B+ and use 6B4G's inplace of the 6550's with cathode bias .

I will be using low noise diodes instead of a rectifier tube , and rectify the 5.2 volts to DC for the input tubes , and use 12AX7 & 12BH7 as the input tubes

Backwards 15V transformer.

The backwards 15V xfmr would deliverabout 46-48 VAC with the 15V side connected to 6.3VAC filament power, that should work well.
It is a "time-honored" method of generating bias voltage. With the backwards xfmr, you could use a full wave bridge for cleaner DC voltage ,
or just go with the standard half-wave rectification, either way is OK.

/edB

You can also use the Eiclone method, using - from memory so check the schematic - a .1uF 630V cap and a 27K, 3W resistor, half wave rectified.

Shannon

The Eiclone method works well (Thanks Shannon!), I've used it in many builds.