by EWBrown » Mon Jan 04, 2010 2:28 pm
First, you will need a separate 5VAC, 3A filament trannie.
The 6AX5 uses pins 3 and 5 for the HV AC input, and pins 2 and 7 for the filament.
The 5U4G uses pins 4 and 6 for the HV AC input, and pins 2 and 8 for the filament.
Given this information, the conversion can be done relatively easily, first cut the traces from the 6.3VAC terminals to the rectifier tube's filament pins (2 and7) in order to isolate them..
Next just jumper the rectifier socket pins 3 to 4, 5 to 6, and 7 to 8. (1 and 2 could also be jumpered, but this isn't really necessary). This makes things a lot easier, as no other etch "surgery" would be required, and allows for "backward compatibility".
Connect the filament (pins 2 and 7/8) to the separate 5VAC, 3A winding, and take the B+ off pin 7/8. The AC from the HV secondary connects to pins 3/4 and 5/6
This should make it compatible with 5U4GB, 5R4, 5V4, and 5AR4, and this also allows reverting back to 6AX5 later on, simply by reconnecting the filament 2 and 7 back to the original 6.3VAC connections, AND removing the jumper between 7 and 8 (this step is VERY important)
Two separate 6.3VAC windings are still necessary for the GSG, to allow the "cathode" resistor biasing.
If you really want to "gild the lily", Edcor could make up a custom power trannie equivalent to a PA774, with an added 5VAC, 3A winding. Once they do this, it becomes a new part number on their lready extensive product listing. Think of it as "Immortality" for cheap ;-)
HTH
/ed B
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