So I bought a 300a last week from ebay. Very clean unit and it was shipped in 2 packages. Unfortunately one package, the supply, was shipped to a previous customer. Im hoping to get it back soon but in the mean time I did some investigating on these supplies and here a c&p of an old copper post I found on the differences between the 2 supplies.
"Pins 7 and 8 supply 120 Volts AC to the transformer primary.
Pin 1 is tied to pin 3, pin 2 is tied to 4. One pair on each side goes to the 12.6 V ac winding. They doubled up the pins to carry the high current feeding the tube heaters. The current required at 12.6 V ac is about 6.5 Amps. The tubes are wired as pairs in series, so each of three pairs of tubes split the 12.6 to get 6.3 V on each tube heater. The tubes draw 2 Amps each, times 3 PAIRS, adds up to 6 Amps, plus another half-Amp or so to run the meter light, relays, preamp and bias voltages.
Pins 5 and 6 carry the high voltage AC to the HV rectifiers. Here's where it gets tricky. The 300A was built with two different transformers. They both look alike, and have the same plug. The rectifier used inside the amplifier for the HV was configured TWO ways, and the transformer HV winding has to match it. The version that had the rectifiers arranged as a voltage doubler calls for 315 Volts AC at about 1.5 Amps. The transformer that matches the BRIDGE rectifier inside the amp box is 630 Volts AC at about 3/4 Amp (750 mA).
If you find an original transformer with the matching plug and cable, it's a 50-50 coin toss as to whether it's the right one or the wrong one. The safest way to tell if it's the right one or not is to trace the rectifier wiring on the power supply PC board underneath, and determine whether it is wired as a 'full-wave bridge' or as a 'full-wave voltage doubler'. Hooking pins 7 and 8 of the transformer to a PROTECTED, preferably isolated AC supply and metering pins 5 and 6 will tell you whether it is the 300-volt or the 600-volt HV winding.
There are two possible WRONG combinations of transformer and amplifier. If you have the 300-volt transformer and an amp wired for the 600-volt version, you'll have an amplifier that is stuck on 'low' side all the time. The tubes would last forever, but you'd be disappointed with the power output. If you plug the 600-volt version of the transformer into an amp with the DOUBLER circuit (needs 300-volt version) you'll get a large POOF either the moment you turn it on, (later version with NO relay underneath) or the moment you first key it. (earlier version had a relay underneath that keyed the HV"
I hope this info is helpful.
