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Einric 10th June 2013 11:44 PM

Stromberg-Carlson AP-50
I just picked up a pair of SC AP-50's for $25 each.
They are missing half their tubes and the Power Traffo is suspected to be fried on both due to a 5U4G failure.
Does anybody out there happen to have an equivalent part # from Hammond or Edcore???
I have hard copy schematics coming by the end of the week with parts list.

I won't take possession of them for a while since they are sitting at my fathers house 250 miles away.

I also picked up the matching rack mount preamp for $10.

Tom Bavis 11th June 2013 03:16 AM

I had one from a similar AP-55 but sold it recently... The same PT was also used in the AU-35 and AU-36 amps if I recall correctly. Common 4 1/2" size, the upgraded ST-70 transformer might be close, though might be shy on heater power.

Sams folder is on my web site: Made In Rochester

Einric 11th June 2013 03:29 AM

The AU-35 is very close, it's only off by 200mA on the heavy 6.3v winding.
What is this upgraded ST70 transformer?
I have no problem picking up a supplementary 6.3v transformer.

Eli Duttman 11th June 2013 12:07 PM


What is this upgraded ST70 transformer?
It is here. Hafler's original was on the ragged edge. The replacement has a thicker lam stack. Check the spec's, as it might have a few more "stones" than the original.

Einric 11th June 2013 01:24 PM

Thank You
I will post more when they arrive.

Einric 12th June 2013 01:39 AM

1 Attachment(s)
Here is a scan of the schematic

Einric 12th June 2013 01:59 AM

What is the little X on the line below the optional relay in the Power Transformer secondary?

Tom Bavis 12th June 2013 02:52 AM

"X" is connection that's broken if the relay is used. Relay opens minus side of plate supply. This uses "back bias" - the 100 Ohm resistor in the negative lead drops 25V at 250 mA, so the center-tap is -25V, used to bias the output stage. It's cathode bias, rearranged a little.

Eli Duttman 12th June 2013 03:37 AM

Heads up! The replacement ST70 power trafo can provide 4 A. of 5 VAC. 2X 5U4s requires 6 A. In order to use the replacement ST70 power trafo, either 5AR4s or 5R4s will have to be the rectifiers. Select between the 2 depending on where you want the B= rail voltage to come in. Forward drop in a 5AR4 is small, while forward drop in a 5R4 is large.

Einric 12th June 2013 04:16 AM

Thanks, I'm almost starting to think going all Toroid with these and using a separate 5V & 6.3V traffo might be the way to go in order to get the amperage I need.
I'll keep my eyes open for a suitable main traffo in the mean time, but....the upgraded ST70 one is on my short list.

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