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Old 10th June 2013, 10:44 PM   #1
Einric is offline Einric  United States
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Default 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.

Last edited by Einric; 11th June 2013 at 12:20 AM.
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Old 11th June 2013, 02:16 AM   #2
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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.

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Old 11th June 2013, 02:29 AM   #3
Einric is offline Einric  United States
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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.
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Old 11th June 2013, 11:07 AM   #4
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Quote:
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.
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Old 11th June 2013, 12:24 PM   #5
Einric is offline Einric  United States
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Thank You
I will post more when they arrive.
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Old 12th June 2013, 12:39 AM   #6
Einric is offline Einric  United States
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Default Schematic

Here is a scan of the schematic
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File Type: jpg AP-50 Schematic.jpg (525.7 KB, 148 views)
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Old 12th June 2013, 12:59 AM   #7
Einric is offline Einric  United States
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What is the little X on the line below the optional relay in the Power Transformer secondary?
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Old 12th June 2013, 01:52 AM   #8
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"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.
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Old 12th June 2013, 02:37 AM   #9
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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.
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Old 12th June 2013, 03:16 AM   #10
Einric is offline Einric  United States
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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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