rookie seeks help w/ ST-35 build

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rookie seeks help w/ ST-35 build

Postby cliffpickle » Fri Sep 12, 2008 8:13 am

Ty- thanks for the tip- Here are my voltages, (all in V):
V1;P1-40.88,P2-0,P3-1.24,P4-2.98,P5-2.98,P6-42.54,P7-0,P8-1.22,P9-2.96
V2;P1-313,P2-40.88,P3-43.33,P4-2.98,P5-2.98,P6-315,P7-42.3,P8-44.48,P9-2.96
V3;P1-(-),P2-.02,P3-15.51,P4-2.98,P5-2.96,P6-(-),P7-374,P8-(-),P9-378
V4;P1-(-),P2-.03,P3-15.4,P4-2.97,P-5-2.92,P6-(-),P7-375,P8-(-), P9-378
V5;P1-(-),P2-.03,P3-16.3,P4-2.99,P5-3.01,P6-(-),P7-375,P8-(-),P9-378
V6;P1-(-),P2-.02,P3-15.4,P4-3.01,P5-2.99,P6-(-),P7-374,P8-(-), P9-378

The line voltage at the unit is 120.05VAC.
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Postby Ty_Bower » Fri Sep 12, 2008 3:15 pm

Well, the output tubes (V3 through V6) look perfect.

I think the driver (12AX7) is V1. Forty volts on the plate seems way too low to me. Shannon says they should run around 115 volts.

The splitter (12AU7) is V2. Again, Shannon says they should be 210 on the plate. You've got over 300. Something is wrong here too.

If you have a Rev D, I'd take a real close look at R7, R8, R9 and R10 (the plate loads). Also check R5, R6, R11 and R12 (cathode resistors). Make sure the values there match the schematic (page 12 of 17 in the .PDF). Maybe you accidentally swapped parts somewhere?

Also check the voltage at the + terminal of C12, or at the end of R33. It ought to be around 320 VDC.
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Postby Ty_Bower » Fri Sep 12, 2008 3:26 pm

Oddly enough, Ohm's law seems to hold true around your driver tube. There's something amiss on the splitter (V2). I'm figuring about .26 mA through the plate load, but 1.31 mA through the cathode resistor.

There must be something wrong with either R9/R10, or R11/R12. Maybe both. Or you're drawing current through the grid. Is that possible here?

Are you sure you put a 12AU7 in V2?
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Postby Ty_Bower » Sat Sep 13, 2008 9:04 pm

You've got the 12AX7 in the 12AU7 hole, and vice versa. Swap the two, and all should be fine.
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