Voltage problem w/ my ST-70

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Voltage problem w/ my ST-70

Postby SDS-PAGE » Tue Jun 19, 2007 9:26 am

I just finished wiring my ST-70 modded with Shannon's board. It seems to bias OK with initial adjustment w/ wire-wound pots at -42.5 VDC max. But when I checked votages across all the tube sockets I am getting around 520 VDC! I checked AC volt feeding 5AR4 at pin 6 and 4, and it's 390 VAC. With the tube in place, I get 530 VDC at pin 8 and 2. Then I checked Volt drop cross resistors on cap lugs and I get 530VDC on D, 530VDC on C and 520VDC on B and 520VDC on A. I don't understand why those resistors aren't dropping more. I am using a mixture of new and original parts for the mod and those resistors are original. Also, 390 VAC on the secondary? Isn't it supposed to be around 360 VAC? I checked the rectifier tube on a tube tester and it's "almost ?/replace". I do have a bad tube? What volt should I get on pin 2 and 8 on a good tube? Thanks in advance.
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Postby mesherm » Tue Jun 19, 2007 9:50 am

What bias current do you have flowing through your output tubes?
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Postby SDS-PAGE » Tue Jun 19, 2007 10:19 am

Ok, I thought those values given on the manual were w/o tubes inserted. I tubed the amp and the volts are where they should be. Yellow_Light_Colorz_PDT_09
Thanks for the "current" question, Mike. It made me realize you needed tubes as a load to drop those voltages. I freaked out when I saw 520+ VDC on preamp tube pins (w/o tubes).
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Postby mesherm » Tue Jun 19, 2007 10:48 am

That explains it then. Those readings were typical of an ST70 with no current load. Eric was just warning people in a separate thread against installing a switch in the tube cathode circuit to put your amp on "standby" for that very reason. With no load from the power tubes the downstream filter caps can see some pretty high voltages.
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