What can be causing the loss of signal in this amp?

Posted by DAK (A) on April 17, 2016 at 09:43:45

This 1 year old amp has a 12at7 input tube cap coupled to a 2A3 which is transformer coupled to a 845. There is fixed bias on the 845 I don't have my notes but -110vdc maybe 120 sticks in my head. There is no schematic for the circuit.
What happens to the amp while on the variac is that the sound comes thru the amp at about 70vac but it is somewhat ragged. Then at higher voltages the sound gets increasingly distorted and weaker. By 90vac there is no signal at all. I have ruled out the tubes as I took them from the other amp which works fine. The source and the speakers have also been ruled out. Deviations of measured voltages from this amp to the other are about 10% higher B+ and the bias voltage of the 2a3 is varying unlike the +50vdc on the good amp. I was trying to track down the bias resistor but could not find it. I checked the RCA manual and 750 ohms was listed. but I have not found something close to that. There was a 25watt 1k? although I am not positive of the value since the designation is not obvious. I have not run across this problem other than relating to assembly issues. But since the amp was working fine before I am unsure where to check for a non obvious fault. If anyone has suggestions on where to look I would be most appreciative. Best regards, Dak

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Topic - What can be causing the loss of signal in this amp? - DAK 09:43:45 04/17/16 (19)

RE: What can be causing the loss of signal in this amp? - Triode_Kingdom 11:33:35 04/17/16 (18)

RE: What can be causing the loss of signal in this amp? - DAK 12:38:22 04/17/16 (17)

RE: What can be causing the loss of signal in this amp? - Triode_Kingdom 13:52:08 04/17/16 (16)

RE: I think i found the problem - DAK 15:40:09 04/18/16 (14)

RE: I think i found the problem - Triode_Kingdom 16:31:16 04/18/16 (13)

RE:The pot is fried. - DAK 15:26:44 04/19/16 (5)

Glad you found it! - Triode_Kingdom 06:04:49 04/21/16 (4)

RE: Glad you found it! - DAK 09:21:30 04/21/16 (3)

RE: Glad you found it! - Triode_Kingdom 12:52:25 04/21/16 (2)

RE: Thanx TK, i will dial it back a bit. - DAK 13:10:21 04/21/16 (1)

RE: Thanx TK, i will dial it back a bit. - Tre' 08:08:07 04/22/16 (0)

RE: I think i found the problem - DAK 19:23:51 04/18/16 (6)

RE: I think i found the problem - Triode_Kingdom 05:35:55 04/19/16 (5)

RE: I think i found the problem - DAK 09:17:57 04/19/16 (4)

RE: I think i found the problem - Triode_Kingdom 11:04:06 04/19/16 (3)

RE: I think i found the problem - DAK 11:55:26 04/19/16 (2)

RE: I think i found the problem - Michael Samra 12:41:40 04/19/16 (1)

RE: I changed out the jensen coupling cap - DAK 13:27:06 04/19/16 (0)

RE: What can be causing the loss of signal in this amp? - DAK 15:26:39 04/17/16 (0)

What can be causing the loss of signal in this amp?

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