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

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Posted on April 17, 2016 at 09:43:45
DAK
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Joined: August 8, 2010
 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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RE: What can be causing the loss of signal in this amp?, posted on April 17, 2016 at 11:33:35
Triode_Kingdom
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Posts: 10049
Location: Central Texas
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 "the bias voltage of the 2a3 is varying unlike the +50vdc on the good amp."

Sounds like the coupling cap to the grid of the 2A3 might be leaking.

 

RE: What can be causing the loss of signal in this amp?, posted on April 17, 2016 at 12:38:22
DAK
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Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 I was wondering about that , it is a Jensen pio. No outward signs of a problem. If I bypass it with known good cap and the voltage goes back to the +50vdc that would definitely indicate a bad coupling cap? regards, Dak

 

RE: What can be causing the loss of signal in this amp?, posted on April 17, 2016 at 13:52:08
Triode_Kingdom
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Location: Central Texas
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 If the cap is leaking, it has to be removed from the circuit to test. You could also monitor the grid of the 2A3 with a DC voltmeter. Note that the 2A3 is at risk if the cap is leaking.





 

RE: What can be causing the loss of signal in this amp?, posted on April 17, 2016 at 15:26:39
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 Hi TK, thank you for the tip. I will disconnect one side and try another cap in the spot and see if the voltage goes back to what it is supposed to be. regards, Dak

 

RE: I think i found the problem, posted on April 18, 2016 at 15:40:09
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 Hi TK, I traced the issue to the negative bias on the 845 grids. It is supposed to be about -125vdc. But I can not get the value to be stable. I adjust the pot a fraction and it goes too negative and a fraction the other way and it goes too positive. What can be causing this? The bias supply is on a cb. Would bad caps cause this? Any place else I should check? Thank you for all the helpful suggestions. regards, dak

 

RE: I think i found the problem, posted on April 18, 2016 at 16:31:16
Triode_Kingdom
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Posts: 10049
Location: Central Texas
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 First, I hope you're not experimenting with the bias circuit while the 845 is plugged in. That could be a costly mistake! About the bias, could it be the pot itself? Maybe the wiper is intermittent?

 

RE: I think i found the problem, posted on April 18, 2016 at 19:23:51
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 I only have the amp at 90vac. The bias voltage is adjustable, it just doesn't stay put. I try to keep it at certain current/voltage and after a moment it start to drift downwards or the -dc starts to go more negative. And when I go more positive it goes too far. There is not a point where it wants to stabilize. There is a current meter on the amp that is working so it is easy to follow. The 845 is supposed to be nearly new so I don't think it is the tube. any ideas? cheers, Dak

 

RE: I think i found the problem, posted on April 19, 2016 at 05:35:55
Triode_Kingdom
Audiophile

Posts: 10049
Location: Central Texas
Joined: September 24, 2006
 The bias supply feeds both the 2A3 and the 845, correct? Pull those tubes and see if it's stable. If not, you'll probably need to sketch the bias circuit to make troubleshooting more logical. It should be pretty simple, unless it's regulated.



 

RE: I think i found the problem, posted on April 19, 2016 at 09:17:57
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 The bias supply is on a circuit board. On the board are a bunch of rectifiers maybe zeners and 2 SS devices. Plus an IRF840 and a Linear LT1083CP and 2 bridge rectifiers 2W10M. Is there any way to test the transistors? I think I know which caps are used in the bias voltage. I was going to change them out.

 

RE: I think i found the problem, posted on April 19, 2016 at 11:04:06
Triode_Kingdom
Audiophile

Posts: 10049
Location: Central Texas
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 Sounds like that PCB might have more than just the bias components on it. In any event, I think you'll either have to sketch a schematic for the board so you can troubleshoot, or design your own bias supply from scratch. Sorry I can't be more help.








 

RE: I think i found the problem, posted on April 19, 2016 at 11:55:26
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 Do you think I should change out the caps in the bias supply? I have found them based on the residual voltage on the metal body. They are some fotoflash type by Elna? You know the ones that they use in the cheap cameras. I know those are not so reliable. regards, Dak

 

RE: I think i found the problem, posted on April 19, 2016 at 12:41:40
Michael Samra
Dealer

Posts: 36118
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 Dak
Take out the signal tubes and measure the negative bias again.I mean take out the 12AT7 and the 2A3 and measure the negative bias.I almost sure it's one of the Jensen caps leaking and like TK says,you have to measure DC leakage with the cap lifted on the side going into the grid of the 2A3 and or 845.Photoflash caps can be questionable because they can have a short lifespan and higher leakage if run near their rated voltage.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: I changed out the jensen coupling cap, posted on April 19, 2016 at 13:27:06
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 Before I took new readings. The bias voltage is still unstable. I will try without the at7 and 2a3 and see what happens. If no difference I think I will change out the 3 caps in the bias supply. thank you for the tips. Dak

 

RE:The pot is fried. , posted on April 19, 2016 at 15:26:44
DAK
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Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 You were right, it was the pot. I changed out the caps and there was no change so I decided to remove the bias adjustment pot and it measures bad. So, next thing it trying to find a replacement. Thanx TK.

 

Glad you found it!, posted on April 21, 2016 at 06:04:49
Triode_Kingdom
Audiophile

Posts: 10049
Location: Central Texas
Joined: September 24, 2006
 If you don't find a replacement pot at the usual sources like Mouser, Digikey, etc., also try Surplus Sales of Nebraska. They have a lot of unusual items that can't be found anywhere else.

 

RE: Glad you found it!, posted on April 21, 2016 at 09:21:30
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 I went with a linear 25k pot by Allen Bradley. It seems to work great. A setting of -150 is about 3/4 travel. An according to the front amp meter is about 100ma of plate current. Is that a good operating point for just above 1000v on the anode?

 

RE: Glad you found it!, posted on April 21, 2016 at 12:52:25
Triode_Kingdom
Audiophile

Posts: 10049
Location: Central Texas
Joined: September 24, 2006
 1KV @ 100 ma - you're at the maximum plate dissipation rating of the tube. That's too much. I wouldn't run it past 80W, or 80 mA if anode voltage is exactly 1KV. Some people would probably consider 70W the limit for good tube life.



 

RE: Thanx TK, i will dial it back a bit., posted on April 21, 2016 at 13:10:21
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 How does one calculate the dissipation for this amp with fixed bias? regards, Dak

 

RE: Thanx TK, i will dial it back a bit., posted on April 22, 2016 at 08:08:07
Tre'
Industry Professional

Posts: 17306
Location: So. Cal.
Joined: February 9, 2002
 Plate dissipation in watts is plate to cathode voltage times current.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

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