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In Reply to: RE: What can be causing the loss of signal in this amp? posted by Triode_Kingdom on April 17, 2016 at 13:52:08
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
Follow Ups:
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?
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Buy Chinese. Bury freedom.
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.
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.
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Buy Chinese. Bury freedom.
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?
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.
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Buy Chinese. Bury freedom.
How does one calculate the dissipation for this amp with fixed bias? regards, Dak
Plate dissipation in watts is plate to cathode voltage times current.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
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
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.
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Buy Chinese. Bury freedom.
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.
Edits: 04/19/16
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.
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Buy Chinese. Bury freedom.
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
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.
A friend you get for nothing,an enemy has to be bought
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
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