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In Reply to: RE: Powered it up WITHOUT the driver tubes...still misbehaving posted by DAK on January 14, 2017 at 13:09:40
It doesn't rule out the coupling caps.
Not that I think that's your problem.
I still think it's filter caps in the negative supply.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Follow Ups:
That would seem to indicate that the bias supply caps are good.
I guess you're down to environment induced oscillation.Grid stoppers and ferrite beads?
Maybe a bad PT.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 01/15/17
I can put a grid stopper no problem but where would i put the ferrite beads?
I would start with the large snap on type on the power cord and audio input cable.
Then the tiny donut type around the wires leading to the grids.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I got a snap on the power cord already. But i can put the bead type on the inputs. Do you think a grid stop resistor is warranted on the input/driver tube? The amount of gain from the driver tube is already on the low side since the amp is only 2 stages. thanx tre.
If the amp behaves in a different location then we are assuming that you have a lot of RF at home so, yes. A grid stop resistor on the input tube might be just what you need.
It's easy enough to try.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
The amp was stable for about 10 minutes then slowly the cathode current of the 6550 started to tick up, and then it picked up speed and i shut it down. I also put in a grid stopper on the input/driver tube. I put the ferrite bead on the wire lead to the grid stopper. it is short not even 2". Maybe i should go with a shielded input wire? The run is short as i said.
Edits: 01/16/17
DAK
Just disconnect the coupling cap going to output tube and run it.You only have to disconnect the cap on one side and you can't play music but you can run the amp at idle to see if it drifts.It may not be your problem but it's quick way to find out.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
If one is bad i can see that as a possibility but both new caps being bad does seem unlikely. Even if 1 cap is bad can it pull the bias voltage down equally in both power tubes?
Even if 1 cap is bad can it pull the bias voltage down equally in both power tubes?Manufacturers do things in runs.If there is an issue with those caps,being they came from the same run no doubt,that can be an issue for both.
It's very simple to do that test and get it out of way.That may not be your problem but newer oil caps have had issues at times,including the Jensens.There may be cases where the oil has become conductive for whatever reason. You have built these amps several times and this is the amp you are having trouble with. I would try the caps. Just disconnect one side of each cap and run it and that way you will know.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Edits: 01/16/17
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