I put 5.6K resistor grid stoppers and the problem is completely solved!!!
Anyway I really dont understand why I've been using this amp for one year without grid-stoppers and suddenly I need them: it's a mystery....
Thanks everybody for your precious help
JTM45 problem
Moderators: pompeiisneaks, Colossal
Re: JTM45 problem
Tubes can get microphonic as they age.
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martin manning
- Posts: 14308
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- Location: 39°06' N 84°30' W
Re: JTM45 problem
So exactly what are we hearing in the clip?
Given the assumed purpose of these resistors would the distinctive ripping sound be a breaking into and out of a parasitic oscillation that is triggered when the signal amplitude is high? And, why did it not occur previously? Do the inter-electrode capacitances change as the tube ages? Presumably it is some electrical characteristic that has changed over time, since altering only the electrical characteristics of the surrounding circuit cured it. Flatart, did you try a different set of tubes an any point?
MPM
Given the assumed purpose of these resistors would the distinctive ripping sound be a breaking into and out of a parasitic oscillation that is triggered when the signal amplitude is high? And, why did it not occur previously? Do the inter-electrode capacitances change as the tube ages? Presumably it is some electrical characteristic that has changed over time, since altering only the electrical characteristics of the surrounding circuit cured it. Flatart, did you try a different set of tubes an any point?
MPM
Re: JTM45 problem
It could have even been an existing bad solder joint that was addressed when he changed the grid stoppers.
Tom
Don't let that smoke out!
Don't let that smoke out!
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martin manning
- Posts: 14308
- Joined: Sun Jul 06, 2008 12:43 am
- Location: 39°06' N 84°30' W
Re: JTM45 problem
Apparently not:Structo wrote:It could have even been an existing bad solder joint that was addressed when he changed the grid stoppers.
The effect of the scope probe, essentially inserting a high value resistance to ground, is interesting too.flatart wrote:I re-soldered almost all the circuit: output tubes pins, all bias circuit, all pots and all solderings that may appear not so good, but nothing changes.
So next try I will put a grid stopper on pins 5 (I will go for 5k resistors).
I will also try to disconnect feedback (never tried yet).
Another strange thing I noticed is that also when I put scope-probe on pins 1 and 6 of PI the problem seems to disappear or at least is quite quite less evident.
MPM
Re: JTM45 problem
Really I did reflow all solders but I still had the problem.
As I said in previous topic I also tried a couple of 6L6GC but nothing changed.
The only thing that fixed the problem immediately was grid stopper resistors. I mind that I tried it at last, but at least now all solders are almost perfect!
As I said in previous topic I also tried a couple of 6L6GC but nothing changed.
The only thing that fixed the problem immediately was grid stopper resistors. I mind that I tried it at last, but at least now all solders are almost perfect!
Re: JTM45 problem
sorry, forgot to say that I tried also to change all tubes in the circuit, one at a time
Re: JTM45 problem
Your Plate wire is too close to grid wire near your powertubes. Try to move the wires..
..And then you can remove the grid stoppers..
..And then you can remove the grid stoppers..