I know....... I was just joking. You already described the obstickles in the "swapping".
Unfortunately (at least as I see it), it´s probably tha last posibility.
I asume, that all the wiring is identical..... same length....... same path in both channels??
BTW.... Take note of post #40. Not at all impossible, that they (Troroidy) could have made an error in wire colors.
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But it still needs a negative resistance to oscillate. He has 1k grid stoppers, so I find it difficult to see where that negative resistance could be.
I am in process of building tube amps with kt90. I get the same response you get bc the grounded side of the secondary Opt came loose under chassis. Strange behavior would work just fine for about 5 seconds and the Osc. at 2k. Measure with dmm. Have you tried other grid stopper values? Go to 4.7k or higher. One other point. Always breadboard first. Don't assume everything is going to work and ty wrap things or shrink wrap a bunch. Wire loosely enough to move things around and give room to test.
He still hasn't shown us the full schematic including power supplies, nor a photo of the physical construction. I suspect the answer will be in one of those.
In triode mode the oscillation is worse, it happens at a lower voltage. A Zobel from G2 to plate will not be a solution.
Having used different power supplies with identical results rules out the power supplies.
Connecting the input capacitors to audio ground makes no difference.
Regards, Gerrit
Having used different power supplies with identical results rules out the power supplies.
Connecting the input capacitors to audio ground makes no difference.
Regards, Gerrit
Did I understand you correctly that this amp oscillates with just the power tubes inserted, hence without any GNFB? In this case the one and only feedback, hence the one and only reason for oscillations are the UL taps. Instead of swapping both OPT's, reverse the screen (or plate) connections. I suspect the xformer manufacturer might have labelled the UL taps erroneously.
Best regards!
Best regards!
Yes that's the case. Its difficult to understand how triode mode can oscillate unless you have got the screen connected to the wrong plate. That's quite numpty unless you are going through a triode/UL switch.
Well, just to react to a few comments here I did swap the UL-wires on the triode/UL switch.
The amplifier is dead silent now, no Zobel, nothing special whatsoever needed.
I agree now that it must have been the transformer. Now I can start to undo all the unnecessary changes and temporary wiring (with a smile on my face…).
Regards, Gerrit
The amplifier is dead silent now, no Zobel, nothing special whatsoever needed.
I agree now that it must have been the transformer. Now I can start to undo all the unnecessary changes and temporary wiring (with a smile on my face…).
Regards, Gerrit
I don't get it...you did what? You swapped what? What switch? Was the thing wired incorrectly? Was the OPT busted?
I didn't understand anything that happened.
Hi Grataku,
I changed the two wires from the transformer on the triode/UL switch. The switch goes to the screen grids. That’s all.
Regards, Gerrit
I changed the two wires from the transformer on the triode/UL switch. The switch goes to the screen grids. That’s all.
Regards, Gerrit
What you forget to mention:
Did you wire the 2 wires to the switch wrongly to begin with,
or was the wire color code wrong from the factory??
On my first and only tube PP amplifier I also incorporated a triode/UL switch. After a few trial and listening I never flipped the switch any more. Eventually I removed it altogether.