KT88 Cathode Current Runaway/Oscillations?

[DF96]

Try to identify where the two different builds might have different stray capacitances at a high impedance point or around a high gain stage. Better to find and solve the problem rather than just put a sticking plaster on.

 

[Matt BH]

Swap the plate(anode) connections and screens at the output transformer. Be sure to swap both plate and screen connections for each output tube. Connect your feedback loop again and see if it's sorted. Or if it's easier, reverse the ground and feedback connections to the secondary of your output transformer.

Try this before thinking about any other modifications. As DF96 says a different layout can make a lot of difference but this seems like positive feedback to me. I have read about but never seen that two apparently identical output transformers may have either primary, secondary or both windings connected to the output leads round the other way to another.

Cheers
Matt.

 

[mbeards]

Thats a great point Matt, Ill give that a try. The only difference in the layout is putting the power supply on a fiberglass eyelet board.

 

[mbeards]

Can I reverse the the ground and feedback connections on a multi tap secondary and not lose the functionality of the other taps? for example, I am using the 8ohm tap for the feedback loop and grounding the "Common" speaker tap.

 

[Miles Prower]

Can I reverse the the ground and feedback connections on a multi tap secondary and not lose the functionality of the other taps? for example, I am using the 8ohm tap for the feedback loop and grounding the "Common" speaker tap.

If you're using 8R speeks, then it makes no difference. However, if you intend on using whatever other taps are available, then it will make a big difference if you ground the 8R tap.

I would still try that to see if Mr. Murphy didn't wire you for positive feedback, which is what this looks like. If reversing the secondary connections fixes it, then you'll have to reverse all primary connections to keep the 8R tap the 8R tap, and the common the grounding point.

 

[richwalters]

Despite all these posts;I'm loosing this as I DON'T like the idea of toiling with an unstable amp with good tubes at 550V....a screen wipe out can come quick and expensive. My experience is work at a lower voltage as an amp which is not running right will show the same instability evidence. Square wave doctoring.....where are those waveforms ?

richy

 

[sajti]

Can I reverse the the ground and feedback connections on a multi tap secondary and not lose the functionality of the other taps? for example, I am using the 8ohm tap for the feedback loop and grounding the "Common" speaker tap.

You will lose it for sure. Change on the primary, that should be better solution.
I'm about 90% sure that the problem is the wrong connected feedback. As You wrote the oscillation happens suddenly, even with low bias current, and not possible to stop it. So it should be very strong oscillation, not just instability.

Sajti