Hiwatt Dr103 clone oscillating

[bxershrts]

So the presence control setting doesn’t affect the issue?
If so, that kinda indicates that the power amp isn’t in the loop.

Yes! You are correct, the presence control does mute the issue with the pot all the was Clockwise just as the treble pot.

Thoughts

[pdf64]

A schematic and layout are needed really for specific suggestions.

[bxershrts]

This is what i have found online from Mhuss.

[bxershrts]

As an update.

I have a pile of papers here on my bench and many of the schematics layouts diagrams for all the hiwatt models - and missed that the phase inverter on the early 70's Dr103 is a 12at7, i had a 12ax7 in there. I have now put in a 12at7 and the amp has zero oscillating.....

[bxershrts]

As an update.

I have a pile of papers here on my bench and many of the schematics layouts diagrams for all the hiwatt models - and missed that the phase inverter on the early 70's Dr103 is a 12at7, i had a 12ax7 in there. I have now put in a 12at7 and the amp has zero oscillating.....

As something to note for my own experience. I have been chasing this oscillating problem for a month now. What I didn't even think about was changing the new tubes. They are all new mesa (Chinese) tubes. I would sub the phase inverter tube from the 3 other mesa tubes and it all did the same thing. My first thought was that it wasn't the tube.. Replaced with a 12at7 and the problem was solved. Looking at things I thought how is that possible when there isn't that much gain from stage to stage.. So I subbed in other brands of tubes and eureka....only Chinese tubes squealed in the phase inverter.

Amp functions as expected now

[pdf64]

It seems strange that the root cause was something in the phase splitter, yet the presence control setting didn’t affect it.

[bxershrts]

It seems strange that the root cause was something in the phase splitter, yet the presence control setting didn’t affect it.

Yes, the presence control did change the issue along with the treble control

[mhuss]

Thousands of Hiwatts were built with the long PI feed wires without a problem. They're not even twisted, just run side by side inside the sleeve.

If the difference between a 12AX7 and 12AT7 'fixes' the oscillation problem, you're just pulling it right below the surface, and it's likely only marginally stable. I have worked on many Hiwatts that had 12AX7s in the PI by accident or with intention, and none had oscillation problems.