Screen drive and other P-P experiments

[SpreadSpectrum]

Those pictures look very similar to the traces that I had right before my triode connected KT88s died. Very rounded tops when clipping. Beautiful.

 

[dupyv]

Hi George,

Very interesting write-up about your screen-drive experiments!
Highly informative as ever, especially when you explore the limits. 🙂

While mowing the lawn, i thinkered about a 6AV5 screen drive amp, and it occurred to me that an OPT with bifilar windings (ala McIntosh)
should be optimum with the screen-drive amp.
The bifilar OPT, in theory, minimizes the distortion near the shutoff-point of a tube being run at low idle-current.

I've clone OPT's of a Mc MA-230 collecting dust on the shelf since 10 years. I think i'll give them a shot using your topology as time permits.

kind regards,

Yves

 

[SpreadSpectrum]

Why did the screen voltage go positive when it was connected only to a negative supply? There is only one explanation that I can think of. The screen grid had become hot enough to emit electrons and begin to act like a cathode. These electrons traveled to the plate causing some serious "secondary emission" raising the screen grid voltage, which in turn caused the cathode to plate current to increase leading to runaway.

This sounds like something I was just reading about in RDH4. Check out page 21, 'grid blocking' if you aren't already familiar with it. Just out of curiosity, what's your source follower load resistor?

 

[Michael Koster]

This sounds like something I was just reading about in RDH4. Check out page 21, 'grid blocking' if you aren't already familiar with it. Just out of curiosity, what's your source follower load resistor?

Pretty sure everyone's figured it out by now, but with a source follower driving a grid there is still a grid resistor needed to pull back down and prevent runaway. Both Primary and secondary emission can cause negative current. The first couple dozen pages in RDH4 cover it well.

Cheers,

Michael