Balanced PI Tube

[Roe]

marshalls with 100k@4ohms negative feedback tend to push v4-5 much harder than the inverted side (v6-7). JTMs have much better balance due to the 27k@16ohms feedback.

the balance can be improved by:
1 decreasing the 1m on v3a to 390k like marshall did on the park 1210
2 increase negative feedback
3 increase tail resistor (ala vox, 47k)
4 adjust plate resistor, usually the 82k needs to be decreased

plenty of 70s 100w marshalls redplate at v4-5 when running flat out. usually, the redplating disappears when matching the PI

[Structo]

Yeah, I remember a few guys at the Metro forum having trouble with V-4-5 redplating.

I thought some of that was from too high of a screen voltage?

Thus the flying resistor before the screen resistors?

[Roe]

Yeah, I remember a few guys at the Metro forum having trouble with V-4-5 redplating.

I thought some of that was from too high of a screen voltage?

Thus the flying resistor before the screen resistors?

yeah, high voltages in general together with unbalanced PI. the flying resistor on the jtm45s reduces stress on screens greatly

[gingertube]

If you want to see an example of gross PI imbalance look at the Subway Rocket circuit.

http://www.webphix.com/schematic%20heav ... rocket.pdf

I had one brought to me with the elegant fault description of "it sounds like s..t". Further discussion revealed that it had always sounded that way. It has 68K and 120K anode load resistors on the PI. I modded it by fitting 110K in place of the 68K PI anode load resistor and the guy loved it, he was happy as a dog with 2 tails.
Cheers,
Ian