Phase Inverter Distortion

[Littlewyan]

If in amp, say a Marshall, a LTPI is used and the value of the grid leaks going to the power valves is lowered, the LTPI will distort more. Why is that? As I believe thats why a PPIMV is effective, because as you lower the signal to the power valves it will make the PI distort more.

[Stevem]

It's a long answer it would seem!
Head to the Aiken amps page and read the" designing long tailed pair "white page!

[pdf64]

The power tube grid leaks are effectively in parallel with the LTP plates and the LTP plate resistors.
So as the total impedance decreases, the LTP gain and max potential output voltage swing will also decrease.

[jjman]

It is increasing the "load." Similar to increasing the load in a truck. Poorer acceleration and deceleration.

[tubeswell]

Also the type of output tube is an aspect to consider in the design. With higher-power output tubes (typically hotter tubes running/designed for higher current), the grid leak resistance has to be lower to allow sufficient electron replenishment to the grid. So if you're running a KT88 (say), you will want a grid leak of 47k to 100k, compared to 220k-470k for a 6V6. As the others have noted, the grid leak resistance of any stage is in parallel (i.e. in terms of the Thevenin equivalent circuit) with the plate resistance(s) of the prior gain stage. This is why amps using multiple KT88s or 6550s generally need a highish-current buffer/driver stage (like a 12AU7 with a 22k to 47k load) between the PI and the output tubes, so that the grid leak resistor for the latter doesn't sink too much of the available driving current (across the impedance bridge between the phase inverter/driver and the output tube grid).

[Littlewyan]

It all makes perfect sense now. Thanks guys!