Leo Fender was very good at producing quality amps. Like most other legends, he didn't adhere too closely to the tube manufacturers' book value tube parameters. He made a gazillion PP 6L6 amps using 220K grid resistors. He also abused the max limits on voltages. Those amps are still alive today and are one of the most cloned amps ever. 220K, you bet I'm gonna use them, just like Leo did. Who cares that the RCA RC-30 Tube Manual says 100K max. 
Bear in mind the rather high HT on these amps; some of the brown series amps had HTs above 500V, notice how that didn't last for long, with the HTs getting reined back to compliant levels with the blackface series.
The grid leak limit is there to prevent grid current creating a significant grid voltage. Grid current, especially positive, tends to increase with gassy tubes, but also increases with overall temperature; hence if the plates are dissipating heavily, (positive) grid current can increase, and if the grid leak value is excessive, then the tube's operating point will get hotter and there's the risk that things can spiral out of control, as the hotter operating point causes yet more plate dissipation, and the tubes redplate. See
http://www.mif.pg.gda.pl/homepages/frank/sheets/084/k/KT88_GEC.pdf how there's a balance between max dissipation and max grid leak, ie if the design can cap dissipation at a lower value, then the grid leak limit can be increased.
And similarly, self regulating effect of cathode bias allows higher grid leak values.
Hence most guitar amps 'get away' with 220k grid leaks, due to the nature of the application not tending to result in max dissipation for extended, continuous time periods, eg things get chance to cool down between notes etc.
But the 135W TR is not most amps, the HT as noted is rather high. Even with the slightly higher p-p impedance these have, and the g2 taps further mitigating stress, these amps seem to have been designed around the Sylvania STR387, which were a super tough 6L6 variant reputedly with enhanced voltage, dissipation and vibration capability.
The higher HT will likely increase the max plate dissipation that the 6L6 are subjected to, and thereby the grid current may well be higher than with a regular TR AB763.
So I suspect that regular / current production 6L6GC would have a hard time in the stock circuit, hence I don't think that 220k grid leaks are a good idea here. The SF circuit seems to have been done by competent engineers, as the range evolved, mitigation for both blocking distortion and tube life were put in place, and it seems a retrograde move to revise all component values to the AB763 equivalents without carefully thinking everything through.
Topic: Biasing a Fender Twin Reverb ultralinear 135 (Read 20929 times)