Rg1 Max?

[dennyg]

Struggling to understand grid circuit max resistance and it’s significance. e.g. an EL34 and 6L6 state Rg1 max for fixed bias is 100k yet a JCM800 and many other PP amps use 220k plus any resistors in series from the bias circuit. Also wouldn’t grid stoppers add to total grid circuit resistance particularly if considering grid current, or is Rg1 max only in context of input impedence? Obviously I’m missing something here.

[PRR]

> Rg1 max for fixed bias is 100k yet a JCM800 and many other PP amps use 220k plus any resistors in series from the bias circuit.

Yes, guitar amps traditionally cheat this spec.

There's a Right Way, and there is "it usually works".

It is a worst-case spec. In The Old Days, 99% of a crate of tubes would be fine with a higher Rg. But a trace of cathode-stuff splashed on the grid might happen a few times per thousand tubes. The spec allows for some light contamination.

Also: the spec covers tubes run right AT Pdiss rating. Many guitar amps run significantly under Pdiss. A little current rise from grid resistor may not be an instant melt-down. Also nobody dies, and guitarists traditionally can change their own tube.

This has caused trouble in the brave new world of lowest-bidder global tube racketeering. Some reisues of very hot-rod amps did seem to have to back-down Rg1.

Yes, the bias divider and grid-stopper counts.