6L6 to el34 conversion - grid resistors

[Guitarist]

So far I've left the stock 47K Ω grid resistors in. The amp sounds killer and not too dark. Is it fine to just leave them in as is? It has a 12at7 in the PI and will henceforth.

[Chuck H]

Grid resistors can be whatever you like. And you seem to like 47k for this amp. If it sounds good, it IS good (respecting safe operation of course). There's no tube specific rules on the value for this component as far as I know. The only considerations would be stability, the sound and maintaining an appropriate grid load.

[HaroldBrooks]

There is something on my EL84 spec sheet termed "Grid No. 1 Circuit Resistance" and it's listed at 1meg for cathode bias operation, and 300k for fixed bias,

but I'm not smart enough at this point to know if that applies here (sorry if it doesn't apply) !

Maybe someone else could chime in who knows what that spec is in the truest sense. In any event, 47k is well under even the 300k fixed bias number.

[Chuck H]

There is something on my EL84 spec sheet termed "Grid No. 1 Circuit Resistance" and it's listed at 1meg for cathode bias operation, and 300k for fixed bias,

but I'm not smart enough at this point to know if that applies here (sorry if it doesn't apply) !

Maybe someone else could chime in who knows what that spec is in the truest sense. In any event, 47k is well under even the 300k fixed bias number.

The "Grid No. 1 Circuit Resistance" is the "grid load". And it would be a combination of the "grid stop" resistor and the grid to reference resistor. "reference" being either ground, as in cathode bias, or the bias supply for fixed bias. But I am only assuming the 47k mentioned above is a grid stop resistor value. The term "grid resistors" is vague and 47k would seem low for a load resistor in that circuit position.

[Guitarist]

Yes thanks I'm referring to the grid stopper. I know higher values are common in preamps. There 47k would be pretty conservative. But on output tubes they seem to always really low: 1.5k, 4.7 k et al.
Thanks HaroldB, el84 are wild little tubes!

Thanks Chuck I can recall you being about here years back too!

[Gingertube]

Grid stop Resistors do 2 things:
1) essential prevention of parasitic oscillations. The required minimum resistance is proportional to the inverse of the tube gm. EL84, EL34, KT88 etc higher gm tubes are happy with 1K5 minimum, 6L6, 6V6 etc lower gm tubes need 4k7 minimum.
2) When you overdrive the power tubes the grid to cathode starts to look like a forward biased diode as the peak voltage from grid to cathode reaches 0V and tries to swing positive. Grid current then flows and the coupling capacitor charges up and starts to cause blocking distortion. To reduce this, we increase the grid stop way above those minimum required values. The more you want to overdrive the outputv tubes the higher you should perhaps make those grid stops, keeping in mind that we do not want to exceed that total grid circuit resistance consisting of the gridstop + grid leak resistances.

Those grid circuit maximum resistances are specified for the tube running at full rated anode dissipation. If you bias for 70% of full dissipation you can safely multiply those max grid circuit resistances by 2, which nearly every Guitar Amp designer has done.

Cheers,
Ian