higher value power tube grid R's?

[daz]

yeah, i know....just try it. I will, but i just wanna ask first to see if it's worth the trouble. In my cathode biased marshall style 2xel34 amp i have 5.6k blockers on the PT grids. If i up the value it will lose highs, correct. How hig do some designs go before it's considered abnormal, how far do you have to go to see a very obvious loss of highs, and what if any are the other effects of higher values there?

[tubby]

check out the aikenamps tech articles. there's a section on grid stoppers.
i think you can go pretty high (30ish-50ish in kohm) if your design calls for such values,
before noticeable high frequency loss occurs. some hiwatts used 22k i think,
and i wouldn't say they're dark sounding amps.

if you're not experiencing blocking distortion with 5.6k, i don't think there's anything to
be gained tonewise by upping the value. having said that, i never bothered using higher Rs than 5.6k
on the grids.

but why don't you...well....just try it

[Merlinb]

yeah, i know....just try it. I will, but i just wanna ask first to see if it's worth the trouble. In my cathode biased marshall style 2xel34 amp i have 5.6k blockers on the PT grids. If i up the value it will lose highs, correct. How hig do some designs go before it's considered abnormal, how far do you have to go to see a very obvious loss of highs, and what if any are the other effects of higher values there?

Sort of, yes. But power valves have such low input capacitance that you can go really high before the corner frequency get's below 20kHz, well over 100k ohms. It's a very sneaky way to get a smoother overdrive though, because it reduces the variation in loading on the PI, which prevents some of the weird PI distortions which can occur.