Why Use Grid Stoppers

From mbaier1–(at)–ail.idt.net Sun Jan 26 23:51:42 CST 1997
From: mbaier1–(at)–ail.idt.net (Karl LaFong)
Newsgroups: alt.guitar.amps
Subject: Re: ‘ 62 Concert mods
Date: Mon, 27 Jan 1997 00:20:03 GMT

mgarvi
>I suppose Weber would remove the 1500 ohm grid stoppers on the power
>tubes, too (No, don’t do it!). They are there for much the same
>reason as the 68k. There is already ‘series resistance’ in the driver
>circuit, in the form of the 12at7’s plate resistance in parallel with
>the 82k or 100k load resistors…so why put in a little 1500 ohm
>resistor? To get some resistance *close to the output tube*! I think
>Fender’s location of the 68k input resistor was a manufacturing
>convenience, as the same holds true for its location (as you have
>pointed out).

Tale from the trenches:
On my Bassman amps, we were noticing a profound :pop: when the standby
switch was turned on. Since we were following the 5F6-A circuit, we
were building them w/o grid stoppers. Desipte the efforts of my very
able techs, we were unable to locate the source of this momentary
parasitic ocsillation. Then one day an amp displayed a wildly
ocsillating 6L6. We tried everything but couldn’t figgure out why this
amp was so unstable. Then like the proverbial 16 ton weight, it hit
us. The amps had no grid stoppers. We put ’em on, and voila! no
ocsillations! We also noticed that the standby :pop; was gone too.
After reflecting on this, we felt like fools for not thinking of this
right from the get-go, but it goes to show ya how important the lil
buggers are in preventing instability. Les(Plopa) my head tech, was
particularly embarrassed for not thinking of it sooner. Evidently the
1500ohm grid stoppers knock the Q out of whack in what is in essense
an tuned circuit going to the power tube grids.

MBaier
>Incidentally, for anyone interested, pentodes are not as prone to
>treble loss due to grid capacitance (as compared to triodes), due
>to diminished Miller effect.

>MGarvin