Gridstoppers in a Guitar Amp

[dbishopbliss]

I want to build a guitar amp so I've been looking at schematics and layouts of various classic designs. I have noticed that both Fender and Marshall have grid stoppers located on the tag board well away from the socket (see the attached image).

This practice continues to be common among people building clones of these designs.  As every bottlehead should know, the grid stopper should be soldered to the tube socket with a very short lead.  Is there a reason why they were built this way?

Just wondering, thanks!

[Maxwell_E]

I would assume that the more components that are on the PCB translates to cheaper production. My amp tech fixed that on my Marshall 50 watt Plexi by putting a 1% Dale straight on the tube socket and then over to the PCB.

[debk]

David

Which amp are you looking at building.  I am in the process of building a clone of the Fender Twin Reverb for my son.

Debra

[JC]

Twin is a great choice.  It can play small rooms, but it can also play some pretty big ones, and its still a manageable size to schlep around.

It's also a reasonably "clean" guitar amp, meaning that you can add about any stomp box you want to customize the sound.  As we always used to say, there ain't no "clean" pedal to use on "dirty" amps.

I'll have to go back through my schematics, though, because I am a little puzzled by the original post.  I just don't seem to recall any extensive use of "grid stoppers" in Fender or Marshall tube guitar amps. Nor Gibson, for that matter.

[2wo]

I don't think they are stoppers per say. looks like there are 2 inputs to the grid...John

[dbishopbliss]

Which amp are you looking at building? 

I'm building a modified version of the Marshall "Plexi" using 2 6V6 output tubes.  This makes it around 20 watts.  There are a number of popular 18-watt Marshall designs out there that use EL84 output tubes, but they have slightly different pre-amp stages and I want something a little different.  Not to mention, I already have two 6V6 tubes sitting around from the amp contest.

Personally, I'm not a big Twin fan - too loud, too clean.  I have a 1967 Blackface Vibrolux that I'm getting ready to part with.  Great amp, but I'm just not using it any more.  More fun to build my own.

I just don't seem to recall any extensive use of "grid stoppers" in Fender or Marshall tube guitar amps. Nor Gibson, for that matter.

I don't think they are stoppers per say. looks like there are 2 inputs to the grid...John

I'm not sure, but based upon their location in the first schematic I was looking at they looked like they were supposed to be grid stoppers to me.  Then I reviewed a different schematic that matched the board layout more closely and they don't look as stopper-ish to me.

They must be providing some sort of attenuation, but I don't know what?

[Paul Joppa]

In both cases, they do not look like grid stoppers to me. The value, at 68K, is much higher than necessary. In the "...question3" circuit I seeone input to two triodes, one of which has a much smaller bias resistance than the other. Without the plate loads it's hard to tell, but quite possibly the bias voltage is much lower on that triode, allowing it to be driven into some grid current on signal peaks. The resistors would prevent that from happening identically to both triodes.

In the "...question2" fragment there are two inputs to one triode. The resistances would prevent interaction between the two inputs, in particular the risk of loading them with the other pickup's impedance. This is a fairly standard approach to multiple inputs which you want to add together.

The 12AX7 is not a very high transconductance tube, and thus grid stoppers are less likely to be needed. I use them everywhere as cheap insurance, but it's the high transconductance tubes that really need them.

[JC]

The examples having two input jacks both look like standard Fender designs.  They used that set up in a zillion amps with two jacks on the input, presumably to isolate the two "guitars'" output pots from one another.

I think the other example uses the 68k resistors to isolate the two tube sections' grids from the common input.  It could have been for a design that used those two triodes to feed two different chains down the line, one triode for "regular" or "clean" and the other to feed a reverb or tremelo circuit, for instance.  I have seen such in guitar amps, sort of dual input preamp stages.  The "effect" chain often ends up getting mixed with the "clean" signal just before the phase splitter.

[dbishopbliss]

I think the confusion comes from "...Question3" schematic does not actually match the layout.  It only shows one input when in fact there are four.  Now that I know they are probably for channel/input isolation rather than grid stoppers, I will leave them on the board.