5E3 grid stopper resistors

[stephenhartley]

Browsing Weber's book and I notice that all the post-tweed circuits have 2x 420K resistors from the power tube grids.

What exactly do they do? Is it worth fitting them to a 5E3 build?

[Chuck H]

I have no idea what circuit your looking at. Can you please post a schematic???

[stephenhartley]

They're on all the Brownface fenders including the Pro 6G5 and the Super 6G4 - I'll try and find an online schematic to post

[JHow]

Screen resistors perhaps? 470 1watt?

[stephenhartley]

Yes -they're the one's. What do they do & how come they're not on the earlier tweed circuits including the 5E3?

[pdf64]

Earlier schemes have rather saggy B+ screen nodes, so excessive screen grid dissipation probably wasn't a problem.
With a choke rather than eg 4k7 resistor, the screen grids are pushed much harder, and so limiting resistors are needed.
Pete

[MWJB]

They are on most modern repros of 5E3, 5E# & 6L6/6V6 amps in general. You can use 1K 5W too.

They are like little 'shock absorbers' for the screens.

The 5E# amps didn't originally have them, but the larger 5E# amps drew B+ & screen supplies from the same node, meaning the screens ran at a higher potential than the plates, which is often considered bad practice. The large B+ to screen supply dropping resistors (5K) in 5E3s negated this issue, to some degree, but it is still a good idea to install the 470ohm resistors.

[stephenhartley]

Thanks - will add some. Do they affect the tone in any way? Have also read that some people replace the 5k dropping resistor with a choke?

This is my second build and my aim was always to reproduce what my vox AC30 & Fender Tremolux do (including a second channel that can be used for vocals at rehearsals and small gigs) but in a smaller, lighter more compact package. I was going to move to a 2 x 6l6 build next but I don't think I need to - the amp sounds awesome and very loud. Plenty of variation in tone (have used a 6G3 tone circuit).

One slight niggle is that the volume comes in pretty loud at 1 - almost too loud - is this just the quality of the pots or is there a way of altering this?

[MWJB]

No adverse effect on tone, only healthy benefits.

A choke is a possibility, of course, but in an amp like this will see you screen voltage rise even further with respect to the cathode biased plates...then you may want to fit even larger screen grid resistors, if this bothers you.

Did you use Alpha 1MA pots? They have a very fast taper. Is the output from the pots (feeding V2) coming off the centre pot tab, or the RH pot tab (I know you said 6G3 style, but you could conceivably wire either way & keep a 6G3 style circuit)?

[pdf64]

If your vol pots are definitely audio taper (rather than linear) have a look at the Brown Deluxe 6G3 vole / tone / mixer arrangement -
http://www.webphix.com/schematic%20h..._6g3_schem.pdf
Suggest you consider swapping the 5E3 arrangement for that.
Pete

[stephenhartley]

Yes - they're 1 meg audio. The RH lug of the pot feeds V2 although looking at the 6g3 schematic it's the other way round. I'll try swapping it and see if it improves the taper.

Thanks again for helpful suggestions.