Grid Resistors on 6G6B Bassman

[JammyDodger]

Hey All,

I have recently acquired a '63 Bassman with a 6G6B circuit. The amp is 90% original but for a couple of mods. One that I am wondering about is that there are 1.5k Resistors hanging off of pin 5 of the power tubes. I think these are called grid resistors. They are not original to the circuit. Is this just to 'tame' the amp / prevent oscillations? What is the risk if I remove them?

Thanks, Mike

[Firestorm]

You're right that the "grid stoppers" are there to prevent oscillations. So the risk in removing them is that the amp may oscillate.

[billyz]

You can probably remove them. You will hear a little more top end. Later Fenders had them , but I have never encountered a problem without them on older fender like your Bassmen. Showmans had one 1.5K per pair of output tubes.

[solderstain]

Bassmans started having 1.5k grid stoppers with the AA864 model. If it were mine, I'd leave 'em there. No harm, no foul, ya know?

[tubeswell]

Why not take 'em away to see what it's like (and if it oscillates then you can put 'em back)?

[Merlinb]

Values as small as 1.5k have absolutely no effect on top end, especially where pentodes/tetrodes are concerned. Removing them will only make the PA more unstable and increase the chances of blocking distortion, while giving no rational benefit.

[billyz]

Values as small as 1.5k have absolutely no effect on top end, especially where pentodes/tetrodes are concerned. Removing them will only make the PA more unstable and increase the chances of blocking distortion, while giving no rational benefit.

I agree there may be a benefit regarding stability with the 1.5K grid resistors in place. However, You can hear the top end increase with out them. Don't take my word for it. Do a little experiment, bridge across them with a wire. Don't just use theory, experience is a good teacher too.

[Merlinb]

Do a little experiment, bridge across them with a wire. Don't just use theory, experience is a good teacher too.

Believe me I have, and the practice has always agreed with the theory so far. Bandwidth extending to hundreds of kilohertz is no more audible to me than bandwidth extending to 20kHz, or even 10kHz where guitar speakers are concerned!

[billyz]

Do a little experiment, bridge across them with a wire. Don't just use theory, experience is a good teacher too.

Believe me I have, and the practice has always agreed with the theory so far. Bandwidth extending to hundreds of kilohertz is no more audible to me than bandwidth extending to 20kHz, or even 10kHz where guitar speakers are concerned!

Well, you can't argue with experience. I can hear it though, as do the clients I have done it for. If you don't hear a difference I accept that too. I probably can't hear past 14K. I am not saying I am an Advocate of no Grid Resistors as I do use them to eliminate RF and even cell phone interference.
I also know guys who hear the difference between 1.5K and 2.7K .

How do they say in the UK,
"Cheers "

[Baxtercat]

14K, wow. I wonder if I'm 4K at this point [35 years, 2 or 3 gigs a week, earplugs the last 12 years]

[dartanion]

I can definitely hear the difference of no grid stoppers and 1.5K. It's pretty minor difference and you have to really be listening closely. Now when you start moving upward, there is marked differences to me. Take a Marshall 18watt type circuit and change the 8.2Ks down to 1.5Ks. There is a lot more highs present which is easily audible if your ears ain't shot.

FWIW- I have seen that some brown era Fenders did have grid stoppers from the factory. Again, this was probably more so to be able to sell an unstable amp than staying true to the schematic. Leo was a miserly guy, so this makes complete sense.

Mike,
Did you try removing them yet as we talked about earlier this week? Any difference that you were able to notice?