PSE : can we share the grid-stopper and screen grid ?

[kleuck]

Hello
The title says it all, i'm on a PSE (a pair of 6F3P/6BM8/ECL82) and one of my goal is to use the less part i can, if it's not a reliability or sound issue.
So i'm wondering if i need to use a screen and grid stopper resistor for each tube.
I'm sure it would be good practice for Hi-Fi stuff, but it's a guitar amp, and i'm after juicy harmonics, so perfect symmetry is not at all needed (on the contrary)
So can i use without worrying shared resistors ?

[Jazz P Bass]

I think Peavey did it that way a while back.
Try it.

[pdf64]

One of the key functions of grid resistors is to mitigate oscillation, and that's done most effectively when they are mounted on the tube socket terminal; shared resistors can only do that for one tube or the other (or neither).
I've read that the shared screen grid resistor on Selmers can result in oscillation (when using current production tubes), which is stopped when using individual resistors.
Pete

[printer2]

Separate screen resistors is a bit of insurance to prevent one tube that shorts out internally from taking out the other. For example the 18 Watt uses one main screen resistor but adds two 100 ohm resistors to help protect the tubes.

Along with stopping oscillations, grid stopper resistors help reduce harsh clipping.

[tubeswell]

What the others have said. The Rg2 is a good idea on an output pentode to eat up excess screen current. For a couple of cents worth of resistors and a couple of minutes worth of fitting and soldering, its better insurance to have them methinks.

(Having said that quite a few fender tweed amps had no Rg2 on the output tubes - although that doesn't mean that those designs were optimal).

[kleuck]

Thanks all.
But i do not want to avoid to fit either grid stoppers or screen resistors, and it's not a matter of time or cost, just a design choice to use the fewer components possible, and i don't want to make the power tubes behave perfectly, on the contrary ..
For example, the two triodes will share a single cathode resistor, and the low shaping will be done only with coupling caps, etc.
The only part of the amp where i do not count components is the power supply (choke etc)
So, as tube have between all electrodes a somewhat not negligible resistance, i was considering the option to use a single resistor for the two grids, and a single resistor for the two screens.
I am searching schematic of such designs, but i find either some with resistors on each electrode, or not at all
I'm pretty sure i don' need a grid stopper for each power tube, but there can be issue with the screen limiters as pdf64 pointed out.

[Steve Conner]

I'd say that one grid stopper per power tube, mounted right on the socket pin, is a good policy to avoid parasitic oscillations. Even with a small tube like the ECL82.

As far as the screen stopper is concerned, you can often get away with a shared Rg2 or even none at all. However, as the number and size of power tubes increases, the risk of parasitics in the screen circuit goes up. The original Ampeg SVT had grid, screen and plate stoppers, one per tube.

[kleuck]

Thanks for the reply, i can't avoid screen resistor anyway, as i will use a somewhat high voltage for ECL82, the Phillips datasheet says 2K2 at 272 volts (would be about 260-265 i think).
Can be an issue indeed : if i use a 1K one, and a tube dies, the screen of the other won't have enough protection, but i plan to use another 1K before the screen node, and if a tube dies, no more sound, i would shut the amp off, don't think the second tube can be damaged instantly.

[loudthud]

The quandry for the DIYer lies in the possible problems. Without an oscilloscope, you may have an oscillation and not know it. It could be a slight oscillation that only occurs on part to the waveform. It might even make the amp sound good. So without good instrumentation, the amp can have problems that you will never be able to find. You may end up swapping parts and moving wires to no end. Best advise: Install the resistors. You can always take them out later to see if they make a difference.

[kleuck]

Wise advice
I will fit the 4 resistors.

[kleuck]

Wise advice I will fit the 4 resistors.

Actually, i changed my mind : i will try with shared resistors as a friend just gave me a bunch of ECL82 (10, so i now have about 20), will try them all tracking rf oscillations etc. I then will know for sure if it's good practice or not

[kleuck]