audio-talk

What do we think about grid stopper resistors?

When building the 300B's, I remember considering whether to include them or not, as they were not specified in the Lynn Olson schematic or the Reichert circuit on which it was based. I do remember that I decided to build without them in the end, as the *SN7 has a reputation for being well behaved and the following stages had quite low gain. No RF tubes in this design.

However there seems to be a mix of opinion - some say you should include them regardless, some say you should only include if the circuit is unstable without them. I've seen no sign of any oscillation issues - nothing showed up on the scope tests but I may not have seen anything occurring in the Mhz range as I'm unsure how far up the Analog Discovery goes. Also no strange behaviour concurrent with oscillation. The leads from the 12SN7 to the 6V6 are very short - well they are the capacitor lead-outs. The wires to the 300B's are a little longer - between 4 and 5 inches I think. I've tried to keep signal paths as short as possible.

I suppose the question is, does this circuit raise any red-flags in terms of oscillation risk, and is there anything I should be looking for that may point to oscillation that I haven't detected? The output transformers remain cool in operation, and the amps themselves have been working fine the last couple of weeks or so - we use them for several hours every evening. So I'm thinking - leave well alone, but would appreciate opinions, dissenting or not.

Circuit here.

My two penneth worth is that if you can scope things you'll find it ( up to 50 kHz). If it oscillates of course you won't hear it but it makes the amp sound awful. A scope shows the cause. If it's easy to use grid stoppers test and remove, test again, it's all worth knowing. SN7s and 6v6s aren't normally associated with nasty oscillations imho but....

Are you sure you mean 'up to 50kHz' Andrew, I would have thought the effects are often up much higher than that.
However as Dave says, these valves are not high-transconductance , and you may be OK without them. I don't think I have grid-stoppers on my 300B's at the moment, and have not always used them on 300B or 2A3. I have got them on the 4P1L's. Nick probably knows most about what does & doesn't need them.

I wouldn't normally bother with output triodes or sn7, sl7. once the gm goes up with things like 5687 and ecc88 then yes. Pentodes better safe that sorry.

yes, I meant higher of course. I did get some weird oscillation out of push pull 2A3s once; sort of 'in the output transformer, the scope showed 150kHz oscillations - cured by changing the physical layout of the components of the amplifier; topology?

Those 6S45PRs - titanium anode triodes could be a bugger.

andrew Ivimey wrote: ↑Mon Mar 01, 2021 9:22 am yes, I meant higher of course. I did get some weird oscillation out of push pull 2A3s once; sort of 'in the output transformer, the scope showed 150kHz oscillations - cured by changing the physical layout of the components of the amplifier; topology?

Those 6S45PRs - titanium anode triodes could be a bugger.

Yep, they will oscillate given the chance, and as for E810F, oscillate in the box is my way of describing them.

Your 150Khz sounds like a group delay + feedback (possibly via power supply) somewhere issue.

Twas a long time ago now, happy it's gone away.

Thanks all for the reassurance. I'm not going to sweat it too much for now - the 6V6 is a tetrode of course but it's being operated in triode mode. My wife's guitar amp also uses a triode-connected 6V6 without a grid stopper, and that was designed by someone who knows what he's doing!

I only swept to 100kHz when testing but I did do a 10kHz square wave test which didn't indicate any nasties. Next time I get them back on the bench I'll probably do a sweep up to the limit of the AD device, just to check things out.

Grid stoppers are to prevent RF oscillation, so you are nowhere close to seeing it. An AM MW radio is a better way of checking.