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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Grid stoppers?

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Using grid stoppers to cut down the treble might not be such a splendid idea (except maybe in guitar amps), as you would have to use a very big value, in most cases, to get any significant effect (I'm not talking about the subtle changes you might get with different smaller values...). Too much resistance in series with the grid will result in distortion due to modulation of the tiny grid current variations over a cycle; Morgan Jones cites ca 60k (if I remember correctly) as the value where he started to get a measurable increase in distortion (with the valve he used - may have been a 6S45P?).
I think an rc filter before the grid (and, preferably, a grid stopper) would be a better idea (except as noted in guitar amps, where the added distortion due to a huge grid stopper (esp. in overdrive) might be interesting.

Anyway - good luck; whatever works works. :)
 
Ex-Moderator
Joined 2003
Grid-stopper values are very much "suck it and see." They aim to damp the resonant circuit formed by valve capacitances and inductances in the grid/cathode loop. The main thing is that the resistor body should be as close to the valve pin as possible. Carbon resistors are a good idea because the higher resistivity of carbon means fewer turns are needed and they have lower inductance. In general, valves with higher mutual conductance are more likely to oscillate, so directly heated triodes are almost immune, but frame-grid valves designed for UHF use (ECC88, 6C45 etc) are more susceptible. Good RF layout (short wires, ground planes etc) reduce the problem. With really good layout, it's possible to get the grid-stopper down to <100 Ohm, but Mullard used between 1k and 4k7 for EL34 and EL84 power pentodes in ultra-linear mode. Once you start paring the value down, you need a good oscilloscope to monitor the anode of the valve concerned whilst provoking it with a square wave to make sure that it's not oscillating.

Most people just use a larger value than strictly necessary in the knowledge that if it's too big it will definitely damp oscillation.
 
"suck it and see."

You can say that again. I never take design rules for granted and have often found i prefer the sound without a grid-stopper. No idea if this is simply because another lousy resistor is not in circuit or the grid prefers seeing lower driving impedance but even on problematic valves like 6S45P i'd first make sure that grid-stopper is really needed.
 
Yes I can scope it. The negative feedback was acting weird, with a cap in parallel with the feedback resistor it won't work. I switched the output transformer wires on the power tubes and got that great screech, switched them back. I have gone over the wiring in this area and double checked the schematic and all looks well. the only way it will work at all is to take that cap out. What do you think it is?


P.S. Sounds great exept for this
 
diyAudio Senior Member
Joined 2002
Hi,

You can say that again. I never take design rules for granted and have often found i prefer the sound without a grid-stopper.

He, he....You really have to live that far away, don't you?

As an experiment after reading a GA issue many years ago stating without much further context that gridstoppers would improve "definition", I once added gridresistors where none were really needed.
The result was shocking: less "definition" and a dull and sluggish sound...
Hi-hats almost sounded like wood blocks, yikes........

Just like screened interconnects, if you don't need them don't use them.
If you know how to layout your stuff and select your valves you'll hardly suffer from hum or wild oscillations.
If you nonetheless should, just use the smallest value resistor that does the job. Preferably carbon comp with the body of the resistor soldered as close to the grid pin as you can manage.

If you follow that and a dozen other guidelines you'll start to understand what the high-end mags mean when they talk about "air" around the instruments.
In fact you'll even be able to hear the acoustics of the recording venue and occasionally even be able to tell to microphone capsules apart; that and nothing else is what I'd call "definition".

Cheers,;)
 
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