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dsavitsk 14th June 2006 10:58 PM

Grid Stopper
 
What is an appropriate size grid stopper to be using? I have been using 1K, but looking at other explanations and schematics, it seems this might not be ideal. In particular, I am concerned with 5687, 6J5, and 2C22 as those are the tubes I have been playing with lately.

Thanks,

-d

EC8010 14th June 2006 11:32 PM

Who knows? Seriously, although valves with low gm can probably get away without a grid-stopper at all, it's also down to circuit layout. The only definitive answer can be given by exciting the circuit with a >10kHz square wave and looking at the cathode (if it's undecoupled) or the anode with a >100MHz oscilloscope to see if there's any oscillation. Sorry not to be more helpful, but it really is suck it and see.

Geek 15th June 2006 04:51 AM

I've found that it's not critical. 200 ohms up to 1K for small tubes doesn't seems to make much difference on the scope or sound.

Tubes4e4 15th June 2006 05:46 AM

Usual practice values for small signal tubes g1 stoppers are 1-10k. (Donīt ask me why, but values at the upper end of that range are often found in geetah amps, while 1k is found in HiFi amps).

For power tube g1 stoppers, its values usually is at the lower end of that range (typically 1k).

For power tube screen stoppers, typical values are in the 100-270 ohms range. The Telefunken Werkstattbuch recommends not to go much above 100 ohms, so the changing voltage drop effect due to varying screen current across that stopper will stay neglegible (and hence will keep distortion low).

Tom

Miles Prower 15th June 2006 06:53 PM

Re: Grid Stopper
 
Quote:

Originally posted by dsavitsk
What is an appropriate size grid stopper to be using? I have been using 1K, but looking at other explanations and schematics, it seems this might not be ideal. In particular, I am concerned with 5687, 6J5, and 2C22 as those are the tubes I have been playing with lately.

Thanks,

-d

It all depends. The whole point of adding a stopper is to form a first order lowpass filter with the tube's input + Miller capacitance being the reactive element. That gives: w= 1/RC for the cutoff frequency. To find out what you need, o'scope to see what the problematic frequency is. (In my case, it wasn't an oscillation, but rather a 700KHz "clear channel" AM xmtr some 30 miles away. To keep that out, I needed a 4.7K stopper on a 6J5 with Av= 10.0. This gave a cutoff of about 50KHz, well below the 700KHz noise.

For guitar amps, you typically find much larger stoppers being used to shape the frequency response in the upper audio band. That's neither necessary nor desireable for music reproduction.

EC8010 15th June 2006 07:54 PM

I'm afraid I have to disagree with you there. Whilst it's perfectly possible to use a grid-stopper as a CR low-pass filter, the original purpose of the grid-stopper was to lower the Q of the resonant circuit formed by stray C and stray L. Since Q = 1/R sqrt(L/C), fitting the grid-stopper resistor as close to the valve pin as possible reduces L, and hence, Q. That's why it's always recommended that the grid-stopper should be as close to the valve pin as possible, and why some people even go to the lengths of soldering SM resistors directly onto the pin.

Geek 15th June 2006 09:36 PM

But in the process, it does also act as a minor LPF. Have an EL36 amp that told me so on the scope ;)

EC8010 15th June 2006 09:48 PM

Absolutely, but that's an (undesirable) side-effect of the main action.

Geek 15th June 2006 09:56 PM

Indeed. Allen Wright had such a poetic way of explaing why that's undesirable too :D

Merlinb 16th June 2006 10:33 AM

Quote:

Originally posted by EC8010
I'm afraid I have to disagree with you there. Whilst it's perfectly possible to use a grid-stopper as a CR low-pass filter, the original purpose of the grid-stopper was to lower the Q of the resonant circuit formed by stray C and stray L.
That's why it's always recommended that the grid-stopper should be as close to the valve pin as possible,

Well I've never seen that in any old radio handbook. I've only seen them advised to limit forward grid current (power valves) and to block RF (input stage in general). Soldering it directly to the pin ensures you won't be picking up RF after the stopper.
You can pretty much omit the RF blocking resistor completely if you add a capacitor from anode to grid, whose value acts with the Miller C and ouput impedance of the devide you're connecting, to produce a low pass filter.


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