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-   -   Values for grid stoppers for 6922... (http://www.diyaudio.com/forums/tubes-valves/12966-values-grid-stoppers-6922-a.html)

JoeBob 26th March 2003 11:25 PM

Values for grid stoppers for 6922...
 
I've got some 6922s as common cathoed amplifiers, driving a cathode follower, in turn driving a source follower, and my MOSFETs keep blowing since I redesigned the enclosure for my tubes. My bet is on oscillations. I haven't probed it with my scope yet, but I'll be willing to bet my money it's the 6922. I've got a 30R resistor on the grid of the tube, but I guess I should use a higher value as a grid stopper, but what? Should I use 100R, or something higher like 1k or even 2.2k? Any help would be apreciated, I've only got one extra FET left, hopefully I won't have to order more....

Tube_Dude 27th March 2003 12:37 AM

the grid stoper...
 
Is this 6922 after a volume pot??

If so put 2,2K Ohms

If not try 1k OHms

regards

PS: Have you put gate stopers at the gate of the output Mosfets??
If not try 220 Ohms

JoeBob 27th March 2003 02:11 AM

I do indeed have gate stoppers on the MOSFETs though. And the thing is, this all started when I changed the tubes from their own chasis and put them in the same chasis as the MOSFET follower, so I gather it's oscillations from the change is layout or something. And it is after a volume pot, but I might later add a preamp before it, so I'll try 2.2k like you said to begin. Thanks alot by the way.

fdegrove 27th March 2003 02:20 AM

GRRRR...IDSTOPPERS.
 
Hi,

Need them? Rest assured you have a problem elsewhere.

Need them? Keep'em as small as you can get away with.

Don't need'em?

Enjoy the music,;)

JoeBob 27th March 2003 02:26 AM

I know I should not need them, but what could cause such a tube to oscillate? In a seperate chasis, it was stable and caused no problems, but when I transported it to it's new home, it went all wacko, I gather is layout, but what should I look out for?

Tube_Dude 27th March 2003 03:27 PM

The pot...
 
Hi Joebob!!

What is the value of the pot??'

Joel 27th March 2003 03:38 PM

Joebob,

Use a 10k. This is an acceptable and somewhat standard value for any modern voltage amplifier tube. But also it will prove the point beyond a doubt, no?

Series resistance (within reason) is not going to hurt your fidelity, to spite what Frank may alude to.

fdegrove 28th March 2003 12:38 AM

THERE'S MORE TO IT...
 
Hi,

Quote:

Series resistance (within reason) is not going to hurt your fidelity, to spite what Frank may alude to.
Yeah...we try to make the signal path as short as possible and to top it off we add a few KOhm in series with the most sensitive element of all, the grid.

Not that I expect you to hear a difference of course, we also choose low noise tubes where possible and naturally we mess that up by adding series resistors.

As I said it before, if the circuit doesn't oscillate then don't use them...If it does oscillate than either the tube is very sensitive to instablility or the circuit design sucks...

Take your pick, saying that adding a gridstopper doesn't have an audible (usually degrading) effect on the sound is utter rubbish.

If you need to combat RFI than use a ferrite as I suggested to Apass_gear before, it's effective and doesn't degrade the sound, quite to the contrary.

Cheers,

Tube_Dude 28th March 2003 12:50 AM

The grid stopers...
 
Grid stopers are a must after a volume pot...

The volume pot in a typical valve amp is 100K so the maximal resistence ( pot is at the midle position) driving the grid of the first valve is +- 25K..in the others positions less ..so 25K more 2.2K(of the grid stopeer) doesn't make much diference!!

Only at near the 0 position the resistence driving the grid is near 0 Ohms...and driving a grid with a so low resistence gives a very wide bandwith...so the usual parasitic oscilation at the 0 position of the volume pot!!!

So put it after a pot!!...Please!:nod:

316a 28th March 2003 12:56 AM

Gridstoppers Again
 
I agree with Joel about gridstoppers for the 6DJ8/6922 family . I have often seen little puffs of oscillation picked up on the scope with some 6DJ8 types , almost intermittantly . Some of the harshness often attributed to 6DJ8 could be due to this also . From experience I find it best to use a 1k grid stopper , but this may depend on whatever operating point is used . Fit them : better to be safe than sorry , expecially if different 6DJ8 are to be used .

316a


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