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Old 16th June 2006, 10:40 AM   #11
EC8010 is offline EC8010  United Kingdom
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I don't have very many radio handbooks, so I can't comment, but one of the Mullard audio books mentioned the resonance thing.
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Old 16th June 2006, 10:59 AM   #12
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Hi,

I can see the need for making a low pass filter on the input on apower amplifier, it is a good idea to filter out any frequency that is higher than what the amplifier can handle, it will eliminate any chance of TIM or DIM type of distortion that is produced by a feedback loop that have lower bandwidth than the input signal.

However I agree with EC8010 that the main purpose of a gridstopper is to reduce any chance of oscillation that can happen in an amplifier stage as some tubes can oscillate for certain values of input impedance when the relations between Cgp and stray inductance meet certain conditions.

However some tubes will never oscillate and I therefore think a bad idea to use gridstoppers as default as it is un-neccesary and often less than ideal to reduce bandwidth of an amplifier stage.

In my preamplifier I only use a gridstopper on the input of the line stage as I found that there was a risk of oscillations when the volume control is close to 0. In my power amplifier I use a gridstopper that make dual service as a lowpass filter so it is quite big at 4.7kohm, it give me a -3dB frequency of about 300kHz.

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Old 16th June 2006, 12:02 PM   #13
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I agree with EC....
even if I can't recollect my memory where I read about grid stopper for G1, I read that ; besides-there are numerous places where we can read exactly the same for G2 for power ,even signal,pentodes ; what's worth for G2 ,it's worth for G1 too
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Old 16th June 2006, 02:10 PM   #14
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A few more words on grid stoppers. I usually use the smallest value possible for a grid stopper. I start with a few hundred ohms and raise it if necessary. I have seen a few stubborn cases where there was an oscillation that only lasted for a few cycles at about 80 MHz just as the input signal crossed through zero. As stated earlier it was occurring only at certain positions of the volume control. This seems to happen most often when RF triodes are used as small signal audio amplifiers. I found it with a storage scope while searching for a vague raspyness on some vocals. The tube was a 12AT7 this time (look it up, the 12AT7 was developed for the tuner section of an FM radio receiver). I have seen the same effect with 6BQ7, 6BK7, 6AQ8. and 5670. In the 12AT7 case I went from 150 ohms to 470 ohms.

Another use for large grid stoppers is on the signal grid of an output pentode. When fed by the typical coupling capacitor, and grid bias resistor, it is common to encounter a "blocking distortion" where the grid is driven positive and upsets the charge balance on the coupling cap. The overload "recovery time" is determined by the magnitude of charge drained off of the coupling cap by the conducting grid, and the RC time constant of the coupling cap and the grid bias resistor (often 100K or larger). The amp will distort when hit with a large transient and continue to distort a normal signal for the "recovery time" after the transient passes. I have seen guitar amps thet were modded by an "expert" that distorted badly for up to a second after a blast of a 10 db overdriven signal. This overload is common in guitar amp applications.

If the grid stopper is big (22 to 75K ohms) the grid will not be able to suck much charge out of the coupling cap, so the recovery time is very fast. The amp will softly clip the transient and continue operating without distortion as soon as the transient passes.

Why can't I do this with an output triode? The Miller capacitance will form a nice low pass filter against this resistor and kill all of your treble. This is why you see so many DC coupled DHT circuits. There are other ways to solve this problem, PowerDrive, intrestage transformers, and cathode followers are a few.
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Old 16th June 2006, 02:26 PM   #15
lndm is offline lndm  Australia
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Quote:
Originally posted by tubelab.com
I have seen a few stubborn cases
Have you heard of output triodes that won't behave with less than 10k on the grid?
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Old 16th June 2006, 02:42 PM   #16
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In my amplifiers I get away with a few hundred ohms on the grid with DHT's. I can say that I have only used a few different types of DHT's though. Mostly 45's, 2A3's, 300B's, 211's, 845's (all low cost varieties) and a few military surplus tubes. I have found that some of the high Mu triodes (particularly the 811A) will oscillate if you give them a chance. It often helps to add a snubber on the plate cap of these tube types. I make it by wrapping about 10 turns of enamel covered wire (about 20 gauge) around a 100 ohm 1 or 2 watt CARBON COMP resistor. The wire is soldered to the resistor leads so that it is in parallel with the resistor. This is also useful on 807's and some sweep tubes.
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Old 17th June 2006, 02:03 AM   #17
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It's also worth noting L in series with a grid's Miller C is still a low-pass filter, just second-order and probably underdamped.
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Old 17th June 2006, 06:52 AM   #18
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What about ferrite beads?
Will they work inplace of a grid-stopper?
Perhaps you could eliminate the resistor completely.

I've read here that some have used them in thier projects,what are your thoughts and observations?
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Old 17th June 2006, 07:27 AM   #19
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Quote:
Originally posted by EC8010
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.
Yes.

For those who wonder where the stray L might come from: Just remember that grids are wound like coils

(Strictly speaking it is not "stray" L, just like interelectrode capacitances are not "stray".)

Tom
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Old 17th June 2006, 04:24 PM   #20
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Hi,

Stray inductance and capacitance you usually say is what you get in addition to what the tube have and is unwanted. It comes from the tube socket and from the components you connect to the tube, a capacitor with say 10cm leads probably have more inductance than the grid itself and the same goes for capacitance where the socket sometimes have more capacitance than the tube cap between grid and cathode. BTW the grid is not a true spiral in most tubes, it is made like a spiral but the turns are short circuited at 2 places along each turn, so the inducatnce is much less than it would have been if it had been a spiral, (how would a tube like ECC81 work at several 100s of MHz with that kind of inductance?)

Quote:
It's also worth noting L in series with a grid's Miller C is still a low-pass filter, just second-order and probably underdamped.
What is important is that the combination of gain, tube capacitance, inductance and stray additions can make the grid to be seen as a negative resistance with some sort of reactance, if then you connect some components which at some frequency look like the conjugate to this negative resistnace and reactance combination you will have created an oscillator at that frequency. Adding a series resistance will give the result will be that the conditions for oscillation are no longer fulfilled and oscillation will stop, a ferrite bead can give the same result but it will be a case by case situation.

As not all tubes, (at least not with all kind of circuit values) can create a negative resistance at the grid there is no reason to add gridstoppers to all tube amplifiers as some do, it think is better to add them only if they are needed.

Regards Hans
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