Carbon comp is obsolete in every aspect. Only the low value metal film resistors (under 10 ohms) have any inductance. Above that they barely have more inductance than a length of wire. The resistance is what stops oscillations. Ham radio guys used carbon comp because in those days it was the most commonly available type.
Food for thought,
A three inch wire is somewhere between 30 to 75nH of inductance.
It can resonate with the stray capacitance, and tube capacitance.
The question is, at what frequency; will you have an oscillator, only your circuit can tell you.
That is why a grid stopper is most effective, when it connected to the tube grid with a very short wire.
A three inch wire is somewhere between 30 to 75nH of inductance.
It can resonate with the stray capacitance, and tube capacitance.
The question is, at what frequency; will you have an oscillator, only your circuit can tell you.
That is why a grid stopper is most effective, when it connected to the tube grid with a very short wire.
I got a bag of really small 2,2k CC resistors in the mail today, 1/8-watters if I remember correctly. I should be able to hide a pair inside the grid connectors.
Yes, Ham radio, amateur radio was the origin of grid stopper use in audio, and the suggestion that carbon composition should be used .
But the amateur transmitters chose carbon composition for good reason: the transmitter grids carried radiofrequency current, sometimes substantial. A resistor having non-negligible rising impedance over VHF would be a bad choice.
Reliable information on this subject can still be found in the old radio handbooks. My copy of Radio Society of Great Britain Handbook (fifth edition, paperback 1982) pp 6.38 has very valuable analysis and debug hints. Interestingly, grid stoppers are third choice in the countermeasures hierarchy here, behind wirewound resistor in the anode, and ferrite bead in the screen lead. It suggests trying to achieve stability with as little as 10 ohm as grid stopper, to calibrate the constraints on impedance for their application.
With LF and audio, any film resistors will work fine, a little inductance will do no harm if the resistance is sufficient to depress the Q of the unintended oscillator.
I find their preference for low values of R to work well, and have never needed the high values often seen (a stopper does not act as a low pass filter, if correctly implemented).
All Audio Vacuum Tubes work at RF frequencies.
Just look at all those preamp, line amp, driver, and output tubes that are used in RF transmitter service.
That is the reason we use grid stoppers on audio amplifiers.
If your audio amplifier oscillates, before you attempt to fix it, study up on Colpitts, Buttler, and other common RF oscillators (and then look for similarities in your circuit that you just built into your "audio vacuum tube amplifier").
Just a hint from my varied experiences.
Just look at all those preamp, line amp, driver, and output tubes that are used in RF transmitter service.
That is the reason we use grid stoppers on audio amplifiers.
If your audio amplifier oscillates, before you attempt to fix it, study up on Colpitts, Buttler, and other common RF oscillators (and then look for similarities in your circuit that you just built into your "audio vacuum tube amplifier").
Just a hint from my varied experiences.