Shared cathode resistor question

[catalin gramada]

Hello, I just read somewhere here about potential instability problems caused by a triode which share the same resistor in cathode. Just wonder how old PA amps deal with that? I have two six channels PA amps (Sound City and Simms Watts) which have no less than 6 cathodes linked by a single bypassed resistor, there are pretty stable. The tubes are anything but no matched at all, it shows different currents tested in other circuit. So, how instability ideea comes from, please? Thanks
And one more,please. A half value resistor for two shared cathode it expect to perform the same than use full value for each. That is expected for static condition. What is happen when one start to draw current? I don talk about common mode circuits but just about two parallel stages into v1 position,specially if is a tube which draw semnificative amount of current, thanks.

[eschertron]

As long as the bypass cap is up to the task, no amount of small-signal AC should affect the operating point for the triodes as a group. How much each tube draws at idle doesn't matter except to establish what that 'collective bias point' will be. The bias point will not shift without a DC component.

If there's a DC shift due to blocking or other extreme input conditions, then there will be interactions. This is a first-guess/ basic theory answer. The real world may be slightly different, but not enough for me to notice. Hope that helps.

[catalin gramada]

Hey, thanks for the input. My opinion is the bias will shift anyhow as time the supply is not regulated or ccs used in respect of how much current will be draw on the rail. But of course how much is affected by the amount of current draw by one triode from group is different story.Absolutely forgotten about the bypass cap which clearly should be as large as it can be, evidence forgotten Thanks

[pdf64]

Hello, I just read somewhere here about potential instability problems caused by a triode which share the same resistor in cathode...

May have been referring to the V4 cathode in BF 2 channel reverb amps, eg node 'E' https://el34world.com/charts/Schemat...b763_schem.pdf
The issue being that the 'shared cathode' stages are in cascade; when the bypass cap eventually develops sufficient ESR, there's a positive feedback path and the preamp may oscillate above a certain point setting of the reverb control.
For non cascaded stages that issue won't arise.
Don't forget that LEDs etc can be used for biasing (rather than resistors and bypass caps).

[catalin gramada]

Thank you. Well now the problem is how large the bypass cap should be in respect with cathode. I don't think it is relate just from frequency point of view as time remember saw 'ridiculous' values used in some as large as 220uF or 330uF. I bet they did it with a reason as time all published datasheets claims for order of tens uF to decouple everything down to limit of Hz.

[Helmholtz]

An individual (bypassed) cathode resistor stabilizes the DC operating point against fluctuations of supply voltage and variations of tube parameters by negative DC feedback (cathode degeneration). A shared cathode resistor forces all tubes to run with the same bias and stabilization of individual DC operating points gets lost. Also stabilization against supply voltage fluctuations is reduced because the resistance value has to be smaller.

[eschertron]

Thank you. Well now the problem is how large the bypass cap should be in respect with cathode. I don't think it is relate just from frequency point of view as time remember saw 'ridiculous' values used in some as large as 220uF or 330uF. I bet they did it with a reason as time all published datasheets claims for order of tens uF to decouple everything down to limit of Hz.

As a mixer, the inputs can produce difference frequencies below what you might normally expect from a single-input guitar amp. Engineering to allow "all that" may be a good idea.