Understanding cathode caps and cut off frequencies

[julkke]

reviving this great topic.... just to be sure of my math : so a .68uF+820r (=285 Hz) combo is "brighter" than a .68uF+2.7k one (=86 Hz) ???

Yes. It also has more gain.

[antosimoni]

reviving this great topic.... just to be sure of my math : so a .68uF+820r (=285 Hz) combo is "brighter" than a .68uF+2.7k one (=86 Hz) ???

Yes. It also has more gain.

thanks... Im astonished, I didnt hear such a huge difference (3 times) when I swapped both combos on my amps, thinking also that late 60s SL have another .68+820 combo on V2 it would make them brighter than later JMPs (.68+2.7k and no cap on V2 820r) but to my ears its the opposite... well, food for thought

[julkke]

reviving this great topic.... just to be sure of my math : so a .68uF+820r (=285 Hz) combo is "brighter" than a .68uF+2.7k one (=86 Hz) ???

Yes. It also has more gain.

thanks... Im astonished, I didnt hear such a huge difference (3 times) when I swapped both combos on my amps, thinking also that late 60s SL have another .68+820 combo on V2 it would make them brighter than later JMPs (.68+2.7k and no cap on V2 820r) but to my ears its the opposite... well, food for thought

Yeah it's not a night and day difference. 70s heads are brighter for many reasons: nfb, voltages, output transformers, filtering... v1b cathode combination makes little difference compared to those.

[antosimoni]

thanks, do you know if this formula applies also to mixer combo (470k+560pF x instance) ??

[julkke]

thanks, do you know if this formula applies also to mixer combo (470k+560pF x instance) ??

Yes it does. That combo gives a rolloff frequency of around 600hz if I remember right.

[antosimoni]

good, many thanks

PS : youre right is 604 hz, for 460pf+470k I got 736 hz while JTMs 460pF+270k is 1281 Hz (?)

[julkke]

You got it.

[Reeltarded]

Imma tauist.

Yes.

[metalhead3ecr]

Would anyone happen to know the equation if there was no cap. For example, just using a 10K resistor on a gain stage. What would the cutoff frequency be?

[toner]

Would anyone happen to know the equation if there was no cap. For example, just using a 10K resistor on a gain stage. What would the cutoff frequency be?

Without a bypass cap there is no frequency-dependent boost happening so there is no cutoff frequency. In other words, the gain will be equal across the entire bandwidth, but it will be much less than if a bypass cap were used, especially with a relatively large resistor like 10k.

[metalhead3ecr]

oh ok that makes sense! thank you

but what exactly does that 10K resistor to ground do?

[Reeltarded]

Cathode? The 10k cathode resistor to ground?

[neikeel]

The cathode resistor sets the bias point for that particular triode. 10k is considered cold (ie less gain) but 820R as per typical plexi is quite hot. But you will need to consider the bypassing effect of the capacitor if one is used too as the frequency response curve will be shifted depending on the size of the cap relative to the cathode resistor - hence the calculations/tables.

Have a read:http://www.aikenamps.com/CommonCathode.htm

[metalhead3ecr]

Thanks! I read that before. But that biasing info was what I was looking for.