Plate to cathode cap

[35L6]

How do you calculate the corner frequency for the 100p from plate to cathode on V1a of a 2204 ? I assume it is 1/ 6.28 RC but what is R ? Plate resistor parallel with cathode resistor ? And does this create negative feedback ? I've downloaded the schematic from Schematic Hell but am having trouble attaching it .

[Helmholtz]

No NFB involved.

With a cathode bypass cap, the cathode is an AC ground.
Relevant source resistance is internal plate resistance (around 60k) in parallel with the plate resistor and in parallel with the (AC) load resistance (around 1M, can be neglected).
Corner frequency with a 100p cap is just above 40kHz.

[tschrama]

R = plate resistance // plate resistor .. e.g. 66k//100k . I dont think the relative small AC voltage on the catho does anything significant to the plate voltage or vice versa.

[Helmholtz]

R = plate resistance // plate resistor .. e.g. 66k//100k . I dont think the relative small AC voltage on the catho does anything significant to the plate voltage or vice versa.

See post #2.

[35L6]

" just above 40k " That's about what I was thinking . At first I thought it was to take a little top end off it , but then I thought 100p was kind of small . That's what led me to " what is R " . I had taken some gain at frequency measurements . This is on my " low power Marshall " . It has 20EZ7 which is a 12AX7 with 20V heaters . Specs are the same except slightly lower grid to plate capacitance . Supply is 150V . At 500Hz I had a gain of 42 , 33 at 200Hz , 24 at 100Hz . Makes sense with a partially bypassed cathode . Going the other way I had a gain of 42 at 1kHz , 40 at 2kHz , 32 at 4k2Hz . After considerable head scratching and staring at schematics I noticed that the following grid resistance is lower for high frequencies . For low frequencies you have the 470k and the 1M pot . At high frequencies the 470k is bypassed and also the portion of the 1M pot bypassed by the wiper . I don't recall exactly but I think the gain was at 1/4 to 1/2 . The load at high frequencies could have been as low as 250k . I'm thinking this could reduce gain and account for the roll off . Does this seam reasonable ?

[Helmholtz]

Going the other way I had a gain of 42 at 1kHz , 40 at 2kHz , 32 at 4k2Hz . After considerable head scratching and staring at schematics I noticed that the following grid resistance is lower for high frequencies . For low frequencies you have the 470k and the 1M pot . At high frequencies the 470k is bypassed and also the portion of the 1M pot bypassed by the wiper . I don't recall exactly but I think the gain was at 1/4 to 1/2 . The load at high frequencies could have been as low as 250k . I'm thinking this could reduce gain and account for the roll off . Does this seam reasonable ?

At which circuit point do you measure the gain?
What equipment do you use?

There are a number of (partly hidden) interacting HP and LP filter effects.
E.g the Miller effect will cause a capacitance around 100p between grids and and ground.
Also your meter will add some capacitance and load resistance.

For simplicity I would measure with the vol. pot fully up, which shorts the 1n "bright" cap and takes it out of circuit.

What change do you see if you lift the 100p cap?

Do you have a problem with the amp regarding gain or frequency response?

[35L6]

No problems , it actually sounds pretty good . I was trying to see how much gain it had and what effect that cap had and also the partial bypass . I believe I was measuring after the 22n and before the 470k 470p combination . I should probably test again and vary gain pot to see if it influences gain . I could lift the 470p also . Is it better to measure gain directly at plate ? I'll take a look at it tomorrow .

[Helmholtz]

Before or after the 22nF cap shouldn't make a noticeable difference. Corner frequency is 4.8Hz with a 1.5M load.

[pdf64]

There may be a theoretical benefit in mounting such a cap at the valve socket terminals, as that eliminates parasitic inductances, eg in wire, coupling cap, as much as is feasible.

[35L6]

OK , new data . Tenma handheld signal generator . It doesn't sweep it's stepped . signal applied to the Hi jack . I kept the signal level low so as not to clip either triode . Scoped V1b to look for clipping . Measured VAC with Amprobe DMM , 10M impedance . Measured after 22n coupling cap and before the 470k 470p treble peaking circuit . Gain knob full Gain knob "0" Hz in out gain Hz in out gain 1k 31mV 1.41V 45.5 1k 29.4mV 1.28V 43.5 1k2 31.7mV 1.38 43.5 1k2 30.2mV 1.30 43.0 1k5 29.4mV 1.35V 45.9 1k5 28.4mV 1.23V 43.3 2k 29.3mV 1.35V 46.1 2k 28.1mV 1.19V 42.3 2k8 28.2mV 1.30V 46.1 2k8 26.9mV 1.09V 40.5 3k2 27.4mV 1.26V 46.0 3k2 26.1mV 1.03V 39.5
I don't believe that43.5 for the 1k2 full gain . Also that signal generator is not as rock solid as I thought . It seems the gain at "full" is pretty steady . At "0" it falls at high frequency . Probably due to loading from the bypass . I should repeat with the caps lifted to confirm no change .

[35L6]

Dang it ! I typed that out in chart form and the computer put them back in line .I hope you can read it .

[Helmholtz]

What is the bandwidth of your meter?

Many DMMs are only true up to 400Hz or 1kHz.

I'd use a scope (or my Velleman Bode-plotter).

[loudthud]

HZ	In	Out	Gain
1k	31mV	1.41V	45.5
1k	29.4mV	1.28V	43.5
1k2	31.7mV	1.38V	43.5
1k2	30.2mV	1.30V	43.0
1k5	29.4mV	1.35V	45.9
1k5	28.4mV	1.23V	43.3
2k	29.3mV	1.35V	46.1
2k	28.1mV	1.19V	42.3
2k8	28.2mV	1.30V	46.1
2k8	26.9mV	1.09V	40.5
3k2	27.4mV	1.26V	46.0
3k2	26.1mV	1.03V	39.5

35L6, Is this what you wanted the table to look like ?

I too have been bitten by the automatic formatting that text undergoes on this forum. There is a tool that allows you to create a table on the right of the tools menu above the text window in the quick reply box. Hover the cursor to find it. It takes a little practice, this was my first try.

[35L6]

Thank you so much ! I'm an analog man trapped in a digital world . More data . I just looked at higher frequencies because thats where all the action is .Looking at 3k2 , with no 100p gain came up a tiny bit not sure if its real . With no 470p gain came up a tiny bit at full and had a much smaller drop at "0" , just as we would expect . At 5k gain rose from 45.1 to 46.4 with 100p lifted . At 10k from 36.8 to 42.4 . At 20k from 28.9 to 34.8 . I'm stating 3 digits but I don't know if I trust the last one . It would be nice to know the bandwidth of my meter , I think the manual is on line , I'll have to look it up . So it seems the 470p does what we would expect and the 100p has an effect starting somewhere around 10k . Don't know why it starts so low and again I wonder whats "R" . Maybe there is some more "C" in there . To put this in context the gain of V1a has a mid band emphasis but the signal at V1b grid has a definite high frequency emphasis and I don't think guitar speakers have much output at 10k .

[Helmholtz]

Thank you so much ! I'm an analog man trapped in a digital world . More data . I just looked at higher frequencies because thats where all the action is .Looking at 3k2 , with no 100p gain came up a tiny bit not sure if its real . With no 470p gain came up a tiny bit at full and had a much smaller drop at "0" , just as we would expect . At 5k gain rose from 45.1 to 46.4 with 100p lifted . At 10k from 36.8 to 42.4 . At 20k from 28.9 to 34.8 . I'm stating 3 digits but I don't know if I trust the last one . It would be nice to know the bandwidth of my meter , I think the manual is on line , I'll have to look it up . So it seems the 470p does what we would expect and the 100p has an effect starting somewhere around 10k . Don't know why it starts so low and again I wonder whats "R" . Maybe there is some more "C" in there . To put this in context the gain of V1a has a mid band emphasis but the signal at V1b grid has a definite high frequency emphasis and I don't think guitar speakers have much output at 10k .

All problems here are analog.

From the table of post 13 I'd say your gain is essentially constant on average between 1kHz and 3kHz.

A corner frequency of 40kHz means the gain is down by 3dB or around 30% at that frequency. Of course the drop gradually starts at lower frequencies.
But don't forget that a gain change of less than 1dB or 12% is typically considered insignificant/inaudible.

Also try to find the input capacitance of your DMM.