parallel cathode bypass caps of different values?

[chuggy]

A harp mod kit specifies adding a 22 mfd cap in parallel with a 4.7 mfd cathode bypass cap rather than replacing the 4.7 mfd cap. 
What is the sonic effect of the parallel caps compared to replacing the lower with the higher value cap?

Thanks
Jim R

[LooseChange]

It all depends on what the cathode resistor value is but if it's 1k5, there is no sonic difference that you'll hear.

[chuggy]

The resistor is 1.5K. 
In general, what is the sonic effect of parallel placement of cathode bypass caps of greatly different values (say .01 mfd and 50 mfd in parallel) compared to use of just the higher value cap?

[thermion]

caps have many operating parameters at play other than farads and voltage rating. da, df, esr, for example. these other things vary based on cap design and construction. the biggest result I hear is when bypassing a larger value electrolytic with a smaller value poly or mica cap, especially in and around the preamp. I've seen uf ratios of 1:100 to 1:5000 thrown around in discussion. I did 1:2000 and it was nice and the apparent uf seen by the circuit was essentially unchanged, so I must assume it's these other factors.
remember though you can introduce phasing issues, not always pleasing, when doing this kind of thing. let your ear be the judge.

[Willabe]

+1 on what thermion is saying.

It's not the value of the smaller cap, it's the type of cap (construction) that their going for. Hi-Fi guys have been doing this for a long time.

Does it work?              Only one way to find out.


               Brad      

[tubeswell]

If you are paralleling 22uF and 4.7uF you have 26.7uF. (capacitances in parallel sum up)

With a 1k5 cathode resistor and 4.7uF the frequency rolloff point is:

1/[(44/7) x 1500 x 0.0000047F] = 22Hz (which is below Low E on a bass guitar E-string, so won't make much difference in the volume in a standard tuned guitar fretboard, however it will knock out some of the low harmonic 'boominess' if used in V1)

With a 1k5 cathode resistor and 26.7uF, the frequency roll-off point is:

1/[(44/7) x 1500 x 0.0000267F] = 3.9Hz, and this is well below the audible hearing range, but you will hear more bass harmonics.

Now if you had 1k5 cathode resistor and switchable bypass caps of 22uF toggling with a 1uF, then the frequency roll-off would be between 106Hz (for the 1uF bypass) and 4.6Hz (for the 23uF). 106Hz is between G# and A on the 4th fret of a bottom E string on a standard tuned guitar, so if you have this configuration at the V1 cathode, you would notice a reduction in volume on the bottom three or so notes on a standard tuned guitar when the cathode bypass cap was toggled to 1uF.

(Ed: all fixed up after HPB's review :)

[HotBluePlates]

With a 1k5 cathode resistor and 4.7uF the frequency rolloff point is:

1/[(22/7) x 1500 x 0.0000047F] = 45Hz ...

You got the right answer, but with 2 offsetting errors.

Don't forget that you don't use pi (22/7), but 2*pi (or 44/7). The difference is the roll-off point is an octave lower than predicted.

This formula also doesn't account for the resistance looking into the cathode, which is effectively in parallel with the cathode resistor. For the moment, assume a typical 12AX7 stage has a resistance looking into the cathode of ~1600 ohms. That makes the total 1500 ll 1600 = ~774 ohms. This is about half the resistance you used, which would move the predicted roll-off point an octave higher. -1 + 1 = 0
The resistance looking into the cathode is (rp+Rl)/(u+1).
rp = internal plate resistance of the tube at the operating point.
Rl = plate load resistor
u = mu at the operating point (but nearly a constant)

The effect of of the resistance looking into the cathode can vary depending on the tube used and what current (what operating point) it is operated at.

[tubeswell]

With a 1k5 cathode resistor and 4.7uF the frequency rolloff point is:

1/[(22/7) x 1500 x 0.0000047F] = 45Hz ...

You got the right answer, but with 2 offsetting errors.

Don't forget that you don't use pi (22/7), but 2*pi (or 44/7). The difference is the roll-off point is an octave lower than predicted.

ah the 2Pi - feck it all. Thanks HPB :)

[HotBluePlates]

PRR got me with the same correction about 2 years ago... 

[PRR]

> What is the sonic effect of the parallel caps compared to replacing the lower with the higher value cap?

I'll say "NONE".

4.7u is maybe a bit small for full musical bass (in this circuit).

22u or 26.7u is plenty big for all your bass.

How you get 22-27u, whether one cap or 4.7+22 or two 13u or a dozen 0.5u caps, is meaningless for tube audio.

Yes, for w-i-i-d-e band amplifiers, the large cap which passes 2Hz may choke-up just a smidge at 20KHz. Hi-fi fanatics (and some well-engineered systems) will sometimes bypass a 10u with a 0.1u to be effective over a wider range. Also an audio-modulated radio transmitter may be shaking at both 50Hz and at 1,500,000hz, and need a big cap for audio and a sleek cap to stabilize the radio frequency.

However the sluggish nature of tube audio amplifiers makes this unnecessary for guitar amps.

He put the 22 across the 4.7 because the 4.7 was already there and adding 22 was easier than ripping out the 4.7.

[LooseChange]

"NONE"

Hey, That's what I said up top. :-)

He put the 22 across the 4.7 because the 4.7 was already there and adding 22 was easier than ripping out the 4.7.

+1