Cathode Bypass Capacitor Options

[jammybstard]

Thank you all; lots to think about.

One last question; is a fixed bias arangement all about power, or is there a tonal difference as well?

[brewdude]

I don't pretend to really understand it, but the explanation I recall was that the large bypass cap, say 1000uF, stabilized the cathode voltage swing and enabled it to behave similar to a fixed bias amp.

47uF is enough to stabilise the cathode for all the audible frequencies that the human ear can detect in an EL84. You won't notice any difference really on an EL84 geetar amp between 47uF and 1000uF bypass cap. But in reducing (say halving or quartering) the value of the 47uF bypass cap, you will start to notice reduction in the bottom end.

On the other hand, what fixed bias does, is increase the overall gain of the tube by increasing the voltage differential between the plate and the cathode - by putting the cathode at ground potential instead of at 10V above ground potential. But to do that you have to have the grid now set at -10V with a negative bias voltage supply. That is how fixed bias give the stage more power.

I don't intend dispute you. You seem to know what you are talking about. However, I can say that I definitely hear (or is it feel?) a difference between 47uF and 1000uF on my 18 watt. I like it better with 1000uF. It seems to be tighter or less flabby in the bottom end.

[tubeswell]

I don't pretend to really understand it, but the explanation I recall was that the large bypass cap, say 1000uF, stabilized the cathode voltage swing and enabled it to behave similar to a fixed bias amp.

47uF is enough to stabilise the cathode for all the audible frequencies that the human ear can detect in an EL84. You won't notice any difference really on an EL84 geetar amp between 47uF and 1000uF bypass cap. But in reducing (say halving or quartering) the value of the 47uF bypass cap, you will start to notice reduction in the bottom end.

On the other hand, what fixed bias does, is increase the overall gain of the tube by increasing the voltage differential between the plate and the cathode - by putting the cathode at ground potential instead of at 10V above ground potential. But to do that you have to have the grid now set at -10V with a negative bias voltage supply. That is how fixed bias give the stage more power.

I don't intend dispute you. You seem to know what you are talking about. However, I can say that I definitely hear (or is it feel?) a difference between 47uF and 1000uF on my 18 watt. I like it better with 1000uF. It seems to be tighter or less flabby in the bottom end.

Well maybe 100uF or 220uF would pick up as much bottom end as you could distinguish with say a 100R cathode Resistor if you were wanting to hear 10Hz - remembering that I have gone partially deaf from a lifetime of rock music. But if you are wanting to hear 75Hz (which is about as low as a Dropped D tuning on the bottom E string on a geetar) 42uF would do it, which in common cap parlance is 47uF.

Ck = 1 / (2 * pi * f * Rk)

= 1/ (2 x 3.1428 x 75 x 100R)

= 42uF

or 28uF for 150R (meaning you could get away with 33uF)

[jammybstard]

Ck = 1 / (2 * pi * f * Rk)

= 1/ (2 x 3.1428 x 75 x 100R)

= 42uF

or 28uF for 150R (meaning you could get away with 33uF)

f is the top of the 6db curve yes?

[marcoloco961]

Ck = 1 / (2 * pi * f * Rk)

= 1/ (2 x 3.1428 x 75 x 100R)

= 42uF

or 28uF for 150R (meaning you could get away with 33uF)

f is the top of the 6db curve yes?

I believe "f" is the frequency of 75 Hz.

[tubeswell]

Ck = 1 / (2 * pi * f * Rk)

= 1/ (2 x 3.1428 x 75 x 100R)

= 42uF

or 28uF for 150R (meaning you could get away with 33uF)

f is the top of the 6db curve yes?

Its an approximation of the 'half-boost point'*

* the frequency at which the gain is halfway between min and max

[Merlinb]

I like it better with 1000uF. It seems to be tighter or less flabby in the bottom end.

It is not just about frequency response, but also crossover distortion and blocking distortion.
The bigger the cap, the longer the valve will maintain its normal bias (avoiding the above problems) when heavily driven. Although, once the bias does eventually start to increase, it will take longer to recover of course, so it's still a trade off.

Has no one mentioned removing the cap altogether? That's the way I usually prefer things.

[tonewood]

Merlinb-

Do you prefer no bypass cap with both single ended and PP amps?
If so, could you discuss the advantages with SE?

Thanks

[Merlinb]

I don't have so much experience with SE amps, but so far yes, I usually prefer things without the bypass cap. The factors are basically the same as with PP, apart from crossover distortion of course.
There is practically no loss in max volume, and you gain headroom, but this also means you lose some ability to overdrive (so I tweak the driver stage to really slam the PA, since I like to have lots of distortion on tap), but the freedom you get from blocking is worth this price, I think. The overdrive just sounds "cleaner"; the notes don't smear into each other as much. Plus it's the simplest mod you can possibly have, and so easy to put on a switch! YMMV.

[crbowman]

Steve over at Angela has a SE - 2X6v6 design listed on his site. He puts the cathode bypass cap on a switch. Never built this one before, but I always thought it looked pretty cool.

http://angela.com/angelasupersingle-end ... oject.aspx

[Structo]

Interesting about the cathode bias cap.
Never tried that before.

[guitardude57]

Here is an interesting bypass cap chart. Pick your Rk and frequency, draw a straight line, there is your cap to use.

[KellyBass]

I was just reading this thread and thinking about a chapter I recently read in Merlin's fantastic book "Designing tube preamps for guitar and bass" and lo and behold he's here! How cool is that!?

Anyway, chapter 1 has a really good section on cathode bypass caps. It really helped me get a new design sorted out.

[PCollen]

I've quite new to valve amps and I've been playing around with some of the components in my ampmaker SE-5a, so far mainly the bypass capacitors:

Any thoughts on what could be achieved here? your experience would be welcome.

First thing I'd do is increase the second valve Rk from 820 to 1.5K or 2.2k., and put a 22uF cap across it. 820 Rk just biases the tube too hot, in my opinion, and makes it less articulate than I prefer.

[jammybstard]

In the end i've suck a 3-way switch in and fitted 2.2uF, 47uf, and 560uf selectable; I'm really supprised at the difference in tone between the 47uf and 560uf; I thought it'd be marginal, but it's actually quite distinct. And the 2uf is very different again. differet settings seem to work better with clean/distorted and bridge/neck. It really is a very usefull modification!

Cheers guys