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Old 17th July 2002, 12:28 AM   #1
SHiFTY is offline SHiFTY  New Zealand
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Question Cathode bypass caps?

What is the rule of thumb with cathode bypass caps, both in the preamp and power amp sections of a tube amp? I am talking about the caps which are in parallel with the cathode-to-ground resistor.

I am rebuilding someones homemade amp from the 70s, and it uses 100uF electros for cathode bypass caps everywhere. However, I have read that you dont really need these, and they may just be screwing with the sound. The amp sounds good, but the bass is a bit "rumbly" and there is a little bit of distortion i am trying to hunt down.

Could I just try swapping the caps for smaller values? Or just removing them all together? What would happen if i used 1uF polyester (non-polar) caps?

All answers appreciated.
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Old 17th July 2002, 01:45 AM   #2
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These caps are used to couple the ac signal current to ground while still letting the bias voltage be dropped through the resistor.
If you remove the caps or reduce their value, the audio signal will also partly be dropped by the resistor causing negative local feedback thus reducing the gain a lot. If you remove these caps, the amp will still work but the gain will be much lower and you might get really weak bass response. They should be kept at 100uF to keep the audio gain good. You'll find that if they are too small, the bass response will suffer, definately don't change to 1uF or you'll get no bass with your highs and mids. If you wan't to enforce bass stability, you could even increase the value of hte caps but too much capacitance will make for more needed time to stabilize even after the filaments heat up. Also, make sure your caps are rated for the same or even a higher voltage than that of the originals!
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Old 17th July 2002, 05:09 AM   #3
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Default bypass cap

Hey Hi Shifty:

Try reading this link for a couple of pages.

www.tpub.com/neets/book6/20h.htm

Cheers,
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Old 17th July 2002, 05:35 AM   #4
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Default Re: bypass cap

If you use a cap it should be big and as good as you can get. If you don't you eliminate a cap from the signal path (a good thing) but you get 100% regenerative feedback so less gain.

Sometimes it sounds better w the cap sometimes not. Try it with & without. Use the method one that best suits.

dave
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Old 17th July 2002, 08:43 AM   #5
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If he got 100% regen feedback, he'd have a good oscillator. Actually, if you remove the bypass cap from a class A amp, you introduce degenerative, or negative feedback. Also, it won't be 100%, if you have 100% negative feedback, you have a unity gain amplifier, which makes a pretty useless VAS!!
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Old 17th July 2002, 11:12 AM   #6
SHiFTY is offline SHiFTY  New Zealand
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Well, I tried disconnecting the caps altogether, the ones on the power tubes had NO discernable effect on the sound when removed, so i left them off!

The caps in the preamp had a slightly strange effect when removed: the bass seemed to get a bit "boomy" and the sound quality was... changed. Quite what i dont know, but it wasnt entirely nice sounding. So anyway i got some 3.3uF polyester caps (big suckas) and chucked them in: the amp now sounds quite good. I have noticed another set of 100uF electros in the negative feedback loop, so i think these are the next to go!!!

BTW, i did not find that the gain was changed at all. The volume control was left at the exact same position and the amp did not appear quieter!! Also the bass sounded much the same with the caps removed; maybe a bit "tighter" even.

Tweak o rama time!!
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Old 17th July 2002, 03:44 PM   #7
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Hi ShiFty,

Ya done good for yourself. The name of the game it try it and see. However, one could do some calculations and try to see if one gets as good a result without such a dramatic change.

Here are some hints for either this project or the future:

First off, leave the one in for the power output. Why? It aids in maintaining the class AB function and maintaining the bias at idle. If it is 100 or more bypassing a 100-300 ohm resistor, better to use a smaller value like 22 to 33F instead. This way one can have a quicker dynamic recovery.

There is something about capacitors called a time constant. That is the time it takes for a given capacitor to charge and discharge in the circuit. A larger time constant means that the capacitor will take longer to charge and discharge. So making this smaller rather than eliminating it will allow the bias to remain constant while giving them the ability to change in step with music demands.

As for the preamp stages, I usually go with making the bypass cap's reasctance 1/10th the value of the cathode resistor, at the desired frequency. So, if I want the bypass cap to have that value at 20 Hz, then I take the resistors value, divide by ten and lug the numbers into the formula for capacitive reactance.

For example, for a cathode resistor of 2.2K, I want 220 ohms. I want the frequency to be 20Hz (that gives a -3dB at 4 Hz).

The formula for capacitive reactance, turned around to get a capacitor value, is:

C=1/2PifXc

So

1/2(3.14159)20(220) = 49F

A 47F cap will do fine.

Even smaller, but no less than 10F (for resistances of 1K to 4.7K), will also do well without degrading the overall sound. In fact, smaller values might improve the high frequency content since the larger capacitors have larger internal inductance. But that is a whole other issue better left for another thread.

Hope this helps.

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Old 17th July 2002, 06:09 PM   #8
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Quote:
Originally posted by Duo
If he got 100% regen feedback, he'd have a good oscillator. Actually, if you remove the bypass cap from a class A amp, you introduce degenerative, or negative feedback. Also, it won't be 100%, if you have 100% negative feedback, you have a unity gain amplifier, which makes a pretty useless VAS!!
You are right, for some reason i had cathode follower in my head...

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Old 18th July 2002, 02:58 AM   #9
SHiFTY is offline SHiFTY  New Zealand
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Wow thanks for teh info....

I am going to take the amp to be analysed with a scope and see what i can find... Hopefully I can get a tube guru to give me some advice... I guess any techs who work on guitar amps et al should be able to help me. I want to fine tune this amp so it sounds not just good but great.
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Old 18th July 2002, 06:37 PM   #10
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I usually bump that cap up to 470uf or larger, parrallel in a nice poly to "clean" the sound and it acts more in the way of a low pass and your bass is improved. And those have been usually 10mF caps I replaced. If you had 100 in there trying a 1,000 would not be out of line. A lot of guys don't like tubes because of the lack of punch in the bass. This seems to help.
Here is a real suggestion; next time you ask a fairly specific question it is very helpfull to the people you ask to tell them just what it is you are playing with and what you are trying to accomplish. If you had been doing this to a SE amp I would suspect that you would now have a very nice looking oscilator. So I am guessing you have a PP, but still only know that is a hombrew from the seventies not even if it is a stereo to listen to or an amp to play a guitar through.
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