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Old 10th December 2007, 02:48 PM   #1
PRNDL is offline PRNDL  United States
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Default preamp cathode bypass cap benefits

I've designed a fairly simple guitar amp that has uses a 12AX7 preamp tube with the tone stack in the middle, a 12AU7 long-tail-pair phase inverter, into a pair of 6V6's.

With cathode bypass caps (25F with 1.5K) on the two preamp stages, there's too much gain. I could put in a simple resistive voltage divider.

Without the bypass caps, the gain is just right.

I've heard that the bypass caps help reduce noise, but I really don't hear much noise without them.

Are there benefits to the cathode bypass caps, which compel me to use them?

Thanks!
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Old 10th December 2007, 03:45 PM   #2
kevinkr is offline kevinkr  United States
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Default Re: preamp cathode bypass cap benefits

Quote:
Originally posted by PRNDL
I've designed a fairly simple guitar amp that has uses a 12AX7 preamp tube with the tone stack in the middle, a 12AU7 long-tail-pair phase inverter, into a pair of 6V6's.

With cathode bypass caps (25F with 1.5K) on the two preamp stages, there's too much gain. I could put in a simple resistive voltage divider.

Without the bypass caps, the gain is just right.

I've heard that the bypass caps help reduce noise, but I really don't hear much noise without them.

Are there benefits to the cathode bypass caps, which compel me to use them?

Thanks!
You indicate that noise performance is acceptable so they aren't needed for this reason anyway. Tone and compression behavior under deliberate overdrive may change, but again if you like the result then it is acceptable. There is no mandatory reason they have to be there - so if performance is satisfactory without them they can be left out.
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Old 10th December 2007, 04:03 PM   #3
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The bypass caps bypass AC signals which allow higher gain through the tube (as you as well have found out). In combination with the cathode resistor you can forum a hi pass filter to allow only higher frequencies to pass. So if you have too much bass you can adjust the cap appropriately.

The formula is:

cut-off freq = 1/ (2*pi*R*C)
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Old 10th December 2007, 04:24 PM   #4
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by d1camero
The bypass caps bypass AC signals which allow higher gain through the tube (as you as well have found out). In combination with the cathode resistor you can forum a hi pass filter to allow only higher frequencies to pass. So if you have too much bass you can adjust the cap appropriately.

The formula is:

cut-off freq = 1/ (2*pi*R*C)
Perhaps not quite that simple, you need to know the thevenin equivalent source resistance of the cathode resistor plus the cathode's internal resistance. Internal cathode resistance is typically fairly close to 1/gm.. IIRC

Pretty sure none of this is relevant to PRNDL's original question about omitting the cathode bypass caps..
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Old 10th December 2007, 04:25 PM   #5
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Perhaps you can be a little more helpful as to how to calculate the cutoff frequency? You reply is a little obscure...


Oh, and it is relevant to his original question - he asked the purpose of the caps. In amp design we use this all the time to adjust to tone of amp stages.
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Old 10th December 2007, 04:53 PM   #6
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by d1camero
Perhaps you can be a little more helpful as to how to calculate the cutoff frequency? You reply is a little obscure...


Oh, and it is relevant to his original question - he asked the purpose of the caps. In amp design we use this all the time to adjust to tone of amp stages.

Just calculate the parallel equivalent resistance using the value derived from gm (the tube's transconductance) and the cathode resistor you are using. IIRC for the typical common cathode 12AX7 it is about 1K, (assuming gm of ~1000 uMhos) so using a 1K cathode resistor would give you 500 ohms thevenin equivalent resistance and then use the equation you posted previously. Most people not "tuning" tend to just deliberately overkill on this value rather than calculate it I suspect.

Point taken on voicing tone, was not thinking of the small value caps sometimes seen in guitar pre stages cathode circuits to fine tune the sound.
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Old 10th December 2007, 07:17 PM   #7
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Default Re: preamp cathode bypass cap benefits

Quote:
Originally posted by PRNDL
Are there benefits to the cathode bypass caps, which compel me to use them?
No.

If you have too much gain with these cathode bypasses, and enough without them, it's a good deal easier to eliminate them than to be making either voltage dividers or adding NFB networks to make anode followers. Removing cathode bypasses also improves linearity and reduces distortion.
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Old 10th December 2007, 07:29 PM   #8
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Miles, he is building a guitar amp. With guitar amps you want both amplitude and harmonic distortion - bypass caps are one of the techniques we commonly use to color the tone of the amp.
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Old 10th December 2007, 07:34 PM   #9
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rephrase, surely you dont always want harmonic distortion?
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Old 10th December 2007, 08:01 PM   #10
7N7 is offline 7N7  United Kingdom
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Default Re: Re: preamp cathode bypass cap benefits

Quote:
Originally posted by Miles Prower


[...]Removing cathode bypasses also improves linearity and reduces distortion.
Miles, I don't understand; surely one of the main causes of distortion is variation of Ra; the cathode bypass capacitor is supposed to address this.

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