Cathode bypass with Long tail pair

Printer2 · 15th October 2011, 09:54 PM

Hopefully a quick question. With a long tail pair, 12AX7, 82k plate resistors, 620 ohm cathode, 33k tail, would there be much effect by having the 620 ohms capacitor bypassed?

SY · 15th October 2011, 09:57 PM

More gain, but you'll need very large bypasses if you want to extend that gain into the bass.

Printer2 · 15th October 2011, 10:32 PM

22uF, are we talking about the same gain increase as you would get in a single stage? I do not recall seeing it used before, not that I get around all that much.

Printer2 · 25th October 2011, 12:48 AM

Hate to second guess the only reply I got but I thought about this a bit more and am a little confused. I should have thought more on it earlier rather just before I wire up the circuit but better late than never.

Thinking this through I get the feeling that if I put a capacitor on the cathode resistor in a Long Tailed Pair inverter I would not get any signal in the second triode. I see the LTP as a self-split output stage that needs a ac voltage developed across the cathode resistor in order for the second triode to have a signal to amplify. By using a bypass capacitor the gain on the first triode is increased but the second triode grid and cathode is at the same potential as each other and you would not get any signal on the plate.

If someone can ease my mind over this I would appreciate it.

Merlinb · 25th October 2011, 09:55 AM

If you bypass the bias resistor you will get fractionally higher input impedance (assuming an AC coupled circuit), but apart from that there is very little to be gained; the tail resistor still dominates the performance.

Printer2 · 10:48 PM

Quote:

Originally Posted by Merlinb

If you bypass the bias resistor you will get fractionally higher input impedance (assuming an AC coupled circuit), but apart from that there is very little to be gained; the tail resistor still dominates the performance.

That is good to know. I am trying a switched PI (cathodyne, paraphrase, LTP) and did not want to switch out the capacitor when using LTP but wanted it for the other two modes. Thanks.

And since you are partly to blame for me wanting to try it...

http://i406.photobucket.com/albums/p...uckAmp17-1.jpg

15th October 2011, 09:54 PM	#1
Printer2 diyAudio Member Join Date: Apr 2010	Cathode bypass with Long tail pair Hopefully a quick question. With a long tail pair, 12AX7, 82k plate resistors, 620 ohm cathode, 33k tail, would there be much effect by having the 620 ohms capacitor bypassed?

15th October 2011, 09:57 PM	#2
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	More gain, but you'll need very large bypasses if you want to extend that gain into the bass. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

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15th October 2011, 10:32 PM	#3
Printer2 diyAudio Member Join Date: Apr 2010	22uF, are we talking about the same gain increase as you would get in a single stage? I do not recall seeing it used before, not that I get around all that much.

25th October 2011, 12:48 AM	#4
Printer2 diyAudio Member Join Date: Apr 2010	Hate to second guess the only reply I got but I thought about this a bit more and am a little confused. I should have thought more on it earlier rather just before I wire up the circuit but better late than never. Thinking this through I get the feeling that if I put a capacitor on the cathode resistor in a Long Tailed Pair inverter I would not get any signal in the second triode. I see the LTP as a self-split output stage that needs a ac voltage developed across the cathode resistor in order for the second triode to have a signal to amplify. By using a bypass capacitor the gain on the first triode is increased but the second triode grid and cathode is at the same potential as each other and you would not get any signal on the plate. If someone can ease my mind over this I would appreciate it.

25th October 2011, 09:55 AM	#5
Merlinb diyAudio Member Join Date: May 2006 Location: York	If you bypass the bias resistor you will get fractionally higher input impedance (assuming an AC coupled circuit), but apart from that there is very little to be gained; the tail resistor still dominates the performance.

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