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Old 20th October 2005, 10:59 PM   #1
Danko is offline Danko  Hungary
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Default LongTailedPair gain -> how to calculate?

Hello!


I have built this schematic:
Click the image to open in full size.

On the schematic, there's 6SL7GT, but I used ECC85.

How can I calculate the voltage-gain of this stage?


Thanks in advance!
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Old 20th October 2005, 11:29 PM   #2
SY is offline SY  United States
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The gain will be one-half that of the same circuit in grounded cathode.

Gain = muRp/2(Rp + rp), where Rp is the plate load resistor and rp is the plate resistance.
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Old 20th October 2005, 11:47 PM   #3
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Quote:
Originally posted by SY
The gain will be one-half that of the same circuit in grounded cathode.

Gain = muRp/2(Rp + rp), where Rp is the plate load resistor and rp is the plate resistance.
WOW! That is the most concise explaination of LTP gain calculation that I have ever heard. Good one!
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Old 21st October 2005, 01:06 AM   #4
SY is offline SY  United States
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Well, to be fair, what I said is only true for very long tails like current sources. But for a tail of 47K in this circuit, it will be close.
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Old 21st October 2005, 06:20 AM   #5
Yvesm is offline Yvesm  France
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As a rule of thumb: /4

You may have good surprise

Yves.
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Old 21st October 2005, 06:37 AM   #6
Danko is offline Danko  Hungary
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Thank you SY!

Oh, thanks for mentioning current source! Wich parameters will (will?) improve of the LTP, if I use a LTP in the "tail" of the LTP, instead of a resistor?

And also thank you, Yvesm! I have surprised! :-))
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Old 21st October 2005, 11:20 AM   #7
SY is offline SY  United States
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There are a lot of good current source circuits floating around. Morgan Jones shows both bipolar and tube versions in "Valve Amplifiers." Gary Pimm has some more exotic examples on his web site. In the one long tailed pair amp I have at the moment (a rebuilt Eico HF87), I used a pretty simple cascode bipolar sink. The performance is excellent and it cost only a couple of dollars in parts.

If it's possible for you to use a small negative supply (say, -12V or so), you can greatly simplify your circuit and improve the balance by using a CCS tail.
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Old 21st October 2005, 12:48 PM   #8
Danko is offline Danko  Hungary
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Thanks! I will search google for some good performing CCS.
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Old 21st October 2005, 07:58 PM   #9
Danko is offline Danko  Hungary
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Why do people mostly use semiconductor CCS, and not tube-CCS? I have seen many tube-amplifier schematics, but just a few of them use tube based constant current source. Is it only, becouse of a MJE340, or BC546 is far more cheaper than a tube?
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Old 21st October 2005, 08:21 PM   #10
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As a side note, if you use the tail resistor instead of a CCS, don't forget that the sum of the 2 tube's current across the tail will eat your voltage at the plates down by it's value. It's an obvious mistake that I made once and plotted my loadlines about 60v too high, had quite a surprise when I built the thing up and tested it.
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