LongTailedPair gain -> how to calculate?

Danko · 20th October 2005, 10:59 PM

Hello!

I have built this schematic:

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

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

Thanks in advance!

SY · 20th October 2005, 11:29 PM

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.

aletheian · 20th October 2005, 11:47 PM

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!

SY · 21st October 2005, 01:06 AM

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.

Yvesm · 21st October 2005, 06:20 AM

As a rule of thumb: µ/4

You may have good surprise

Yves.

Danko · 21st October 2005, 06:37 AM

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! :-))

SY · 21st October 2005, 11:20 AM

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.

Danko · 21st October 2005, 12:48 PM

Thanks! I will search google for some good performing CCS.

Danko · 21st October 2005, 07:58 PM

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?

aletheian · 21st October 2005, 08:21 PM

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.

20th October 2005, 10:59 PM	#1
Danko diyAudio Member Join Date: Apr 2004 Location: Hungary	LongTailedPair gain -> how to calculate? Hello! I have built this schematic: On the schematic, there's 6SL7GT, but I used ECC85. How can I calculate the voltage-gain of this stage? Thanks in advance! __________________ Best regards, Danko

20th October 2005, 11:29 PM	#2
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	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. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

21st October 2005, 01:06 AM	#4
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	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. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

21st October 2005, 06:37 AM	#6
Danko diyAudio Member Join Date: Apr 2004 Location: Hungary	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! :-)) __________________ Best regards, Danko

21st October 2005, 11:20 AM	#7
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	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. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

21st October 2005, 06:20 AM	#5
Yvesm diyAudio Member Join Date: Jul 2004 Location: Ardeche	As a rule of thumb: µ/4 You may have good surprise Yves.

21st October 2005, 12:48 PM	#8
Danko diyAudio Member Join Date: Apr 2004 Location: Hungary	Thanks! I will search google for some good performing CCS. __________________ Best regards, Danko

21st October 2005, 07:58 PM	#9
Danko diyAudio Member Join Date: Apr 2004 Location: Hungary	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? __________________ Best regards, Danko

21st October 2005, 08:21 PM	#10
aletheian diyAudio Member Join Date: Apr 2004 Location: Philly	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. __________________ Heghlu'meH QaQ jajvam!

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
How to calculate gain and output impedance?	RogerGustavsson	Solid State	1	7th June 2008 05:09 PM
How to calculate gain?	lumanauw	Solid State	10	29th July 2007 12:57 AM
Have to calculate gain for typical small JFET preamplifier	supernet	Solid State	4	11th August 2006 11:22 PM
How to calculate the gain of an input-driver stage?	Antonio Tucci	Tubes / Valves	0	8th October 2004 12:25 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read