Question re: Distortion in triode gain stage - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 14th July 2012, 04:13 PM   #1
rongon is offline rongon  United States
diyAudio Member
 
rongon's Avatar
 
Join Date: Jul 2009
Location: Across the river from Rip's big old tree...
Default Question re: Distortion in triode gain stage

I noticed something in the discussion on distortion in medium mu triodes in Morgan Jones' Valve Amplifiers 3rd Ed.

There's that part where he compares 6SN7 and its cousins against 12AU7/ECC82, E182CC (7119), and 5687.

The test circuit has a triode wired D3a as an active load (mu follower) for the lower triode under test. The plate current for all the tubes under test is set to 8mA.

Now, I understand that 8mA plate current is a nearly ideal condition for getting the most out of a 6SN7 or similar triode. I noticed that on the graphs for 6SN7 ra vs. Ia, 8mA puts the tube in a place where its ra remains constant through much of the output swing.

When you look at a 5687 with Ia = 8mA, the picture is not so rosy. The 5687 never has as constant an ra vs. Ia line as 6SN7 does, but it is much better at 15 to 20mA than it is at 8mA. The same holds for E182CC.

Isn't it unfair to compare 5687 at Ia = 8mA to 6SN7 at Ia = 8mA? Wouldn't it be a more fair comparison if Ia = 15mA for 5687 and Ia = 8mA for 6SN7?

--
  Reply With Quote
Old 14th July 2012, 04:42 PM   #2
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
You can get comparable results to his by loading the tube with a CCS, then measuring through a buffer or high impedance input; that's what I did when I repeated and extended slightly his 6SN7 survey. I'd be interested in what you find with the 5687.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 14th July 2012, 05:40 PM   #3
rongon is offline rongon  United States
diyAudio Member
 
rongon's Avatar
 
Join Date: Jul 2009
Location: Across the river from Rip's big old tree...
Well, I lack the gear, and probably the skill, to make comparable measurements.

The basic question is: Will a 5687 with a CCS plate load, or as a mu follower, and 15mA Ia, have comparable distortion to a 6SN7 in a similar circuit, but with 8mA Ia?

I suspect the 6SN7 will still have lower distortion (its ra curve looks impressively flat from about 7.5mA and up), but if you look at the 5687 data sheet, the 5687 looks like it's in a pretty bad spot with only 8mA of plate current.

Here are a couple of pics to illustrate...

To me, it looks like 5687 doesn't really come into its own until it has 18mA of plate current.
Attached Images
File Type: jpg 6SN7_ra_vs_Ia_chart.jpg (218.6 KB, 172 views)
File Type: jpg 5687_ra_vs_Ia_chart.jpg (156.5 KB, 174 views)
  Reply With Quote
Old 14th July 2012, 05:47 PM   #4
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Remember that this circuit is running constant current, so variations in rp with current are second order issues. The main issue is variation in mu with plate voltage.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 14th July 2012, 06:31 PM   #5
rongon is offline rongon  United States
diyAudio Member
 
rongon's Avatar
 
Join Date: Jul 2009
Location: Across the river from Rip's big old tree...
I always learn something new from you (and many others)!

Ah, that is very interesting...

From those charts, the 6SN7 with 150V on the plate and at least 9 or so mA of plate current has an impressively flat curve for mu.

The 5687 mu curve is always ever so gently sloping, from lower currents to high.

Mu for 6SN7 with Va of only 50V is not so constant at all. Actually goes down with higher current. 5687 mu curve stays pretty flat all the way down into cutoff. Is that due to 5687's "high perveance"?

Is a very flat mu curve in the above charts predictive of low distortion performance?

Also... When using a resistor as the plate load, does the variability of rp become more important? Or as important as variability of mu?

--

Last edited by rongon; 14th July 2012 at 06:33 PM.
  Reply With Quote
Old 14th July 2012, 06:33 PM   #6
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Well, again, that's mu versus current, not mu versus voltage- you're running constant current so the former doesn't matter. At the current you choose, you can pick off the points at different voltages, but for most datasheets, it will be three points spaced very far apart in voltage (like 50, 150, 250V).
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 14th July 2012, 06:55 PM   #7
rongon is offline rongon  United States
diyAudio Member
 
rongon's Avatar
 
Join Date: Jul 2009
Location: Across the river from Rip's big old tree...
I guess what you're saying is that running the tube with a CCS in the plate makes it run in a linear fashion at a wide variety of plate currents, because mu will be fairly constant. So the voltage can do what it will and the mu remains fairly constant, as the current is forced to be constant.

Therefore, the test is a valid one and the 5687 at plate current of 8mA should be as linear as it is at plate current of 16mA, so long as it is loaded with a CCS (and not called on to swing outside of its linear window of operation).

Right?

--
  Reply With Quote
Old 14th July 2012, 06:59 PM   #8
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
A tube will generally show different distortion at different currents, even with a CCS. But... at any given current, the distortion will be minimized FOR THAT CURRENT with a CCS load.

Pick your current, draw a vertical line. It will intercept the mu curves at a few different voltages (depending on how kind the datasheet writers were). The closer those mus are to one another, the lower the distortion.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 14th July 2012, 08:16 PM   #9
rongon is offline rongon  United States
diyAudio Member
 
rongon's Avatar
 
Join Date: Jul 2009
Location: Across the river from Rip's big old tree...
Quote:
Pick your current, draw a vertical line. It will intercept the mu curves at a few different voltages (depending on how kind the datasheet writers were). The closer those mus are to one another, the lower the distortion.
Hmmm....

So if the mu lines are very close to each other, you're looking at a very linear spot to run a tube?

I've marked up the charts I uploaded before, but now with red lines showing what you were saying there (I hope).

If I got it right, a 6SN7 running really hot at Ip = 15mA will be extremely linear, because its mu will be almost exactly the same at Vp = 150V as it is at Vp = 250V.

Looking at the 5687, its mu is always different at different plate voltages, no matter which plate current you choose.

The 5687's difference between mu from one plate voltage to another stays constant no matter what plate current you choose.

However, the more current you put through a 6SN7, the less difference in mu at different plate voltages. So the 6SN7 gets more linear the more current you put through it.

No?

--
Attached Images
File Type: jpg 6SN7_ra_vs_Ia_chart_mu.jpg (219.1 KB, 156 views)
File Type: jpg 5687_ra_vs_Ia_chart_mu.jpg (157.5 KB, 151 views)
  Reply With Quote
Old 14th July 2012, 09:17 PM   #10
diyAudio Member
 
Michael Koster's Avatar
 
Join Date: Jan 2008
Location: Eureka, CA
"Isn't it unfair to compare 5687 at Ia = 8mA to 6SN7 at Ia = 8mA? Wouldn't it be a more fair comparison if Ia = 15mA for 5687 and Ia = 8mA for 6SN7? "

"A tube will generally show different distortion at different currents, even with a CCS. "

Right?
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
2 stage Stereo/4 Stage Mono Triode Preamp with independent drive control & inverters sampleaccurate Tubes / Valves 10 9th March 2011 10:31 AM
Transformer based voltage gain + unity gain stage? soundcheck Solid State 2 10th August 2010 02:44 PM
Low gain triode recommendation plz zigzagflux Tubes / Valves 17 19th June 2008 10:40 PM
Favorite noval hi gain dual triode? leadbelly Tubes / Valves 0 11th October 2007 10:28 PM
Heretical Unity Gain line stage PLUS a little gain ? yagas Tubes / Valves 7 24th January 2006 01:30 PM


New To Site? Need Help?

All times are GMT. The time now is 07:10 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2