Triode plate resistance == internal negative feedback? - Page 6 - 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 8th March 2011, 12:28 PM   #51
diyAudio Member
 
ruffrecords's Avatar
 
Join Date: Jun 2007
Location: Holt, Norfolk
Quote:
Originally Posted by SY View Post
When you don't make a tube with an rp of 60-70k drive a 100k load, the linearity (not surprisingly) is 10-20 times better than that.

That's the point of using a mu follower.

Quote:
Interestingly, in a test where the tubes weren't asked to drive a heavy- for them- load, the no-name ultra short plate 12AX7 (innards look like 12AT7, Eastern European source) tested as well or better than the much-vaunted smooth plate Telefunkens.
Can you provide figures and describe your test circuit?

Quote:
I'm surprised that the 6CG7 doesn't do better than that.
Morgan Jones in his tests with a CCS load got figures only 3dB better than this for the intrinsic distortion in a 6SN7. Most tubes do worse.

The good thing about a 6SN7 is that it does as well driving a 10K load but the 12AX7 obviously does not.


Cheers

Ian
  Reply With Quote
Old 8th March 2011, 12:36 PM   #52
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Oops, sorry, I had a senior moment. When you said mu follower, for some reason my brain thought anode follower.

With the mu follower, the distortion depends on load impedance so you would need to check that you had not inadvertently favoured one valve type.
  Reply With Quote
Old 8th March 2011, 12:43 PM   #53
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Quote:
Originally Posted by ruffrecords View Post
Can you provide figures and describe your test circuit?
Guess That Tube

Also, see post 52 in that thread.
__________________
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 8th March 2011, 02:15 PM   #54
diyAudio Member
 
ruffrecords's Avatar
 
Join Date: Jun 2007
Location: Holt, Norfolk
Quote:
Originally Posted by SY View Post
Guess That Tube

Also, see post 52 in that thread.
OK, that ties up well. Since distortion is directly proportional to output level your 8.5V rms is -7.5dB relative to my 20V rms. I got 2H at -60 which means at your level I would get -67.5dB cf your -70db. And as I said before, the mu follower seems to be no more than 3dB worse than a CCS.

So, the key question is exactly which 12AX7 was it you tested?

Cheers

Ian
  Reply With Quote
Old 8th March 2011, 02:22 PM   #55
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Did you look at the other post I referenced? That gave results at close to 20VRMS out.

That particular spectrum was from a Telefunken. As I said in the thread, the tiny plate Eastern European versions gave me almost the same results (within a dB or so).
__________________
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 8th March 2011, 03:27 PM   #56
diyAudio Member
 
ruffrecords's Avatar
 
Join Date: Jun 2007
Location: Holt, Norfolk
Quote:
Originally Posted by SY View Post
Did you look at the other post I referenced? That gave results at close to 20VRMS out.
Yes, you got -65dB 2H cf my -60dB so that's a bigger 5dB difference - more than I would have expected.

Quote:
That particular spectrum was from a Telefunken. As I said in the thread, the tiny plate Eastern European versions gave me almost the same results (within a dB or so).
Which does not tally with my tests where the small (less tall) plates gave distinctly worse results. That said I only tested two tubes of each type all current production. I also tested a couple of EH 12AX7 WA and one gave the amazing result of -68dB 2H at 20V rms - but this was at the expense of higher 3H. I am not quite sure what I was seeing there but I have noticed some other tube examples that have a better/worse 2H component accompanied by a similarly worse/better 3H component.

You mentioned the effect of mu follower load on distortion. This is quite true but also seems to exhibit this trade off between 2H and 3H.

For example, I tested some 6CG7 at 20V rms into loads varying from 100K down to 10K. At the extremes the 2H was identical but 3H increased by about 10dB at 10K load. There was a pronounced 2H minima of several dB at 15K load but again at the expense of 3H (although THD was also at a minimum). Generally from 100K load downwards to 15K, 2H decreased at the expensive of 3H (though THD decreased too) thereafter both increased. I also measured higher order harmonics which were not present at the 100K load but began to appear with heavier loads. I repeated the tests at 10V rms and there were no higher harmonics than the 3rd detectable at any load.

Cheers

Ian
  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
PSPICE and negative resistance rtarbell Software Tools 2 16th May 2009 08:34 PM
NPN negative collector resistance?? darkfenriz Parts 7 16th September 2007 05:45 PM
Automotive relays and internal resistance? pjpoes Everything Else 2 9th February 2007 04:18 PM
Two port negative resistance element rtarbell Parts 9 18th March 2006 09:27 AM
811A tube internal resistance ? Zoran Tubes / Valves 38 11th May 2004 07:33 AM


New To Site? Need Help?

All times are GMT. The time now is 01:27 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