Plate load impedance - 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 24th September 2010, 07:30 AM   #1
diyAudio Member
 
Join Date: Feb 2010
Location: Helsinki
Default Plate load impedance

I was just wondering, how could I calculate the plate load impedance of the attached stage?
Attached Images
File Type: png 6SN7 driver.png (15.5 KB, 219 views)
  Reply With Quote
Old 24th September 2010, 09:24 AM   #2
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
It will depend on the resistance at the other end of the line going to the right. The plate load contribution will be rp + (mu + 1)Rk, with rp and mu being the plate resistance and amplification factor for the top tube. This will be effectively in parallel with whatever load is off to the right.
__________________
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 24th September 2010, 10:31 AM   #3
diyAudio Member
 
Join Date: Feb 2010
Location: Helsinki
It is direct coupled to a 1626 grid. How do I figure out the the load impedance of that?
  Reply With Quote
Old 24th September 2010, 10:56 AM   #4
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Is there a resistor to ground? If not, it's just the input capacitance of that stage.
__________________
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 24th September 2010, 01:11 PM   #5
diyAudio Member
 
Join Date: Feb 2010
Location: Helsinki
Both triodes are 6SN7. Data quotes rp as 6.7k at 90V / 10mA and 7.7k at 250V / 9mA. Both triodes have about 150V across them with 9.6mA, so I'll guestimate that the rp is about 7.5 k.

mu = 20
rp = 7500
rk = 312 (the bottom triode has a diode in there, but the voltage drop is equal to this resistance)

rp + (mu + 1)Rk = 7500 + (20 + 1) x 312 = 14052

Now I must have misunderstood something, this can't be - with 14k plate load the DC conditions don't add up; at 9.6mA the voltage drop measured is about 150V, but calculated it's only 134.9V - and I've understood that this kind of arrangement has a higher AC load than DC load, at least it should be equal, right?

Where did I go wrong?
  Reply With Quote
Old 24th September 2010, 02:23 PM   #6
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Quote:
Originally Posted by MrCurwen View Post
with 14k plate load the DC conditions don't add up; at 9.6mA the voltage drop measured is about 150V, but calculated it's only 134.9V - and I've understood that this kind of arrangement has a higher AC load than DC load, at least it should be equal, right?

Where did I go wrong?
It's an AC load, not a DC load. But you've calculated it correctly; a triode run that way is an amazingly poor CCS. That's why people have added extra bits to run mu or beta followers. Or used MOSFETs...
__________________
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 24th September 2010, 03:34 PM   #7
diyAudio Member
 
Join Date: Jul 2003
Location: Ann Arbor, MI
hey-Hey!!!,
One solution is to deliver a voltage reference to allow a significantly higher cathode R OTO 5-6k Ohms, which when multiplied by the mu of ~20 delivers some significant plate resistance. Or get a higher mu tube, or both, and/or stack a pentode on top of the triode to make a tube cascode...but that will take two biasing voltages...
cheers,
Douglas
__________________
the Tnuctipun will return
  Reply With Quote
Old 24th September 2010, 06:49 PM   #8
diyAudio Member
 
Join Date: Feb 2010
Location: Helsinki
Ok, so I put some extra bits in there; now it's a mu follower. How do I calculate the plate load for this? (Still direct coupled to 1626 grid.)

Sure does sound a lot better!
Attached Images
File Type: png mu follower.png (18.4 KB, 124 views)
  Reply With Quote
Old 24th September 2010, 07:00 PM   #9
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Same formula as before. All you have done is separate the AC and DC arrangements for the upper triode. Rk is now 10.1K.
  Reply With Quote
Old 24th September 2010, 07:16 PM   #10
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Quote:
Originally Posted by DF96 View Post
Same formula as before. All you have done is separate the AC and DC arrangements for the upper triode. Rk is now 10.1K.
Yes, but that increases the source resistance. It's still mediocre, but 200k is better than 14k, and is close enough to a true CCS (in relation to the rp of the lower tube) that the gain will be very close to mu and the distortion very close to minimum.
__________________
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

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
Pentode Plate Load dsavitsk Tubes / Valves 20 13th March 2014 07:38 PM
Plate load for push-pull Jeb-D. Tubes / Valves 5 9th April 2008 08:53 PM
diodes before plate load alexg Tubes / Valves 2 3rd March 2008 01:26 PM
CCS Plate Load Jack Thomas Tubes / Valves 4 21st May 2007 10:39 PM
bypassing the plate-load memphissound Tubes / Valves 3 12th December 2006 12:46 PM


New To Site? Need Help?

All times are GMT. The time now is 06:37 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