Pentode Plate Load - Page 2 - 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 6th June 2007, 10:53 PM   #11
abj1 is offline abj1  Australia
diyAudio Member
 
Join Date: Jan 2005
Location: Melbourne
Totally concur with Jeff, this is pretty much the process I follow when working up a pentode for input duty - which I commonly do. Stable screen grid voltage is high priority (VR's are great and simple to use), stay away as far as possible from the knee with the smallest useable load for required gain (ie as vertical a load line as suitable for the gain required) and try to keep max grid voltage to around -1V max (not always possible) to stay away from decreasing input impedance as grid current starts.

Used this way pentodes can do great service. I have 3 amps set up this way using 77's, 6J7's and 24A's as input pentodes/tetrodes.
  Reply With Quote
Old 2nd July 2007, 05:34 AM   #12
diyAudio Member
 
dsavitsk's Avatar
 
Join Date: Jan 2005
Location: Hartford
Reviving an old thread ...

Just how low can a plate load on a pentode go and what does this do to Gm? For instance, on a 5879 (datasheet), a load of 2K5 would result in a gain of about 2.5 or so. Is this reasonable to do? And, does this cause gm to go way up as the change in current is high for a small change on the grid?
  Reply With Quote
Old 2nd July 2007, 06:00 AM   #13
diyAudio Member
 
Miles Prower's Avatar
 
Join Date: May 2005
Location: USA
Blog Entries: 6
Quote:
Originally posted by dsavitsk
Just how low can a plate load on a pentode go and what does this do to Gm?
Depends. Reducing the plate load will reduce the gain and the possible output voltage swing. How much of both do you need? That'll usually set the floor for Rp values.

Quote:
For instance, on a 5879 datasheet, a load of 2K5 would result in a gain of about 2.5 or so. Is this reasonable to do? And, does this cause gm to go way up as the change in current is high for a small change on the grid?
It might be reasonable to do that especially if what you're trying to achieve is a wideband response. When combined with Co + Ci (next stage) + Cstray, a small Rp will give a higher f(h) cutoff. Getting Rp down as far as possible is used in the design of video amps, for example. Of course, you're not gonna get the full 4.0MHz BW you need just from reducing Rp, and so you'll require compensating coils to extend the BW. (If using BJTs, you can reduce the Rc to 50R -- 100R, still have useable gain, and BWs into the VHF region without needing any compensating coils.)
__________________
There are no foxes in atheistholes
www.dolphin-hsl.com
  Reply With Quote
Old 4th July 2007, 06:49 PM   #14
diyAudio Member
 
dsavitsk's Avatar
 
Join Date: Jan 2005
Location: Hartford
Quote:
Originally posted by Miles Prower
Depends. Reducing the plate load will reduce the gain and the possible output voltage swing. How much of both do you need? That'll usually set the floor for Rp values.

But assuming I don't need much swing or gain, say for preamp duty, but do need a low output Z, then a plate load in the 500R to 1K range could be reasonable?


For another question, I am using some 6688's as triodes which means that g2 is tied to the plate and g3 is internally tied to the cathode. On the pentode curves, they say that g2 is at 150V and g3 is at 0V. Now, I have 150V on the plate, which means that I have 150V in g2 as well -- so what has made this magically switch from pentode to triode? The ony differences I see in the datasheet (http://www.mif.pg.gda.pl/homepages/f...30/e/E180F.pdf) are that the voltages are slightly dfferent (150 & 180 for pentode, 150 & 150 for triode.)
  Reply With Quote
Old 4th July 2007, 07:47 PM   #15
diyAudio Member
 
Miles Prower's Avatar
 
Join Date: May 2005
Location: USA
Blog Entries: 6
Quote:
Originally posted by dsavitsk
But assuming I don't need much swing or gain, say for preamp duty, but do need a low output Z, then a plate load in the 500R to 1K range could be reasonable?
Again, it depends. A better solution might be to use a cathode follower for a Lo-Z output.

Quote:

For another question, I am using some 6688's as triodes which means that g2 is tied to the plate and g3 is internally tied to the cathode. On the pentode curves, they say that g2 is at 150V and g3 is at 0V. Now, I have 150V on the plate, which means that I have 150V in g2 as well -- so what has made this magically switch from pentode to triode? The ony differences I see in the datasheet
What makes a pentode into a triode is the fact that both the plate and screen see the same AC voltage. Now, if you were to run the 6688 with Vsgsg= Vpp, and you maintained the screen at AC ground, then it would still operate just like a pentode. Indeed, that is frequently done in cases where you don't have a whole bunch of voltage, such as a TV IF strip that runs off a power xfmr-less DC supply.
__________________
There are no foxes in atheistholes
www.dolphin-hsl.com
  Reply With Quote
Old 10th March 2014, 09:12 PM   #16
diyAudio Member
 
Join Date: Jun 2008
Quote:
Originally Posted by the_manta View Post
... . . . .
A good pentode working in inductive load is one of the best stages ever. With a high gm tube and larger currents you are able to drive evrything. A C3M can drive a 300B without problems and with very low distortion.
I've been thinking about the C3M to drive a 6336 and am interested in your experience with inductive loads on pentodes. I'm guessing the L value would need to be pretty high. Can you say more about your experience?

Thanks
  Reply With Quote
Old 10th March 2014, 11:58 PM   #17
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
Might have a look at some of the Lundahl chokes like the LL1667/1668. I've not tried this but can see no reason why it would not work provided that there is not excessive capacitance in the choke. Note that the resistance in the grid circuit will define the gain based on value chosen and there may be some LF interaction between the choke and coupling capacitor. Best to model in spice.
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote
Old 11th March 2014, 01:47 AM   #18
diyAudio Member
 
Join Date: Mar 2003
Location: NYC
The inductance also needs to be sized to be a multiple of the chosen load at the lowest frequency of interest. 10K is borderline, 100K is out of the question.

dave
  Reply With Quote
Old 11th March 2014, 08:13 PM   #19
diyAudio Member
 
Join Date: Jun 2008
Quote:
Originally Posted by kevinkr View Post
Might have a look at some of the Lundahl chokes like the LL1667/1668. I've not tried this but can see no reason why it would not work provided that there is not excessive capacitance in the choke. Note that the resistance in the grid circuit will define the gain based on value chosen and there may be some LF interaction between the choke and coupling capacitor. Best to model in spice.
Alas, I mostly work on Mac and have never learned to use LTSpice beyond occasional noodling with stock library parts. I got the impression from Simon's post that he had experience with the topology and was looking to hear what he had discovered. It was an old post though and perhaps he has moved on in his thinking.
Quote:
Originally Posted by dave slagle View Post
The inductance also needs to be sized to be a multiple of the chosen load at the lowest frequency of interest. 10K is borderline, 100K is out of the question.

dave
I was thinking of starting with the oft' quoted 12K , putting the first multiple at ~190H for 20Hz. Whatdaya say Dave? is 2x worth looking at ?
  Reply With Quote
Old 11th March 2014, 08:40 PM   #20
diyAudio Member
 
Join Date: Mar 2003
Location: NYC
Hey,

the 12K and 190hy will give you a -1dB point of 20hz but you have to also take into consideration the load at 20hz will be 8K which will reduce the output even more. (edit.... spice shows simply a -1dB point at 20hz???

dave

Last edited by dave slagle; 11th March 2014 at 08:57 PM.
  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
how to connect an EL34 pentode into a triode and 100ohm resistor between plate and g2 jarthel Tubes / Valves 15 26th October 2013 02:44 PM
How do I design a pentode constant current source plate load? Bitrex Tubes / Valves 16 22nd January 2009 08:10 AM
CCS Plate Load Jack Thomas Tubes / Valves 4 21st May 2007 10:39 PM
Low plate voltage pentode (<40v) for diff pair? Rescue Toaster Tubes / Valves 3 2nd September 2006 12:56 AM


New To Site? Need Help?

All times are GMT. The time now is 01:29 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