Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

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

Importance of Screen Voltage of Pentode???
Importance of Screen Voltage of Pentode???
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 17th September 2020, 08:15 AM   #51
50AE is offline 50AE  Bulgaria
diyAudio Member
 
Join Date: Jul 2011
Location: Sofia, Bulgaria
Importance of Screen Voltage of Pentode???
I wonder what is your folks opinion on g2 max voltage? There are some tubes that are mentioned with a 250Vdc max in their datasheet, but in triode mode, it can be higher. What we're looking at is g2 current and power dissipation when the plate swings well below g2.

I'm asking this because I'm designing UL+CFB output stages. The problem is, for some tubes to conduct enough quiescent current, I need to increase g2 a bit above 250V. In the UL+CFB mode, the g2 swings a bit by the overall UL percentage, so it should be okay to increase it a bit, yes? Also, we're talking about music, so g2 overload should be happening at peaks only. What is your opinion on this?
  Reply With Quote
Old 17th September 2020, 10:34 AM   #52
indaco is offline indaco
diyAudio Member
 
Join Date: Jun 2016
Location: Pizza country (Italy)
How about g2 in a power tube run on triode mode? I've read that V plate can be high and consequently the V screen (apart a standard often used 100R resistor), so well over its max specs without the screen itself being damaged.
Anyone could explain that? And what would be the right behaviour than one must follow on U.L. mode?
  Reply With Quote
Old 17th September 2020, 12:52 PM   #53
Koonw is offline Koonw
diyAudio Member
 
Join Date: Apr 2013
Food for thought. Just plot screen current of triode mode with Vg2 225V and 450V. The kinks are more pronoun when Vg2 is at the plate potential, Ig2 is exceeding power dissipation (unconfirmed), triode curve shifted to the left by about 35v
Attached Images
File Type: png Triode curve Vg2 225v 450v compared.png (116.5 KB, 112 views)
File Type: png Triode mode Ig2 225v 450v compared.png (84.6 KB, 111 views)
File Type: png Triode curve Vg2 225v 450v compared-2.png (113.2 KB, 105 views)

Last edited by Koonw; 17th September 2020 at 01:10 PM.
  Reply With Quote
Old 17th September 2020, 01:40 PM   #54
OldHector is online now OldHector  Sweden
diyAudio Member
 
Join Date: Jun 2018
Location: Täby, Sweden
Quote:
Originally Posted by indaco View Post
How about g2 in a power tube run on triode mode? I've read that V plate can be high and consequently the V screen (apart a standard often used 100R resistor), so well over its max specs without the screen itself being damaged.
Anyone could explain that? And what would be the right behaviour than one must follow on U.L. mode?
Some tubes have a triode-connected value quoted. For example, with an 807 G2 and plate can be 400VDC.

I am also a little mystified by all the criteria to consider when G2 may exceed limits, for example in an ultralinear configuration. Since G2 current is proportional to plate current, it should be possible to infer dissipation, and verify that is within limits, and then using a screen resistor keeps the voltage in check.

What is really confusing for me is that some makes of tubes tolerate abuse better than others, so there is an element of suck it an see.

I want to build this circuit ...
Brimar 25P1
... but the B+ is 440VDC, and the 5B/255M tube is meant to be an 807 on an Loctal base, so I don't see how it works. But it came from the manufacturor of the tubes.
  Reply With Quote
Old 17th September 2020, 01:49 PM   #55
rdf is offline rdf  Canada
diyAudio Member
 
rdf's Avatar
 
Join Date: Jun 2004
Location: big smoke
Quote:
Originally Posted by 6A3sUMMER View Post
My first rule for Hi Fi playback of recorded sound:
If it is clipping, it does not sound good, turn it down..
Almost every North American FM station is constantly "clipping off" high frequency content and has for decades. Including classical and jazz stations. It's built into the processing architecture.
While not ideal minor short term clipping of dynamic material can be inaudible if the circuit is otherwise stable. There is no valid engineering reason for accepting poor circuit behaviour under overload, especially with the inherent limitations of a 2 watt amp. Unless you own K-horns.
__________________
Ears aren't microphones.
  Reply With Quote
Old 17th September 2020, 02:27 PM   #56
AmadeusMozart is offline AmadeusMozart  New Zealand
diyAudio Member
 
Join Date: Oct 2009
Location: New Zealand
History of the EL84 series is in VTV08.pdf

www.tiffe.de/roehren/VTV/VTV08.pdf

Cheers!
  Reply With Quote
Old 17th September 2020, 05:54 PM   #57
Koonw is offline Koonw
diyAudio Member
 
Join Date: Apr 2013
These plots are more accurate than previous, I need to revise the simulation, using zener diode is easier than using equations.

Last edited by Koonw; 17th September 2020 at 06:06 PM.
  Reply With Quote
Old 18th September 2020, 08:20 AM   #58
Koonw is offline Koonw
diyAudio Member
 
Join Date: Apr 2013
The screen grid2 current has been adjusted downward, dissipation appears with 9W limit. If the screen voltage is reduced the dissipation is lower, there is no Vg2 current curve in the datasheet, but it said not more than 15mA, it depends on the operating points.
Attached Images
File Type: png Triode Ig2 curve Vg2 diff 225V compared.png (124.2 KB, 23 views)

Last edited by Koonw; 18th September 2020 at 08:24 AM.
  Reply With Quote
Old 18th September 2020, 02:11 PM   #59
indaco is offline indaco
diyAudio Member
 
Join Date: Jun 2016
Location: Pizza country (Italy)
Quote:
Originally Posted by OldHector View Post
Some tubes have a triode-connected value quoted. For example, with an 807 G2 and plate can be 400VDC.

I am also a little mystified by all the criteria to consider when G2 may exceed limits, for example in an ultralinear configuration. Since G2 current is proportional to plate current, it should be possible to infer dissipation, and verify that is within limits, and then using a screen resistor keeps the voltage in check.

What is really confusing for me is that some makes of tubes tolerate abuse better than others, so there is an element of suck it an see.

I want to build this circuit ...
Brimar 25P1
... but the B+ is 440VDC, and the 5B/255M tube is meant to be an 807 on an Loctal base, so I don't see how it works. But it came from the manufacturor of the tubes.

Ok, for what I understood until now in triode mode plate surface has the major area than screen and if they are tied together (=same V) electrons are most in the plate. So the screen can tolerate some more V than specs.
In case of UL things appears to me more complicate, because also of UL% contribution...I don't know.



For the 5B/255M circuit you posted indeed the tubes run at +40V than max operating values, I can't say how it comes out. They can be "pushed" over without negative effects.
  Reply With Quote
Old 18th September 2020, 04:31 PM   #60
Koonw is offline Koonw
diyAudio Member
 
Join Date: Apr 2013
The triode composite curve becomes very non-linear instead of straight line when Vg2 is lower by 100V or more so, may means much higher distortion and output impedance.
Attached Images
File Type: png Triode composite curve diff 150v.png (95.3 KB, 13 views)
  Reply With Quote

Reply


Importance of Screen Voltage of Pentode???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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Importance of resistor working voltage spec? leadbelly Parts 15 20th July 2014 03:13 AM
Pentode cathode follower with regulated screen voltage? G Tubes / Valves 14 18th November 2013 11:01 PM
Output Pentode Screen at half Plate voltage Andrewbee Tubes / Valves 32 2nd May 2012 11:41 PM
Do EL34s in pentode mode PP AB1 sound better with lower screen voltage? ray_moth Tubes / Valves 11 1st December 2010 02:59 PM
Setting Pentode Screen Grid Voltage dshortt9 Tubes / Valves 1 19th January 2003 11:22 PM


New To Site? Need Help?

All times are GMT. The time now is 08:35 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2020 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2020 DragonByte Technologies Ltd.
Copyright ©1999-2020 diyAudio
Wiki