Screen driven sweep tubes - the return of the evil kink? - 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 25th April 2013, 10:40 PM   #11
diyAudio Member
 
Join Date: Mar 2013
Location: Central Texas
Howdy, All: since my next amp will be using sweep tubes (right now my pick is the 6CB5) and G2 drive is under consideration, though I'm only wanting 30-50W per channel, this thread caught my eye.
The post by Tom is interesting. I'm not here to try to give a theoretical explanation at the moment, that will come later, but I did pull up the 6KG6 curves as published by Amperex. The plate current curves for G2 at 160V do show a pronounce kink at G1 negative biases in the -30 region and even more at G2=175 so it follows that one might see some kinking at other voltages. It does not appear to be the classic tetrode kinking, though, which actually displays a negative resistance. Rather, it is a non linearity which I've seen in other tube curves, including the KT88. But one normally does not operate in those regions (meaning per published curves, not the highly biased G1 with G2 drive) so it may not be an issue. Perhaps the thing to do is to run with either 0 G1 or only a few volts negative for G2 drive.
BTW, snivets as the TV engineers called them, were controlled by bringing G3 positive with respect to the cathode by a few volts. Interesting! The reason why some of the latter sweep tubes separated the G3 connection (OK, beam forming plates) from the cathode.
Anyway, just my two cents worth, I'll be looking at other posts here with a lot of interest.
Rene
  Reply With Quote
Old 26th April 2013, 07:41 PM   #12
diyAudio Member
 
Tubes4e4's Avatar
 
Join Date: Jul 2005
Location: Leverkusen
Gentlemen,

as it looks like so far by plots show not all screen driven sweep tubes tend to show such extreme kinks like types EL509 / 6KG6 / PL509 / 40KG6 or even mor, EL36 / 6CM5 / PL36 / 25E5.

I think more measurements would be needed to verify or falsify. Series of measurements with varying control grid prebias and even more interesting, with variation of the beam forming plates potential (if the tube under test allows for that) should give more insight.

I'll put the later (doing series of plots with variation of Eg3 voltage) on the to-do list, but please donīt hold your breath.

Thanks to all who contributed so far!

Regards, Tom
__________________
If in doubt, just measure.
  Reply With Quote
Old 27th April 2013, 01:39 AM   #13
diyAudio Member
 
Join Date: Mar 2013
Location: Central Texas
Please do look at the published curves for those tubes. The 6KG6 shows no "kinking" at all for its family of plate current curves vs G2 voltage when G1 is held at or close to zero, but shows large non linearities when G2 is held constant but G1 is taken way negative. You may not have any kind of an issue if you simply refigure your G2 drive load lines for G1 bias at or close to zero.
Even the venerable KT88 shows a LOT of plate current non linearities when G2 is held constant and G1 is taken way negative. But that combination, low plate voltage and very negative G1 is not the region where we operate these tubes.
You may not have any issues at all if you simply allow G1 voltage to be not quite so negative as you've been running.
Still, I'd love to see more measurements anyway, it's how we learn!
Good luck!
Rene
  Reply With Quote
Old 27th April 2013, 03:47 AM   #14
diyAudio Member
 
smoking-amp's Avatar
 
Join Date: Dec 2001
Location: Hickory, NC
I also suspect the large negative g1 bias is the problem here causing the kinks. With a negative g1, the g1 wires cause a sheet beam focusing effect between grid1 wires, which is normally used (at fixed g2 volts) to avoid hitting the g2 wires (aligned grids effect normally for lowering g2 current). The sheet beam focal length changes with g2 volts and plate volts, and some combination will cause the electron sheets to defocus enough for them to hit the screen grid wires. The kinks are likely accompanied with peaks in g2 current. Putting g1 at cathode potential should eliminate the sheet focusing effects. Anything left would then likely be secondary emission effects.

It might also be possible to dynamically correct the g1 wire focusing (with a neg. bias on g1) if the g1 voltage is varied as some fraction of the g2 voltage. Then g2 current could be held to a minimum as intended (for the normal pentode mode). Perhaps something near 1/Mu of g2 versus g1. This approach has been tried before (on a curve tracer) with interesting results, since it can cut the required voltage swing of g2 in half for the same plate output (if the two grids can be adding, but would double the required g2 swing if they have to oppose each other for focusing, which seems more likely here).
__________________
Without MS, how would we live without all those hackers?

Last edited by smoking-amp; 27th April 2013 at 04:06 AM.
  Reply With Quote
Old 27th April 2013, 08:55 AM   #15
diyAudio Member
 
Tubes4e4's Avatar
 
Join Date: Jul 2005
Location: Leverkusen
Hi smoking-amp,

interesting explanation! Unfortunately it wouldnīt be wise to omit the (automatic, cathode) prebiasing in real world amps to circumvent what you think is the cause of the kinking.

Mr. Danielak (EL509 Svetlana app report) and Mr. Paravincini (EAR) used the auto prebiasing to keep these fat sweep tubes from thermal runaway, both used about Eg1= -33V, thatīs why I used this also for the EL509 plot shown above. If prebiasing really is the cause of the kink, the (computed) "plot" from the Svetlana app report shown above is far from reality.

Screen drive plots from datasheets usually donīt use any control grid biasing (g1 simply strapped to cathode), indeed.

Regards Tom
__________________
If in doubt, just measure.
  Reply With Quote
Old 27th April 2013, 03:30 PM   #16
diyAudio Member
 
Join Date: Mar 2013
Location: Central Texas
It's hard to see from the published data that runaway is a big monster here. With a mild G1 bias, something in the -5 to -15 range, the idle condition is some G2 voltage value which would be quite low, producing very low plate current. Some people have used idle currents in the only very few milliamps range. I suppose one could still use a combination of fixed plus cathode self bias. My point is, at least at first glance, I don't see that G2 drive is necessarily a breeding ground for runaway conditions. IMHO, go less negative on the G1 bias, limit your desired power by restricting the swing at G2 with the added benefit of making it easier to design your driver and go for it.
Enjoy!
Rene
  Reply With Quote
Old 27th April 2013, 06:52 PM   #17
diyAudio Member
 
smoking-amp's Avatar
 
Join Date: Dec 2001
Location: Hickory, NC
Seeing as how these screen drive amps are usually run near class B, I would think that the usual automatic bias schemes would not work very well, causing tube cutoff after a loud signal passage. What you need for biasing is one of those fancy minimum current sensing (for class B) Op. Amp bias controllers for each tube. (minimum peak detector into an RCRC filter to act as a sample/hold, then compared with an adjustable Vbias reference level, then Vg1 bias gets applied ) Then you only need a small current sensing resistor in the cathode circuit for each tube.
__________________
Without MS, how would we live without all those hackers?

Last edited by smoking-amp; 27th April 2013 at 07:01 PM.
  Reply With Quote
Old 27th April 2013, 10:26 PM   #18
ilimzn is online now ilimzn  Croatia
diyAudio Member
 
Join Date: Feb 2005
Location: Zagreb
Tom, does your setup employ G2 current limiting?
Keep in mind that in any pentode and especially BPT the screen grid current rises dramatically when the plate voltage gets near the screen grid voltage (what 'near' means is a function of the utbe geometry, for sweep tubes the plate has to get well below screen grid voltage for this to happen). Could that be the problem? Also, if it is, you might be getting combinations of voltages and currents that match the conditions at multiple places. Also, DF96 has a good point re beam plate voltage. Unfortunately not all tubes have a separate beam plate connection
Are you measuring plate or cathode current? I have a Tek curve tracer but it's basically designed for semiconductors although it goes up to well over 1kV. Measuring pentodes is a problem as I can only employ pulse measurements because it always starts the curve tracing from what would be Vak=0, at which point a dangerously high current is passing through G2...
  Reply With Quote
Old 28th April 2013, 04:36 PM   #19
diyAudio Member
 
Tubes4e4's Avatar
 
Join Date: Jul 2005
Location: Leverkusen
Hi ilimzn,

Quote:
Originally Posted by ilimzn View Post
Tom, does your setup employ G2 current limiting?
Yes, the regulated PSU used for g2 feed is limited to 200 mA.

Quote:
Are you measuring plate or cathode current?
I am measuring directly at all electrodes. F.e, Ia is measured at the anode and Ig2 is measured at the screen grid, indeed.

Regards, Tom Schlangen
__________________
If in doubt, just measure.
  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
GU50 Screen Driven, DC Coupled matejsirk Tubes / Valves 3 1st March 2013 09:10 PM
6GB5 and Variants as Screen Driven Amps wrenchone Tubes / Valves 26 26th September 2012 08:05 PM
Has anyone screen driven an EL84? Lingwendil Tubes / Valves 11 22nd April 2009 04:04 AM
Screen driven output stage nhuwar Tubes / Valves 2 1st December 2008 06:10 PM
Horz Sweep amp Pent or screen drive mashaffer Tubes / Valves 21 20th July 2008 01:15 AM


New To Site? Need Help?

All times are GMT. The time now is 06:25 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright Đ1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2