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

Doubts on cathode stripping in tubes
Doubts on cathode stripping in tubes
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 12th May 2006, 07:13 AM   #11
ray_moth is offline ray_moth  Indonesia
diyAudio Moderator Emeritus
 
ray_moth's Avatar
 
Join Date: Jan 2004
Location: Jakarta
Another concern, apart from the risk to cathodes, is that of turn-on surge. If you turn on the HT quickly, by using SS diode rectifiers, the tubes are not yet heated and cannot conduct. There will be no current drain on the PS due to the tubes in the amp, so all the current through the rectifiers ia available to charging the smoothing and reservoir caps. This gives a great surge through the caps and some people think that's bad.

The remedy is to use a delay and a soft start, i.e. let the tubes get hot first, then allow the HT to build slowly. This can be achieved by using thermionic indirectly heated rectifiers, which can take up to half a minute before they start conducting, depending on type, and turn on quite slowly. This solution avoids the turn-on surge and also deals with any risk of cathode stripping, either real or imagined. It also results in a relatively
"clean" HT, free from SS diode "hash".

To minimize the impedance which is incurred with thermionic rectifiers, with the resulting voltage drop and "sag", thermionic TV damper diodes can be used. These have a long warm-up time and are quite rugged. They work well in either a "full-wave" circuit (center-tapped power transformer secondary) or in a Gratez hybid bridge, in which they make up the positive-going half of the bridge and SS diodes make up the negative-going half. TV damper diodes tolerate high PIV, high currents and usually high heater-to-cathode voltages.
  Reply With Quote
Old 12th May 2006, 08:46 AM   #12
kathodyne is offline kathodyne  Netherlands
diyAudio Member
 
Join Date: Feb 2005
Location: leeuwarden, netherlands
my diy-amp has 4xpy500A for rectification.....
I make sure i always preheat the py500A

Nevertheless, within 2 years they have developed a 'mirror-blackish' on the top of the tubes...at the top-cap. (that is in this case the cathode connection)

i suspect this is.....'moved' getter and a residu of other materials
could this be cathode stripping also??

or is it just conatmination???

greetings
__________________
emission over emittor
  Reply With Quote
Old 12th May 2006, 09:01 AM   #13
EC8010 is offline EC8010  United Kingdom
diyAudio Moderator Emeritus
 
EC8010's Avatar
 
Join Date: Jan 2003
Location: Near London. UK
kathodyne: It could be removed cathode, but that could be down to your ripple current.

ray_moth: I thought TV damper (or efficiency as they're known in Europe) diodes would be slow warm-up too, but when I measured 12CL3, they took exactly the same time to start conducting as any other thermionic diode - about 11s.

gingertube: What makes you think helium? If your photomultipliers are as expensive as the ones we used in telecines (1500 each in 1992), I can see why you'd want to protect them.
__________________
The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...
  Reply With Quote
Old 12th May 2006, 10:38 AM   #14
SY is offline SY  United States
On Hiatus
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Doubts on cathode stripping in tubes
EC, I've chewed this one around and I still have some trouble with the mechanism you propose. Here's the problem: the concentration of gas in a tube is quite low. Now, when there's a significant electron flux, there could indeed be a significant cross section for collision and ion formation. Problem is that at start-up, the electron flux is miniscule. The probability of electron-ion collision will certainly be proportional to the electon flux and the gas molecule concentration- if both are low, that cross-section will be very, very close to zero.

Things are different when there's a very high electric field. But at normal operating voltages, the field between cathode and plate is not horribly big, in fact, no bigger than the field between an insulated wire carrying B+ and the grounded chassis a few mm away.
__________________
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
  Reply With Quote
Old 12th May 2006, 10:45 AM   #15
Burnedfingers is offline Burnedfingers  United States
diyAudio Member
 
Join Date: Nov 2002
Quote:
Post #8
Thread) Solid State delay on turn on relay.

Frank has a good point I believe.
  Reply With Quote
Old 12th May 2006, 01:49 PM   #16
ray_moth is offline ray_moth  Indonesia
diyAudio Moderator Emeritus
 
ray_moth's Avatar
 
Join Date: Jan 2004
Location: Jakarta
Quote:
when I measured 12CL3, they took exactly the same time to start conducting as any other thermionic diode - about 11s.
I don't know about 12CL3 but 6D22S warm up in 30 seconds, and that's pretty typical of dampers. Their warm-up time is a lot longer than the other tubes in an amp, so it's an effective solution. It's also substantially longer than more commonly encountered tube rectifiers, such as GZ34/5AR4.
  Reply With Quote
Old 12th May 2006, 03:59 PM   #17
kathodyne is offline kathodyne  Netherlands
diyAudio Member
 
Join Date: Feb 2005
Location: leeuwarden, netherlands
@ray_moth: how are your experiences withe 6d22s??????? they're not too expensive and could maybe very well substitute for my py500A's???
__________________
emission over emittor
  Reply With Quote
Old 12th May 2006, 04:04 PM   #18
smoking-amp is offline smoking-amp  United States
diyAudio Member
 
smoking-amp's Avatar
 
Join Date: Dec 2001
Location: Hickory, NC
Default Ionization potentials

I just looked up some gas ionization potentials in the "Handbook of Physics"

Hydrogen: 13.598 eV
H2: 15.427 eV
OH: 13.18 eV
H2O: 12.60 eV
CO2: 13.79 eV
He: 24.587 eV
N: 14.534 eV
N2: 15.51 eV
O: 13.618 eV
O2: 14.01 eV
Ar: 15.759 eV
Kr: 13.99 eV
Xe: 12.127 eV
Rn: 10.745 eV

Neon bulbs and VR tubes strike over around 65V up to 100+V or so .

Maybe the important parameter is when the ions have sufficient energy to actually cause damage to the cathode coating. Which might be more like the voltages necessary to cause secondary emission or higher.

On page 1085 there is a secondary-electron emission table for the elements,
and these seem to be roughly 300 V to 600 V. No listing for any oxides though.

Don
  Reply With Quote
Old 12th May 2006, 04:53 PM   #19
SY is offline SY  United States
On Hiatus
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Doubts on cathode stripping in tubes
The interesting thing is how the IPs are determined. For nitrogen and oxygen, it's electron impact. For water vapor, it's photoionization (at least in the CRC book).
__________________
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
  Reply With Quote
Old 12th May 2006, 07:37 PM   #20
Merlinb is offline Merlinb  United Kingdom
diyAudio Member
 
Merlinb's Avatar
 
Join Date: May 2006
Location: Lancashire
Here's an article addressing cathode stripping and other mythical failure modes:
http://ozvalveamps.elands.com/standby.htm
  Reply With Quote

Reply


Doubts on cathode stripping in tubesHide 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
Cathode stripping in low voltage tubes engels Tubes / Valves 1 26th March 2008 04:05 PM
Doubts regarding a 1Watt FM TX Kit xitronics Solid State 8 23rd May 2007 08:19 PM
Why don't we use output tubes as cathode followers? dsavitsk Tubes / Valves 15 29th January 2007 05:32 AM
VR Tubes as Cathode Bias of 2A3 SRPP Loftin-White snoopyma Tubes / Valves 8 16th June 2006 04:00 AM
Resistors & doubts ;( Duck-Twacy Pass Labs 11 15th May 2003 07:55 AM


New To Site? Need Help?

All times are GMT. The time now is 06:23 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.79%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio
Wiki