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Lets settle the b+ on cold tubes issue!
Lets settle the b+ on cold tubes issue!
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Old 21st June 2010, 05:35 PM   #1
SemperFi is offline SemperFi  Wake Island
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Default Lets settle the b+ on cold tubes issue!

Sorry folks, this got a little longer than I intended, but the heater issue is something riddled with myths and ambiguity.

We all want our precious tubes to last forever, and I know this matter of b+ on cold tubes is filled with different 'facts'.

This is regarding indirectly heated tubes, not filament tubes.

I used to include a standby switch on all my tube gear so I could apply the b+ after the tubes were warmed up. Sometimes I got fancy and included a time delayed relay to do the job. For more than 10 years of building amps I'd been petrified of applying high tension to cold tubes.

Last year there was a discussion regarding the standby switch and I was proven wrong!!! Can u believe it!
Basically I was given a link to the Wizard:

How to design valve guitar amplifiers

...read the article on standby switches. It makes sense that the standby switch enables lower value caps to be used. I've actually used capacitors under-rated the no-load voltage and had to be sure the circuit was loading the psu before applying power. In the old days every penny counted, and those mass produced guitar amps had to save where they could.

After reading the article, I started searching the books for proof that I was right and they were wrong...all those downloads from Pete Millet,
Technical books online
I spent hours and hours reading various books and manuals, and I finally realized I had been wrong.

There is no harm done when applying high voltage to cold tubes!!!

There is a voltage level where it starts to get dubious, but that's a much higher voltage than 90% of us ever go to. (I can't remember the voltage).

So can we all find the right answer to this and decide on it once and for all? Please contribute...

Btw, indirectly heated cathodes are heated by the 'heater', not 'filament'. Directly heated tubes have 'filaments' (which is the cathode as well).

Secondary:
I ended up reading 'Getting the Most Out of Vacuum Tubes' and basically concluded that the only thing that can stress the tubes is the inrush on cold heaters. The potential damage is minor to most tubes; the inrush is relatively harmless and only of interested to those who want to cover every corner. I did an experiment, and on some 6922s I got peak (cold) currents of about 1amp! Obviously with many tubes in parallel this can seem alarming, but I think the internal resistance of most heater supplies limits this to some extent. But including a current limiting function on the heaters can be beneficial.

Third:
indirectly heated tubes cannot bleed noise to the cathode and circuit if the heater potential is above the cathode potential. This is why it is better to bias the heater to a positive voltage than to gnd the heater. Center tapped heater windings is a take off from the filament days.
Biasing the heater to 20-60volts works wonders on any heater induced noise issues, even on Phono preamps. I use this trick on my gutar amps with more than 80dB gain, and it is just as noise free as DC regulated heaters.

I may be rambling on here, but I find it interesting, and seeing some posts here tells me I shouldn't be the only one.

Please feed back what you know/think/wish/want...cheers
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Old 21st June 2010, 05:50 PM   #2
Wavebourn is online now Wavebourn  United States
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Lets settle the b+ on cold tubes issue!
Quote:
Originally Posted by SemperFi View Post
There is no harm done when applying high voltage to cold tubes!!!
Congratulations!

One more woke up from propaganda-induced dreams.

If you examine datasheets carefully they specify max voltages for hot tubes, for cut-off tubes with high negative bias, and for cold tubes. Max voltage for cold tubes is the highest one. It is given bor the case when the equipment is switched on, and rectified voltage is on it's maximum because no current is drawn yet by cold tubes.
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Last edited by Wavebourn; 21st June 2010 at 05:52 PM.
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Old 21st June 2010, 06:10 PM   #3
TheGimp is offline TheGimp  United States
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At 550V (or thereabout) and above I'd be concerned with cathode stripping. For most tube work I consider it to be a non-issue.
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Old 21st June 2010, 06:20 PM   #4
Svein_B is offline Svein_B  Norway
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I only started to understad cathode stripping and cathode poisoning after reading this paper by Morgan Jones.

Baking valves by Morgan Jones (pdf file hosted by Vacuumstate)

It explains the nature of the electrons, ions, and gas molecules within a tube.


After I started to understand, why should I ever want to install a standby switch or delayed B+.
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Last edited by Svein_B; 21st June 2010 at 06:23 PM.
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Old 21st June 2010, 06:33 PM   #5
Wavebourn is online now Wavebourn  United States
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Lets settle the b+ on cold tubes issue!
Quote:
Originally Posted by TheGimp View Post
At 550V (or thereabout) and above I'd be concerned with cathode stripping. For most tube work I consider it to be a non-issue.
800V B+ on cold GU-50 tubes does not bother me at all. Abrupt B+ switch on like in case of time delay switches is more harmful because of currents that charging coupling caps create grid current spikes that create anode current spikes.

However, magnetrons in radar stations is completely different issue... When they start conducting due to uneven emission from surface of cathode some spots on it start heating faster, that heats them even higher, and so on.

Density of cathode current matters.
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Old 21st June 2010, 07:40 PM   #6
Sch3mat1c is offline Sch3mat1c  United States
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Quote:
Originally Posted by Svein_B View Post
I only started to understad cathode stripping and cathode poisoning after reading this paper by Morgan Jones.

Baking valves by Morgan Jones (pdf file hosted by Vacuumstate)
Wowww, I *hope* that's not actually written by Mr Jones, at least not directly. I have seen few terribly formatted documents, and I am sad to say, this is one of them! Headings are not indicated; Omega seems to have been replaced with Sigma; the font is too small or the column too wide, and I'm willing to bet it was typed in MS Word using only default settings.

I wonder if you know of a better written article? I would like to read it, but these things are just too distracting.

Tim
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Old 21st June 2010, 09:29 PM   #7
SY is offline SY  United States
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Lets settle the b+ on cold tubes issue!
This looks like a poor pdf conversion. On my computer, it appears fine except for the omegas and mus. The photo of the ruined CV1988 was heartbreaking, but I can't blame Morgan's software for that.

I don't think he uses Microsoft Word- far too modern. A quill is more his speed.
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Old 21st June 2010, 10:32 PM   #8
Allen Wright is offline Allen Wright  Switzerland
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Quote:
Originally Posted by SY View Post

I don't think he uses Microsoft Word- far too modern. A quill is more his speed.
He normally uses Word Perfect, and it took years for me to get anything out of him that I could even put up on a screen. I suspect what he finally sent was the WP original converted (badly?) to MSW. Which I then converted to PDF in Mac OSX.

I've done my best, if it offends anyone, too bad - you are invited to do better. But whatever, the data is correct and I consider, very valuable.

Regards, Allen
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Old 21st June 2010, 11:00 PM   #9
jlsem is offline jlsem  United States
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Other than the ∑ instead of Ω it's perfectly fine. It's a slight keyboard error that can be easily overlooked.

John
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Old 21st June 2010, 11:14 PM   #10
Wavebourn is online now Wavebourn  United States
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Lets settle the b+ on cold tubes issue!
It does not matter did MJ write that article, or somebody else. The article is about gas and getter. B+ delay myth is completely different story, totally irrelevant to this one. It is obvious that if to heat up a getter flash gas will be absorbed by it, but if to heat up cathode gas will be absorbed by cathode.
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