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Cathode stripping question.
Cathode stripping question.
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Old 11th June 2018, 09:50 AM   #11
Free grid is offline Free grid  United Kingdom
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Greetings to the forum.

Well well.

Old dogs can learn new tricks!

Thanks for the replies.

Just to recapitulate on the fact supported information that I've gleaned here (and elsewhere). Cathode poisoning and stripping is only a problem in valves with coated cathodes/filaments. Cathode poisoning is only an issue if a valve heater is lit and no inter-electrode current is allowed to flow....over a very, very long period. Cathode stripping, on the other hand occurs when you attempt to draw more current from the space charge than the cathode can support, AND, only then when the valve is being operated at high electrode PD's.

As underrunning a valve heater is not a mainstream technique, there seems to be no accepted method of working out what's good and what's not. A member of another forum answered and stated that once he had achieved zero bias by underrun heaters, he then found at what HT the valve would pass rated anode current, into a shorted load, and used that voltage as a guide to a safe maximum operating at that cathode temperature.... No maths or other evidence were offered, so purely anecdotal.

Quote:
Originally Posted by Markw4
We used to do perveance measurements of thoriated tungsten cathodes which might be the kind of procedure you are interested in? There is a section on this paper on that which might give you some ideas: https://www.illinoisacceleratorinsti...rs/Siqi_Li.pdf
Thanks, would've been interested in reading your link but just got a forbidden message.

Quote:
Originally Posted by pcan
26A7 audio output. They should work at 24V.
I must admit i didn't know this range of devices existed, the 26A7 should give 500mW in PP according to the literature, so will be the choice for this project, thanks!
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Old 11th June 2018, 10:53 AM   #12
DF96 is offline DF96  England
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Quote:
Originally Posted by Free grid
As underrunning a valve heater is not a mainstream technique, there seems to be no accepted method of working out what's good and what's not.
As I said, the reason there is no method of working things out is sample variation. Underrunning heaters has been used for decades for certain purposes, so I would not describe it as 'not mainstream'.
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Old 11th June 2018, 12:16 PM   #13
Free grid is offline Free grid  United Kingdom
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Quote:
Originally Posted by DF96 View Post
....the reason there is no method of working things out is sample variation
Quite so, also I've found that it's not a time stable method either, requiring relatively frequent adjustments as the valve ages.
Quote:
Originally Posted by DF96
....Underrunning heaters has been used for decades for certain purposes, so I would not describe it as 'not mainstream'.
Sorry, for my ignorance The only examples I have come across of heaters being extensively used for altering a circuits operating conditions was in the early days of radio when gain control of amplifiers or regenerative detectors used filament rheostats with tungsten bright emitter filamentary cathode valves. The same methods can be employed with indirectly heated types, but the heating time lag makes that less useful, I'd be most interested in any more recent example of the technique.

Best Regards.
Norman.
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Old 11th June 2018, 01:51 PM   #14
DF96 is offline DF96  England
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Underrunning heaters has been used to improve the small-signal performance of AM envelope detectors. Rarely seen in domestic receivers, but sometimes seen in amateur or professional receivers. I think this was where I first came across it: wondering why an EBC90/6AT6 had a small dropper resistor in series with the heater.
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Old 11th June 2018, 03:42 PM   #15
Markw4 is offline Markw4  United States
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Quote:
Originally Posted by Free grid View Post
Thanks, would've been interested in reading your link but just got a forbidden message.
Strange, but I get the same thing from here. Too big too attach too. I will put it up at dropbox for a few days. Dropbox - Siqi_Li.pdf
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Old 11th June 2018, 04:35 PM   #16
Free grid is offline Free grid  United Kingdom
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Quote:
Originally Posted by Markw4 View Post
.... I will put it up at dropbox for a few days.
Much obliged, thanks!

Norman.
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Old 11th June 2018, 04:52 PM   #17
Free grid is offline Free grid  United Kingdom
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Quote:
Originally Posted by DF96 View Post
Underrunning heaters has been used to improve the small-signal performance of AM envelope detectors.
Oh snap! The forerunner of the Schottky junction!
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Old 14th July 2018, 10:19 PM   #18
Free grid is offline Free grid  United Kingdom
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Greetings to the forum.
Just a quick update...

I've found time to play with this again, and have breadboarded a little PP circuit with two PL81's transformer driven, with an ht of 28volts, and bias controlled by heater power, (14volts, instead of the 21 spec'd, the two heaters in series across the supply ! ), home butchered transformers, driver one gapped for se drive by another PL81 with LED bias. Suboptimal, but already getting near half watt out of clean audio, (ear test) and flat(ish) from 120Hz to 7kHz (no NFB).

Will continue along these lines and if it develops into something nice, I'll post a scheme. Would probably make a nice practice amp?
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Old 15th July 2018, 02:07 AM   #19
ArcticBrew is offline ArcticBrew  United States
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You might consider a boost converter to get a B+ voltage more appropriate for tube amp and a buck converter for the filaments.

They are available on ebay for not too much money.

Steve
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Old 15th July 2018, 04:46 AM   #20
trobbins is offline trobbins  Australia
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You may identify some useful details and references in linked 1955 M.Sc. thesis by Walter Johnson:
https://calhoun.nps.edu/bitstream/ha...pdf?sequence=1
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