Greetings to the forum.
With valves, there is always manufacturer data available to refer to with respect maximum anode current for the specified cathode heater power.
Now, put the case that we go off piste and want to run the cathode at a lower (critical) temperature, say, because we want to use a very low HT supply and/or achieve a near zero bias condition, then the question is, is there some empirical method of determining a safe anode current for the new condition, that will avoid cathode damage, ie. "Stripping"?
Any thoughts appreciated, thanks.
With valves, there is always manufacturer data available to refer to with respect maximum anode current for the specified cathode heater power.
Now, put the case that we go off piste and want to run the cathode at a lower (critical) temperature, say, because we want to use a very low HT supply and/or achieve a near zero bias condition, then the question is, is there some empirical method of determining a safe anode current for the new condition, that will avoid cathode damage, ie. "Stripping"?
Any thoughts appreciated, thanks.
^ Thanks for the observation, and yes, I would agree that given an ultra low power application, survival will be good. My application however is at the other end of the signal chain where I am told that even dragging modest power levels from valves with underrun cathodes can damage them permanently. (?)
If you're reducing the plate voltage you negate any of the risk you think is there. No HT, no current.
That's kind of what I'd initially thought, but the naysayers started with talk of "cathode stripping" and "poisoning", the most eloquent opinion was that the space charge around the cathode protected it from bombardment and if it were removed by drawing too much current, then the damage would occur.
Its my belief that that would only happen at high voltages and there must be some kind of safe operating area as found in semiconductor devices.
I think I may just build what I have in mind and spank it hard to see if it dies young.
Thanks.
Norman.
Its my belief that that would only happen at high voltages and there must be some kind of safe operating area as found in semiconductor devices.
I think I may just build what I have in mind and spank it hard to see if it dies young.
Thanks.
Norman.
I have a friend who's grandson is a woodsman and lives off grid. He joked that he would like a boutique valve radio that would run directly from his 24v PV/wind generated supply but give a modest loudspeaker output without using a (noisy) inverter. That sounded like a challenge but has now turned into a wager....the car radio valves may be a good idea for the signal stages Alan, though I've had some success modifying mains circuits and standard valves. The sticking point is audio output, the 12K5 audio driver tetrode is a little disappointing, even two in pp, but, two PL36 in pp with underrun heaters are showing promise.... Though it doesn't mater to the end user, the setup is hideously wasteful of power....even under run the heaters are consuming about 8W with an audio output of around 250mW! Silly project, I know, but doing stuff like this keeps the brain from going soft in an old man.
Norman.
Norman.
The cathode damage would depend on how gassy the valve is, what the gas is, and how much cathode space charge is left in place, as well as how high is your 'high' voltage. As none of these is easily predictable for any particular valve sample it is unlikely that any manufacturer would offer a guess about a 'safe operating area'. All you can do is try it and see what happens.Free grid said:
There was a full lineup of tubes designed for aircraft communications receiver at 26 volts DC power supply, such as Collins R-392. They are: 26D6 pentagrid converter - 26A6 and 26FZ6 rf amplifier - 26C6 audio triode/diode - 26A7 audio output. They should work at 24V. The PL504 sweep tube should also work as output tube at 24-30V B+, and I've read that the 12SX7 low voltage double triode is actually a respecified 6SN7.
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.illinoisacceleratorinstitute.org/2012 Program/student_papers/Siqi_Li.pdf
Regarding gassiness, sometimes it may be possible to blow the gas out of the area where electron flow in the tube is and get the gas to stick to the cooler glass tube envelope for some time. Don't know how well it would work with something like this but it is a routine procedure for warming up x-ray tubes. It involves pulsing them a few times at low anode voltage and current, increasing it a bit and doing it again, until full power is reached. Once warmed up it doesn't have to be done again unless the tube is left sitting idle for a few hours. Of course those are rather different tubes than than small receiving tubes, don't know if anything like that can help with them.
Regarding gassiness, sometimes it may be possible to blow the gas out of the area where electron flow in the tube is and get the gas to stick to the cooler glass tube envelope for some time. Don't know how well it would work with something like this but it is a routine procedure for warming up x-ray tubes. It involves pulsing them a few times at low anode voltage and current, increasing it a bit and doing it again, until full power is reached. Once warmed up it doesn't have to be done again unless the tube is left sitting idle for a few hours. Of course those are rather different tubes than than small receiving tubes, don't know if anything like that can help with them.
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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.
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.
Thanks, would've been interested in reading your link but just got a forbidden message.Markw4 said:
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!pcan said:
Quite so, also I've found that it's not a time stable method either, requiring relatively frequent adjustments as the valve ages.
Sorry, for my ignoranceDF96 said:
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.
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.
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
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?
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?
You may identify some useful details and references in linked 1955 M.Sc. thesis by Walter Johnson:
https://calhoun.nps.edu/bitstream/handle/10945/14102/effectofoxidecoa00john.pdf?sequence=1
https://calhoun.nps.edu/bitstream/handle/10945/14102/effectofoxidecoa00john.pdf?sequence=1
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