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Doubts on cathode stripping in tubes
Doubts on cathode stripping in tubes
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Old 13th May 2006, 12:04 AM   #21
ray_moth is offline ray_moth  Indonesia
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@ray_moth: how are your experiences withe 6d22s??????? they're not too expensive and could maybe very well substitute for my py500A's???
My experience, having used them for a couple of years, has been very good. Forward voltage drop is about 15v at 250-300mA. The heater requires 6.3v @1.9A but, according to Svetlana, they will work well even with only 5v. Maximum heater-cathode voltage is 600v, which means the heater can be safely fed from the normal grounded 6.3v supply used for other tubes, if you wish. As you say, this is an inexpensive rectifier and, given its superior ratings in all respects, it makes more sense to me than something like a GZ34 if you need high current.

The 6D22S is a glass-based single diode (i.e. you need 2 of them). It needs a magnoval socket, which is like an octal-sized B9A socket. Its only disadvantage is the top cap cathode connection, which means that you have to be careful to ensure that it can't be touched accidentally. The ceramic-shielded cap supplied for this purpose works well. I bought my 6D22S tubes, sockets and top caps from Amplimo in the Netherlands.
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Old 13th May 2006, 10:46 AM   #22
kathodyne is offline kathodyne  Netherlands
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Thanks Ray-moth!, i had also seen them at amplimo's site....

i'll keep them in mind for replacing the py500A's (42v, 300ma heater) when they are worn out....!
got the sockets etc...

(even on this bad picture you can see the 'blackish' residu around the top caps, sorry don't have a close-up)
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Old 13th May 2006, 01:33 PM   #23
Bandersnatch is offline Bandersnatch  United States
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Quote:
Originally posted by teocc_1308
To confuse this issue a little.
I just read on following

Source: http://yarchive.net/electr/tube_time_delay.html

"One common argument used by tube rectifier aficionados is the so-called cathode stripping effect where high voltage applied to the plate of a tube before the cathode has warmed up can strip the cathode of emitting material.

Unfortunately, this effect only occurs at high voltage, typically above 10
kilovolts. It is not a factor in small receiving or transmitting tubes.
If it really were a problem, it would destroy the tube rectifiers which have plate voltage applied immediately at turn-on.


It's only affect tubes typically >10 KV, while not affecting small signal tubes or transmitting tubes.

Just wondering is there any official research or paper publication to precisely define this concern ?
Thank & regards
That's exactly where I was going...and I am currently running some 40 year old pulls that still test like NOS ones in my amp's ps( they are f32-type GZ34's ). This cathode stripping thing seems like a booogey-man story for frightening small children...
cheers,
Douglas
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Old 13th May 2006, 01:55 PM   #24
SY is offline SY  United States
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Doubts on cathode stripping in tubes
Well, here's what's missing: a wide-ranging statistically valid comparison for some typical usage. Everything published is either very narrow and specialized or purely anecdotal. For me, the long life of rectifiers is a pretty strong strike against the cathode stripping hypothesis at less-than-kV-levels with metal oxide cathodes.

My anecdote is that I've not seen any significant difference in tube life between things I've built/owned that turned on rapidly versus slowly. If something ate tubes (and I built a LOT of gear that ate tubes!), it always turned out to be for a different reason.

I say this just as I finished doing some comparative measurements in my EL84 amp using a set of new JJ EL84 and some ancient United Electron 6BQ5/EL84 (date code 1966). The UEs were pulls from a salvaged Dynaco SCA35, solid state rectification, instant on- folks my age will fondly remember the mechanical thump at switch-on. They'd seen a lot of on and off in the twenty years or so in that amp. On test, they showed the same power out and quite similar 1kHz THD compared with the new JJs. The old tubes' idle current was a bit higher at the same bias voltage. Cathodes would seem to be intact.

Sample of one. But still, one more data point.
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Old 13th May 2006, 06:00 PM   #25
tubetvr is offline tubetvr
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Hi

As most others members contributing to this thread I am doubting, that cathode stripping happens during warm up. If it would be the case many tubes would have a shortened life and practically all tube amplifiers using solid state rectifiers would have short tube life which is as I understand not a common experience.

However, cathode stripping can happen in other cases like when the cathode current rating is exceeded, it seems to be so that different tubes have very varying margins when it comes to cathode emission and some tubes seem to be operating very close to safe values.

My father who at one time was responsible for maintaining the tube database for the Swedish navy, (maintaining which tubes that was recommended to use and recommended operating conditions) one time experienced the phenomena of cathode stripping. The tube in question was 807 and it was used in a public address system aboard some newly built battle cruisers, (this was in the early fifties).

The amplifier was designed to operate in a certain load impedance which of course a cathode current within safe range, (class B). What happened was that some one who was responible for the public address system, (but obviously not with any knowledge of electronics) added more speakers in parallell reducing the load impedance until the point where probably the anodes was glowing red and the cathode surface started to break up.

The amplifiers failed after very short time in use and the cause of the problem was investigated, during this investigation some tubes was dismantled and photos was taken of the damaged cathodes. I have somewhere one of these photos but due to me moving around I have currently no idea where it is, If I find I will scan it and post it here.

Regarding different tubes having varying safe margins I believe that 6C33C is a good representative of a tube with very good margins, the tube is designed for a continous max current of 600mA but I have run this tube at 2.5A peak current, (BTW this is not the saturating limit) without any ill effects. The heater power of a 807 is 5.7W and the safe operating current is 125mA, so the relative heater power is 0.0456W/mA, for the 6C33C the heater power is 41W and a cathode current is 600mA so relative heater power is 0.068W/mA this difference together with the fact that 6C33C is a much more modern tube could possibly explain the difference.

Regards Hans
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Old 13th May 2006, 07:10 PM   #26
SY is offline SY  United States
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Doubts on cathode stripping in tubes
One more argument that tubes do not deteriorate from non-delayed B+: the number of data sheets that rate maximum plate and screen voltages for the cold tube.
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Old 14th May 2006, 01:43 AM   #27
ray_moth is offline ray_moth  Indonesia
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Quote:
the number of data sheets that rate maximum plate and screen voltages for the cold tube.
SY, I've never noticed those ratings. Are there any examples on the web?
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Old 14th May 2006, 01:49 AM   #28
SY is offline SY  United States
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Doubts on cathode stripping in tubes
Sure. Look for example at the Mullard data sheet for the EL34. Or Mullard's EL37 datasheet. Or EL84. (All available at Frank's site)

The relevant ratings are denoted by the parenthetical "b" subscript in the maximum ratings sections.
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Old 14th May 2006, 11:11 AM   #29
Burnedfingers is offline Burnedfingers  United States
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Quote:
The amplifier was designed to operate in a certain load impedance which of course a cathode current within safe range, (class B). What happened was that some one who was responible for the public address system, (but obviously not with any knowledge of electronics) added more speakers in parallell reducing the load impedance until the point where probably the anodes was glowing red and the cathode surface started to break up.
Being that I am employed in the commercial sound field I have experienced many instances where the load was way beyond the power capabilities of the amplifier. One such occurance was a newspaper company that had a 70 volt commercial Bogan amplifier with a load of 10 ohms on it. That translates to a 500 watt load. The amp was a 50 watt Bogan. No cathode stripping occured with its 807 output stage.

I repaired the fault which ended up to be an offending speaker down the line without a transformer. The amplifier to this day continues to run, transmitting music and pages. Still has its 807 tubes in it.

My thought is cathode stripping might occur when signal is
applied before warm up has occured.
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Old 14th May 2006, 03:49 PM   #30
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Doubts on cathode stripping in tubes
Afaik, cathode stipping occurs only with directly heated tubes, if it occurs at all. Indirectly heated tubes are rather immune to this disorder, afaik.

I think the issue is that the filament has thorium embedded on the surface, not alloyed into the tungsten. If the thorium is "stripped" from the surface, "she's-a-no-work".

An old timer's ham radio "trick" for tubes with low emission was to overheat the filaments for a short period of time, to bring more active thorium to the surface. It didn't always work. But was worth a shot.

At least this is my foggy understanding...

_-_-bear

PS. fwiw, many transmitting tubes have in their data sheets the requirement to preheat the fils for 60secs or more... they are directly heated. Many do use B+ in the kv range.
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