Yes - though on the face of it the issue of hot spot cathode degeneration might still develop without actual arcing perhaps ?
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Interesting example. Advice to do this is not usually given for rectifiers of this general type. The W in the type number indicates the tube is for military applications and as been specially tested and/or charcaterised to a greater extent than for the standard consumer/industrial type.
RCA 5R4GB data in their databook RC30 does not have this graph, but it does have this advice:
If hot-switching is required choke input circuits are recommended.
The 5R4 (without the W) was considered by RCA and industrial type, not a domestic/consumer type. In industrial and military use, reliability and service life has greater emphasis.
In the absence of any warning in a specific datasheet, I don't think this suggests any HT delay is needed for power tubes intended for audio.
Rectifiers are designed for high perveance - this means very close cathode/filament - anode spacing to minimise voltage drop. Power tubes are made, for various reasons with considerably greater distance from cathode to anode. I don't know the spacing for the 5R4, but from looking at similar tubes it is likely to be around 3 mm at most. A high power audio tube rated for much the same average anode current would be around 15 to 18 mm.
Also, in power tubes, the grid structure reduces the anode's electric field strength in the vicinity of the cathode considerably. All up, the maximum electric field strength at the filament during warm-up in volts/mm in the 5R4 would be of the rough order 30 to 50 times what it would be at the cathode in an audio power tube.
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Yes it might, but from the information at hand I still see nothing sufficiently worrying to warrant a delayed B+
Of course, others may worry themselves about it and just for that reason a delayed B+ will be a good idea for their peace of mind.
Further to my Post #222:
In any case the problem with rectifiers isn't just damage to the cathode surface, which may not be that significant. Its that the gas produced allows an arc to form between the two anodes, increasing power dissipation from 20W or so to 500 W or more, becuase there is nothing other than the transformer to limit the current. So the rectifier is immediately destroyed. And it may damage the transformer and explosively take out the fisrt electrolytic cap as weel.
In any case the problem with rectifiers isn't just damage to the cathode surface, which may not be that significant. Its that the gas produced allows an arc to form between the two anodes, increasing power dissipation from 20W or so to 500 W or more, becuase there is nothing other than the transformer to limit the current. So the rectifier is immediately destroyed. And it may damage the transformer and explosively take out the fisrt electrolytic cap as weel.
An extreme example is the 5V4GA which has a punishingly small anode-cathode spacing of 0.5mm, and can operate with a peak blocking voltage of 1400V. The data sheet does not mention pre-heating but only states the usual "If hot-switching is required in operation, the use of choke input circuits is recommended." So nothing new there. Along similar lines, the RCA Transmitting Tubes manual No. 4 (p65) states that receiving tubes do not require pre-heating. http://www.tubebooks.org/tubedata/tt4.pdf
In other words, a current limit in the external circuit is required to prevent destructive sparking during warm-up.
The sparking behaviour of the 5R4G series is very easy to demonstrate. I have seen it a number of times: the arc flows from a point near the tip of the filament to the anode.
Anyone can easily verify it themselves, even at normal voltages. I am surprised, Keit, that you have not seen it yourself.
Set-up where I have seen it most often:
Trafo 370-0-370 with low impedance windings.
Output cap 68uF 500V
dc Load: eg 1x KT88 running 60mA, already warmed up.
Apply the rectifier voltages together, and admire the A->K spark that occurs within the first second. If it doesn't do it the first time, it soon will.
If a 200-700 ohm series resistor is added in the anode leads, no sparking at all.
OK the test will cost you a 5R4GY - but I can't believe I am the only one here to have already seen it.
68uF is well beyond the rated cap value, I know, but this is a demonstration of the effect of instantaneous excessive current demand during warm-up.
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You've misunderstood. By Hot Switching, RCA mean powering up with the filament already hot. As with the standby switch in the transformer input provided in high power public address amplifiers, with a separate heater/filament transformer. In this situation, the filter input electrolytic tries to draw massive peak current. When powering up with a cold filament, this massive peak doesn't occur - the rectifier filament warmup brings the capacitor up gently.
Thus, the advice is driven by a completely different problem to powering up without HT delay.
There's several good reasons why I've never seen it particularly in a 5R4, though I have certainly read about it:-
1) The 5R4 was not made in my country, Australia, and is a rare tube here. I've seen it in professional equipement though.
2) As a professional engineer, I have always followed manufacturer's instructions - ensure there is sufficient resistance in the transformer to meet the datasheet specified minimum, never use a filter input capacitor larger than permitted by the tube manufacturer. That's what Pro's do - if nothing else it prevents getting in trouble with the boss!
3) I began my career in electronics in the late 1950's, not long before solid state rectifiers took over in consumer audio, and in much professional equipment.
Your test with a 68 uF cap is so far outside the datasheet permitted maximum (20 uF) it's not funny.
I recall a problem that happened from time to time with a certain early Tektronix storage CRO. I can't remember the type number. An accidental dead short (as in a slipped multimeter probe) on the raw HT would instantly write off the rectifier tube (similar to 5R4). It wasn't from arcing though. Nor cathode stripping. The heavy current surge apparently took out the internal cathode lead like a blowing a fuse.
If that were true, it would be completely at odds with the (much more explicit) instructions in the 5R4WG data sheet.
In the 5R4WG DS we are instructed to preheat for ten seconds before applying a heavy load There can be no different interpretations about that.
Therefore, I suggest that you are misinterpreting what "hot switching" means. you'll have to explain the discrepancy with the 5R4WG data sheet, otherwise.
Actually, the maximum capacitance allowed for the 5R4GY is 4uF.
This observation did not take place in equipment for a professional application, though I am a design professional.
It was actually an experimental piece of tinkering, using a chassis designed elsewhere.
Think of it as "Destructive testing" if you have to put things into a jobs-worth box.
28MV/m, wow. Yes that RCA advice is confined to sequencing of non-receiving rectifiers. I suppose that is a function of operating current and blocking potential - though Harmmann/Waganer Vol1 pp106-118 describes sparking process as high operating current@low cathode temperatures kicking-off changes to both cathode surface and gassing which are the seed for a physical arc. And the physical cathode-anode arc can happen later during blocking (presumably) or subsequent turn-on.
I suppose one issue is 'can conditions for cathode hot spot development occur during warm-up of receiving valves?' ie high operating current and low cathode temperature. Much seems to depend on what current density say an audio power valve cathode is operated at power up, versus say a rectifier? But dc/transient conditions during warm-up seem worth looking out for I think in any event, which is good practice anyway. And many things have to come together for the phenomenum to happen, not least the cathode construction itself, so even if external conditions are fertile it still might not happen.
Once 'armed', whether arcing physically occurs at cold start is a matter of whether fields ever get strong enough due to spacing, cold B+ and current limiting. What are physical a-k spacings in common power valves, BTW ? Does it vary with grade markings ?
There seems a possibility again that this is a source of advice to sequence B+/heaters over the years domestically perhaps. Whether or not it has a basis in practice is a different matter. I have to say it seems plausible to me, under some circumstances, that it might apply in practice to receiving valves such as power triodes/pentodes especially where transient conditions at power up support high initial operating current.
There's no descrepancy. If you want to hot switch, as RCA calls it, a 5R4GB you advised to use a choke input filter. If you want to cold switch a 5R4WGB, to have some HT delay. If you are operating it in Area II, you are advised the delay must be at least 10 seconds.
As I said before, the W suffix means its made for military applications. Generally this means special or more comprehensive testing, but can means changes to the materials used in the tube. In other words a 5R4WGB is not the same as a 5R4GB.
I don't know what specific changes there are in this case, but I do know that in many cases the directly heated W code tubes have a pure tungsten filament (with the oxide coating) whereas non-W tubes had cheaper tungsten-molybdenum alloy filament wire.
Accodring to my copy of RCA Tube Databook RC30-1975, under heading 5R4GB on page 147, it says the maximum permitted filter input is 20 uF, unless the plate supply impedance is sufficient to limit the peak current to the maximum rated value. RCA30 and the previous RC26 have no data for a 5R4GY or 5R4GYB, as they are discontinued types.
My Sylvania 1951 databook lists 5R4GY, and has the same advice except that it cites 4 uF. As does my 1962 Miniwatt (Philips) catalog.
You can easily estimate a-k spacings by visual inspection, and for commom power valves it's in the region 10 to 18 mm or so.
However, for a power valve, the A-K spacing is not directly relavent to dicussing stress on cathodes, because the presence of grids means that the electric field strength seen in the vicinity of the cathode is very very small compared to what it would be without the grids. This applies both during normal operation and during warm up.
In fact, the electric field strength in the vicinity of the cathode may well be near zero, if not in opposite sense to the anode field once the cathode temperature comes up to the design value. The only reason electrons make it to the grid is due to their inertia they acquired before leaving the cathode surface.
A-K spacing, in general, does not vary with grade markings. That would alter the tube characteristics, which would require a complete new tube type number.
A complicating factor in this is that tubes whose original design was a pentode sometime got manufactured by other manufacturers as a beam tetrode (or a pentode if the original design was a beam tetrode). So long as the tube still meets the datasheet specifications, manufacturers are free to do that, and it will change the spacings. The most famous example of this is the battery power tube 3V4. As originally designed in 1938 by RCA, its a pentode. But all the 3V4's I've ever seen are actually beam tetrodes.
I've alsoo seen the odd 12AT7s that looks very different to the common American and European brands.
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There's a typo on that particular data sheet. It's supposed to say operation in Area I operation is permissable without filament preheating (see other data sheets for the same tube, e.g. http://www.r-type.org/pdfs/5r4gy.pdf).
Rod, the meaning of 'hot switching' is well understood and explained in the literature; it always meant switching on the HT when the cathode is already heated. And you are right, it is at odds with the 5R4WG advice for pre-heating! Hmm..
2.8MV/m, surely?
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There is a discrepancy, as Merlin has just noted.
Choke input Filters substantially reduce the peak current for a given dc load.
What I can say for certain, is that a 5R4WGY, or 5R4GY with a warmed-up load, and excessive output-capacitance - will spark visibly if anode voltage is applied without preheating, and not otherwise.
On the face of it, yes, as the datasheet Merlimb posted is labelled 5R4GY. It is strange that the graph was not included in either edition of the RCA databook RC- series. Presumably then if it's a 4R4WGB or 5R4GY, you must have a delay. If its a 5R4GB you should not have a delay. Either that or it would sem that you are nor suppose to operate it in Region II at all, as either you will violate the injunction against cold switching, or the injuction against hot switching. I suspect the inclusion of the Area II injuction in the 5R4GY datasheet was an error. It is the option that makes the most sense.
That's why they tell you to use choke input filters. Fundamentally, tube rectifier with oxide coasted filaments or cathodes don't like capacitor loads.
Thanks, yes as we've been discussing as to fields. The arc would be anode-grid in a triode/pentode I think? To the grid post if lucky, but arcs are surely bad in a triode/pentode. But yes, the effect of the grid would surely mitigate electrostatic tearing of the coating in triodes/pentodes as a mechanism, I agree.
Physical arcing is the last stage of the disease, apparently. I suppose gassing and cathode degeneration due to hot spotting alone might be chronic in the long term. Depends on so many things I think its hard to generalise as to extent, if any it happens at all in practice. But I get why the advice for sequencing rectifiers might have been transcribed and applied to oxide cathode power triodes/pentodes, if current density of cathodes can be similar at start-up, and there is a perceived possibility of degeneration associated with hot spots anyway?
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Interesting ! - is this cathode arcing the reason why these valve rectifiers have limits on the capacitance they can drive ? - I had thought it was to do with peak current limits.
Yes, the capacitance limit is determined by the peak current.
When fully warmed up, the heat dissipated in the rectifier is determined by the rms current through it. The rms current is higher for large peak currents, even when the dc load is the same. Increasing the cap value increases the peak currents.
But you can use larger cap values, provided you increase the resistance in the anode circuit, to limit the peak currents to the design value.
That's one limitation.
A separate limitation is the amount of peak current the rectifiers can withstand during warm-up, without actually arcing.
Overall: if you exceed the rms (or repetitive peak) current only while the filament is hot, you'll probably risk overheating the rectifier.
If you exceed a critical value when the filament is below working temperature you'll get arc-over.
A serious arc-over event renders the rectifier useless.
A less serious arc makes the rectifier weak and given to arcing in the same position as before (from actual experience).
The only data sheets which specify pre-heating in Area I and II are the 5R4WGA and WGB:
http://www.shinjo.info/frank/sheets/127/5/5R4WGB.pdf
http://www.shinjo.info/frank/sheets/127/5/5R4WGA.pdf
I suspect this was simply a typo "with" instead of "without".
The 5R4GY, GYA and GYB specify pre-heating only in Area II.
http://www.shinjo.info/frank/sheets/127/5/5R4GYA.pdf
http://www.tubebooks.org/tubedata/HB-3/Receiving-Type_Industrial_Tubes/5R4GYB.PDF
http://www.shinjo.info/frank/sheets/127/5/5R4GY.pdf
http://www.shinjo.info/frank/sheets/074/5/5R4GY.pdf
That is very interesting. Is it reversible ? Does it happen straight away or after how many starts ? What is the dc load current/blocking voltage ? Is Ia limited ? Thx.
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