A very old story.
Long time ago in the early sixties, we constructed Amps with a
coaxialcavity on UHF 70 cm . We where happy with 4X150A pulls from the army.
We had often extern flash over, inside the cavity, these samples where
throwed away. Untill we find out that tubes which had been cleaned with
alcohol to get some greas of, they never flashed. Even manipulating the
tubes with dirty hands can produce some external flashs over.
Its a very old story.
Could be Ham hands are cleaner nowadays hi hi .
----- Original Message -----
From: "Traian" <firstname.lastname@example.org>
Sent: Monday, May 03, 2004 3:03 PM
Subject: [Amps] power vacuum tube hi-pot and "conditioning"
> Hi all,
> Time ago I was testing the vacuum tubes for leakage current and
> breakdown voltage using the focus connections from CRT monitors,
> so the max. available voltage was 7500V from the very high output
> impedance FBT's focus source. The leakage current for different
> tested NOS 3 kV tubes was beginning to rise at HV levels between
> 6 and 7.5 kV and because of high output impedance, I could never
> get a real breakdown between tube's electrodes.
> I have bought few GU43 power tubes, '79 and '82 manufacture
> date and found that their data sheets specify the allowed storage
> period of time as being 8 years. Found by others local hams
> also, this info was the source for some controversial discussions
> here. For trying to find the right answer, few weeks ago I have built
> a better HV source, using a switching AC HV supply after which I
> added a tripling output rectifier block from an old TV set.
> The tripling rectifier include capacitive filtering and the resulted
> output impedance is much lower now. By varying the PS's output
> switching transformer DC voltage, I can get an output DC HV from
> about 300V to over 17 kV.
> The measurements were made between the anode and closest
> element (screen/suppressor grid), without any other previous
> conditioning of the tubes cathodes/gettering.
> HV was measured directly, by a 30 kV 1000:1 probe and the
> leakage by inserting a 10k 2W resistor on the B- lead (plus
> the needed protective parallel 5.1 V Zener), using two DMMs.
> Now the interesting part...
> I tested again some of the NOS tubes I had, plus others.
> I have tested 3 GU46, 8 pcs of GU74, a GU78, a 4CX1500B,
> SRL460 (4CX250), RS1009, two used 4S040 (4-400C), a used
> At some high voltages, the leakage got high and then few discharges
> could be heard from inside of each of the tested tubes (sparks could
> be seen inside for the low power RS1009 glass tube). After few
> discharges, the tube's breakdown voltage was increasing and the
> leakage went low. For every important increase of the HV level
> (for every few kV up), this phenomenon was repeating for two or
> three times up to very high voltage when no more improvement in
> breakdown HV level could result.
> This was the case for most of the verified vacuum tubes, except for
> the used 4CX1000 which presented no improvement and for the
> used 4S040 (4-400) for which the improvement was less important
> than for the other tubes (the all others were NOS).
> I did not know what was happening inside the tubes, but I suspected
> that somehow a similar process to a "gettering" took place.
> One friend here confirmed my findings from its experience regarding
> the use of NOS low power glass tubes (SRS551/RS1003) for his RF amp.
> Searching the reflector again I have found the following messages:
> These are explaining what was happening and my findings seems to
> confirm the respective statements also. The tripling rectifier's
> filtering seemed to help more for "debarnacling" or "conditioning" the
> tubes (because of higher spark/discharge energy available?!)...
> Most interesting was that two of the NOS GU74 (1985 manufacture)
> could withstand very high voltages of over 13 kV. More than this, they
> did not breakdown internally, but one did it externally at about 14 kV,
> the other one at 15 kV (also outside of the tube), directly from the
> cooler to screen's connection ring. The GU74's data sheet specify
> the limit anode voltage of only 2kV DC and kV peak. For the
> two NOS SRL460 (4CX250B) which have similar specs, (2 kV) the
> breakdown voltages were found to be at about only 50% from
> the GU74's measured ones. These two GU74s are the only tubes
> for which I have found (until now) that are limited by the external
> (outside of the tube) rather that internal breakdown.
> Other results:
> NOS 4CX1500B at 10 kV and leakage not high until close to top HV
> two of the GU43's and the GU78 breaks at about 14 kV,
> third GU43 at 9 kV,
> one GU46 at 12 kV,
> other two GU46 at 14 kV,
> others GU74 at 12 to 14 kV,
> used 4CX1000 - high leakage: 170 uA at 9 kV and no improvement by the
> 500pF/5kv vacuum variable cap at 6 kV and no improvement by
> My findings seems to confirm the results reported by some other
> amateurs which are using the GU74 beyond the specs, at 3kV,
> and the GU84 and GS35s at 4kV with no big problem, and this may be
> a proof for the respective tubes ruggedness...
> Of course, the tube's cathode conditioning and their emission
> may be the subjects for other stories...
> Obviously, some questions arising...
> The conditioning is usually needed for large tubes, but the lower power
> (1k) ones may need it too?!
> May these tubes to need for similar "conditioning" again, after some
> long time of unuse?
> (I think they may do, at least after a long period of unuse, even if
> another test after two weeks has shown here no detectable deterioration
> during this time).
> Can some NOS tubes to arc when first high RF level is aplied because of
> missing such previous "conditioning"?
> I don't want to start a controvercy, but any other such
> experiences/comments/thoughts ?!
> All the Best from Bucharest,
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