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Old 15th May 2009, 12:24 AM   #21
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Location: Pretoria, South Africa
Quote:
Originally posted by adamus

Pulled the valve with the bright spot in it..... and there is my black spot. All this after i was promised they were replaced. grrrrr
Good for you, Adamus!
Mistake?? Hmmm .... I should say name and shame about due.

Regarding heaters protruding from cathodes, this is not abnormal, though not the rule. I have had that in (original) Mullards and Telefunkens.

Then, also the heater white spot. I have had that on a few occasions without that valve burning out prematurely, but only during heating up. It disappears gradually as the heater heats and should go down to normal dull orange after say 5 or so seconds. You will notice an insulating covering on the heater where it is visible, and often a 'naked' piece is sticking out where the heater 'turns'. This will light up brightly on turn-on because of the large current drawn initially.

About the 330V, this is not too high. The important thing is the dissipation. EL84s are rated at least 450V absolute maximum on G2 and 600Va by some manufacturers (Philips, Mullard). This is conditional, but at least indicates that no sparking should occur. Electrode separation distances are not that small. Perhaps you could inspect tube construction with a magnifying glass. Neat 'wiring' and spot welding with no sharp points close to each other usually indicate proper manufacture (but sadly not always).

Good luck!
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Old 15th May 2009, 12:26 AM   #22
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Quote:
Originally posted by adamus
bleugh.....

got my replacement tubes..... and one is exhibiting the indredibly bright heater again. instead of the normal orange glow its really whitish yellow in the section you can see through the hole in the plate.
FYI, that glow is not the heater, that's likely the screen grid glowing. Sometimes those can get misaligned by a hard whack in transit.

Also, the heater "exposure" variation has been going on for a very long time, many old stock tubes showed similar variations.

I saw your other post, I hope it was just an error too.
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Old 16th May 2009, 01:26 AM   #23
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Quote:
Originally posted by Jim McShane

.....that's likely the screen grid glowing. Sometimes those can get misaligned by a hard whack in transit.
Jim,

I am slightly puzzled here, but you seem to have had experience. Do you mean that the screen gets misaligned, how? In the EL84 the G1 and G2 pitch is different, so there is not a factor there as with beam tubes. Is that with regard to the cathode then? If so, I thought that the end 'isolators' (mica) would be robust enough and the hole tolerances close enough to firmly anchor whatever it is supporting. (Question, not contradicting you.)

Then slightly off-topic, if Adamus will allow.

We recently encountered a special Genalex KT66 locally, 'specially made (glass envelope) to resemble the classic model - special price too; almost 50% more. Is the present thread a wake-up call to be careful of Genalex in general?

Experience please, other members.

[PS: Apart from these, the only other so-called KT66 available here is a Russian monstrosity in a 'Coke-bottle' envelope, innards exactly a 6L6GC - but at twice the price of 6L6EH. Naturally nobody wants them.]

Thanks, Adamus for allowing.
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Old 16th May 2009, 01:12 PM   #24
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Johan,

Thank you for the kind words, but I'm certainly not the most qualified expert on tube making!

But I do know a couple things:

1. The internals of a tube are fairly easily disturbed. I can take a power tube (new production or old stock), run it on my match rig, and note cathode current. If I then take that tube and give it a whack it will quite often change the current flow - especially if the tube is hot when I do so. I've done this accidentally more than once by dropping a tube, etc. It only takes minuscule movements of internal parts to cause changes in electrical characteristics.

2. Apparently tube screen wires can sag when they heat up; also the support posts themselves can deform.

3. It must be quite difficult to get screen alignment correct in production. Some years back the (then called) Svetlana 6550 had round plate holes which then changed to square. The square holes were designed to allow use of a special fixture in production that allowed precise screen alignment. Before that went into production the Svetlana 6550s were notorious for having hot spots on the plate. It was the screen alignment that was off. This is according to Eric Barbour who was the tech guru for Svetlana's US distributor at the time.

Ei in the last years had terrible screen alignment on the KT-90. It made most of them unusable - or only usable at a fraction of their normal ratings.

So clearly this is an area a tube maker must be extremely careful on.
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Old 17th May 2009, 12:14 AM   #25
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Thanks Jim,

Makes sense, even when sadly so. Yes, one does not always realise how much minute differences in grid construction can influence matters. From that point of view, perhaps it should be appreciated what kind of technology needs to go into construction with the temperatures encountered near a power cathode.
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Old 17th May 2009, 06:19 AM   #26
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Quote:
Originally posted by Poindexter
First, EL84 is a 300V tube, so yes; you are out of rating.
Many modern guitar amps run the EL84 at 350V to 375V and no meltdown.

I've taken several NOS anodes to 550V.. as long as the screen is kept @ 250V or lower and a higher load, magic happens

EL84 max power for HiFi Amp
http://www.diytube.com/phpBB2/viewto...er=asc&start=0
(scroll down to Roger A Modjeski's post)

Cheers!
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Old 19th May 2009, 06:46 AM   #27
adamus is offline adamus  United Kingdom
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the replacement arrived and works beautifully.
They sound very different to the 6p14p's they are replacing.

They sound slightly 'distant', not sure how to explain it properly but the sound is wider and deeper.

which do i prefer.... not sure as yet. The 6p14's had stronger bass, but the gold lions have only just gone in so need some burn in time.
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