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Old 31st August 2007, 04:27 PM   #1
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Default Tube flashes on tester... Bad ?

Well I tested a 7868 on my b&K 700 the other day . It has the novar socket on the board where you have to dial in the individual pins. I had it set wrong and turned it on by accident...

The tube Flashed brightly, and now it doesn't read anything! The other 7868's still read good..

I popped it back in the amp and it still appears to work though... What could I have possibly done? The amp is AB1, but can AB1 operate on one tube and still put out sound that doesnt sound distorted? The flash looked to me almost like the screen grid burning out. From my experience, a bad screen grid obviously wont allow the tube to conduct...
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Old 31st August 2007, 04:32 PM   #2
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Sounds like you burned out the filament actually. I'd start looking for another 7868 now..

The amplifier will still play although distortion at anything above low volumes should be pretty obvious.
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Old 31st August 2007, 04:33 PM   #3
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Thats what I thought, however the filiment still lights up just fine.
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Old 31st August 2007, 05:18 PM   #4
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HMMM, that is odd, perhaps it did arc internally and you may be right that the screen is burned out.. Some testers actually do use the screen as the plate for testing power tetrodes and pentodes.

If things melted the tube may now be a bit gassy, and bits and pieces of the roached electrode might soon be in a place to cause further mischief. Were it me I'd not use this tube at all under any circumstances.

In any case my recommendation to get a new 7868 - one that preferably is the same brand and vintage as you've currently got, stands.
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Old 1st September 2007, 02:34 AM   #5
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The grid and screen have two terminals each, so there's a jumper wire inside the tube connecting them. If you get heater voltage across this jumper, it may blow! The grid may still have a connection to one pin or the other, so it might work if you amp is wired to match. I've come across a few 7868 tubes with a grid pin open, so I always wire pins 2,6 and 7,1 on the socket together.
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Old 2nd September 2007, 04:09 PM   #6
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Oh! this is making sence! I might have just popped an internal jumper? This would probably explain why the tube still works, and the filiments glow, but the tube does not test good on the tester. I will try wiring those pins together with some jumper wires and throw it on the tester and see if I can get it to register.


BTW: it is an old sylvania... their pretty expensive new ($54 NOS).


REALLY hoping its not bad, as it tested @ 92 on the GM reading.
EDIT: my amp does have the mentioned pins jumpered on the sockets, so this might explain why its working ! Will let you know what I find out .
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Old 2nd September 2007, 05:26 PM   #7
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amp still works... tube still does not register on the meter. The other 3 7868's all test at 97.

I tried jumping 1,7 and 2,6 but it did nothing. The internal connections for the grid appear to be intact. On some of my other tubes some are bad, but as long as one is there the tube still works.
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Old 2nd September 2007, 09:35 PM   #8
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I've also done wrong by turning the screen up higher than the plate voltage on a power pentode and it suddenly flashes. The ironic thing, is checking the tube afterwards with normal volts it appears to behave normal. However, a check with the tube in UL pent amp with signal, the distortion has notched up higher. So screen has been damaged but not enough to upset conditions. A good rap on the glass when hot can often dislodge the busted wire into permanent s/c.
Buy a new one.

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Old 3rd September 2007, 03:25 PM   #9
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aye, I was pretty sure it would come to that, I havent been using it and just decided to see what was up. I'll have to remember to be more careful next time when setting the dials

for the price of NOS , I'm just going to get some EH's. My tubes were a matched quad @ 97 each. The exception being that two are GE and two are sylvania. ( I think GE, was it GE or RCA that had the smooth top?)
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Old 3rd September 2007, 06:51 PM   #10
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Originally posted by ThSpeakerDude88
aye, I was pretty sure it would come to that, I havent been using it and just decided to see what was up. I'll have to remember to be more careful next time when setting the dials

for the price of NOS , I'm just going to get some EH's. My tubes were a matched quad @ 97 each. The exception being that two are GE and two are sylvania. ( I think GE, was it GE or RCA that had the smooth top?)
Be careful! The pins on the EH tubes are larger diameter than the old stock. Once you use the EH the sockets won't make good contact with old stock anymore. I'm not saying don't use them, but I do want you to know what will occur.

Your tubes may have been matched at 97 on the tube tester, but it doesn't mean they were current matched. With the EHs you can get current and Gm matched tubes, and the people I've sent them to are quite pleased with the tone.

On the other hand I do happen to have a couple good used old stock ones here - drop a note if you decide to stay with old stock. One is a Sylvania, one is an RCA.
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