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...
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...
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.
The amplifier will still play although distortion at anything above low volumes should be pretty obvious.
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.
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.
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.
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 .
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 .
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.
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.
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.
richj
Buy a new one.
richj
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?)
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?)
ThSpeakerDude88 said: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.
thanks for the tip! I don't think the sockets will have any trouble, most people haven't had problems with this. the GE's that I have are identicle to the EH's, the sylvanias have the longer skinnier pins.
ThSpeakerDude88 said:
Believe me, there HAVE been problems with this. Here is the text of a label I include with every 7868 EH purchase:
Be careful! When installing 7868 tubes be sure to line up the pins and center the tube over the socket. The fit is very tight! A bit of Caig DeOxit or similar product on the pins will ease installation. Without careful installation the tube glass can be broken!
EH tube pins measure about .050" - USA old stock (including GE) measure about .040" - I just re-measured to be sure. That's a 25% difference, and that's an issue.
As I said, the EH are fine, and as long as you don't feel you'll go back to old stock the stretched out socket is not a problem. But you really can't go back.
Well this is an update.
I just received a minty Knight KG-600A tester from a friend and recalibrated it. (wasn't off much.) I went through it and checked all resistors, which were apparently gold-band 5% and they all tested fine. I checked the only capacitor in there and it was fine as well.
So I went through and retested all my tubes and they seem to test pretty accuratly, so I decided to try this particular 7868 again.
Well this time I am getting a reading for a good tube, meter reads around 80 and theres no gas. I checked for all shorts and other than one bad pin ( 1 and 7 are usually tied together for control grid) it tested fine. You can visibly see that pin 7 has become disconnected inside the tube, but pin 1 is still connected. It is my thought that maybe the B&k 700 used pin 7 of this tube to test for GM and if it was missing ( due to being burnt) it would not give me a reading? This would also explain why it seems to function in an amplifier just fine.
I am wondering also, if when that pin went bad, that the peice of metal could be floating around in the tube and shorted something to cause a bad reading, and now the peice has become dislodged and is no longer shorting?
I've been running this amp through a pair of hammond 1650F's and the power output is a lot better than the dinky transformers the original amp had. These transformers have taps for UL so I think I will try what rich walters said by running it in UL.
I just received a minty Knight KG-600A tester from a friend and recalibrated it. (wasn't off much.) I went through it and checked all resistors, which were apparently gold-band 5% and they all tested fine. I checked the only capacitor in there and it was fine as well.
So I went through and retested all my tubes and they seem to test pretty accuratly, so I decided to try this particular 7868 again.
Well this time I am getting a reading for a good tube, meter reads around 80 and theres no gas. I checked for all shorts and other than one bad pin ( 1 and 7 are usually tied together for control grid) it tested fine. You can visibly see that pin 7 has become disconnected inside the tube, but pin 1 is still connected. It is my thought that maybe the B&k 700 used pin 7 of this tube to test for GM and if it was missing ( due to being burnt) it would not give me a reading? This would also explain why it seems to function in an amplifier just fine.
I am wondering also, if when that pin went bad, that the peice of metal could be floating around in the tube and shorted something to cause a bad reading, and now the peice has become dislodged and is no longer shorting?
I've been running this amp through a pair of hammond 1650F's and the power output is a lot better than the dinky transformers the original amp had. These transformers have taps for UL so I think I will try what rich walters said by running it in UL.
well I tried it in the amp again. Blue glow ( like normal glow, only visible in bright daylight..) and the inside has a...yellow plasma cloud? passes DC trhough the speaker and wants to melt the cathode resistor. Its toast 🙁 I am thinking that the screen grid was damaged, but what would cause it to read GOOD on this tube tester? Does this one ground the screen in pentodes and test them as triodes? 
Most tube testers don't test tubes with operating conditions (plate current or voltage) anywhere close to what they actually encounter in service. It is not all that unusual to encounter power tubes that test ok and actually don't work in the amplifier.
- Status
- Not open for further replies.