Delta Blues blew up

[Randall]

A 2011 version Delta Blues. I think it is a redesign of the truly awful three board sandwich nightmare. Anyway, it went up in smoke on stage. I can't find a schematic, but the 1K 5w screen resistor on one of the EL84s got real hot and charred the board real good. It now reads 646 ohms, and has signs of distress. OT passes the neon test, and I find nothing else amiss. My first inclination is the tube shorted, but it doesn't look that way on my recently acquired military surpluss Hickok tester. I mention I just got it because I cannot confirm I have ever seen it detect a short before, so there is that. PLus, the tubes all look very similar and good in the other tests.

So, what to think here? Bad tube or how likely is it that resistor just started to fail and caused the heat issue? I usually expect a resistor to fail by increasing in value, not decreasing.

[Enzo]

So stick a different tube in there. It doesn't have to match. Even if you prefer matched, it need not match to tell you if the other tube was bad or not. Tube testers do not put real world conditions on tubes.

Peavey customer service has all the schematics.

[nosaj]

A 2011 version Delta Blues. I think it is a redesign of the truly awful three board sandwich nightmare. Anyway, it went up in smoke on stage. I can't find a schematic, but the 1K 5w screen resistor on one of the EL84s got real hot and charred the board real good. It now reads 646 ohms, and has signs of distress. OT passes the neon test, and I find nothing else amiss. My first inclination is the tube shorted, but it doesn't look that way on my recently acquired military surpluss Hickok tester. I mention I just got it because I cannot confirm I have ever seen it detect a short before, so there is that. PLus, the tubes all look very similar and good in the other tests.

So, what to think here? Bad tube or how likely is it that resistor just started to fail and caused the heat issue? I usually expect a resistor to fail by increasing in value, not decreasing.

You could always use your DVM to check for shorts where they shouldn't be. Don't use the tube tester till you know you can rely on it. Go through it calibrate it.

nosaj

[Randall]

Jason, shorts don't always show up on a cold out of circuit tube. I bought the tester from a known vendor who went through it and says it is working well. I just haven't experienced a shorted tube myself. Doesn't mean it isn't working.

Enzo, I don't think sticking another tube in there with the charred board and damaged resistor is a good idea. Gotta fix the damage first I reckon. Just wondering what to think here.

Anyone have any other options other than the tube or a self destructing cement resistor? What would make that 5 watt decrease in value?

[dstrat]

Hi, The tube shorted a fried screen R is not that unusual. if your asking why it changed value?
heat? would be my guess, 5w resistor tried to eat more then 5watts.
maybe its all ok or maybe not, replace and throw a tube in.

[Enzo]

You fix the damage of course, but your old tube is probably shot, sub a different one to see if it all works after you repair the charcoal.

Your tester probably works fine.

Tubes fall into three groups: good, maybe, and bad. Tube testers are good at finding BAD. They are not good finding good or maybe. Just like a cap that tests fine at 20v can be awful at 400v, a tube that tests OK at a tube tester can be worthless in a real circuit. it isn;t the tester's fault. You just have to understand its limitations.

[dstrat]

yeah tube testers were mostly for TV repair people seems to me, but as Enzo said not really applicable for a guitar amp.

edit- the best tube tester for guitar amps are guitar amps.

[Randall]

This Hickok TV 3b/U has a 5 position short tester. The very first thing the instructions tell you to do is test for shorts before anything, and then test again at the end of testing. Are these not real world conditions? I don't know what voltages the tester is working with.

[dstrat]

This Hickok TV 3b/U has a 5 position short tester. The very first thing the instructions tell you to do is test for shorts before anything, and then test again at the end of testing. Are these not real world conditions? I don't know what voltages the tester is working with.

I have heard those are good testers, no idea about what kind of voltage it provides to the tube under test, you can find out with a VOM if you wanted.
the grid drive is likely very low compared a guitar amp tho is what I was saying.

[Sowhat]

I've had a few tube testers in my day and as Enzo say's they don't put tubes under real world test conditions. I embarked upon making my own pro grade tube tester based on the British AVO testers from the 1950's. It was quite the chore but even spending over $1000.00 on it the thing still can't do burn in tests, too much power required. My B+ is a little anemic maxing out at just under 400 volts and there's a host of other little nagging problems with it like sensing the AC riding on the B+ without blowing up the mu meter with a shorted grid or something. It does however check everything and every combination for shorts, it's good at that, it's good detecting leaks as well. A really good tube tester is hard to find.

[Enzo]

This Hickok TV 3b/U has a 5 position short tester. The very first thing the instructions tell you to do is test for shorts before anything, and then test again at the end of testing. Are these not real world conditions?

And if the machine says a tube is shorted, I believe it, so toss any shorted tubes.

Real world conditions means putting full voltages on tubes, like 400-500v or whatever. Pentodes should be tested as pentodes, where a lot of testers wire the screen to the plate and test it like a triode for emission. We use tubes with large idle currents, that whole 70% thing and all. Your tube tester does none of that.

Imagine working on a car. In the garage you can start the engine, and rev it to see that it works. You could lift the driven wheels and put her in gear and make the tires spin. You can measure alignment. But that is not remotely the same as driving the car around the block and a couple miles down the interstate.

I have a tube tester. I get it out when I get a box of random tubes. I use it to weed out clearly bad tubes. But until I hear the tube work in a circuit, I never trust the "good" ones. There are very good top end testers, they are not cheap. But most testers check for emission and shorts. They don't check for microphonics or noise. They never treat power tubes they way your amp does when you crank it up and shred.

[Jazz P Bass]

How do you measure 'screen sag'?

Sometimes you will never know what happened inside a failed power tube because of the power.

Small weld flecks can short something and vaporize in the process.

[Justin Thomas]

I got an Eico 666 with all the original paperwork. Yes, their tube charts were known for being riddled with errors, but I haven't blown up any tubes from that yet. More interesting, though, the previous owner was a guitar player/amp tech, and had made a rather detailed extra chart of all of the popular (and some NOT-so-popular) amp tubes, with new specs for the plate, grid, and screen. Basically, from what I can tell from the tester schematic and theory of operation, it basically jacks up the plate and screen voltages, while adjusting the grid bias for more "real-world-amp-bias" scenarios. I've only been willing to try the former owner's charts on new JJs or Sovteks rather than my NOS tubes, but again, so far so good. I've weeded out a few shorts with it, and some weak preamp tubes.

Justin

[g1]

Many power tube shorts are intermittent. The tube tester is not likely to find these, especially if they are triggered by higher voltages than the tube tester uses.
If you are going to try the short tests with the tube tester, you need to be tapping on the tube while you do the short tests, and that still does not guarantee it will be detected.

[Justin Thomas]

If you are going to try the short tests with the tube tester, you need to be tapping on the tube while you do the short tests, and that still does not guarantee it will be detected.

Just replace "be tapping on the tube while you do the short tests" with "give the entire tester with the tube under test in it the old Enzo Whack," and it might get them all...

Maybe I should rewrite the instruction manuals for tube testers for guitar players, not unlike the guy did for my spec sheets...

Justin