Bad, new 12ax7 - what is actually broken?

[thelonious]

I had just about finished building an amp for a friend and was doing my startup tests on it. I was only getting strange fizzy/RF sounds out of it, and I got that all-too-familiar sinking feeling... you know, the one you get when something is wrong with your new build? Thankfully, it turned out just to be a bad tube in V1. It was a brand new 12ax7. When I measured its voltages, I got 5 volts on one of the plates. On the other side of the plate resistor, the supply voltage was around 250 volts, so something was seriously screwy for the tube to be dropping 245 volts across a 220k plate resistor. When I replaced the tube, all was well. I tested the tube on my tube tester just to see what it would read. One triode read 120 (high good), while the other triode maxed out the needle past good into the red "? area".

This all got me thinking - when we say a tube is "bad", we mean different things by it. This one wasn't reading as shorted or having low transconductance, and it wasn't gassy---it was reading as way past good on the tester, but not functioning in the circuit.

What was actually wrong inside it, and why did that cause the plate voltage to dip so low in the circuit? Why did the tester read ? instead of bad or shorted?

And there's a lesson I learned in this: Test your new builds with known good tubes!

[jojokeo]

Either you testor's "short" test function isn't working or adjusted properly or the tube wasn't shorted to the point that your tester deemed it to be shorted. The internal resistance of your tube was low enough to drop a large amount of voltage similar to a low value resistor would. See if you can return the tube?

[eleventeen]

It's certainly not possible for one half of a dual triode to be gassy and the other half good. Sounds like a defective weld where the pins that go thru the base go to the little metal strip that goes to the plate, in other words, an internal connection is NG.

[thelonious]

Thanks, guys. I sent the tube back to be exchanged for a new one.

Sounds like I better double check my tube tester functions. It's a Jackson 648 I picked up at a flea market, and I've never serviced it.

[HotBluePlates]

... When I measured its voltages, I got 5 volts on one of the plates. On the other side of the plate resistor, the supply voltage was around 250 volts, so something was seriously screwy for the tube to be dropping 245 volts across a 220k plate resistor. ...

You already know what's the problem.

Balloon-Rock Theory:
If you have a device like a triode with a plate load resistor, when the triode draws current the plate node will sit at some voltage between B+ and ground, because current through the triode causes a voltage drop across the plate load, and the resulting plate voltage is something less than B+.

If the triode is open circuit, no current flows through the triode, so no voltage flows through the plate load resistor, and it cannot drop any voltage. The plate node drifts upward like a balloon to the B+ voltage.

If the triode is a short circuit, it is as though the tube plate were connected to ground, and current through the triode is limited only by the plate load resistor (and cathode resistor, if present). The large current drops all or nearly all the supply voltage across the plate load, and the plate node drops like a rock to ground (0v).

You measured 5v at your triode's plate, so somehow it was drawing excessive current and dropped nearly all the supply voltage across the plate load.

... I tested the tube on my tube tester just to see what it would read. One triode read 120 (high good), while the other triode maxed out the needle past good into the red "? area". ...

Ultimately, the meter in ALL tube testers is a current meter.

It may measure current as a result of voltages applied to tube elements (emissions tester) or it may measure a current variation as a result of a step in grid voltage or applied grid signal (static- and dynamic-mutual conductance testers).

Each type measures how much current the tube passes under the tester's operating conditions, and compares that current to an expected value for a good tube, as denoted by the meter calibration.

So your tube passed more current than the expected "good value" on one triode. But you could know this would happen based on your in-circuit measurement of 5v at the plate (too much current).

Exactly why, well that's a different problem.

It's certainly not possible for one half of a dual triode to be gassy and the other half good. ...

Yes, if the problem is excess gas in the tube.

But one triode could have cathode coating contaminating the grid of one triode and not the other; that might lead to grid emission, a loss of bias and excessive current. Or there could be some other fault in the tube such as a relatively low-resistance leakage path where there should be no connection.

Ultimately, There's some kind of contamination or partial short within the tube affecting one triode. Snce you can't get into the tube to fix it, the precise cause may be academic.

[PRR]

> 5 volts on one of the plates. ...the supply voltage was around 250 volts

Plate-cathode short.

I bet if you check the cathode voltage, it was also 5V.

Using Fender values, 100K and 1.5K, a dead-short tube with 250V supply will put 3.7V on the cathode resistor. Your values seem to be different, but simple math will compute the cathode votlage for a dead-short tube. (Were you using 4.5K cathode resistor?)

How does that happen? Loose bits in the guts. Or just made wrong, though you'd think they did a basic do-it-work? test before shipping.