New Electro Harmonix 12AT7 in reverb driver (Fender Deluxe), won't hold 410v plate?

[Enzo]

If you are tempted to use steel wool, try a Scothbrite pad first.

[Chuck H]

+1 on the Scotchbright pads. They come in white, green and red. All for different hardness substrates. I think red is the one you really want, but green is so common and cheap I use it for all kinds of stuff all the time. Steel wool is conductive and full of manufacturing grime. Great for some things though. Another great cleaning tool for your bench is a brass wire toothbrush (*note* "toothbrush" denotes the shape-do not brush your teeth with it).

[Enzo]

I have both brass and stainless wire brushes for my Dremel. I use them all the time, and shining oxidized tube feet is one application.

[nosaj]

I have both brass and stainless wire brushes for my Dremel. I use them all the time, and shining oxidized tube feet is one application.

You call them feet I call them udders

nosaj

[mikepukmel]

Thanks for tips on cleaning brushes. I had a brass tooth brush sized thing, have to hunt for it.

Back from short trip. Tried out the old 12AT7, crackles like crazy. <Big frown> Put the replacement in, amp runs OK. Fairly sure it wasn't a bad solder joint since I jammed too many leads into the top of a turret there, and it was MISERABLE to get them unsoldered and pulled out.

Thinking now: bad caps killed tube #1, and would have done so to tube #2 if I had run the amp longer. Maybe.

While doing diagnostic work, I had the replacement tube grid shorted, and the wire I had clipped to the chassis came loose, which let the tube go running wild. Luckily when it happened I had the volt meter on the post, and saw the voltage across the cathode run way up. Shut the amp off before the cathode resistor melted anything, but it got toasty for a few seconds.

I don't have a capacitor tester. Is there a way to test for DC leakage with a meter and an old beat up scope? Id rather not put this cap back on the plate of V1, since it could be what caused the trouble in the first place.

[nickb]

Hook one end of the cap to b+ and then measure the DCV from the other end of the cap to ground. Turn on to allow the reading to settle, ideally the reading should be just about zero for non-electrolytics. Once you are sure its not leaking like crazy you can switch the meter to read current and measure the leakage directly. Electrolytics will require a resistor across the meter to allow them to charge in a reasonable amount of time, say 100K. You can check the manufacturer's data sheet for the expected leakage.

[mikepukmel]

Hook one end of the cap to b+ and then measure the DCV from the other end of the cap to ground. Turn on to allow the reading to settle, ideally the reading should be just about zero for non-electrolytics. Once you are sure its not leaking like crazy you can switch the meter to read current and measure the leakage directly. Electrolytics will require a resistor across the meter to allow them to charge in a reasonable amount of time, say 100K. You can check the manufacturer's data sheet for the expected leakage.

Thanks Nick. If it is leaking a lot, should I start out with some kind of load between the B+ and cap or cap to ground?

[nickb]

Thanks Nick. If it is leaking a lot, should I start out with some kind of load between the B+ and cap or cap to ground?

When on DC volts the meter is the load, something like 1 to 10 meg ohms depending on the meter. When you have a higher current you put a resistor in parallel with the meter and measure the drop across it. If the resistor is less then say 1/10 the meter resistance then you can calculate the current from I=V/R.

BTW, I really should have said that measuring DCV across the resistor is preferable to using the current range in the case where a cap has excessive or intermittent leakage as you run the risk of damaging the meter.

[mikepukmel]

When on DC volts the meter is the load, something like 1 to 10 meg ohms depending on the meter. When you have a higher current you put a resistor in parallel with the meter and measure the drop across it. If the resistor is less then say 1/10 the meter resistance then you can calculate the current from I=V/R.

BTW, I really should have said that measuring DCV across the resistor is preferable to using the current range in the case where a cap has excessive or intermittent leakage as you run the risk of damaging the meter.

Been there. (not with the tube amp, though). Some people learn the hard way.