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

[loudthud]

If it's a problem with a cap, there will still be DC if the 12AT7 is removed. If it's a problem with the 12AT7 running too hot causing grid current, the only cure is to lower the grid leak resistor from grid(s) to ground.

[mikepukmel]

If it's a problem with a cap, there will still be DC if the 12AT7 is removed. If it's a problem with the 12AT7 running too hot causing grid current, the only cure is to lower the grid leak resistor from grid(s) to ground.

Thanks. I replaced the 0.02uf cap off V2 pin 6. I have a reason, the cap is one of those "vintage tone" yellow caps. They obviously don't have very good quality control, this cap is pretty grossly misshapen. If you look down one end, its round, but at the other end it has an oval shape, almost to a point at one part It looks like they squeezed the casing when it was being potted. I think these things are some kind of foil hand wrapped into a cylinder, some wires soldered on, then they put it in a plastic tube and shoot some potting material in.

Replaced that, and also replaced the mica cap, restarted and no crackling at all. Reverb on 1 or 10. I can't get the amp up too loud, too late tonight will try next weekend.

THe 12AT7 reverb driver glows a lot brighter than other tubes even with no HV on (standby ON). I measured the heater voltage and its right about 6.8.

I re-measured the grid on the reverb driver, and it was something like a steady 0.05, pretty much in noise range?


So, if this tube is running hot and its causing grid current how can I measure that? The grid leak is 1m, this is fairly standard? If it is a problem, what would you try the first couple of shots, say -10%, -15%?


***
The replacement 0.02uf cap is an orange drop looking thing. I didn't have any more "boutiquey" caps the right size. The amp *is* brighter now. Not unreasonably so, but maybe due to the other problems with that cap this is the way its supposed to sound.

Thanks everyone, for all the help (and fielding my often stupid questions)

[mikepukmel]

Is it possible for a capacitor to arc inside?

[The Dude]

Is it possible for a capacitor to arc inside?

Absolutely.

Edit: Also, on the tube glow, it's not uncommon for tube filaments to have varying degrees of brightness from one tube to another. If the filament is bright, I wouldn't worry about it. If the glow is redplating, that's another story.

[loudthud]

An old RCA 12AT7 is rated at 2.5W per plate so you are well within that spec. Problem is with some tubes coming from who knows where, you can never be sure if the tube can take it or not or how it will bias up. That's why I ask the question about plate dissipation. Anyway, it sounds like you have found the problem.

[Enzo]

So after worrying the tube was somehow overstressed, we find a cap or two at fault. Meaning the tube was happy in the circuit. That is to be expected because plain old 12AT7s from the box have worked fine in this circuit for over 50 years now. If the thing works, stop fixing it.

[mikepukmel]

Thanks Enzo, didn't know, so for me part of the debugging/diagnostic process. Also, you have to admit, tubes made today are a far cry from what they were 50 years ago!

[mikepukmel]

Absolutely.

Edit: Also, on the tube glow, it's not uncommon for tube filaments to have varying degrees of brightness from one tube to another. If the filament is bright, I wouldn't worry about it. If the glow is redplating, that's another story.

It just looks like bright heater as far as I can tell. I will check the bias once it settles in and the smoke and sparks clear (Barrump bump!!!)

What is really scary to me is how bright the GZ34 glows. It is far brighter than the other tubes, but maybe that's due to the construction?

[mikepukmel]

An old RCA 12AT7 is rated at 2.5W per plate so you are well within that spec. Problem is with some tubes coming from who knows where, you can never be sure if the tube can take it or not or how it will bias up. That's why I ask the question about plate dissipation. Anyway, it sounds like you have found the problem.

eek I never even looked up the data sheet. Should have been the first thing.
If its accurate: the link on the tubedepot site to dta sheet for Electro Harmonix 12AT7

Plate dissipation power of each triode, W, not more 3.25
Plate voltage, v, not more 330

But I got some cool vintage tubes in there now

[Chuck H]

Since there was a perceived change in tone it's possible there was a problem with the caps, but, a perceived change in tone may also be ear fatigue resulting from resting your ears between the time of the last listen and the installation of the new caps. I've experienced this many times. So on that note, did the original 12at7 actually fail? It's in the trash now because it doesn't work? If that's the case I think it's possible that the crackle could have been an intermittent fault, likely a cold solder joint that caused the tube to lose bias and fail and the cap replacement inadvertently fixed that.

[pdf64]

It just looks like bright heater as far as I can tell. I will check the bias once it settles in and the smoke and sparks clear (Barrump bump!!!)

What is really scary to me is how bright the GZ34 glows. It is far brighter than the other tubes, but maybe that's due to the construction?

I know it's the first thing we may notice when inspecting the back of a misbehaving amp, but how bright the visible glow from a tube is irrelevant, it's not a specified characteristic, eg there's no bogie value of lumens etc for any of our tube types.
So it really doesn't matter if one glows more brightly than even its type neighbour, nevermind a completely different type.
From that, matched tubes can't be expected to have matching visible glow.
The visible glow is unrelated to any tube characteristic of interest.
What matters is that the heaters get the tube up to operating temperature reasonably quickly; the glass envelope of a power tube may be expected to be too hot to touch, longer than momentarily, within a minute or 2 of powering up; rectifiers within a few seconds.

[mikepukmel]

I know it's the first thing we may notice when inspecting the back of a misbehaving amp, but how bright the visible glow from a tube is irrelevant, it's not a specified characteristic, eg there's no bogie value of lumens etc for any of our tube types.
So it really doesn't matter if one glows more brightly than even its type neighbour, nevermind a completely different type.
From that, matched tubes can't be expected to have matching visible glow.
The visible glow is unrelated to any tube characteristic of interest.
What matters is that the heaters get the tube up to operating temperature reasonably quickly; the glass envelope of a power tube may be expected to be too hot to touch, longer than momentarily, within a minute or 2 of powering up; rectifiers within a few seconds.

Thanks. When things go wrong, my imagination starts running wild.
"What was that sound??"
"Is that the odor of burning bakelite?"
...

[mikepukmel]

Since there was a perceived change in tone it's possible there was a problem with the caps, but, a perceived change in tone may also be ear fatigue resulting from resting your ears between the time of the last listen and the installation of the new caps. I've experienced this many times. So on that note, did the original 12at7 actually fail? It's in the trash now because it doesn't work? If that's the case I think it's possible that the crackle could have been an intermittent fault, likely a cold solder joint that caused the tube to lose bias and fail and the cap replacement inadvertently fixed that.

Hi Chuck, thanks. yeah, my sound memory isn't so good I suppose. I don't think I toasted the old tube. With the first 12AT7 amp had a crackle. Got some replacement tubes in, put one in, still had a crackle, exactly the same. Swapped out two caps, noise gone. I usually err on the over-soldering side, rather than under. I cleaned off all of the turrets with fine steel wool, and cleaned the part leads and turrets with alcohol before soldering. I usually put little GOOT clips on the parts and make sure the solder flows pretty well. That said, Im pretty far from a competent electronics tech, so yeah, could very well have been a bad joint.

Now you have me really curios, thinking about putting the old plate cap back in to see if it was the cause of that fizzle/crackle.
Also, have to check the tube, pretty sure since the amp behaved about the same with either tube, its OK. I don't have a tube tester, though, so can't tell much.

If it was a bad solder joint that ran teh tube bias too high, is there something I can measure on the tube, or its just something Id have to wait to fail?

[nosaj]

Hi Chuck, thanks. yeah, my sound memory isn't so good I suppose. I don't think I toasted the old tube. With the first 12AT7 amp had a crackle. Got some replacement tubes in, put one in, still had a crackle, exactly the same. Swapped out two caps, noise gone. I usually err on the over-soldering side, rather than under. I cleaned off all of the turrets with fine steel wool, and cleaned the part leads and turrets with alcohol before soldering. I usually put little GOOT clips on the parts and make sure the solder flows pretty well. That said, Im pretty far from a competent electronics tech, so yeah, could very well have been a bad joint.

Now you have me really curios, thinking about putting the old plate cap back in to see if it was the cause of that fizzle/crackle.
Also, have to check the tube, pretty sure since the amp behaved about the same with either tube, its OK. I don't have a tube tester, though, so can't tell much.

If it was a bad solder joint that ran teh tube bias too high, is there something I can measure on the tube, or its just something Id have to wait to fail?

You should really be careful using steel wool for anything in electronics, it tends to break into small pieces which can create shorts. Just an FYI.
nosaj

[mikepukmel]

You should really be careful using steel wool for anything in electronics, it tends to break into small pieces which can create shorts. Just an FYI.
nosaj

Thanks, Nosaj. I was careful to clean the turrets before the board was installed into the amp and cleaned it off well before that.