another failing tube question

[michael_dba]

My JMP clone is starting to make some low freq grumblings about every 2-3 min, bassy static, for just a few seconds. Not really loud, more background.

It also does this when I turn the power switch off and leave the stand-by on as the amp drains/fades away.

The power tubes have red-plated in the past (bias way too high) so they got some wear on them.

Any way to tell if this is a preamp tube or power tube without starting to swap tubes one at a time?

[surfnorthwest]

You can tap on the preamp tubes with a pencil eraser head, if there is a bad one you will know it. However what you describe is classic power tube issue. I am seeing a lot of this of late. Not only in forum posts but from people I deal with. Tubes today are really hit and miss.

[michael_dba]

Thanks for the tip Surf.

I just got done messing around with another chassis and finally got it all buttoned up in the cab....wasn't itching to get into another chassis but I guess it's giving me a warning it's gonna take a dump.

[michael_dba]

Hey Surf, I switched cabs, this one is 16ohm....I don't hear any of that noise when the amp is switched to 16 instead of 8ohms....strange.

Maybe the negative feedback tap is on the 8ohm tap...?

I'll switch power tubes next week and check it out again.

[surfnorthwest]

Hmm, I am just not technical enough to answer that, it would be a guess.

The good thing is that the amp stopped doing it. I would leave it be unless you do not like that 16ohm cabinet.

[ClassicRock88]

I have been having a similar problem. It is like a very soft, low frequency crackling sound. Not the "frying bacon" bad preamp tube sound, but more like a soft hiss with the occasional muffled popping sound. I thought it was just my JCM 800, because it tends to eat preamp tubes (I assume that this is the gain mod that is the cause). However, I noticed that the new Granger was doing the same thing. The common denominator is the cabinet. Both amps were running through my old G12-65 cab. As far as I can tell it is a 1981 cab, so it is getting up there. When I took the back off it looked perfect inside. I am using a good quality oxygen free speaker lead to connect it. The only things that I can think of are... it is technically a 15 ohm cab. Could the slight mismatch be bothering the amps? Because the output impedance and speaker impedance supposedly fluctuate as the amp sends a signal and the magnets move within the coils I wouldn't think that a 1 ohm mismatch would upset anything. The other thing I notice is that the input jack on the cab is a bit loose. It isn't so loose that the cable falls out but it is not the positive "clunk" when I push a cable in that my other cabs do.

Any chance that either my noise or Michael's noise is the cab?

[surfnorthwest]

Would be worth a look, especially if its a used cab. I have seen my share of cabs wired wrong and ohm mismatch.

Also the speaker jack has been known to fail.

[ClassicRock88]

Sorry to hijack the thread... What reading would I look for on a multimeter? Since the multimeter is DC I know it will not e 15 ohms.

[michael_dba]

This is a different cab, and a longer cable...those two things changed, so maybe something there.

Also changed the ohm selector setting so that is why i mentioned the NFB tap location possibility.

You description is exactly what mine sounds like when it happens.

If you're in the US you can buy a cheapo digital meter for < $10 from Harbor Freight.

[Anitoli]

Generally a 16 ohm cab will read about 13 on a dc meter an 8 ohm about 6. This shouldnt affect anything as long as the cab matches the amp. By any chance have you checked the tightness of all the mounting screws in the cab?

[ClassicRock88]

It is not the flubbing sound of a loose back panel. It is actually only noticeable when the amp is idling and I'm not playing anything. As soon as I start playing it either goes away or you can't hear it, any more than you can hear regular cable crackle when I move around.

[Anitoli]

Hey Surf, I switched cabs, this one is 16ohm....I don't hear any of that noise when the amp is switched to 16 instead of 8ohms....strange.

Maybe the negative feedback tap is on the 8ohm tap...?

I'll switch power tubes next week and check it out again.

The NFB tap is generally tied to one side of the output coil. It shouldnt matter what tap its on but stranger things have happened. Im curious if the noise would go away with the suspect cab if you installed a new phase inverter?

[Anitoli]

It is not the flubbing sound of a loose back panel. It is actually only noticeable when the amp is idling and I'm not playing anything. As soon as I start playing it either goes away or you can't hear it, any more than you can hear regular cable crackle when I move around.

What youre describing about the low hiss then a pop sounds like you might need to look at the filter capacitors. One could be going. That sound is like a current leak. How long have the caps been in there?

[ClassicRock88]

The 800's caps are about 7 years old I believe. I have had it for about 2 years and the owner told me he thought the caps were 5 years old when I got it.

The Granger I have no idea. I assume that they are however old the amp is but I do not have a way to date it.

Wouldn't it be an odd coincidence for me to have filter caps going out on two separate amps?

[Anitoli]

Are you using a gibson?