Volume drop after warm-up

[walker]

I've noticed a strange occurrence with my tube preamp. (w/ 12AX7s) When I first power up, everything's all hunky-dory initially, but after about 20 minutes, I notice that the volume has dropped by about 20% at the gain stage. (Loss of grit is noticeable - it's not just a matter of lower volume of the same gain). Does this just mean that my tubes are bad? Or could there be a problem with the voltage regulator? (If there is such a thing in my preamp) I've gone through many sets of tubes before and never noticed this power drop even with tubes that I had used well beyond their average life expectancy.

[johnallg]

I've noticed a strange occurrence with my tube preamp. (w/ 12AX7s) When I first power up, everything's all hunky-dory initially, but after about 20 minutes, I notice that the volume has dropped by about 20% at the gain stage. (Loss of grit is noticeable - it's not just a matter of lower volume of the same gain). Does this just mean that my tubes are bad? Or could there be a problem with the voltage regulator? (If there is such a thing in my preamp) I've gone through many sets of tubes before and never noticed this power drop even with tubes that I had used well beyond their average life expectancy.

Check the driver tube(s). I'd wonder about that one.

[walker]

Is there a tube checker tool/device to do that?

[johnallg]

[walker]

Holy krap! Why, I'm sure that can't be expensive! And I'm sure it's super simple to learn how to use!

[johnp]

There's two things that could be happening here. One is your preamp valves are going a bit sick, and the other is your biasing may have some temperature dependency. Have you tried changing the preamp valves (one at a time)?

Cheers
John

[walker]

Forgive my novice understanding, John, but are "valves" what the tubes are seated in? And I'm not sure about the biasing as it relates to temperature dependency. All I know is that I've never altered the bias since I've owned it. But the preamp does get pretty hot because of the 12AX7s, regardless of the room temperature.

[cjj]

"Valves" are what folks from the UK call "tubes". It's short for Thermionic Valves, which is a reasonable explanation of what they do, regulate the flow of electrons similar to a water valve.

The bias voltage could be temperature dependent, especially if it's regulation is controlled by another tube that's starting to fail. It's also possible to have things like resistors that start varying their value as they get hot, which can also lead to bias changes with temperature...

[walker]

Derrrr. Thanks, Cjj. I should have known that, but wasn't sure.

Resistors going bad as they heat up is a possibility I'm curious about. Something other than a faulty tube might explain why this is the first time since 1993 that I've had this particular problem. But I'm not ruling out the bad tube/valve option yet. I just put in a new(er) set and am going to test out the unit this afternoon.

[johnp]

Yep valves = tubes. I'm British, so habitually use UK terminology. Or the correct terminology as I like to think of it

Anyway, start with the easy things first, swap out one valve (tube) at a time and see what happens after each change. If the problem goes away you're done, and if not you know where to start looking next.

Amps sitting unused for long periods of time can result in electrolytic capacitors failing, although these tend to go short circuit and melt something.

Also the usually caveats about high voltage apply and there is zero shame about taking your amp to a tech. Being electrocuted is not fun.

Cheers.
John

[cjj]

Not knowing how the circuit is designed makes it hard to guess what could be wrong, but tubes going bad is always a good place to start.

As for the bias being off, it's possible that it has drifted just a bit (due to component changes with age) and is now sitting at a level that works fine while things are cool, but is not really right when everything warms up. So, it's possible that nothing has actually failed, but rather is just in need of a bit of adjustment...

[johnallg]

....It's also possible to have things like resistors that start varying their value as they get hot, which can also lead to bias changes with temperature...

I was going to suggest that also. Also, if you use it a lot and haven't changed tubes since you got it in 1993, that is probably what is wrong. You got one starting to die, probably the heater or cathode.

[walker]

I tested out the preamp with the newer tubes, and after about an hour, it had lost a little "steam", but had only diminished by about 5%, v the 20% with the older tubes. And yes - I have indeed changed the tubes several times since '93! But whenever I've changed out to a new pair, the difference in tone has hardly been distinguishable, like it is when putting on new strings. But I guess the difference btw old & new is more detectable in noise or hiss that may come up over time, or power issues like I just discovered here. But even with this improvement with the tubes, I still think the preamp could stand a good bias tune-up from a pro.

[walker]

And by the way, thanks for the input, guys. That was good insight to have.

[cjj]

Yeah, a bias tune up from time to time is a good thing to do. I'm guessing that there's probably a pot or two in there to make the adjustment, and those being electro-mechanical devices can build up a bit of dirt/corrosion in the wiper contact. This can cause their setting value to go off a bit. Moving the contact around a bit usually gets 'em cleaned up, but without actually measuring things, it's pretty difficult to know if you wiggled 'em and got them back to the right spot...