Tube amp starts fuzzing after a minute.

[KidAmnesia]

I have a tube guitar amp, a Laney cub 10. Great little amp for what I paid for it some 12 years ago and in all this time it had no issues at all.

But now it started acting up. I turn it on, warms up, sounds good for a minute or so and then suddenly the volume drops and it starts to sound very distorted/fuzzy.

I suspect the tubes might be dying but there's no abnormal glow or anything like that in any of them (2 12ax7 and 2 6v6).

I know enough about amps to replace the tubes and to know I shouldn't touch anything inside that could be electrically charged, but is there any point in opening it and looking for any burnt/damged components? Should I get a new set of tubes just to see if that's what's failing? Or is there anything else I can test before throwing money at the problem?

 

[agedhorse]

Not imo.

First, try changing the preamp tubes one at the time and see if one of them is the cause.

If not, you can change both power tubes and try it again.

If neither resolves the problem, I recommend a qualified service tech.

 

[eniac]

I have a tube guitar amp, a Laney cub 10. Great little amp for what I paid for it some 12 years ago and in all this time it had no issues at all.

But now it started acting up. I turn it on, warms up, sounds good for a minute or so and then suddenly the volume drops and it starts to sound very distorted/fuzzy.

I suspect the tubes might be dying but there's no abnormal glow or anything like that in any of them (2 12ax7 and 2 6v6).

I know enough about amps to replace the tubes and to know I shouldn't touch anything inside that could be electrically charged, but is there any point in opening it and looking for any burnt/damged components? Should I get a new set of tubes just to see if that's what's failing? Or is there anything else I can test before throwing money at the problem?

While the amp is off, GENTLY wiggle the tubes in their sockets 2 or 3x, on the suspicion that one or more tube pins or sockets are corroded. Doing so might scrape away enough corrosion to put things right (if corrosion is the problem of course). At the same time make sure the tubes are fully seated. Then power up and see if the problem is still there.

Also, have you tried warming up the amp for 10 minutes w/o playing, and THEN playing through it?

 

[Wasnex]

The schematic I found for the Cub 10 shows it has adjustable fixed bias. So I don't really consider the output tubes user replaceable. I would not install different output tube in it without checking the bias. The amp has no bias indicator circuit or specific test points to check the bias; so setting the bias will require bias probes or a procedure that exposes you to dangerous voltages.

IMHO if neither reseating the tubes nor replacing the preamp tubes fixes the problem, I suggest taking the amp to someone who is qualified to work on it.

 

[KidAmnesia]

Thank you guys, I'll try wiggling and pressing the tubes and letting it warm up without playing.

If that doesn't work I'll take it to someone else.

 

[logdrum]

The schematic I found for the Cub 10 shows it has adjustable fixed bias. So I don't really consider the output tubes user replaceable. I would not install different output tube in it without checking the bias. The amp has no bias indicator circuit or specific test points to check the bias; so setting the bias will require bias probes or a procedure that exposes you to dangerous voltages.

IMHO if neither reseating the tubes nor replacing the preamp tubes fixes the problem, I suggest taking the amp to someone who is qualified to work on it.

I thought the small Laneys are cathode-biased

 

[Wasnex]

I thought the small Laneys are cathode-biased

I don't have any experience with the Laney Cub 10.

Here's a link to the schematic I referenced: https://www.thetubestore.com/lib/th...sfcb19fdGQQOVD3nfWaqg4AX68KZx4VNz1Gg_KjqfuBXK

Here is the bias supply copied from the schematic:

This shows the wire that feeds bias to the bias bleeders (junction between R13 and R28):

The cathodes of V3 and V4 are tied to ground.


Google says The Cub Super 10 is cathode biased. AFAIK this is the current model. Looks like it runs a single EL84.

 

[logdrum]

I don't have any experience with the Laney Cub 10.

Here's a link to the schematic I referenced: https://www.thetubestore.com/lib/thetubestore/schematics/Laney/Laney-Cub-10 (2009)-Schematic.pdf?srsltid=AfmBOoo1ou_sfcb19fdGQQOVD3nfWaqg4AX68KZx4VNz1Gg_KjqfuBXK

Here is the bias supply copied from the schematic:
View attachment 7489448

This shows the wire that feeds bias to the bias bleeders (junction between R13 and R28):
View attachment 7489451
The cathodes of V3 and V4 are tied to ground.


Google says The Cub Super 10 is cathode biased. AFAIK this is the current model. Looks like it runs a single EL84.

Ah I did not see the 6v6, The TGP guys are saying there is an trim pot but yeah you probably need to measure voltages - they do not provide LEDs Many do say that they just get a stout 6V6 like a JJ or EH and all is good. without touching the trim pot. Also I was thinking H and K rather than Laney,

 

[KidAmnesia]

While the amp is off, GENTLY wiggle the tubes in their sockets 2 or 3x, on the suspicion that one or more tube pins or sockets are corroded. Doing so might scrape away enough corrosion to put things right (if corrosion is the problem of course). At the same time make sure the tubes are fully seated. Then power up and see if the problem is still there.

Also, have you tried warming up the amp for 10 minutes w/o playing, and THEN playing through it?

I forgot to report back. I did this and it worked!

I'll take it to a tech anyway because I discovered a different issue. There's a weird background hum that sounds modulated, like if the noise had a flanger on. It doesn't seem to affect the actual signal, and I can only notice it when I stop playing. But it probably indicates the amp is due a proper service.

 

[eniac]

I forgot to report back. I did this and it worked!

I'll take it to a tech anyway because I discovered a different issue. There's a weird background hum that sounds modulated, like if the noise had a flanger on. It doesn't seem to affect the actual signal, and I can only notice it when I stop playing. But it probably indicates the amp is due a proper service.

I agree, wouldn't hurt. Sounds like an easy problem to chase down. Very simple topology.