Amp sounds darker after the gig's second set

[BobW]

I have a problem with my 50W ODS/SANO accordian amp. (see files section, based on the dumblelite) After a few hours of playing in the OD mode, the OD experiences a tone shift from a nice bright singing dumble quality to a darker Marshall type tone. It doesn't happen in the clean mode with or w/o PAB. Unfortunately this is the second time it has happened during a gig. This amp has been used for over 30 gigs, however when I try to repeat the problem at home, on the bench, it doesn't cooperate. I have yet to see any voltage shift on any of the filters caps or plates, so any suggestions are welcomed.

[ayan]

I have a problem with my 50W ODS/SANO accordian amp. (see files section, based on the dumblelite) After a few hours of playing in the OD mode, the OD experiences a tone shift from a nice bright singing dumble quality to a darker Marshall type tone. It doesn't happen in the clean mode with or w/o PAB. Unfortunately this is the second time it has happened during a gig. This amp has been used for over 30 gigs, however when I try to repeat the problem at home, on the bench, it doesn't cooperate. I have yet to see any voltage shift on any of the filters caps or plates, so any suggestions are welcomed.

Begs the question: did you replace any tubes after it happened the first time? If so, don't know what to suggest. If not, it might be a tube that goes apes*@#$% on you and then recovers. I would replace V2 (12AX7) and observe the amp's behavior.

Gil

[jaysg]

Something is heating up...probably tubes, as Gil pointed out. Might also want to set a fan behind it, aimed at the tubes...remove the back panel. Relays, not LDRs, right?

[BobW]

Begs the question: did you replace any tubes after it happened the first time? If so, don't know what to suggest. If not, it might be a tube that goes apes*@#$% on you and then recovers. I would replace V2 (12AX7) and observe the amp's behavior.

I tried that already, and it may have fixed the problem, since I was unable to repeat the problem on the bench. thanks Gil.


Since the amp is top loaded and the chassis gets warm to the touch, I agree a fan would help reduce issues, and extend tube life. thanks Jay.

[BobW]

Yes relays. no LDRs.

[rfgordon]

I like to use fans, and my fave is the Silent Eagle, available at places like www.xoxide.com. It's the quietest out there, with an 80mm fan rated at 17 dB, which is pretty darn quiet. The fan blades are dimpled like a golf ball, so they take advantage of the boundary effect for noise reduction by reducing the aerodynamic drag of the blades as they spin. I only wish they made one in AC power version!

[Bob-I]

Another thought.

Speakers can heat up and change charactor. A few years ago I was having some trouble with my PA cabs while playing outside. I thought it was my power amp heating up so I put a fan blowing over a bag of ice and it was cool to the touch. Then I opened up a cab and touched the woofer, it burned my hand it was so hot. I put a fan in the vent and they cooled off and the tone returned.

What are your speakers rated at? Are you pushing them to the edge of their rated power?

[Tubetwang]

Thanks for the fan link Rich!

[BobW]

Bob, I am using the stock 15" Utah speaker that was part of the original amp. Model and wattage unknown. It does have the magnet cover and based on the magnet and voice coil size, it may have been sufficient for that amp but dumblizing it may have pushed it over the edge. Although the speaker was in excellent condition, and sounded smooth, I will check the magnet/frame after another burn-in. I have been kicking around to install a new speaker anyway, and just need to do it regardless of the results.

That was a good suggestion, thanks.

[CaseyJones]

That's your ears, bro.

[d95err]

That's your ears, bro.

That's what I was thinking too (but I didn't dare write it... )

Are you sure the problem with the amp is not just your ears loosing treble sensitivity after being exposed to loud music for a few hours? If it is, then it explains why the problem doesn't appear when you test the amp at home.

EDIT: Then again, if it happens only with this amp, and not with your other amps, then I guess it's the amp after all...

[Bob-I]

Are you sure the problem with the amp is not just your ears loosing treble sensitivity after being exposed to loud music for a few hours? If it is, then it explains why the problem doesn't appear when you test the amp at home.

EDIT: Then again, if it happens only with this amp, and not with your other amps, then I guess it's the amp after all...

Your ears actually lose sensitivity in the range of 4-5000Hz, not the very high frequencies we normally think. We can only detect actual pitch up to about 8000Hz on a good day, so the rest is "sweetening" but we don't hear the actual sound, that for dogs only.

Threshold shift, or your ears getting tired, is more across all of our range so it's unlikely that you're hearing is to blame.

[BobW]

That's your ears, bro.

Nope, Sorry try again,

Although I've been playing for over 35 years, it ain't my ears, since this is the only amp that acts this way. I also religiously use the top end Hearos earplugs during my gigs.

Thanks for all the inputs, will let you all know the results.

[JimiB]

I experienced this recently during a gig and the amp felt really hot. I checked the bias when I got home and it was way hot! It had drifted almost 15mA.

[BobW]

Problem solved. One of the 6L6s drifted up to 40mA after a two hour signal/dummy load test.

Swapped out the pair with a fresh set of JJ 6L6s. Sound sweet again.

Thanks to all for your input.