what's happening to my amps?

[teleking36]

I have two topics to address, and I could really use some help:

1) I have a Peavey Classic 20 that I've been using as a practice amp for the past 3-4 years. It's a great little amp and hasn't really required much maintenance except for the occasional tube change.

But recently, after a tube swap, I've noticed a problem. The amp starts up fine, and sounds fine. But after about 3-5 minutes of being on, the amp gives off an electrical burning odor. I can pinpoint what this is, as my technical experience with amps is limited. My guess is that it may be the transformer, but I'm not sure.

2) My main rig is a Dr. Z Prescription ES head and Z Best cab. The amp sound incredible and is very loud, but I use an airbrake to tame it most of the time. I normally use pedals for my OD tones, but this amp is designed to have some great natural tube OD, hence the "Overdose" option on the amp. I've just recently been starting to try and get that natural overdrive from the amp, using the Airbrake to manage the volume levels.

Here's the problem. When I crank the amp up around 7-8, with the overdose engaged, and the Airbrake at full attenuation, I get some of the most horrible sounds out of the amp. It sounds like the tubes are being spit out and the speaker cones are being stomped on. It gets worse the more aggressively I play, too.

The amp is loaded with new tubes, with the EL84s being about 2 1/2 months old, and the preamp tubes being anywhere from 6-8 weeks old. I've been playing mostly in my apt. with the volume low, and put in maybe about 5-6 hours a week on it. What could the problem be?

[donnyjaguar]

(1) Are you sure this isn't dust and other chemical residue from the factory being burnt off the new tubes?

(2) I have never used an "airbrake" but it would appear to not be a good match with your amplifier.

[EADGBE]

How does the first amp sound?

Also I'm not a fan of attenuators. As I think they can mess amps up. How about using a distortion or overdrive pedal into the front of the amp instead? A lot of pros have used them to good effect.

[hasserl]

As for the Dr Z & Airbrake combo, I'd bet there is an impedance mimatch. The Airbrake attenuators use a generic impedance load, not specific per amp. I'm not a fan of this approach. When you really load it down the mismatch is causing problems, keep it up and you'll likely be buying an output tranny and/or tube sockets and tubes. I would only use that attenuator to roll off a couple of dbs, and that with out cranking the the amp up. Sorry, that's probably not what you wanted to hear. But if you want to get apartment volume out of that amp with the volume cranked I'd suggest you get a Hotplate or MASS, impedance matched to the amp.

[danieldroukas]

for your Peavey, make sure that the power and output transformers aren't cooking too hot and putting out that odor. Put your hand on the transformer after you've played it for the 3-5 minutes you suggest it takes the odor to appear. The transformer will likely be warm, but if it's HOT you've got trouble.

Otherwise, are your tubes glowing red on the plates? If I recall, the classic 20 is cathode biased. Even though it is cathode biased, you still should check the static dissipation wattage since you retubed it, and make sure that the cathode biasing resistor(s) hasn't(haven't) drifted.

If you don't know hot to read the static dissipation wattage, do the math:
[(voltage on pin 7) - (voltage on pin 3)] x [(voltage on pin 3) / (cathode resistor value in ohms)]. This method is for your EL84 tubes only, and your cathode resistor value should be somewhere between 110-130 Ohms for a 2xEL84 amp. The absolute max you should get is somewhere around 12W per tube, but you should be somewhere lower than this.

If these two aren't the culprit, it may be as simple as burning chemical residue on the tube perhaps (maybe Blue Strat or Timbre Wolf can chime in), or maybe it's cooking something in the power supply. Either way, it'd be best to look into it and be sure.