My Ampeg Jet reverb has been having this problem for the past month or so. The decay on notes seems quicker than it should be and there's sort of a dirty, bassy distortion as they fade. It happens more with some intervals than others - especially major thirds.
I'm pretty sure it's not a speaker problem as I've had 3 different ones in there. New preamp tubes, still no change. Power tubes are 5 month old GTs (the fancy European ones). Filter caps were just changed. It sounds like an electronic type noise rather than mechanical.
Any ideas?
Amp Prob - quick, dirty decay
Moderated By: mods
If it sounds something like the farty distortion you get if you switch off and then keep playing as the amp discharges (or if you switch off standby before the amp's fully warmed-up), I'd suspect a power supply issue. Not necessarily anything drastic, it might just be a loose or dirty socket. Maybe clean all tube pins and sockets, and re-tension the sockets so they grip the pins more firmly. If you can, check the heater voltage, bias supply voltage, and tube bias. Just a wild guess, but at least you can rule that stuff out afterwards.
It does sound a bit similar to that. Hmmm... When I got it the main ribbon connector bringing the power to the board was broken off. I resoldered it, but it's possible there was a loose connection somewhere else I didn't notice. All the insides were pretty fucked when I got it.
It's cathode biased, so I don't think that's an issue. I'm using middle-rated tubes as well. I put some old Mesa EL84s in tonight and the offending noise lessened.
I'll check the connections and whatnot tomorrow. What should the heater voltage be after the draw of four tubes?
It occurred to me I also run this amp really hard all the time - it's dimed 3 hours at a stretch 2-3 times a week. Is it possible I've just started to kill the tubes already? 5 months is a good while under that type of abuse.
It's cathode biased, so I don't think that's an issue. I'm using middle-rated tubes as well. I put some old Mesa EL84s in tonight and the offending noise lessened.
I'll check the connections and whatnot tomorrow. What should the heater voltage be after the draw of four tubes?
It occurred to me I also run this amp really hard all the time - it's dimed 3 hours at a stretch 2-3 times a week. Is it possible I've just started to kill the tubes already? 5 months is a good while under that type of abuse.
If this schematic matches your amp, it looks like the heaters are being provided with 8.6 volts AC "peak-to-peak" during use (test points 13 & 14). I think this is actually 4.3 volts AC, because it's alternating between +4.3V and -4.3V to get that 8.6 number. I've always heard 6.3V as the magic number for heaters, but there you have it. Maybe it's 6.3V unloaded, and 4.3V under load -- another guess.
BTW, the electrolytic bypass caps on the tube cathodes should be changed along with the filter caps. If one of these goes bad, it might conceivably cause what you're hearing.
The output tubes won't get damage from dime-age unless they're biased abnormally hot.
BTW, the electrolytic bypass caps on the tube cathodes should be changed along with the filter caps. If one of these goes bad, it might conceivably cause what you're hearing.
The output tubes won't get damage from dime-age unless they're biased abnormally hot.
OMG Thank you for finding that schematic - have been looking for MONTHS.filtercap wrote:If this schematic matches your amp, it looks like the heaters are being provided with 8.6 volts AC "peak-to-peak" during use (test points 13 & 14). I think this is actually 4.3 volts AC, because it's alternating between +4.3V and -4.3V to get that 8.6 number. I've always heard 6.3V as the magic number for heaters, but there you have it. Maybe it's 6.3V unloaded, and 4.3V under load -- another guess.
BTW, the electrolytic bypass caps on the tube cathodes should be changed along with the filter caps. If one of these goes bad, it might conceivably cause what you're hearing.
The output tubes won't get damage from dime-age unless they're biased abnormally hot.
the 6.3v figure is unloaded, so 4.3v makes sense - will check.
I didn't change the cathode bypass caps cause they look fine - I really should, though. I need to make a Small Bear order anyway, so I'll swap those out and call it maintenance.
Thank you for for all the help, guys!
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That explains it, thanks. That Ampeg schematic is interesting, because it's really set up for troubleshooting with an oscilloscope. They even show the wave shapes you should be seeing at various test points.
Good about the filament supply voltage, because that means the supply winding on the transformer is OK. As long as the rest of the heater circuit (wires/sockets/pins/filaments) is clean & solid, that'll rule that out.
That explains it, thanks. That Ampeg schematic is interesting, because it's really set up for troubleshooting with an oscilloscope. They even show the wave shapes you should be seeing at various test points.
Good about the filament supply voltage, because that means the supply winding on the transformer is OK. As long as the rest of the heater circuit (wires/sockets/pins/filaments) is clean & solid, that'll rule that out.
It's a moot point now, because you've arrived at more or less the same fix on your own -- but I was going to say, when troubleshooting an amplifier, the following usually works:
1. Snip the heavy, boxy part of the amp off the power cord.
2. Thread a different boxy part onto the power cord, and solder in place.
3. Switch on and test.
Whenever you've got a mysterious amp problem, 99 percent of the time it turns out that the boxy part's gone bad. It's the Achilles' heel of virtually all amplifiers.
1. Snip the heavy, boxy part of the amp off the power cord.
2. Thread a different boxy part onto the power cord, and solder in place.
3. Switch on and test.
Whenever you've got a mysterious amp problem, 99 percent of the time it turns out that the boxy part's gone bad. It's the Achilles' heel of virtually all amplifiers.
I've checked most bits of the Ampeg now. I'm gonna replace all the electrolytics (cause they all look dodgy) and get some more rugged mil-spec tubes. That should probably fix it. A little research: the caps are in the same age range and maker as the infamous "capacitor plague".
The Yamaha is awesome - was able to tweak the EQ so it sounds cool and buzzy with my fuzz face. My band is about to get very psychedelic!
My house has terrible wiring, so my '57 Dano always had a loud 60-cycle buzz. I dialed the parametric to the narrowest "Q" I could and sucked that buzz right out! It's like paradise now.
The Yamaha is awesome - was able to tweak the EQ so it sounds cool and buzzy with my fuzz face. My band is about to get very psychedelic!
My house has terrible wiring, so my '57 Dano always had a loud 60-cycle buzz. I dialed the parametric to the narrowest "Q" I could and sucked that buzz right out! It's like paradise now.