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Ampeg PF 20t not working, halp

S

Southtwin

Nov 11, 2019
55
42
#1
Hey all, I am by no means an amp/electronics guru, but I know enough to isolate issues and occasionally solder. My beloved Ampeg PF-20t recently quit working and I don't know why. I generally run it at very low volume (gain cranked, volume at 1, Caline 10 band eq pedal with gain all the way down) so as to get rich creamy distortion without pissing off nearby neighbors. The last time it was working, I accidentally left the amp on (not standby) for a few hours with instrument volume down before going back to practice room and turning it off.

Ever since, it's had the following status: power switch works (I hear slight normal electrical hum in amp), the red LED in front comes on, but no change when I flip standby switch. No output to speaker cab (verified cab is fine), no signal from DI out (preamp or power amp), none of the tubes glow at all, and LED color doesn't change (I think it's supposed to switch to blue when not on standby, I covered LED with tape long ago and don't remember for sure). Had a ceramic fuse so I can't tell if that's the issue, just ordered glass replacement fuses but I'm assuming it wouldn't come on at all if fuse was blown. I don't currently have a multimeter or tube tester but might be willing to buy if need be. Tried searching the forums but no luck. Any help for next steps or possible culprits is appreciated!
 
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#6
Southtwin said:
If the answer is "sounds like you should switch out your tubes" or "replace easily accessible transformer(s)" that's a pretty simple and non-dangerous fix I can do. I have no intention of getting into the PCB or messing with capacitors, if it's that bad I'll call it a loss and sell for parts. Again, I'd really appreciate help diagnosing, not admonition
Click to expand...
It could very well be a bad tube somewhere, and in tube amps they're always my first suspect, but unless you can turn it on and test the tubes, then you're just guessing. Still, it wouldn't hurt to have a spare tube or two, and it would settle the question of whether you have a bad tube or not. I can pretty much assure you it's not a transformer, though. Rarely do they go bad and it's very uncommon to lose one.
 
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#7
Great, thanks for the input. Would it make sense for none of the tubes to glow if just one has gone bad? Also am I right in thinking a blown fuse wouldn't allow the amp to come on or front LED to glow? Finally, is running it low volume like I've been doing potentially bad for the amp or harder on the tubes?
 
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#8
Southtwin said:
If I had the cash or lived anywhere near qualified techs I wouldn't be posting on here, no offense but I'd really appreciate it if people only post if there's something useful/productive to add
Click to expand...

If you don't have the electronics experience, test equipment and skill set to DIAGNOSE the problem, how would you propose to fix the problem?

Southtwin said:
If the answer is "sounds like you should switch out your tubes" or "replace easily accessible transformer(s)" that's a pretty simple and non-dangerous fix I can do. I have no intention of getting into the PCB or messing with capacitors, if it's that bad I'll call it a loss and sell for parts. Again, I'd really appreciate help diagnosing, not admonition
Click to expand...

Why would you swap out a part without knowing that it was actually bad?

Diagnosing do you have a scope, signal generator and the knowledge to use them effectively (safely)

Southtwin said:
Great, thanks for the input. Would it make sense for none of the tubes to glow if just one has gone bad? Also am I right in thinking a blown fuse wouldn't allow the amp to come on or front LED to glow? Finally, is running it low volume like I've been doing potentially bad for the amp or harder on the tubes?
Click to expand...

It depends entirely on the circuit, and where the fuse is in the circuit. If it's the AC primary fuse, you wouldn't have any lights or tube heaters (or anything else)
 
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#9
Southtwin said:
Great, thanks for the input. Would it make sense for none of the tubes to glow if just one has gone bad? Also am I right in thinking a blown fuse wouldn't allow the amp to come on or front LED to glow? Finally, is running it low volume like I've been doing potentially bad for the amp or harder on the tubes?
Click to expand...
It could very well be that one tube keeps it in permanent standby (though it's also possible that it's not tube related), and yes, a blown AC fuse would shut the amp down entirely. But no, running it at a low volume is perfectly fine for the amp, and what kills tubes quickly is running it super hot and distorted. Tubes are a bit of a wild card, though. I've had new tubes go out quickly and I've had them last for years, even decades.
 
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#11
BioWeapon said:
You know how it goes.

If you have to ask... now cut that out before you hurt yourself. If you don't even have a multimeter or a tube tester you're not even slightly kinda close to maybe equipped for this job.
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Look, I think we can all agree that spare tubes are not a bad idea. And there are so few tubes in the PF-20T that it's not going to deny the average person meals for the kids. If it doesn't work, back to the drawing board. But since I've found that tube amp problems are often caused by tubes, I don't see the harm in trying it. The ARE user serviceable parts, after all.

HOWEVER...

beans-on-toast always recommends pulling all your tubes and reseating them in the sockets first before buying new ones. This often fixes things more than you'd think and it's free as long as you already have a Philips screwdriver.
 
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#13
agedhorse said:
If you don't have the electronics experience, test equipment and skill set to DIAGNOSE the problem, how would you propose to fix the problem?

Why would you swap out a part without knowing that it was actually bad?

Diagnosing do you have a scope, signal generator and the knowledge to use them effectively (safely)

It depends entirely on the circuit, and where the fuse is in the circuit. If it's the AC primary fuse, you wouldn't have any lights or tube heaters (or anything else)
Click to expand...

I'm really surprised at your response Andy!
You've been here long enough to know that nearly every tube amp problem is related to a tube or a fuse; major tube amp users on this board suggest this on nearly every thread.

Then again, they are the same ones that are quick to mention that tubes last a very long time, which has been my experience for 40 years. And when the do fail on their own, it is almost always a slow process. Thankfully the usually only die like a light bulb when another component fails.

JimmyM said:
It could very well be that one tube keeps it in permanent standby
Click to expand...

I don't have the schematic for this amp but am curious.
How could a bad tube could keep the amp from coming out of standby?
Especially since the OP stated that the tubes aren't seeing any voltage (by not heating up).
 
#14
Jim C said:
I'm really surprised at your response Andy!
You've been here long enough to know that nearly every tube amp problem is related to a tube or a fuse; major tube amp users on this board suggest this on nearly every thread.

Then again, they are the same ones that are quick to mention that tubes last a very long time, which has been my experience for 40 years. And when the do fail on their own, it is almost always a slow process. Thankfully the usually only die like a light bulb when another component fails.



I don't have the schematic for this amp but am curious.
How could a bad tube could keep the amp from coming out of standby?
Especially since the OP stated that the tubes aren't seeing any voltage (by not heating up).
Click to expand...
My experience says that it's much more likely something else that a tube.

I see so many simple repairs made much worse and more expensive with "Wild West" style repairs.
 
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#15
JimmyM for the win! Followed your/beans-n-toast's advice and pulled and reseated all the tubes, working like a charm now (LED does switch to purple when working properly btw), thank you sir. I just hope others searching forums on issues like this in the future see this and don't follow the advice of immediately sending it off and ending up with a totally unnecessary bill
 
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#16
Southtwin said:
Hey all, I am by no means an amp/electronics guru, but I know enough to isolate issues and occasionally solder. My beloved Ampeg PF-20t recently quit working and I don't know why. I generally run it at very low volume (gain cranked, volume at 1, Caline 10 band eq pedal with gain all the way down) so as to get rich creamy distortion without pissing off nearby neighbors. The last time it was working, I accidentally left the amp on (not standby) for a few hours with instrument volume down before going back to practice room and turning it off.

Ever since, it's had the following status: power switch works (I hear slight normal electrical hum in amp), the red LED in front comes on, but no change when I flip standby switch. No output to speaker cab (verified cab is fine), no signal from DI out (preamp or power amp), none of the tubes glow at all, and LED color doesn't change (I think it's supposed to switch to blue when not on standby, I covered LED with tape long ago and don't remember for sure). Had a ceramic fuse so I can't tell if that's the issue, just ordered glass replacement fuses but I'm assuming it wouldn't come on at all if fuse was blown. I don't currently have a multimeter or tube tester but might be willing to buy if need be. Tried searching the forums but no luck. Any help for next steps or possible culprits is appreciated!
Click to expand...
Any electronics store nearby can fix it for real cheap. Probably just fried a transistor or something. Dude fixed my guitarists blues master in an hour. Same thing was happening. No sound, no glow, new tubes.
 
#17
Southtwin said:
JimmyM for the win! Followed your/beans-n-toast's advice and pulled and reseated all the tubes, working like a charm now (LED does switch to purple when working properly btw), thank you sir. I just hope others searching forums on issues like this in the future see this and don't follow the advice of immediately sending it off and ending up with a totally unnecessary bill
Click to expand...

Sometimes you get lucky, sometimes not but you cause no additional damage. Then there's the guys who get into something replacing parts that have nothing to do with the problem because they read about it on the internet, and create additional problems that become even more expensive because a tech needs to figure out what else got screwed up.

JeezyMcNuggles said:
Any electronics store nearby can fix it for real cheap. Probably just fried a transistor or something. Dude fixed my guitarists blues master in an hour. Same thing was happening. No sound, no glow, new tubes.
Click to expand...

Probably not a transistor... there aren't any that would cause this problem in that amp. ;)
 
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#20
CryingBass said:
80% of auto electronics I used to sell in the 70’s went to mechanics that were obviously guessing. Because they always came back for the “next” part. Some of them had 10 fingers.
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I see otherwise beautiful (and expensive) amps (and other things) ruined beyond economical repair all too often by "well intentioned" attempts.
 
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