PT-20 Troubleshooting

dfrattaroli

dfrattaroli

New member
Hey all,

I have a PT-20 that I bought used about 2 1/2 years ago. Today I noticed that after 15 minutes or so of playing, the amp start making noise all of sudden. Basically hiss. Sounded like white noise. I turned off the amp and turn it right back. Noise was gone and then re-appeared about 15 minutes later. This time, I turned it off left it alone for about 20 minutes. Turned it back on and played for 30 minutes and heard no noise. This time, the only thing I thought I noticed was a slight volume change after 20 minutes. Sounded like it got a hair louder.

As I said, I bought it used and in truth, I haven't played it that much. I'm going to pull the tubes and clean the contacts. If that doesn't change anything, I'll change tubes, etc. Ultimately, I have a great local tech I can bring it to.

Just thought I'd post here in case anyone has had similar issues with this amp.

PS, other than being a used amp, it looks absolutely mint so I doubt it was abused by the previous owner. Looks totally stock as well.

Thanks!
Dave
 
Before you pull tubes, I suggest waiting for the hiss. At that point, twiddle the front-panel knobs to see if he hiss changes level and/or tone. That'll help you determine where the problem is arising.
 
V2a":1oy0ulsa said:
Before you pull tubes, I suggest waiting for the hiss. At that point, twiddle the front-panel knobs to see if he hiss changes level and/or tone. That'll help you determine where the problem is arising.
Click to expand...

Just turned the amp on waited for the noise. Front-panel knobs seem to have no effect on it. The only thing that changes is that when the volume is cranked, the "regular" amp noise increase but the "bad" noise/hiss doesn't change.

Dave
 
That tells you that the noise comes after the master volume. Likely the EL84s, so swap 'em out with known good ones if you have some on hand. You might have to read your manual to know if you need to pick certain tubes. The amp is cathode biased, so you don't need to set that.
 
V2a":2gh0rzpi said:
That tells you that the noise comes after the master volume. Likely the EL84s, so swap 'em out with known good ones if you have some on hand. You might have to read your manual to know if you need to pick certain tubes. The amp is cathode biased, so you don't need to set that.
Click to expand...

Thanks very much. Love your FB page by the way. Your amps look beautiful.
 
Thanks.
You might also want to try swapping V1 for V3 (PI). Mark one of them with a dot or something so that you can get the V1 tube back in its place after the swap. The PI comes after the MV, and so there is a chance that it is acting up.
 
dfrattaroli":17j7qni3 said:
Hey all,

I have a PT-20 that I bought used about 2 1/2 years ago. Today I noticed that after 15 minutes or so of playing, the amp start making noise all of sudden. Basically hiss. Sounded like white noise. I turned off the amp and turn it right back. Noise was gone and then re-appeared about 15 minutes later. This time, I turned it off left it alone for about 20 minutes. Turned it back on and played for 30 minutes and heard no noise. This time, the only thing I thought I noticed was a slight volume change after 20 minutes. Sounded like it got a hair louder.

As I said, I bought it used and in truth, I haven't played it that much. I'm going to pull the tubes and clean the contacts. If that doesn't change anything, I'll change tubes, etc. Ultimately, I have a great local tech I can bring it to.

Just thought I'd post here in case anyone has had similar issues with this amp.

PS, other than being a used amp, it looks absolutely mint so I doubt it was abused by the previous owner. Looks totally stock as well.

Thanks!
Dave
Click to expand...

Check the solder joint of the 220k (red/red/yellow) resistor voltage divider on the left lug of the Master Volume. This quiets and allows the amp to perform quieter at low volumes. If it breaks down the amp will get louder and you WILL hear more power hiss. If the joints are good measure it with a mm when acting up.

Please remember these amplifiers contain lethal voltages and can injure or kill you if you do not know what you are doing.

My $.02, David
 
Finally got a chance to test with new tubes and check some things. NR, that resistor looks fine. Solder joints look perfect. Dave F. emailed me back and suggested a new set of JJ Power Tubes so those when in today.

At first the amp seemed quieter than before. After about 5 minutes or so, I heard a bit of static but that could be anything. Twenty minutes in, the full on noise came back. Just as loud as before. The only difference was that a minute or so after it came back there was kind of a pop, like when you change channels on a footswitch for example and then the noise went away. I played for another 30 minutes and everything seemed fine.

I'm thinking a cold solder joint. Time for a tech no? Or do I try change the preamp tubes first?

Thanks all.
Dave
 
Just to follow up, Dave suggested changing the PI tube. I pulled the ARS out and put in a JJ ECC81 I had in my stash and that seems to have done the trick. I played for about 30 minutes and it stayed quiet. I may have heard a "spit" or two but I think that's normal tube amp stuff. My DSL does that too from time to time.

Dave
 
fwiw - you'll get varying troubleshooting opinions online, so it's usually best to go to the designer (Dave in this case) if possible. Always suspect tubes first and try to swap them around before opening up an amp.
 
V2a":qw5ka3ft said:
fwiw - you'll get varying troubleshooting opinions online, so it's usually best to go to the designer (Dave in this case) if possible. Always suspect tubes first and try to swap them around before opening up an amp.
Click to expand...

Agreed. Thanks for your help.

Dave
 
V2a":22njxqbl said:
Before you pull tubes, I suggest waiting for the hiss. At that point, twiddle the front-panel knobs to see if he hiss changes level and/or tone. That'll help you determine where the problem is arising.
Click to expand...

Agreed
 
V2a":2zxq1z2h said:
fwiw - you'll get varying troubleshooting opinions online, so it's usually best to go to the designer (Dave in this case) if possible. Always suspect tubes first and try to swap them around before opening up an amp.
Click to expand...

Just got my PT-20...awesome. You seem to be a valuable resource.....will watch for valuable tips.
 
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