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PR20 tone cab problem

 

Having an issue with my PR-20 and was hoping for some guidance.

It plays fine and sounds great, with one exception: every so often I get a random surge of noise (static/crackle, pops, slight distortion, sometimes a kind of whistling), that goes on for 20-30 seconds or more.? This seems to happen particularly when I hit the amp with significant volume.

I think I have isolated the noise to the output tube (V11, 6BQ5) for the Reverb channel.

When the noise starts, I can lightly tap the top of that tube, which makes a loud thump, but it usually stops the ongoing noise.? Then it plays clean for awhile, sometimes as long as maybe 15 minutes before it acts up again.

Sometimes switching the reverb level with the knob on the side of the cabinet will make the noise stop as well.

I have tried changing that tube with no effect.? Could this be a problem with the tube socket?? I have hit it with contact cleaner several times, but the problem hasn't gone away.

I know guitar guys like to change tube sockets when they rebuild amps, but I've never had an issue with one.??So, should I suspect maybe C49 or C50?

I recapped this amp with the kit from Tonewheel General a few years back:??.? So hopefully that rules out a filter cap issue.

Thanks for any advice.? When it's not acting up, the PR20 sounds great - especially in tandem with my "new" JR20!
 

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The problem may be either coupling capacitors, bias or control grid types. ?
When a control grid capacitor goes bad the 6BQ5 output vacuum tube will be allowed to over conduct, or “run away”. ?When this happens the plate of this particular 6BQ5 will glow red hot. ?When this happens the vacuum tube will most likely be damaged.

Coupling capacitors can also cause noise. ?That and a driver vacuum tube. ?

These are filled with paper and ceramic capacitors. ?They may last a lifetime, but not indefinitely.

Don Resor

Sent from someone's iPhone

On Feb 7, 2024, at 9:54 PM, scott.brand@... wrote:

?Having an issue with my PR-20 and was hoping for some guidance.

It plays fine and sounds great, with one exception: every so often I get a random surge of noise (static/crackle, pops, slight distortion, sometimes a kind of whistling), that goes on for 20-30 seconds or more.? This seems to happen particularly when I hit the amp with significant volume.

I think I have isolated the noise to the output tube (V11, 6BQ5) for the Reverb channel.

When the noise starts, I can lightly tap the top of that tube, which makes a loud thump, but it usually stops the ongoing noise.? Then it plays clean for awhile, sometimes as long as maybe 15 minutes before it acts up again.

Sometimes switching the reverb level with the knob on the side of the cabinet will make the noise stop as well.

I have tried changing that tube with no effect.? Could this be a problem with the tube socket?? I have hit it with contact cleaner several times, but the problem hasn't gone away.

I know guitar guys like to change tube sockets when they rebuild amps, but I've never had an issue with one.??So, should I suspect maybe C49 or C50?

I recapped this amp with the kit from Tonewheel General a few years back:??.? So hopefully that rules out a filter cap issue.

Thanks for any advice.? When it's not acting up, the PR20 sounds great - especially in tandem with my "new" JR20!

 

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There may also be a cracked carbon comp resistor creating noise.

At this point you may have to shotgun all components connected to that particular output stage, or more.

Don Resor

Sent from someone's iPhone

On Feb 8, 2024, at 5:18 AM, Don R <organlists1@...> wrote:

?The problem may be either coupling capacitors, bias or control grid types. ?
When a control grid capacitor goes bad the 6BQ5 output vacuum tube will be allowed to over conduct, or “run away”. ?When this happens the plate of this particular 6BQ5 will glow red hot. ?When this happens the vacuum tube will most likely be damaged.

Coupling capacitors can also cause noise. ?That and a driver vacuum tube. ?

These are filled with paper and ceramic capacitors. ?They may last a lifetime, but not indefinitely.

Don Resor

Sent from someone's iPhone

On Feb 7, 2024, at 9:54 PM, scott.brand@... wrote:

?Having an issue with my PR-20 and was hoping for some guidance.

It plays fine and sounds great, with one exception: every so often I get a random surge of noise (static/crackle, pops, slight distortion, sometimes a kind of whistling), that goes on for 20-30 seconds or more.? This seems to happen particularly when I hit the amp with significant volume.

I think I have isolated the noise to the output tube (V11, 6BQ5) for the Reverb channel.

When the noise starts, I can lightly tap the top of that tube, which makes a loud thump, but it usually stops the ongoing noise.? Then it plays clean for awhile, sometimes as long as maybe 15 minutes before it acts up again.

Sometimes switching the reverb level with the knob on the side of the cabinet will make the noise stop as well.

I have tried changing that tube with no effect.? Could this be a problem with the tube socket?? I have hit it with contact cleaner several times, but the problem hasn't gone away.

I know guitar guys like to change tube sockets when they rebuild amps, but I've never had an issue with one.??So, should I suspect maybe C49 or C50?

I recapped this amp with the kit from Tonewheel General a few years back:??.? So hopefully that rules out a filter cap issue.

Thanks for any advice.? When it's not acting up, the PR20 sounds great - especially in tandem with my "new" JR20!

 

I have had problems with old tube sockets. The pins had pretty much lost their springiness so that they were not clamping the pins tightly. No amount of contact cleaning was able to fix that, and I had to wholesale replace the old sockets. Your problem sounds suspiciously similar - especially the loud thump, presumably as one of the tub pins loses and remakes contact with its socket pin. Likely the significant volume connection is that the tube starts to shake and loses contact with the socket again.


Having an issue with my PR-20 and was hoping for some guidance.

It plays fine and sounds great, with one exception: every so often I get a random surge of noise (static/crackle, pops, slight distortion, sometimes a kind of whistling), that goes on for 20-30 seconds or more.? This seems to happen particularly when I hit the amp with significant volume.

I think I have isolated the noise to the output tube (V11, 6BQ5) for the Reverb channel.

When the noise starts, I can lightly tap the top of that tube, which makes a loud thump, but it usually stops the ongoing noise.? Then it plays clean for awhile, sometimes as long as maybe 15 minutes before it acts up again.

Sometimes switching the reverb level with the knob on the side of the cabinet will make the noise stop as well.

I have tried changing that tube with no effect.? Could this be a problem with the tube socket?? I have hit it with contact cleaner several times, but the problem hasn't gone away.

I know guitar guys like to change tube sockets when they rebuild amps, but I've never had an issue with one.??So, should I suspect maybe C49 or C50?

I recapped this amp with the kit from Tonewheel General a few years back: <>?. So hopefully that rules out a filter cap issue.

Thanks for any advice.? When it's not acting up, the PR20 sounds great - especially in tandem with my "new" JR20!

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