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Post subject: '78 Princeton(non-reverb) Humming
Posted: Thu Feb 20, 2020 12:50 pm
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My Princeton has recently developed a 60hz hum when powering up. The hum "surges" shortly after the amp is turned on, then it quiets down to being barely audible. It isn't affected by the volume or tone controls; however, the hum is pulsing and can be altered by turning the intensity knob. Knowing the little that I do about the tremolo circuit in these amps, I'm wondering if there is a problem in the bias circuit or possibly a coupling capacitor. The tremolo seems a bit weak as well. Any help would be greatly appreciated. I'm new to the tube amp world, and trying to learn as much as I can about them.

Also, I've measured the bias a few times over the last 2 years and noticed a trend. The resistance of the OT has been dropping evenly on both halves, and the plate voltage of the 6v6's is slowly on the rise. Not sure if this has anything to do with the recent hum or not.


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Post subject: Re: '78 Princeton(non-reverb) Humming
Posted: Sun Feb 23, 2020 11:39 pm
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Joined: Wed Mar 31, 2010 12:48 am
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The bias-supply circuit would be logical place to begin troubleshooting since you mentioned an issue with the tremolo as well. Be sure to check the diode -- if it's heading south it could cause the 60 Hz hum you hear.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: '78 Princeton(non-reverb) Humming
Posted: Tue Feb 25, 2020 4:19 am
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Joined: Fri Feb 05, 2010 4:31 am
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Location: Province de Québec, Canada
Hum;
1- Check all tubes . When tubes are at their working temperature you have no hum.
Use know good tube, tubes tester may not be reliable. Unbalanced output 6V6 could also be the issue.
Filter caps could be a issue too.

2- Weak tremolo ; most of the time, you have to replace the 3 capacitors around tremolo 1/2 tube 2 X 0.01 mfd and one 0.02 mfd

3- You can't trust output transformer resistance reading if you use ohmeter. Output transformer is not the issue.
4- Bias issue if unbalanced 6V6 , coupling cap ; see answer 2.

5- Buy this amp second hand ? Check fuse, previous owner may put too powerful fuse and you may damage amp

Knew to tube amp world ; don't do some work if you are not sure what you are doing.


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Post subject: Re: '78 Princeton(non-reverb) Humming
Posted: Tue Feb 25, 2020 7:10 am
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Be sure it's 60Hz. This is below the frequency of open low-E string, standard tuned guitar. Versus 120Hz (which is about equal to an open A-string, standard tuning).

Figuring out whether it's 60Hz hum or 120Hz hum will change your fix-it strategy.

Good luck!

:)


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Post subject: Re: '78 Princeton(non-reverb) Humming
Posted: Tue Feb 25, 2020 10:34 am
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Just to rule out issues with other equipment:

1. Remove all stomp boxes
2. Remove any external pedals that aren't required to make the amp work
3. Remove any extension cabinet(s) and speaker cables
4. Do not plug the guitar into the amp. Leave the input empty.

Does it still hum during power-on now?


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Post subject: Re: '78 Princeton(non-reverb) Humming
Posted: Tue Feb 25, 2020 10:39 am
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Joined: Fri Feb 05, 2010 4:31 am
Posts: 14045
Location: Province de Québec, Canada
ghost_of_strings wrote:
Just to rule out issues with other equipment:

1. Remove all stomp boxes
2. Remove any external pedals that aren't required to make the amp work
3. Remove any extension cabinet(s) and speaker cables
4. Do not plug the guitar into the amp. Leave the input empty.

Does it still hum during power-on now?


+1

Amps test noise must always be done with no gear plug to the amp; N-O-T-H-I-N-G.

If any gear is plug to the amp, test may be false.


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