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Post subject: Re: extremely noisy pro jr. III
Posted: Thu Jan 31, 2019 12:50 pm
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Hi TimsAudio, really interesting. Question about your comment "So to get a target for reduction, The Pro Jr, with controls centered and a 1khz, .005vrms input, will put out @2volts rms". Is there something missing in that statement at @2volts rms? Regards.


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Post subject: Re: extremely noisy pro jr. III
Posted: Thu Jan 31, 2019 4:12 pm
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$250-300 used

New $499

On a hot biased el35 amp with no negative feedback.

Complaining about a hiss?

That's like me complaining about getting too much cream in my coffee.

Look on the positive side, for the money one is hard pressed to buy an amp with the same cool vibe as a Pro jr.

Make sure the caps are drained (lots of info on the web about this) re-dress the leads and get some decent tubes and you should be good to go... but remember amps with no negative feedback that are biased hot like a Pro Jr are going to his some... it's the nature of the design. (IMO... that's part of the allure of amps like this)

have fun enjoy it


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Post subject: Re: extremely noisy pro jr. III
Posted: Sat Feb 02, 2019 9:10 am
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The numbers are the standard input level I use for all amp measurements.
2 volts into an 8 ohm dummy load is the stock Pro jr output at 1khz. It will vary with frequency.
Fender specs 20mv input with 6.9v output into 8 ohms, but the gain is still comparable.
The feedback resistor R27, is behind the input jack. You can just add a variable resistor to adjust feedback on top of it.
Solder a 10k resistor onto one lead. Solder a 100k pot in series with it so you have a minimum of 10k raising to 60 k when its in parallel with r27.
This will drop the gain of the main stage to accomodate microphones and reduce hiss.


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Post subject: Re: extremely noisy pro jr. III
Posted: Wed Feb 20, 2019 6:07 pm
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Hi. Just as an update, I contacted Fender requesting a copy of the schematic for a aPro Junior Version IV as they claim it is quieter etc .... Here is the exact response I received “Hey Graeme,

I just heard back from out tech department and it looks like the Pro Junior IV schematic is the same as the Pro Jr III, so that is the only one we will have on file. I included a copy here in PDF format.”

Interesting. So, apparently I’m to believe my Pro Junior III (mod F) is the same circuit as the new IV. Hmmm


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Post subject: Re: extremely noisy pro jr. III
Posted: Wed Feb 20, 2019 11:52 pm
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Both can be true.
The same schematic can be much quieter by simply specing quieter tubes in production runs.
Better tubes = Better music


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