I am working on a later version Pro Jr... Oops, had to correct Freudian typo ProbJr...I think the PCB had a 2002 (c) on it.
His complaints, multiple types of noise including sound with obnoxious but low-level noises other than hiss at 0 volume setting, rapidly-failing output tubes (but he kept calling them 12AX7's...he's musician, not a tech...I assured him he could not be blowing up 12AX7's and had misidentified them.
I put 1 ohm WW resistors in series with the EL-84 cathodes to pin jacks on the rear panel to measure(derive) bias current, and a pot to make bias adjustable.
I found the amp to be the noisiest (hiss & amp gastric noises) amp I've encountered.
If you look at the earliest version (schematic- there seem to be 6 revs but 4(?) Series). The earliest had no cathode bypass capacitor on the first stage. 2nd schematic and beyond there is a 25 uF. You can also see the reference signal levels on the later schematics are higher after the bypassed first stage.
If you compare R6 on first and later rev. schematics, it was 56k and became 22k. Apparently they had to discard more gain the bypassed 1st stage produced.
I exchanged some emails with Ulrich Neumann, and he sent me some suggestions without having played or experimented on one.
He also commented on the strange gain management. He created a sim file of the preamp only and noted two dominant hiss sources...the 470k at the volume control, and the input 10k resistor, which is already relatively low for guitar amps. He suggested going as low as 1k. I asked about the RFI-protection attributed to that 'lambda network'. He has never had an RFI problem, so lowering it worked in LTspice...raise it if necessary.
He also remarked the tone control didn't do a lot...reminded him of a hi-fi-leaning Bode plot.
His suggestions:
Reduce R1 as discussed.
Change the 470k R5 and 22k R6 (56k on original) both to 100k to improve s/n. This requires an additional resistor before the PI to 'normalize' the increased signal level back to what was there (by design, desire or destiny-my comment). Lastly, change the 22 pF C2 to 3300 pF to give the tone control a tilt function.
The sim file duplicates the stock and proposed changes and the Bode plots of the 2nd preamp stage can be compared, via stepped pot values
I am accustomed to making one measurement at a time and calculating gain by hand...so I found the number of plot traces for stepped volume and tone controls a bit overwhelming. I personally could not make decisions based on the plots alone.
I haven't described where the fixed extra 'gain normalization' resistor is because I have to find the sim file to describe where it was. I put a smaller fixed value plus a trim pot on a piece of PCB.
At this point the amp was gutted on a bench in my garage and I was doing non-signal-related changes.
I began trying to isolate the multiple chassis grounds and began clip-leading them experimentally. Despite being non-ideal wiring I found quieter ground/return results and tried a 5751. The hum and noises got much worse! This was annoying and made me think there was more than just gain distribution to blame.
In the PJ Rev. evolution (based on photos I found on the internet) it seems Fender made 1-2 or more grounding changes. This one has a metal chassis standoff grounding the main PCB to the chassis roughly (physical location) between the input resistor and the front panel controls. Another version had a pigtail ground lead from somewhere (? based on a web photo) to chassis.
This specimen clearly has excessive grumbling & whining with volume at 0 (before I gutted it), so I think they didn't finish their homework.
I detoured to some other distractions like mounting 800V MKP 60+20+20 uF box caps on perfboard in a subchassis, changing the output transformer and adding a screen+ preamp B+ choke.
It all fits but there is minimal clearance from the upgrade speaker (he did it so I have to look to say what it is) magnet...success or failure TBD.
So I'm way out in the weeds on goals without even running a guitar thru it yet!
I am in exterminator mode now and decided to apply Merlin Blencowe's grounding techniques with shielded twisted-pair wiring...going to cut some ground traces when I am 110% confident where, install local 'ground'/return terminals, and determine how they will be star-grounded.
I found isolating the output jack from chassis also helped so there is something non-ideal in the grounding.
I uploaded some screen shot traces to a TDPRI website discussion on stock PJ simulation but there was little response and I didn't share the sim file.
If anyone would be interested in seeing it, I can share it.
A lot of unrelated personal things have happened it the last 2+ years so I haven't finished the project. The friend can't use the amp at home & is playing thru a PA at church so he's patiently waiting.
I'm ditching the isolated plastic input PCB jack and using an isolated Switchcraft type with STP wiring.
Murray
