Fender Pro Junior hiss (question about potential source, seeking advice)

[soapfoot]

…have those god-awful wax impregnated cardboard eyelet boards that start to go conductive, leaking voltage all over the place to adjacent eyelets. Not to mention that they buckle after they absorb moisture and crack solder joints. Eyelets are also a major pain to rework (albeit slightly easier than newer Fender PCB's).

To be fair, nobody could have projected that the service live of those devices would be measured in centuries.

I think it’s to their credit that they remain mostly serviceable and reliable after all of this time, even under heavy use. I can’t think of many examples of mass-produced electronics that I’d expect to hold up as well over that time

 

[abbey road d enfer]

I may be in minority, but I haven't seen a single well built Fender, no matter the era.

I have a recent Fender amp that is well-built and cost-effective.
Guess what's the catch...
Incredible value for money. Paid $250 10 years ago, now I see them on Reverb asking $750!




BTW I recenly bought a Harley Benton Tube 15. Also incredible value for money.

 

[murrayatuptown]

Maybe a cathodyne inverter would be better
And/or
Lower gain tubes like CJ posted
And/ or changing the plate voltage divider like Abbey Road d enfer posted

IIRC years ago at ampage and the blue guitar web site there were ideas posted about using EL84 output tubes and limiting the grid drive and setting the gain in the stages. I think the blue guitar site no longer exists. It also had stuff about trainweck amps.
https://web.archive.org/web/20180324093219if_/http://blueguitar.org/

 

[murrayatuptown]

I am going to try a 12AU7 with different resistors for the PI along with lowering the NFB resistor. The lower LTP gain can't hurt. I already have 100k and 100k replacing the 470k and 22k (that was 56k in first version), along with that 22pF>>>3300 pF to try a tilt.

Is this the thread where someone commented about having one foot on the brake pedal and one on the accelerator simultaneously? I had that exact thought at some point, between some less polite ones with there being no 0 volume...like it really does go to 11...or 12.

 

[forp]

s

 

[abbey road d enfer]

?

 

[forp]

Here is my answer, i just suscribed to give my twist to you guys. First put a 12ay7 in v2 phase inverter. 2nd change the 15k bias resistor to 18k r29.3rd put a 33k over the v1a (second triod) 100k plate resistor and a 22uf over the 1.5k cathode resistor.dont use 12ay7 or 12au7 in v1 now because you may ear a pop on shut down.use 12ax7. After we can do 3 thing that i will describe in the next post.

 

[forp]

We can first make it a very good amp for clean and still good at overdrive. You need to modify the r5 r6 c2 network in the preamp to create a TMb regular tone stack there. First remove r5, on the bottom side put something like a 100k then solder a 250k trimpot to control bass wire it as a variable resistor having the middle lug solder to the right one. Over it put a 47n capacitor. The left lug of the trimpit goes to the 100k the right lug goest to a wire that goes to a 25k pot that goes to ground after.You can drill a hole ìn your chassis at the right end there is room for it.Replacing C2 use a 250pf followed by a 250k trimpot (you solder on the right lug the capacitor and the middle lug goes in the upper hole were was c2.You use a wire from the 100k/250k bass trimpot junction to the left lug of the 250k treble trim.R6 cuts the volume before the volume control, it limits overdrive especialy wuth a TMB .You can simply remove r6 or use another trimpot here.I used a 22k fixed resistor followed by a 3M trimpot to ground.

 

[forp]

The next thing you can do is disconnect a leg of c5 .1uf the presence capacitor and leave it like that or having it ajustable with a 25k trimpot soldered from the leg of the capacitor on one side to the hole were the leg was on his other side. It is wired as a variable resistor.You can also change the 100k resistor r27 of the negative feedback . You need to solder another 100k over it for 50k or try a 22k for 19k total, that relly cuts on hiss but on drive too so try both.

 

[forp]

The last thing you can do is to disconnect the purple wire from the speaker socket so you dont have negative feedback anymore (tape the metal end of the loose wire). And then you may try to put a vox cut control using a 250k trimpot wired as a variable resistor followed by 4700pf. You wire this between the top lug of r20 and r23. Having it dial low relly cuts on hiss(like 25k,11k,8k,0). I used stiff lamp cables covered by fabrics and heatshrink soldered at each resistor leg and the 2 part circuit hangs in the air between the 2 resistors.

 

[Scott Harmonic]

the current version pro junior:

Fender Pro Junior IV SE Limited-Edition Guitar Combo Amp Black​

is different from the available schematics-including all the revisions
and:
For me, no noise issues at all.....fwiw, I have a number of 'real' amps here and yet i find this amp, when modified to fit a 12" Vintage 30 Speaker, to sound just great-Crisp, live dynamic response etc etc without the compression and overblown preamp gain that was typical of the first generation.) - Liked it so much i bought a spare to store away. (whereas I sent the first version back first time i tried them in 1990's .) if i had an older one now, I'd button that 'pup up, move it to Ebay, and start with the one above instead, while they're still available... ;-)

 

[abbey road d enfer]

the current version pro junior:

Fender Pro Junior IV SE Limited-Edition Guitar Combo Amp Black​

is different from the available schematics-including all the revisions
and:
no noise issues at all.....fwiw, I have a large number of 'real' amps here and yet i find this amp, when modified to fit a 12" Vintage 30 Speaker, to be one of the best sounding amps on the planet, ( none of the overblown preamp gain that was typical of the first generation.) - Liked it so much i bought a spare that's still unopened etc- whereas I sent the first version back first time i tried them in 1990's ... if i had an older one now, I'd button the thing up and get rid of it and buy the one above instead, while they're still available...

It would be interesting to know what are the differences between the older and newer generation.
Since most of the strategic components (xfmrs, tubes, pots) are probably the same it should be possible to turn the older ones to new specs.

 

[Scott Harmonic]

I traced thru' just the first stage and volume control , there were 2-3 changes just that far-enough for me to decide it was different and the schematics didn't really apply- I'd be wary that transformers etc may not be the same at all- it's not like Fender is in the habit of updating info/schematics/part numbers to reflect what they're actually selling ;-)... ..from memory only, it is a version with a cathode bypass cap - but different value for that cap and the resistor/signal dropping network ahead of the volume control itself , which they claim is changed and works better etc etc..... the speaker made the greatest diff, the feedback loop has a 'presence' cap that i made switchable to different degrees of effect but that's the only mod i heard or thought might be better.

 

[Matador]

With 16mV on the PI input, a hot pickup through a discrete JFET source-follower could drive the amp to full volume, and you wouldn't even need preamp tubes.

 

[Scott Harmonic]

I believe you! but a basic part of the plan is to have a couple tube stages driven hard enough , and then apply that asymmetric/compressed signal, and it's second order harmonic to yet another similar tube stage etc. - I'd guess that all tube guitar amps have far more preamp level than necessary to just drive the power amp , Si? after all, this ain't HiFi, it's simplified 1950's tech, then gone wrong through excessive knob-twisting ;-)
> After that first cranked-up day, just enough preamp level to drive the power amp was probably never the plan ;-)

 

[Scott Harmonic]

p.s. my loudest pickup, on the guitar , fed into a 1 meg shunt resistor across the input of my (vacuum tube ) oscilloscope, was able to generate 6 volts, peak to peak...that was hitting the strings as hard as i could...but it was still easy to peak at about half that level... we could say that we don't really need a preamp at all ;-)

 

[forp]

Scott Harmonic , can you please make us a shematic of the fist preamp stage of this pro junior 4 of yours ?

 

[Scott Harmonic]

well, yes, sometime ;-) (i don't expect to be back inside that amp soon) I said what i can remember :
that it was most like the later revision with cathode cap ,etc - but with different values in the cap, and the resistors forming the voltage divider before the volume control - (and maybe the coupling cap was different ?)
I didn't go further- or even take the board out and look etc, ...they describe something reworked in the 'volume control'- i don't know if that was the pot/ taper/value itself , or if they were describing the resistor network ahead of it...? my interest and my point was mostly that it sounds and works better overall than the old version, right out of the box, -maybe they're using better tubes as well ? ;-)

 

[forp]

Scott Harmonic, a hi definition picture of the right side of the main board will answer most of my questions on what was changed. It needs to be close so i can tell each color lines on the resistors

 

[Scott Harmonic]

I dig. i can do that at some point. - for that matter I have another in storage that I could just sell for US450+ shipping... ( brand new, still in the box, etc)That's probably before I'll get around to taking apart the one that I modified, ( 12" speaker.. -considerably more work than one would think)