Tweaking my Pro Jr rewire.

[toomanyslurpees]

Ok, so I'm finally getting back to this and get it tweaked up. I did this for a learning experience (and 'cause minus the issues with it running too hot and whatnot I actually do like the amp) Anyway, it's up and running, wired as per the schematic except and adjustable bias supply and the NFB on a switch. It's BRIGHT, ice pick bright... I went back through everything to make sure I didn't goof up and put the wrong value somewhere but that's not the case. Could I be right in guessing that the design of the amp compensates for cheaper components or the speaker? (Upgraded to a Weber)

So my first question is: Where's a good place to start experimenting as far as mellowing this thing out a bit? (Possibly related to this but I do get a squeal out of the thing when I turn the tone right down)



Second question is in regards to adding this one tube reverb, I'm looking at the blues amp example and I've assumed using my B+ for B and my Z for E will work but I don't have the 150K resistor to put R and R2 across, can I throw one in before C6 without unwanted effects? I was thinking about moving the bias supply to a new small board as I've had it brought to my attention it might be causing some noise there and work the reverb bits to it's old location, and possibly moving the bias pot and putting the reverb tube there (is this a goofy idea having that on the far side on the power tubes? Would shielded wire be enough to compensate? I've also thought about putting the reverb guts/tube in a small enclosure and mount it outside the chassis.

http://www.el34world.com/Forum/index.php?topic=7957.0

The Pro Jr:

http://www.ampwares.com/schematics/pro_jr.pdf

Thanks for your thoughts, I feeling like I'm slowly starting to learn some stuff from hanging around there (and those of you who gave me advise on lead dress from the last time, I do intend to take your advise and shorten that stuff up!)

[RicharD]

>It's BRIGHT, ice pick bright

Remove C2 - 22pF.

[toomanyslurpees]

Yeh, I originally left C2 out because I was going to put it on a switch based on what I had read about removing it, but popped in in out of curiosity after firing it up, so it's stinking bright without C2.

[RicharD]

There's a few thangs you can try.  I always start with the obvious but you already beat me to that one.

Right now, R1 = 10k.  This gives you a f-3dB of 164kHz for the first stage, way out of the range of hearing.  Changing this to a 33k (ala Fender) puts your f-3dB down to 49kHz, still out of the range of hearing, but you'll probably notice it.  A 68k gets you f-3dB @ 24kHz, you will notice that.  A 100k gets you 16kHz and you've put a sock in it.

C1 looks like a common value for interstage coupling.  I often use .015uF myself.  You could try a .022.  This won't cut highs, but will allow more low frequencies to pass.  .01uf = 30Hz f-3dB, .015 = 20Hz, .022 13Hz, .047 = 6Hz.

You could yank the snubber (C10), but I don't think that's causing the problem.  You could also yank the Zobel network (R26 & C11) but that's probably not the issue either.

-Richard

[toomanyslurpees]

Awesome, thanks, that gives me a good starting point

[conger]

toomanyslurpees - I hope you don't take this the wrong way, but as someone what has modded and modded a PJ and then rebuilt it, I have been through some of this pain.

The PJ is just not the greatest of amp designs out there because it is a low production cost mass produced amp that uses extra components to tame the production limitations.  The OT for example is tiny and poor quality and needs the extra CF filter to makes it sound less harsh.  A VOX AC15, Matchless Spitfire or an 18Watt LiteIIb all sound a world away using less components.

Your tag board is a visual work of art, but it will still be a PJ even when you sort it all out.  Why not remove the unwanted components from the board and build something the amps mentioned above?  Also use an existing layout as a guide on how to wire the board to the tubes etc.

As far as the reverb tube goes I seem to remember that the PT is on the limit of what it can supply and an extra tube might be too much.  Certainly my PJ wired as a Spitfire runs very very hot.

[RicharD]

Conger makes a very valid point.  I have an old Monkey Wards amp that was the same deal.  A cheap Valco made amp.  I gutted it, except for the reverb circuit, added a DC30 front end, a Champ back end, and tweaked the power supply.  It's a jaw dropper now.

[tommytornado]

I would put a 25uF/25v cathode bypass cap in parallel with R4 if you play strat's and tele's for the most part.  Try a 5 to 10uF/25v if it's humbuckers most of the time..  Test the amp and then try the same thing with R11.  You'll be surprised how much the bypass cap across the input preamp's cathode will add.   

[phsyconoodler]

So this is a 'hand wired' Pro Jr?

  Nice work by the way.I would concentrate on cathode bypass caps and treble caps.The pro jr is not particularly trebly stock,so it's odd to have a hand-wired version to be more trebly.

Can we see the faceplate/control layout?

[VMS]

There is a newer revision of Pro Jr. schematic at fender.com with few changes made. I don't know if those will mellow the amp, but here is the link:

http://support.fender.com/schematics/guitar_amplifiers/Pro_Junior_Schematic.pdf

[phsyconoodler]

I'm looking at the preamp cathodes and I don't see ANY bypass caps.That amp should not be trebly at all with no bypass caps.
  Can you post a picture of your layout?That would be easier to follow than the Fender schematic.

[alerich]

Right now, R1 = 10k.  This gives you a f-3dB of 164kHz for the first stage, way out of the range of hearing.  Changing this to a 33k (ala Fender) puts your f-3dB down to 49kHz, still out of the range of hearing, but you'll probably notice it.  A 68k gets you f-3dB @ 24kHz, you will notice that.  A 100k gets you 16kHz and you've put a sock in it.

I did not know this. I know you need that resistor to control radio interference from getting into the input and I have always read/assumed that lower values simply yield a slightly hotter signal. I didn't know that different values affect high response like this. Good stuff.

Another question: Looking at the inputs of the two schematics attached what are the tonal implications of the placement of the 33K resistor in those two manners? I've seen it done both ways.

[RicharD]

As far as I know, it doesn't matter if the grid leak is in front of or behind the grid stopper.  I've seen and done it both ways.  I typically hang it right on the input jack out of convenience. 

A tube has an internal input capacitance.  If you stick a resistor in series with a capacitor you've created an RC circuit so frequency is now a factor.  I used TubeCad to get those frequency results.

[toomanyslurpees]

Thanks for all the tips, I'll let you know once I get the chance to do some experimenting. I'm going to go on the advise at ditch the reverb idea. Like I say it's a learning experience and one of the things I probably learned it gutting this one may not have been worth the trouble, but I think I'll be happy with this one for what it is once it's mellowed out a bit. I have an empty bassman head with a brand new aluminum chassis without a single hole, what I've been working up to, a completely blank slate, still trying to figure out exactly what I want to do with it.

[phsyconoodler]

Can I ask to see the layout you followed?That would make it very easy to troubleshoot.

[toomanyslurpees]

Here's my layout, sorry it's a little rough, I went through it a second time to make sure I didn't deviate from the schematic but that doesn't mean I could have missed something. I tried the tweaks that have been suggested, I like the .022 in C1, I convinced myself I didn't like 68Kohms in place of the 10K R1, though I didn't have anything in between on hand to try out. I have 10uF in parallel with R4 and R11. I think these must be a move in the right direction because I'm for the first time actually digging the sound of the amp but I'm still keeping the tone between 1-3 and it goes to 12. When I do turn it up I start getting a weird sound that I'm trying to think of a good disciption for on lower notes with the tone right down that does away when I turn the tone up a bit (controls are still just volume and tone, an on switch)

Oh, yeh, and I thought I was being smart saving space by putting R1 and R2 on the input just but yep, that's dumb. won't do it again.

[toomanyslurpees]

Sweet! Found it! Finally realized I should look at the old PCB and check the component values against the schematic, butterylicious, you get points for mentioning C1 could be .22uF, it was on the old board, and R6 was the other one, it's 56K on the schematic but 22K on the old board, changed that and it's sounding like it should now. I'm just happy because I can call this done and move on to a new project. Thanks for the help.

[phsyconoodler]

What does it sound like compared to the PCB version?I'm curious because I didn't find the pro jr to be a bad sounding amp at all.
  I'd consider doing one with a 12" speaker and large cabinet.And of course bigger output transformer.

[toomanyslurpees]

Yeh, it's been so long since I ripped it apart it's hard to tell the differences, I have to find myself a stock one to A/B it. It's not doing all the weird random popping and whatnot that it did before, but I would assume just putting in adjustable bias on a stock one would change that, I figured it was noises assosiated with it running really hot. I like the sound of it for a little amp. I haven't plugged it into my openback 112 cab yet, will have to do soon.