NAD: Pro Jr that's been modded, Need Help to Decipher the Mods.

[Commodore 64***]

I hope you don't mind, I cross posted this at TDPRI, jsut cuz there's a lot of people there including BillM. But I know we have a small but POWERFUL community here, and I wanted to share. I promise to try to get some sound clips, you gotta hear this to believe it. And I wouldn't dare post sound clips of my playing anywhere but here. But for tonight, pics is all you get.

My wife bought me a present today. She went down to the local store and offered $200 on used tweed, Pro Jr. They took it. It's a Made in USA Version with the blue speaker. It's been modded.


Looking at the back, it's definitely been modded. It has a spare pair of electrical sockets! My own power supply for my pedal board? Kinda cool.


I turned it on. It has no hiss,no hum, no buzz. Dead quiet. I turned the amp to 11. And proceeded to play my tele. It's slightly above a loud bedroom level. It's dirty, breaks up like crazy. Sounds really, really good just not loud. Look at the back. Only one of the power tubes is lit. V1 and V2 have some glow too. Not a great pic, but it's in the dark. See only 1 power tube is lit.


Well, at this point, I had to get a gut shot. Yep, the first power tube socket has been altered. Altered to do what, I have no freakin' clue.


And here's another gut shot. I can't easily tell what's been modified (if anything). Any ideas as to what I'm dealing with? Right now this thing seems to be a FANTASTIC bedroom amp. And I'm OK with that, since I have a Peavey Classic 30 too. I'd just like to know what's goin' down here. If you look really close you can see at R29, there's 2 resistors wired together. Like maybe a bias mod of some sort. It's the same location where BillM puts a trim pot for his mods. http://home.comcast.net/~machrone/bjr/pjmods.htm

[guitartango]

Nice Amp (i have the Blues JNR version) but I would take it to a amp tech to check over the power mod. You don't know if this has been done by a pro or just someone who likes to tinker.

[duhvoodooman***]

I'm thinking it may have been modded to run single-ended output, i.e. just one power tube. That would definitely drop the wattage down and make it more of a bedroom amp.

[Commodore 64***]

Would that make it 7.5 watts? Because I had a Black Heart Little Giant (5w) and a Champ (5w) and I couldn't max the volume on those without peeling off the wallpaper in my room. This seems somewhere in between the Black Heart Killer Ant I had (1/4watt) and the Champ.

[duhvoodooman***]

I would guess somewhere in the 5W area, which is where the Epiphone Valve Jr. runs with a single EL84. As far as max. volume goes, would depend upon what else has been done to the amp.

Question: In that close up of the power tube socket connections, has the coating just been scraped off of the copper tracing on the PCB, or are the traces themselves scraped away, too?

[Robert****]

Congrats! I have a Pro JR too and I LOVE it. Fantastic sounding amp.

If it's been made into a single-ended amp however, it might sound quite a bit different I imagine.

[Commodore 64***]

I found a gutshot of a stock Pro Jr, of the same year as mine. I circled the parts of mine that appear to have been changed. It's all resistors, as far as I can tell.

Stock Pro Jr.


My Pro Jr.

[Commodore 64***]

Question: In that close up of the power tube socket connections, has the coating just been scraped off of the copper tracing on the PCB, or are the traces themselves scraped away, too?

I don't know. I'll have to crack it open again and check.

[Commodore 64***]

Billm replied over at TDPRI. He says that tube socket is fried, it looks like it was attempted to be repaired. I may have to get dirtier than I've ever done on an amp before.

BillM:

The tube board is obviously fried. You're looking at repairs there, not intentional mods. The easiest fix is to cut all of the traces except heater, separate the ribbon cables, and solder directly to the socket pins. Repair the traces to the heater as shown on my site. But you also have to keep the R3000 suppression diodes connected to pin 7 and ground.

If it were mine, I'd cut away the tube board and install chassis-mounted sockets. You'd need to relocate the small portion to the left that supplies the LED pilot light, carefully remove the R3000 diodes, and attach them directly to the sockets, between pin 7 and pin 3, which is also cathode/ground. The band faces pin 3, as you can see from a PJ schematic.

You need to extend the heater connections to the remaining piece of board that holds the 12AX7s. I do this all the time with Blues Juniors when I do the octal conversion mod.

The other mods are bias voltage and larger screen resistors. You'd want to measure the bias voltage at the tops of R21 or R22 to see if it's in the ballpark (-10.5 stock to -12.5 cool bias).

The input mod over on the right is probably unimportant. C2/R6 is just a little voicing circuit that bleeds off some highs. Fender has messed with these values, along with R5, over the years.

[Algonquin]

Is returning it an option?... Maybe you're up for it, but looks like a lot of work to take on.

If you're up for it, and can do it... great!

[Commodore 64***]

I'm up for it. Not sure if I can do it, but I will sure as Hell find out.

[Eric***]

I understood like, maybe 30-40% of BillM's post. I hope you keep us apprised of your progress -- it's fun to watch projects take shape.

[tunghaichuan***]

I could be wrong, but it looks to me like the left-hand power tube is not working. The heater is not lit up (orange glow.) If this is the case, the amp will function marginally. You'll get low output as the other tube is still working, but it won't sound good. The picture looks like damage, and if repaired, poorly so.

Billm's idea of using chassis mounted sockets is a good one. PCB mount power tube sockets are a bad idea, IMHO. Especially EL84 based amps. Most designs run them very hot, which can cause problems with cracking the traces on a PCB.

[duhvoodooman***]

Ah, the unintentional single-ended mod! No wonder the amp isn't putting out much sound. I wonder what additional effects this burnt-out socket might be having on the amp when it's run? Could it possibly induce a more expensive failure, like a transformer blow-out? If it was mine, I wouldn't run it in this condition, just to be careful.

Was the amp sold to your wife "as is"? Because if the amp was represented as fully functional, which it clearly is not, then you have a legitimate beef with the store.

Take it from somebody who has done an extensive set of mods to the closely-related Blues Junior (including the octal tube mod that BillM mentioned), what he describes entails a LOT of work, some of which is of a fairly advanced sort. IMO, it shouldn't be undertaken by a beginner. I don't know what your skill set & experience level is with respect to electronic repair work and tube amps in general, but this doesn't look to me like a good "learner's project"!

BTW, if you were to do the repairs successfully (or had them done by somebody), the amp will end up a HECKUVA lot louder!

[duhvoodooman***]

Just to provide a bit more information on this part of BillM's response:

"If it were mine, I'd cut away the tube board and install chassis-mounted sockets. You'd need to relocate the small portion to the left that supplies the LED pilot light, carefully remove the R3000 diodes, and attach them directly to the sockets, between pin 7 and pin 3, which is also cathode/ground. The band faces pin 3, as you can see from a PJ schematic.

You need to extend the heater connections to the remaining piece of board that holds the 12AX7s. I do this all the time with Blues Juniors when I do the octal conversion mod."

Here are a couple of photos from the thread I linked above, showing how the left side of the tube board has been cut off and chassis-mounted tube sockets installed. The ribbon connectors for both power tubes have been removed and replaced with individual wires from the main PCB to the appropriate socket pins. The tube heater wiring is green coming off the PCB to the first power tube heater lugs, and then the brown wires connect the heater voltage supply to the second power tube and on to the remaining section of the tube board to fire the heaters for the three preamp tubes. Because of layout differences between the Blues Jr. and Pro Jr., there was nothing else on that end of the tube board that had to be relocated.



Close-up of the chassis-mounted octal tube sockets & wiring:

[Commodore 64***]

DVM thank you immensely for the gut shots. Bill M also sent me a Word doc detailing the octal conversion mod. Looking at your pics, and his, this is definitely something I feel that I can do.

However, I am also considering gutting the amp, reusing the PT and doing a 5E3 circuit. I priced out all the components and I'd be in it for another $139. I'm getting a few amp building books from the library. Once I read those, I'll decide what I wanna do.

[tunghaichuan***]

DVM thank you immensely for the gut shots. Bill M also sent me a Word doc detailing the octal conversion mod. Looking at your pics, and his, this is definitely something I feel that I can do.

However, I am also considering gutting the amp, reusing the PT and doing a 5E3 circuit. I priced out all the components and I'd be in it for another $139. I'm getting a few amp building books from the library. Once I read those, I'll decide what I wanna do.

You might consider contacting the guy who runs this site:

http://www.dreamtone.org/PJ.htm (posted in this thread: http://www.thefret.net/showthread.ph...l=1#post214404 )

and see if he'll make you a replacement eyelet board for your amp.

A couple of things come to mind:

Shoehorning a 5E3 circuit into that amp might be a tall order. The 5E3 has four inputs vs one for the Pro Jr. There is also an extra volume control for the second channel. There isn't enough real estate on the Pro Jr.'s control panel for all the controls that a 5E3 has.

You will also have to add a rectifier tube and I don't think the stock Pro Jr.'s power transformer has a 5V winding. You would have to wire in solid state rectifiers. You could use a 6v rectifier like the 6X4 or 6X5, but you would have increased current draw on the 6v filament winding which may prematurely burn out your PT if it isn't up to the extra current draw. Also, if you run a 6V rectifier tube on the same filament winding as the power and preamp tubes, you risk taking out the PT and all the tubes if the recto tube shorts out.

You'd have to wire the amp so that there was only one of the 5E3's channels, which defeats the purpose of having a 5E3. Part of the magic of that amp is having the two interactive channels and four inputs so you can jumper the channels together to get the interactivity.

I'm not saying you shouldn't try, but it might be simpler to replace the PCB with an eyelet board using the exact same circuit. You could even do the octal mod and use 6V6 tubes instead of EL84s.

My $.02.

[Commodore 64***]

Good advice tung. thanks for the sanity check. I think I might do the octal mod, and chassis mount the octal sockets, and see how that goes. I didn't dislike the EL84s in my Blues Jr. and I certainly like them in my Peavey Classic 30. But as long as I have to lop off PCB and mount at least 1 socket, I feel like I should do the octal mod. Other than that, I really don't know why I want 6V6 tubes, lol.

[jim p]

For what it is worth the PCB board looks scorched but maybe still OK although some solder joints look cold. You can do a point to point with an ohmmeter and voltmeter checks on the tube socket to see if it is OK. Reading the history of Zenith they always hand wired the tube sockets because of PCB heat damage, but that was 40-50 years ago and PCBs have changed. Fender posts almost all there amp schematics on there web site so you may be able to find a schematic there found one here http://blueguitar.org/schems.htm#Fender . One quick thing you could try is to reverse the two power tubes or ohmmeter the heater pins on the tubes, if the dull tube is always dull probably the tube. As far as single ended with only one tube the output transformer is not designed for that and will be in saturation with only one tube. A push pull transformers needs the current from both tubes to cancel each other out to prevent saturation. A single ended transformer has an air gap and requires a larger core with the reduced core permeability to obtain a high enough primary inductance for good bass response. On a push pull amp you could unbalance the AC signal (audio) to the output tubes to simulate the even order harmonics of a single ended amp Peavey does it with a control pot on some of its amps. On the mods if you find a schematic that corresponds to the amp and list the components changed that would make it easer to figure what the mods are.

Looking at the schematic I found all the heater connections are 6.3 volts and in parallel to all the tubes so I would suspect the heater filament in the one tube to be bad.