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.
http://lucky-cricket.com/files/Pro_jr.JPG

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.
http://lucky-cricket.com/files/Pro_Jr_Rear_tubes.JPG

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.
http://lucky-cricket.com/files/Pro_jr_tubeglow.JPG

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.
http://lucky-cricket.com/files/ProJr_EL84Socket.JPG

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
http://lucky-cricket.com/files/Pro_jr_Gutshot.JPG

[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.
http://www.lucky-cricket.com/files/Pro_jr_gut_ro.jpg

My Pro Jr.
http://www.lucky-cricket.com/files/My_Pro_jr_Gutshot.JPG

[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 (http://www.thefret.net/showthread.php/14948) (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.

http://www.duhvoodooman.com/miscimages/musical/BJr/BJr_mods_rear-down.jpg

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

http://www.duhvoodooman.com/miscimages/musical/BJr/BJr_mods_octal_tube_sockets.jpg

[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.php/15529-Perpetual-Craig-s-List-Used-Gear-Thread?p=214404&viewfull=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.

[Commodore 64]

I'm getting -11.1 on the R21/R22 measurements. Billm says -10.5 would be biased hot, and -12.5 would be biased cold. So I'm a little on the hotter side. This is acceptable to me.

Can any of you help me to understand what the screen resistors do? By adding larger ones, is it affecting tube compression/ response? I understand they exist to maintain a voltage drop, but I'm still unclear why they were changed in this amp. What was the original owner/modder looking for?

This amp has virtually no clean. It seems like I get compression (and overdrive/distortion?) at very low volumes. I can get a little bit of clean headroom with my Tele single coils, but my Samick Jazzmaster with P90s, there is no clean.

Also, I've got both tubes working for the past couple of days. I've got the amp open right now. I pulled one of the 12AX7 and of course, one of the female pin sockets came out with it. I stuck it back in and it seems OK, but it sure looks like it might be a good idea to replace the tube sockets, all of them.

Yesterday, I popped in 2 new JJ EL84s, and this morning I swapped out both 12AX7 with brand new tubes. Now I'm getting 4 nice lit tubes, and a bit more power. if you look at that dark pic you see that I had 2 decent glowing tubes. I must have had a bad EL84 and a bad 12AX7. Break-up/compression is still really early, but now the amp seems much more like I expected. So after all this...among other things, I simply had some bad tubes! I'm worried about that, because this amp had 2 bad tubes. I'm thinking the sockets are suspect. Or maybe the store put junk tubes in there (or never bothered to test the amp).

Anyways, I do think I've now established that:

1. The Pro Jr. circuit as it exists in this amp is acceptable to me. Very much so.
2. I need to replace all the tube sockets. I will purchase them this week. I plan to chassis mount them all.
3. You guys rock, thanks for continuing to post as I vacillate between all the possibilities. Thank you!

[Commodore 64]

OK, here's a clip of the amp, with my Samick. I had the amp on 8 (out of 12), tone was maxed. The camera mic really compresses and youtube does their thing as well. As a result, you really can't hear much difference between the PUPs or the pedals in these vids, but it's the best I got.


http://www.youtube.com/watch?v=DwLtWe299RQ

[Commodore 64]

OK, here's a clip of the amp, with my Samick. I had the amp on 8 (out of 12), tone was maxed. The camera mic really compresses and youtube does their thing as well. As a result, you really can't hear much difference between the PUPs or the pedals in these vids, but it's the best I got.


http://www.youtube.com/watch?v=DwLtWe299RQ

[tunghaichuan]

Can any of you help me to understand what the screen resistors do? By adding larger ones, is it affecting tube compression/ response? I understand they exist to maintain a voltage drop, but I'm still unclear why they were changed in this amp. What was the original owner/modder looking for?

This amp has virtually no clean. It seems like I get compression (and overdrive/distortion?) at very low volumes. I can get a little bit of clean headroom with my Tele single coils, but my Samick Jazzmaster with P90s, there is no clean.


Basically the screed grid resistors (also called grid stoppers) limit the the screen grid's current draw when the amp is cranked. If the plate draws too much current it red-plates. If the screen draws too much current, it burns out. The screen grid is relatively a lot more fragile than the anode/plate.

Here is a discussion about them here:

http://www.thegearpage.net/board/archive/index.php/t-274440.html

What values are in your amp? If the grid stoppers are too high in value they limit the output power of your amp. It seems to me that 470 ohm is suitable for 6L6/6V6, 1k for EL34 and 2.2k for EL84. Although the rule of thumb I've heard is to use the smallest value you can where your amp still sounds good.

[Eric]

Pretty cool sounds out of that and some good playing. Have you made any mods/repairs to it yet?

[Commodore 64]

I have not made any repairs yet, but I think tung is on the right track. The screen resistors have to be limiting the output. I was sitting right next to that amp recording that vid, I would think it should have been outputting a bit more. I will take the back off again tomorrow and see if I can't get the values of those resistors, by the stripe code on them, then we can narrow down whether I should replace them or not. It's a great bedroom amp, but I don't think I could use it to practice in a band setting (or as a back-up for my Classic 30), and I'd kinda like to have that option if I wanted.

Eric, I posted some other clips, here: http://www.thefret.net/showthread.php/19227-Finally-Posting-Some-Clips

[Commodore 64]

OK I've written down the color codes for the resistors that have been replaced. Most of them are identical to the stock values. He must have just changed to a different resistor type to reduce hum. However, a puzzling one is R 29, which I believe is the Biasing resistor? It's 2, 2.3k resistors wired together. So this would be 4.6K, right? The legs of one resistor are soldered to the other. Can someone sanity check me on the color code?

http://lucky-cricket.com/files/Pro_jr_R29.JPG

I'm using the resistor calculator here. http://samengstrom.com/24614782/en/read/4_Band_Resistor_Color_Codes
It could also be a 22k and a 2.3k resistor, which seems the most likely. But I'm wondering if it's possible that the previous owner/modder got mixed up on a color code? The capacitor underneath looks like a red, orange, red, red to me (2.3k) but it could also be a red, red, orange, red (22k).

The stock value is 15k, BTW. I relatively sure the intended mod is 22k+2.3k. And the voltage across R22 is -11.1. If that was really a 4.6k. The EL84s would be red-plating, right?


From what I can gather, the other value changes are:

R32: 2.7k -->2.1k
R31: 470k-->410k
R5: 470k-->410k

So R5 is unimportant, but R31 and 32 seem to be part of the Power Transformer circuit. What is the purpose of changing those values?

Also just noticed that the casing for C13 is burnt where the solder joint for R29 is.

I've got a schematic, and I'm tracing the traces on the board. It's starting to make sense. I think it's time to pull the chasses and board out and shut this puppy down (i'll have to use my Vox Pathfinder 15r for practice) until I can fix it. Also on further inspection of the repaired socket, that's exposed copper you see. It's not even been coated with liquid electrical tape or anything. I would thing this is an arc accident waiting to happen.

[tunghaichuan]

R29 is the resistor that sets the bias voltage range. It looks like the mod was done to adjust the bias so that the EL84s would not cook.

I would suggest pulling the resistors in R29 and adding a pot, as shown by Bill M.'s site:

http://billmaudio.com/wp/?page_id=916

This video shows you how to set the bias:

http://www.youtube.com/watch?feature=player_embedded&v=KzetPGnmcXQ

I believe R31 is a bleeder resistor. This resistor bleeds off voltage when the amp is turned off. Changing the value from 470k to 410k doesn't really do anything. I would make sure that it is a 2W metal oxide flameproof resistor as it is in the power supply. R32 adjusts the voltage to the screens. The lower value will increase the voltage to the screens to make them closer in voltage to the plates/anodes. Again, use a 2W metal oxide flameproof resistor here.

R25 looks odd as well. It is a 21ohm resistor. This is one of the screen grid resistors. This is 100 ohms on the schematic, and 470 ohms on Fender amps with 6L6s and 6V6s. You can go up to 2.2k here with the EL84 tubes. I wonder if the modder lowered this value so low due to other problems in the amp.

What you might want to do is remove all the mods and put the amp back to stock before you do anything. See how it works and sounds. Then go from there.



OK I've written down the color codes for the resistors that have been replaced. Most of them are identical to the stock values. He must have just changed to a different resistor type to reduce hum. However, a puzzling one is R 29, which I believe is the Biasing resistor? It's 2, 2.3k resistors wired together. So this would be 4.6K, right? The legs of one resistor are soldered to the other. Can someone sanity check me on the color code?

http://lucky-cricket.com/files/Pro_jr_R29.JPG

I'm using the resistor calculator here. http://samengstrom.com/24614782/en/read/4_Band_Resistor_Color_Codes
It could also be a 22k and a 2.3k resistor, which seems the most likely. But I'm wondering if it's possible that the previous owner/modder got mixed up on a color code? The capacitor underneath looks like a red, orange, red, red to me (2.3k) but it could also be a red, red, orange, red (22k).

The stock value is 15k, BTW. I relatively sure the intended mod is 22k+2.3k. And the voltage across R22 is -11.1. If that was really a 4.6k. The EL84s would be red-plating, right?


From what I can gather, the other value changes are:

R32: 2.7k -->2.1k
R31: 470k-->410k
R5: 470k-->410k

So R5 is unimportant, but R31 and 32 seem to be part of the Power Transformer circuit. What is the purpose of changing those values?

Also just noticed that the casing for C13 is burnt where the solder joint for R29 is.

I've got a schematic, and I'm tracing the traces on the board. It's starting to make sense. I think it's time to pull the chasses and board out and shut this puppy down (i'll have to use my Vox Pathfinder 15r for practice) until I can fix it. Also on further inspection of the repaired socket, that's exposed copper you see. It's not even been coated with liquid electrical tape or anything. I would thing this is an arc accident waiting to happen.

[Commodore 64]

If you reverse the color code on R25 you get 100 ohms. If you are sure this is 21 ohms, then perhaps the original owner was confused by the color bands?

[tunghaichuan]

If you reverse the color code on R25 you get 100 ohms. If you are sure this is 21 ohms, then perhaps the original owner was confused by the color bands?

Maybe I'm reading it wrong? It looks like red, brown, black to me. Okay, I got it now. It is brown, black, brown, the red is the 2% tolerance band. The red threw me for a minute.

[Commodore 64]

I took a dremel to the PCB, that went fine. But I drilled through R34 (150-ohm)when I was putting holes in the PCB for the heater wires extensions. I also melted the copper trace that I had exposed. Lifted it right off the board. That sure is easy to do, there's really not much copper in a trace. Gotta be more careful.

Anyways, the lowest resistor I could round up was a big 5w Flameproof 270-ohm from a Crate amp that I took apart. I have to order sockets, so I'll just order some 150 Ohm resistors while I'm at it. I went ahead and soldered on the 5w, on the outside chance that 120-ohm difference isn't a big deal. Do you prefer ceramic or phenolic sockets?

http://lucky-cricket.com/files/ProJr_PCBFrag.JPG

I also checked each and every resistor on the board. They all match up, the only differences are the one's I listed earlier...so none that would make much difference in power to the power tubes. And as you said, so long as I had -11 on R22 and R21, they should be getting enough juice. The PCB is scorched pretty good, though. Maybe that's what led the previous owner to adjust the bias in the first place.
http://lucky-cricket.com/files/Proje_Scorched_PCB.JPG

I don't think I'll put a variable resistor in for biasing, because my board is older than the one BillM used on his PJR, and there's a resistor in the way. I'll probably leave it as is, but replace those 2 electrolytics (C13 and 14, since one had a melt/burn on it). But just for reference, where can I source a 50k 25 turn trim pots? I think DVM told me once upon a time, but AES doesn't carry them.

I went ahead and split the ribbon cables. It might make more sense to go ahead and use new wires for the sockets. But the less I gotta solder on the boards the better. So I might give it a try with the ribbons.
http://lucky-cricket.com/files/Projr_ribbonCable.JPG

[Commodore 64]

I should have all my parts tonight. I'm recapping all the electrolytics and some of the coupling caps. I also got 2 Octal tube sockets, but I don't really know what else I'd have to change to use them (can't be simply changing the bias via R29). So I'll probably put in 9-pin sockets.

Also got some teflon coated wire for this to reduce the chances of me melting the wrong thing with my soldering iron.

There's 2 small electrolytics near R29. The schematic says 10uf, but they are 22uF in my amp. I ordered 10 uF ones before I realized this. What, if any, affect will this have?

[Commodore 64]

Got my new chassis mount 9-pin sockets mounted. Self tapping screws are awesome. I should have gotten #4 and I should have gotten 1/4-inchers. These were #6 3/8-inch.
http://lucky-cricket.com/files/Pro_jr_new9pin.JPG

Recapped the PCB tonight too. New F&T filter caps, a couple of orange drops, and a silver mica (just cuz I had a 22pf one).
http://lucky-cricket.com/files/Pro_jr_Recapped.JPG

Also wired up the little PCB frag which has the heater wires and pilot light resistor/diode. The wire I've got is 20 gauge, teflon insulated. Seems kinda small for the heaters, so I didn't wire everything up yet. I google searched and most people saw 22 ga. is fine for everything but heaters. So I figured 20 gauge should be good, I can carry 11 amps on it.
http://lucky-cricket.com/files/Pro_jr_pcb_pilot_heater.JPG

I'll put the rest of the tube wires on the PCB board tomorrow evening and see if I can't get the heater wire situation sorted out.

[duhvoodooman]

But just for reference, where can I source a 50k 25 turn trim pots? I think DVM told me once upon a time, but AES doesn't carry them.
That would be this one:

http://www.mouser.com/ProductDetail/Bourns/3299Y-1-503LF/?qs=sGAEpiMZZMuz2E8dTn0VFZv4t3Viqhw2F8w7dO8xNvA%3d

[cebreez]

OK I've written down the color codes for the resistors that have been replaced. Most of them are identical to the stock values. He must have just changed to a different resistor type to reduce hum. However, a puzzling one is R 29, which I believe is the Biasing resistor? It's 2, 2.3k resistors wired together. So this would be 4.6K, right? The legs of one resistor are soldered to the other.

Hey Commodore 64, I noticed nobody pointed out but the two resistors at R29 at least in the photo appeared to be wired in parallel therefore the resistance would be calculated " 1/Rtotal=1/R1+1/R2 "

[Commodore 64]

I got everything wired up and put back together. The amp screams now. My ears are ringing and I only had the volume and tone at halfway. Learned a ton doing this, can't thank you folks enough.

Here's a shot of my wiring job. 9-pin sockets are tight quarters. I'll never use anything but teflon wire for these things. I can't imagine the melted mess I'd have made if I didn't have teflon coated wires.
http://lucky-cricket.com/files/Pro_jr_wired.JPG

Here's a gutshot of the finished product. You can see I wired R34 and 35 directly to the common chassis ground over on the right.
http://lucky-cricket.com/files/Pro_jr_after_gutshot.JPG

And here's a gratuitous shot of the stock blue Alnico speaker.
http://lucky-cricket.com/files/Pro_jr_alnico.JPG

cebreez: whatever the resistance is at R29, the bias of the EL84 is a bit lower than stock, which is preferrable (to me). I think that it's a 22k and 2.3k resistor wired in parallel.

[Eric]

Awesome. I'm glad you were able to figure all of it out. I'm impressed and a tiny bit jealous of all of your knowledge now, but I guess that just means you can help me out if I ever have questions about this stuff.

Nice work, C64.

[duhvoodooman]

Well done! That's a lot of work!

Just curious--how quiet is the amp now? Do you get much hum?

[Ch0jin]

Nice work C64! I can't believe I almost missed a thread like this!

[Commodore 64]

The amp is pretty quiet. Idle noise is nil, even with my tube wiring all crazy like that. I used it at rehearsal last week. Volume at halfway, tone almost dimed. Kept up with the drummer with NO problem.

Now I've got a Peavey Classic 30 that I don't know what to do with, heh, cuz I'm giggin' that Pro Jr. This Saturday. Our first gig, a Halloween party at a local Eagles Club. If you've seen me say before that I've got a "first gig", you have. This is our 4th "first gig". All the others have been last minute canceled. This one, I think is gonna go, as we are officially on their calendar.:rockya

[Robert]

Sweet little amp huh? I love mine!

:)

Congats.

[duhvoodooman]

OK I've written down the color codes for the resistors that have been replaced. Most of them are identical to the stock values. He must have just changed to a different resistor type to reduce hum. However, a puzzling one is R 29, which I believe is the Biasing resistor? It's 2, 2.3k resistors wired together. So this would be 4.6K, right? The legs of one resistor are soldered to the other. Can someone sanity check me on the color code?Sorry that I am hopelessly late to the party on this, but those two resistors look like a 22k (red-red-orange-red) and an 82K (gray-red-orange-red), which in parallel would be 17.3K. That would be a much more reasonable resistance change from the stock 15K resistor for making a bias adjustment.

As Cebreez correctly pointed out, the formula for calculating the overall resistance for two resistors in parallel is 1/Rtotal=1/R1+1/R2. If those really were 2.3K and 22K resistors, the overall resistance would be 2.08K!

[Commodore 64]

OK that makes a LOT more sense. I also did that calculation and got 2.08k and I knew that could not be right. This has been bothering me a little bit. I even had a dream about R29 wherein I figured out that they were wired in series not parallel. I had ordered some 18k resistors in case I wanted to replace R29 with a single resistor, but since I was getting 11.1v on the EL84s, I figured i'd leave well enough alone. Those resistor codes can be really difficult to decipher, especially in bad lighting.

I should have just stuck my multimeter on it.

[duhvoodooman]

Yeah, the problem arises when you really don't know what the resistor tolerance value is. Here, most of them appear to be 2% (red band). But it get's pretty confusing when it's red at both ends! Where do you start from? One of the tipoffs for me was that 22K and 82K are both pretty common resistor values. 2.3K is not.

As you're probably aware, measuring resistors when they're installed in a circuit can be very misleading in some cases, if there are parallel paths from the two points you're measuring across. It just depends upon how the resistor is connected and what else is around it.

[Ch0jin]

Yeah I would have posted up the correct formula too, but by the time I found this thread it had already been sorted! Pretty slack for one of the resident EE's I know :(

On color codes though, often the tolerance band is spaced differently to the others, so if the gap between the value bands is say 2mm there might be a 3mm gap to the tolerance band. On 1/8W Metal film resistors with the four bands for value, it can get pretty hard to tell though so I tend to meter them "just to be sure". I once had a whole bag of 10K resistors from Futurlec turn out to be 1K resistors.......oops.

As DVM says though, when you have a lot of resistor experience, it often comes down to just having a feel for standard resistor values and what looks right.

[duhvoodooman]

On color codes though, often the tolerance band is spaced differently to the others, so if the gap between the value bands is say 2mm there might be a 3mm gap to the tolerance band. On 1/8W Metal film resistors with the four bands for value, it can get pretty hard to tell though so I tend to meter them "just to be sure". Yep, sometimes the band spacing is visible, but I find that to be so variable between resistor types and makers. For example, in C64's photo which I've copied below, the carbon comps have a pretty obvious larger gap to the tolerance band, but the band spacing on those gray oxide resistors is the same across the board. Like you said, the only way to be absolutely sure is to measure 'em with a meter--but that can be misleading when they're installed!

http://lucky-cricket.com/files/Pro_jr_R29.JPG

[Commodore 64]

I've been noticing what I think is cone cry. It's worse on the neck pickup and on certain notes. Sounds like there's some resonance that causes some scratchy nastiness. At first I thought it was microphonics or rattlling plates in my tubes. But I'm pretty sure it's cone cry (after googling and reading descriptions of it) from the speaker. I've also got a Valve Jr. that I'd like to try to cram a 10-inch speaker into. And, I'd also like to get some new tubes in this Pro Jr.

So I headed on over to www.parts-express.com this morning because I also wanted a couple 1-Ohm resistors. I ordered up a couple Sovtek low-microphonic 7025 tubes and a pair of JJs, along with an Eminence Ramrod 10-inch speaker. (http://www.eminence.com/speakers/speaker-detail/?model=Ramrod)

http://i00.i.aliimg.com/photo/v0/249910552/Eminence_Red_Coat_Ramrod_10_Guitar_Speaker.jpg

So the next project will be to do a couple mods to the Valve Jr. and hopefully get Alnico Emi from the P Jr. in there.

[Commodore 64]

High pitch squeal is from my One Spot. It's exacerbated mostly by TC polytune, but it's still there if I take it out of the chain. I had my local store order me a BBE SupaCharger (He can't get the Voodoo Pedal Power pedals), but he's giving me a pretty good deal ($116).

I gotta say that I prefer the tone of the original speaker, even with the tear. The Ramrod is crisp, clean and freakin LOUD. But I was digging the early breakup of the original Alnico. Once I fix the original speaker, I'll put it back in the Pro Jr. I finally cleaned out my garage so I think I'm gonna build a cab for my Valve Jr that will house a 10-inch speaker. Either that or maybe I'll just make a 10-inch external cab. It's too bad the Valve Jr 8-inch speaker is 16 Ohm (and my 10s are 8 Ohm) because it would be cool to make a cab with both an 8 and 10 inch speaker.