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

[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.

[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.

[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/arc.../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.ph...ing-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?



I'm using the resistor calculator here. http://samengstrom.com/24614782/en/r...or_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...&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?



I'm using the resistor calculator here. http://samengstrom.com/24614782/en/r...or_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?



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.


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.

[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.


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).


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.


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/...F8w7dO8xNvA%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.


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.


And here's a gratuitous shot of the stock blue Alnico speaker.


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!