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

[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...=sGAEpiMZZMuz2E8dTn0VFZv4t3Viqhw2F8w7dO8xNvA=

 

[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!

 

[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!

 

[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.

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.