Building with junk, workflow procedure?

[JJGross]

OK, maybe not junk, but old salvaged components.

I posted here asking if anyone wants the antique hi-fi radio/amp I trash picked, and no responses means I'm making a tweed princeton out of it. Pretty much everything is there with plenty of spares. I've already tested all the nice RCA, CBS, Sylvania and other old USA tubes it was stuffed with and I'm good to go on those including the 5Y3gt rectifier, 6V6gt power and 6SJ7 required for a Champ/Princeton build. Even my switched-on volume control and tone control 1M pots are already there. There are actually a few of the 6SJ7, 6SK7 & 6SG7s that all tested good as well as receiver tubes & even a 6E5 'Magic Eye' that'll be on ebay soon. A little part of me wants to put that 6E5 running off the power section to display what I'm playing in true Buck Rogers style. I think with my first amp though, I'll keep it simple.

Done plenty of amp tweaks, & other electrical/electronics builds & mods but never a guitar amp build with a pile of parts. What I'd like some pointers on since this will be my first full amp build are workflow procedures I should be following. Aside from the obvious safety warnings and precautions which I'm well aware of, I'm guessing once I completely tear this thing down and inventory my parts pile, I should test the transformer for voltage output first and go from there?

As cool as it would be to build my amp using all those bumblebees and domino caps, CC resistors and other 60+ year old goodies, I'll be sourcing a lot of new (reliable and stable) parts. I'm pretty sure a lot of those have drifted so far from spec that it would be awfully hit or miss using any of them. Also debugging something made with unknown salvaged components would be like herding cats in the forest while blindfolded. Would testing all the caps, especially the ones like the dominos, be worth it to re-use them or are they pretty much all not worth saving from age? I understand the electrolytics are likely junk & even if I want to use the resistors, testing them all will be the only way I'll know what value they really are (not looking up the color codes).

Any other pointers from the salvage junkies around here? Any work flow you'd recommend for my first stab at making a neo-vintage grinder?

Cheers,
- JJ

[bob p]

those domino-looking caps are mica capacitors. IME they're pretty stable and shouldn't cause problems for you. as far as parts go, you can try to recycle old parts if you want to, but in the Tweed princeton circuit I'd recommend the following:

1. throw away all of the electrolytics that you've salvaged and replace them with brand-new caps

2. for coupling caps, you could try to recycle old caps. the micas are probably fine, but if you have old PIO caps they'll probably be leaky. IMO the best approach is to build your amp with all new resistors and caps. this eliminates them as potential trouble spots that might be hard to diagnose.

3. yes, i would check no-load voltages on the PT before doing anything else. you should at least verify that all of the leads on the PT are functional and that the PT actually works before you do something like cutting a hole in a chassis for it. bear in mind that the no-load voltages will be much higher than the under-load voltages.

[JJGross]

That was kind of what I was thinking, but I've never done this before.

What kind of ballpark % should I expect for unloaded higher voltages on the transformer? I looked all over on the tranny and there is what looks like a part or catalog number, but no familiar looking transformer type codes along the lines of ###-######. I haven't pulled anything from the old chassis yet so I may find something when I get it out and in hand.

Thanks,
- JJ

[dai h.]

-I would check old "domino" micas for leakage (they can leak, IME)

-if you find the exact Les Paul Bumblebee (.022uF 400?V) or bees, I would sell it. They can go for a ridiculous amount of money. Use that to buy parts you need or whatever else.

[JJGross]

-I would check old "domino" micas for leakage (they can leak, IME)

-if you find the exact Les Paul Bumblebee (.022uF 400?V) or bees, I would sell it. They can go for a ridiculous amount of money. Use that to buy parts you need or whatever else.

From a quick visual, all the micas look very clean with no swelling or sweating visible. Any other things I could look for on those? There are quite a few of those in there.

I hadn't thought of reselling some of these, but that would probably be the smartest thing I could do. IIRC, don't cap stripes follow the same color code for value as resistors?

Too bad there aren't any tropical fish caps in there as well, some people seem to jump on those whether they need them or not. No matter, I have my own stash of 100 minty-mint N.O.S. fishies here for other things, maybe one or two will find their way into my amp build. Too bad they're all 0.15uf and not a larger selection.

I'll be looking real hard at all those old parts when I start dismantling everything tomorrow. I was going to just tear a lot of it out quickly just to get the goodies I want to reuse, but now I think I'll be de-soldering everything one part at a time.

Maybe I should build that capacitance tester I've been thinking about before I start on the amp. ;D

Cheers,
- JJ

[dai h.]

you'd have to put some voltage on the micas to check. I don't think you'll get a visual indication as with alu electros. I think one of the sources for probs is moisture creeping in the body of those (which are bakelite). Modern epoxy coverings keep out moisture better.

the code for .022uF should be red red orange (plus another colour) IIRC. I don't if it's vintage Les Paul owners trying to restore their ultra expensive vintage instrument or what but some of the prices on the exact bumblebees are pretty crazy.

[JJGross]

Got everything apart and from the looks of them up close & personal, I'm not messing with any of the caps in this. There might be a few good ones in there, but most are so bad that when handled they're actually falling apart. They're probably the reason this thing was trashed in the first place. I also found on ebay someone selling old magazine ads and one guy has one for this exact radio. I also checked several others like it and can definitely say from the advertizing of the day that this was a 1947 Sparton model 10-BW-76-PA. The picture in the4 ad I found on ebay looks exactly like the cabinet I pulled this from right down to the hardware and knobs. Too bad I couldn't salvage the whole thing.

I did do something incredibly stupid while taking this apart. I pulled the transformers first to get them out of the way and after they were sitting on my bench it dawned on me that I never made note of which leads were the primaries. That's going to make it pretty hard to test it now. Any advice on how to figure it out on my own? The only codes on this are the numbers 5A6146 and nothing else.

The wires from it are pretty faded color wise so telling me the red ones are my heaters is pretty much useless since everything has turned either dirty white or black. There are both solid core and stranded wires though. The stranded are smaller and the solid core wires are quite a bit heavier. There are 3 large green wires (solid) and 2 large solid (white maybe) wires. The small are 2 black and 3 'other' (maybe white or red 40 years ago).





Any help is greatly appreciated.

Cheers

[black_labb]

they probably dont have voltage taps for international voltages so id say just put a multimeter to it and test which are connected. that will atleast let you know which wires are connected, and the resistance will tell you what taps they are (but not the order) if you know what the heaters are then you have a start.

[petemoore]

When DMM readings are done [finding winding leads, noting R values etc.],
Because voltages can be multiplied many times by the tranny, starting with a very small voltage AC adapter comes in handy for safety, using the power applied as 'x1', what the secondaries put out as the :'_' side of the windings ratio, this makes it pretty easy to find the wind/voltage ratio of the transformer windings, and if the AC applied is small enough, after being multiplied many times by the tranny, may still be at a safe level for humans.

[JJGross]

That's a good idea. I have a 5v wall wart that I could use.
Worst case scenario is I get a whopping 25v ~ 30v out of it. ;D

Cheers,
- JJ

[Steve Conner]

I have to worry when I see "junk" and "workflow" in the same sentence

[Enzo]

Sounds like you never set at a commercial repair bench. SOmeone brings in a Gorilla practice amp and those two words come together quite naturally. Or for that matter any number of chea... inexpensive products and small items.