Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: eleventeen on November 06, 2010, 09:54:21 pm
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This was $25. and made my day with an ear to ear smile. Cherry condition Radio Craftsmen 500 w/2 qty Genalex KT66. Haven't given it the smoke test yet. 2*6SN7, 2*KT66, 5V4.
(http://i172.photobucket.com/albums/w32/ttm4/ebay_tubes_3001.jpg)
(http://i172.photobucket.com/albums/w32/ttm4/ebay_tubes_3005.jpg)
(http://i172.photobucket.com/albums/w32/ttm4/ebay_tubes_3003.jpg)
Incidentally; the PT is a Stancor PC-8414 (complete with yellow label) 600-0-600 (!) HV winding.
I *believe* this is a triode-connected amp, but I'll have to research further.
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WOW!
I love a great score!
I do hope you consider keeping it a hi fi. Those craftsman amps have a respectable following for good reason, they sound great. the other "tube guy" around here is an audiophile and has a very similar unit he plays regularly. (2 units, I believe they are monoblocks)
another winner in the lottery of life!
Ray'
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Thanks, Ray, been a while since I caught a good one. Wasn't it you that snagged the $5 Epi amp a while back? Actually, this came from my ad "we buy old tube equipment".
I seriously doubt I will "guitarize" this; I'll probably just sell it as is. In the meantime I am trying to do some research. Turns out there was a 500, a 500C, and a few other variations on "500"...and, viewing pix of them so far, they are different, perhaps not importantly so. A fair number of these were made, maybe 10,000 - 15,000, judging by serial numbers.
The differences are the components between the power & output transformers.
One type has a 60 second thermal turn-on delay relay in it, like a 6N060, looks like a tube
Another has a FATT choke, big, like a Super Reverb output trans. (The power supply uses TWO chokes, 2nd one visible in the gut shot)
What was also good about this was, the seller also had the matching (mono) tuner. (It's not really "matching" it's made by the same manufacturer and is an open-chassis 8" tall affair like an old Motorola) But, it was built into their 1952 custom-built home and pulling it out would have left a rectangular hole and 5 knob-holes in their paneling. The seller didn't want to leave that hole. Which is just as well because I really don't have any interest in the tuner & I'd just as soon not pay for it. Win - win. I'm sure I could get it for free if I supplied a reasonable looking & finished piece of ash-style wood to plug up the hole and threw a couple of screws through the pot-holes to keep it in place. I may consider doing that. Or not!
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The receiver would be great eBay fodder.
Something to ponder with the holidays so close.
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Cherry condition Radio Craftsmen 500 w/2 qty Genalex KT66.
Great find. Well worth a full restoration.
Incidentally; the PT is a Stancor PC-8414 (complete with yellow label) 600-0-600 (!) HV winding.
The 500 used an LC power supply, with the first choke connected to the secondary. B+ is only 450 V with the stock 5U4G and 117 V mains.
I *believe* this is a triode-connected amp, but I'll have to research further.
It is. Based on the original Williamson. The US equivalent of the glorious Quad II.
(http://76.76.148.108/~glennl/wiki/craftsmen_500/craftsman500a.jpg)
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Wow!!!! :huh: That is a truly outstanding find!
And please don't guitar-ize this amp; it really deserves to live life reproducing wonderful music.
The striped Sprague caps have a reputation among radio restorers as being universally leaky. That may not be the case in your amp, but I have read an extraordinary number of posts on a restoration forum where an untold number of scopes, test equipment and radios have been fixed by simply replacing every striped cap in the entire unit. They are willing to do that because after finding a handful that are leaky, they know that the others will fail after a short additional period of time.
I don't have to tell you that keep the smoke-glass KT66's seperate for sale is a smart idea.
And you may want to double-check the circuit against FYL's posted schematic. The first-stage plate and cathode resistor values on the schematic seem reversed (470k where 47k should be, and vice-versa).
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And you may want to double-check the circuit against FYL's posted schematic. The first-stage plate and cathode resistor values on the schematic seem reversed (470k where 47k should be, and vice-versa).
The schemo is from Sams Photofact.
And yes, R1ak should be 470 Ω, not 470 K. R1ap is correct at 47 K. Classic Williamson values...
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Thanks, HBP!
When I began running my "we buy tube stuff" ad, this was literally the prototypical type of situation I was imagining. I happen to live in an area where a remarkable number of folks have lived in the same homes for 35+ years. Of course I always mention that I'm into musical instruments; haven't gotten the $125 '55 Strat yet....but a neighbor DID give my niece a brown 1960 Fender Concert that is bigger than she is!
And please don't guitar-ize this amp; it really deserves to live life reproducing wonderful music.
No worries, wouldn't be prudent!
The striped Sprague caps have a reputation among radio restorers as being universally leaky. That may not be the case in your amp, but I have read an extraordinary number of posts on a restoration forum where an untold number of scopes, test equipment and radios have been fixed by simply replacing every striped cap in the entire unit. They are willing to do that because after finding a handful that are leaky, they know that the others will fail after a short additional period of time.
Yes, I have heard the same and would probably shotgun those if I wanted to operate. The issue is whether *I* should embark upon the rehab (the amp, not me!) because, at least in my feeble mind, the kind of fetishist who want this may want PIO caps, or polywhatthehell caps, or turbo-fletcher caps, and maybe it's a better bet just to offer it unmolested.
I don't have to tell you that keep the smoke-glass KT66's seperate for sale is a smart idea.
I'm think I'm just going to test them and confirm they are of decent strength.
And you may want to double-check the circuit against FYL's posted schematic. The first-stage plate and cathode resistor values on the schematic seem reversed (470k where 47k should be, and vice-versa).
There are a number of sort of interesting aspects to the schematic. You know, as examiners of many schematics, we see that mirror-image structure of a typical push-pull stage driven by one or two preamp stages and they kind of look the same after a while until one acquires higher level knowledge. I've been reading about Williamson's and triode amps and etc etc, and absorbing as I can. I *DID* notice some oddities; the HUGE cathode resistor on the first stage; the feed from the heater winding to the cathodes of the KT66's...
What I can tell you is that there was a "500", a "500A", a C500, a 550C, and perhaps one more variant. with rather modest differences...choke size(s), B+ turn-on relay, rectifier tube choice, 5U4 vs 5V4, I think they went to 6L6GA after a while....and other stuff that is either trivial or world-shaking depending upon one's fetish level. You can see from the pix that the one FYL posted has a monster choke betwixt the OT & PT, leaving no room for above-chassis electrolytics, whereas mine has a Twin-Reverb sized choke & two can electro's.
The GREAT thing about this is that I don't have to do ANYTHING about it right now, as I am swamped with family issues at the moment.
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I *DID* notice some oddities; the HUGE cathode resistor on the first stage; the feed from the heater winding to the cathodes of the KT66's...
The 470 K Rk is a labelling mistake, it should read 470 Ω; there's 40 V atop the cathode Rs, connecting them to the heater CT raises them, with 6.3 V AC running atop 40 V DC => less AC noise, as the static DC field is much stronger than the AC one.
Another bonus is reduced heater to cathode voltages. Check the cathodyne: the cathode is at +100 V, a high value - still OK with quality 6SN7GTs which are rated for 200 V but a problem with sub-par tubes, riding the heater voltage atop +40 V reduces this to +60 V - OK with nearly all 6SN7GTs and their derivatives.
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FYL, the 470K, error or not, stood out even to me with my sketchy knowledge.
And yet.....
http://www.xs4all.nl/~ideas/amps/index.html
This ckt has it as a 470K as well. Me? I dunno.
Looking at the one still sitting atop the Maytag, fresh from the hunt, first triode has either a 220 or a 22K resistor to chassis ground. I can't tell whether the color is brown or orange, if I were forced to give an answer I would lean brown = 220 ohms.
There appears to be no "bus bar" mega-ground bus...but the input RCA connector is just about as close to the first 6SN7 as are the tube-socket mounting screws. I mean, jammed right next to it.
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And yet.....
http://www.xs4all.nl/~ideas/amps/index.html
This ckt has it as a 470K as well. Me? I dunno.
Same typo, corrected later on the site. Check http://www.xs4all.nl/~ideas/amps/chapt3.html
European notation is 470R, 470K is very close visually.
Most US techs will read 470R as 470K with a tiny flea dropping atop the K...
Copies of the original WW leaflet are available online, for instance here: http://www.clarisonus.com/Archives/Amp_Design/Williamson%201952%20The%20Williamson%20Amplifier.pdf
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:smiley:
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FYL, thanks for that great link.
Now if I can just convert to rods per fortnight....!
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Now if I can just convert to rods per fortnight....!
I prefer furlongs per fortnight. 1 furlong = 40 rods
:grin:
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:glasses9:
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> The schemo is from Sams Photofact.
SAMS' draftsmen often had a mistake or two.
As already said, first cathode is 470 no K.
Otherwise the 5.1K NFB resistor would set up an unlikely gain of 1.01 instead of ~~10, a reasonable value.
And it is thoroughly Williamson. A tinkerer's dream. Over-done.
Not a guitar amp: way too polite. And of course more valuable as-is.
> feed from the heater winding to the cathodes of the KT66's...
Yup, as said, this is the +40V DC at cathodes TO heater to attract stray electrons away from cathode signal circuit. And reduce stress on the cathodyne H-K insulation.
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nice score eleventeen!
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Well I finally delved into this a little...more than anything, just alcohol & Q-tipping off 50+ years of liquor-cabinet skunge to prepare it for sale more than anything else. The chassis is like a mirror!
The Genalex KT-66's are AWESOMELY matched, within .5 ma of each other, and they test virtually new. This thing probably saw very little use in its almost 60 years of life.
What may well impact the salability dollar amount is that the power transformer appears to have been replaced. I am sure the specs of the replacement tranny are proper, but I can see the solder joints are not factory, and, there is an unused winding which is taped off. You can see it in my gut shot--immediately west of the fusepost, with orangish tape on the wire-ends. It's not that obvious. The first clue was the typical yellow Stancor label on the end bell; A factory tranny wouldn't have that, and none of the pix I've seen of these retain the Stancor yellor label. Underneath which it's stamped "P-8414". It wouldn't surprise me to find that the tranny is identical to factory, just labeled as a replacement part instead of unlabeled as OEM.
So I can't sell it as "virgin untouched" but other than that, I've variac'ed it up to about 85% and nothing blew up, and the B+ was strong and stable.
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why is this post in the tools board?
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:bravo1:
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SCORE, droools* ebay fodder for sure.
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Im green with envy :icon_biggrin: great score my friend.
no doubt what is that doing in tools thats old amp porn give us a parting shot now that its cleaned up if you dont mind.
Thanks Bill
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Wow! Very nice score. A keeper.
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:thumbsup:
Very nice indeed!
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looks like it was your lucky day :laugh:
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CA only has junk . Yard sales here are handled my the Tijuana Mafia garage sale division. socks, shirts, tamarind candies for the youngins.
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actually, it is more appropriate to be "green" with envy today...... which I am..
Nice find!
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Well, it's been a while and I know I will probably disappoint some here with this report of the "fate" of this amp.
But: I determined that the power tranny was not original; and not only was it not original, the mounting pattern did not *quite* match up with the holes in the chassis. The way the tranny-replacement tech addressed this was to "tweak" the legs of the replacement power tranny inwards. This caused two small pieces of ugliness, eg; the core of the transformer hangs out over the edge of the chassis by about 1/8" and the mounting legs got tweaked inwards. This also raised the tranny about 1/8" above the chassis. None of this affects operation, of course, but visually, this amp was in museum-grade condition, or so I thought. The chassis was really remarkable, mirror-like.
The repl transformer also did not have a center tap, while the original did.
I also discovered that one of the can electrolytics was disconnected under the chassis....had to presume it was NG.
Anyway, I bought it for $25 and sold it for $685 on ebay. The end.
I will say this: The Genalex KT-66's were absolutely the most closely matched pair of tubes I have ever encountered, and were literally in new condition despite being 1951-1952 tubes! Gm: 5500, 5600. Cathode currents: Identical. An ungodly matched pair of tubes.
(http://i172.photobucket.com/albums/w32/ttm4/rc-1.jpg)