A not-so-original V1 McIntosh C20 preamp and the quest for a baseline of "factory spec"

[grantray]

Well, I got it Friday and spent the weekend cleaning out 50 years or so of crud with isopropyl rubbing alcohol, cotton balls, q-tips and electronics cleaner. Here are two samples of before the scrub down, complete with crusted cables from decades of buildup...

And here's what the inside looks like now after roughly six hours of non-stop detailing so I could at least make it nice enough that a tech would consider this old fella a pleasure to work on.

I also fixed a broken low frequency filter connection for the right channel. You can see the entire Flickr album I've started by clicking here.

As the Mallory PVCs make clear, she's certainly not original, but the original owner definitely was no goober. The nephew I bought it from (now an older gentleman) said his deceased uncle, the original owner, was in the electronics business and quality of soldering shows, as does the choice in replacement parts, including the Sprague electrolytic filter caps and touches like the silver wire he used to connect the two Spragues.

The seller didn't give me his uncle's name, but I have the serial, and I know the preamp lived most its life in or near Cincinnati, Ohio, so maybe I can track down a little more history if time permits.

Anyway, I'm not interested in making this back to factory-spec. In fact, I sought out this preamp because of just how customizable it was designed to be. But I'd love to know what that spec was, if it ever existed. Does anybody know if there was a standard component spec for the early McIntosh C20 examples?

I ask because I have the service manual with parts list, and it reads more like a build-your-own set of upgrade options, like when you go tot the auto dealership. Interestingly, dang near every tube, capacitor, and control component lists at least two possibilities. Fixed capacitors? Five options. Ditto for the electrolytics.

I've crawled all over eBay and the internet. My image searches have yet to yield two identical sets of parts. So I've developed a healthy sneaking suspicion that no two C20 preamps came off the line the same. Did they come off the line with whatever the line techs had in the grab bag? A Black Beauty here for this guy, a Bumblebee and some Mallory Gems for the next? Because that's what it's starting to feel like.

 

[twiiii]

I saw my share of C-20's at Mac clinics and Davie O. pulled the bottoms off all of them to clean the switches and pots. They all looked the same to me. He also changed the rectifiers which gave Mac head aches at first and why they started the Clinic program. Now when Davie did change caps he didn't use the original black ones. But ones he had preselected, just as he did with tubes he installed at clinics. I couldn't believe the numbers of tubes he carried and went through at clinics. Case after case. One time he was running low n tubes and went to our GE supplier and bought every 6l6 and every appropriate miniature tubes they had. Then after clinic hours he would fire up different amps and install the new GE tubes and measure their performance. If they passed he kept them, if the amps weren't acting right the tubes were put back in their boxes and returned to the dealer the next morning.

 

[nj pheonix]

I wish i had his throw aways
Not the old caps, the tubes!

 

[grantray]

I saw my share of C-20's at Mac clinics and Davie O. pulled the bottoms off all of them to clean the switches and pots. They all looked the same to me.

At first glance, they do look the same. But there are no two that I've been able to see via image searches that have the same Bumblebees, Black Beauties, and Mallory Gems in all the same spots. Mostly they're Bumblebees, but then sometimes the low frequency caps are Black Beauties. Also, the two main caps for the V4 12ax7 cathode follower (C85 and C46) show up as pairs of either bees, beauties, or gems, or unidentifiable silver caps (that I'm guessing from the parts list) are either Aerovox, Elmenco, or Cornell-Dubliner. I also came across a shot where the caps were entirely Bumblebees. Excluding the big filter caps, of course.

There are almost always one or two little differences, which could just be from service and maintenance over the years. But it sure does make it hard to get an idea of the defining, original intent for the C20's sound. The spec feels more like guidelines, tuned to taste and individual preference.

Or, in the late fifties and early sixties, the factory engineers figured anything given on the parts list that met spec was good enough as long as the production line wasn't halted. Which, tinkering with other motorcycles, cars, and musical instruments from the 30s to the 60s, is totally possible.

 

[grantray]

Just found out the original owner passed away 12 years ago. So, while the C20 most likely sat unused since 2005, that isn't terribly long. And such a recent date might explain the rather newish-looking bit of insulated silver wire he used to run between the two Sprague filter caps (see below, and it's shinier than the photo shows). The Asano 2A3 amp won't be finished and delivered for another couple of weeks, which means I still have yet to listen to the C20 do it's thing. Gah. It's like waiting for Christmas when you're four, and the weeks pass like years.

 

[grantray]

I need a favor. Does anybody out there with a 1st gen C20 have the original clips for holding the face plate? Mine are gone, or at least I think they're gone because the face plate has considerable play when all knobs are on. If there should be clips, and you have 'em, do you think you can take some photos for me so I can figure how/where they attach, and from there, make my own replacements? Or at least give me an idea of how/where they attach so I can start working on a solution?

Or, if there aren't clips, give me an idea of what I should be looking for that secures the fitment of the face plate without slop? Thanks!

 

[squeel]

The first version C-20 had a "floating" front panel because it attached to the wood console cabinet. Then preamp bolted in behind it. I have 2: C-20's now.

 

[grantray]

The first version C-20 had a "floating" front panel because it attached to the wood console cabinet. Then preamp bolted in behind it. I have 2: C-20's now.

Squeel, thanks for the info. My C20 is a 2Lxxx chassis, most likely made in late 1959 or 1960, and definitely used brackets to secure the floating face plate to the chassis. For such a short lifecycle, I'm really fascinated by the immense variation I've seen in these C20 preamps.