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Recreating a legend - Fairchild 275

Sam Cogley

Sam Cogley

Last of the Time Lords
Subscriber
The odds of me ever finding a Fairchild tube setup that I can afford is next to zilch, so among the many projects that have long been kicking around in my head is the idea of re-creating the 275 (and maybe the 255A) monoblocks, and a modernized version of the 248 preamp (that project will involve removing some of the now-useless circuitry like the tape equalization, and streamlining a bit of the dual-mono construction). To that end, I have long been trying to track down information regarding the construction of the 275 OPTs, towards the goal of either winding my own re-creations or finding something suitably close that won't end up costing as much as buying the 275s in the first place, were any to become available.

dcgillespie's amazing thread detailing the restoration of a pair of 275s back in 2020 has provided a lot of great information: https://audiokarma.org/forums/threads/rare-beauty-fairchilds-275-amplifier.909896/

Another incredible source of information, referenced by Dave in his amazing thread, is the 1956 article from Todd Transformer, describing the construction of the 275 OPTs, with a photograph that includes some useful information.

First off, the original OPTs:

Fairchild-OPT_R&TN_11-56.jpg

That "AA-15" marking is a great clue, it's an Arnold Engineering part number. It has to be a 15, there is no AA-75 in the 1950s Arnold catalogs or price sheets, and a pair of 15s side by side would line up with the apparent size of the 275 transformer can.

Arnold AA.pngArnold AA-15.png

OK, now we have some good information about the materials used and the dimensions of the twin C cores.

Now, let's look at the response graph that Todd Transformer reported in that article:
Fairchild 275 frequency response.png

Next up, research to see what's out there at a somewhat reasonable price.
 
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That core looks just like the Macintosh MC75's I rewound, Except for being a single core.

Picture from the web.

Edit: Friend owned Fairchild 275's and I listened to them often, They were nice.

Was big on winding his own outputs. Had a single 275 with a bad output that I was buying but passed before we got it together.

iu
 
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The contenders (all should be dual C-core OPTs):

Takumi OHC-80P-4.13K EN.png
Takumi OHC-70P-4.13K. Hi-B steel that should be similar to or an improvement on the old Silectron, the 80W power rating gives a bit of headroom on the 70W listed by Todd, the 4.13k primary and 40% UL taps are a bit more textbook for 6550s/KT88s than the 3500k/25% taps reported by dcgillespie on his tests of the 275s he restored, and the response curve looks pretty good, even if it rolls off a bit faster at the low end than the originals. General Transformer Sales in Tokyo has these currently on sale for $436.75 each. The problem being that I don't see that General Transformer does international sales, and I can't find anyone else who sells them. They're special order, so if I could find a way to order them, there is at least a chance that I could get them to leave the cans off and pot them myself in cans that are more close in shape to the original Lowey design.


Kiawami FM-60P-5k EN.png
Kiwami FM-60P-5k. Hitachi Finemet cores, currently $544.37 each from General Transformer Sales in Tokyo, with the same issues there previously reported. The 60W rating gives me a bit of pause, as Dave's testing revealed that the original 275 OPTs were capable of a bit over 60W of output, but the low frequency response is excellent even if the HF isn't quite as good as the 80W Takumi. The datasheet doesn't report the UL taps, but they're 50% on the 120W Kiwami OPTs below.
Kiawami FM-120P-3.6k EN.png
Kiawami FM-120P-5k EN.png
Kiwami FM-120P-3.6k and FM-120P-5k. Bigger version of the FM-60P-5k, the 3.6k version has a primary winding closer to the original 275, but with 50% UL taps instead of 25%. I wish they were around 40%, 50% is kind of odd. They're about double the price of the 60W version, at $1,001.14 each from General Transformer Sales.

Hashimoto HW-60-5_1024 EN.jpg
Hashimoto HW-100-5 EN.png
Hashimoto HW-60-5 and HW-100-5. Hashimoto is a legendary transformer producer related to the Sansui of old, and the low frequency response reported here is excellent. They roll off on the top end a bit faster than the Takumi. The 60W version has the same reservations about headroom as mentioned above with the Kiwami. They have UL taps, but the datasheets don't give any specifics. I don't have information on the steel used for the cores, but the 60W version is $368.25 each at General Transformer Sales, or there is a pair on eBay for about $1,030 shipped. The 100W version is $441.71 at General Transformer Sales, and a pair is on eBay for about $1260 shipped. Their US distributor, tube-amps.net, lists the 60W at $1056/pr + shipping, or the 100W for $1284/pr + shipping. (Of note, for the audio restorers out there, Hashimoto apparently makes reproduction Sansui 1000A OPTs.)

ISO FC-60-5.jpg
ISO FC-60-5. ISO is the current iteration of what was the legendary Tango Transformer, though the name has been sold on to another company. Pretty decent low frequency response, excellent HF response, unknown UL taps, and the same reservation about the 60W capacity maybe being just a touch low. $301.86 each at General Transformer Sales, $349.21 each at tubesUSA.com.

ISO FC-120-3.5.jpg
ISO FC-120-5.jpg
ISO FC-120-3.5 and FC-120-5. Excellent low frequency response (especially on the 5k version), excellent HF response, unknown UL taps, and the same reservation about the 60W capacity maybe being just a touch low. FC-120-3.5 $487.70 each at General Transformer Sales, $745 each at tubesUSA.com. FC-120-5 $522.62 each at General Transformer Sales, $709 each at tubesUSA.com. I have no idea why the 120W iron is so much more expensive at the US distributor than in Japan, while the 60W version has a much more modest price increase here.


I've run across a couple of other, smaller suppliers in Southeast Asia that look interesting, though I don't have much data on them other than some good reports about their products here and there. SAC Thailand has a 5k C-core OPT with 44% taps at $725 a pair, and J&K Audio Design in Indonesia looks like they do some interesting work, though their updates on their website are kind of sporadic.
 
Buck Rogers said:
That core looks just like the Macintosh MC75's I rewound, Except for being a single core.

Picture from the web.

iu
Click to expand...
A single C-core with one bobbin is a relatively unusual style of construction, from what I've seen. Most C-core transformers seem to be either single core with two bobbins (one on each side), or dual core with one bobbin.
 
I've been curious about C core output transformers. I decided to try out a Lundahl output transformer once I have the $$$ to get them.
 
The high US price could be because the pricing has not been updated since the extreme devaluing of the yen these past few years. We're at 160 yen to the dollar now. Ten years ago, it was 100 yen to the dollar.

Very interesting project and here's to your success!
 
cicada_shell said:
The high US price could be because the pricing has not been updated since the extreme devaluing of the yen these past few years. We're at 160 yen to the dollar now. Ten years ago, it was 100 yen to the dollar.
Click to expand...
High end transformers were never cheap. That is why a lot of audiophile tube amp manufacturers went out of business. But you have to shop around because there are middleman that like to make extra off of them and some of those overprice the part too much online. If you go through official channels, then you get real prices. Even though other things like Tariffs apply (VAT, US Tariff, etc). In the US, a 15% Tariff is imposed on electronics parts from Sweden. However Sweden imposes a 25% Tariff (VAT) to their consumers on audio electronics imported from the US. Sometimes certain transformers are overpriced from the manufacturer due to certain finished products made from them. Like the Triad HS-29 ($411 in the US after a 12.5% Tariff from the Philippines ) that is in a $5000 mono EQ. But the other transformers in that line from Triad range from $260-$375. Which is more in range for high end innerstage. Sowter's equivalent (1195) is $225. A lot of people don't understand Tariffs in the US because they never exported anything. But before the US Tariffs were around, exporting finished audio electronics from the US to Sweden was around 30%. And Buying parts from Sweden to be shipped to the US was a 35% VAT.
 
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Besides any thoughts on output iron (I’m very open to discussion of these or any other double C-core OPTs that may approach or match the performance of the original Todd parts, if any of you lucky 260/275 owners would be so kind as to provide some measurements of the chassis and the transformer covers, I’d be very grateful.

The numbers that I’ve seen thrown around are 12.25” x 8” for the chassis, and 7.25” for the overall height. It looks like there is about 3/4-1” between the transformer covers and 1/4”-1/2” between the transformers and the sides/back. Considering that the front badge is around 1-7/16” talk, the chassis depth looks to be about 2”, maybe a bit more.

Based on those assumptions, the transformer covers appear to be about 5.5” wide, 3-3/4” deep, and 5.25” tall. I’ll be interested to see how closest is to the real measurements. From what I’ve seen in some online shopping, getting the right style of transformer covers is going to involve buying the biggest ones possible in that rounded style, buying some extras, and cutting them up and welding the pieces back together to get something that’s the right size and shape.

Obviously in need to decide on power transformers as well, but that will come after I sort out the first steps.
 
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