Fairchild 255 Restoration

multichamp12

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Hey everyone! A few weeks ago I received my pair of Fairchild 255 amplifiers, which had some previous work done to them. When popping the covers off I knew everything had to go since the craftsmanship was of low quality. Huge mundorf capacitors were installed, the gain pots were removed, different rca jacks were installed, the bracket that holds the balance pot in place was gone to fit the large capacitors, the soldering was poor, among many other things. So I tore everything out to bring it back to an original state.

Here is what they looked like when I got them.
Fairchild_1.jpg

Since those mundorf filter caps installed are fairly expensive, I of course kept them for another project. I did keep the mundorf silver coupling capacitors in the amps since they are nice, I just cleaned up their placement. A new can capacitor was installed, along with sprague atoms for the bias supply.

The resistors in the power supply were replaced with metal films and wirewounds, and I decided to replace the majority of the other resistors (since almost all were out of spec) with NOS Allen Bradleys, to keep the original sound, and the original look. I was able to match the critical resistors within 2%. I had never used Allen Bradley resistors before, but I must say I am pleased!

Since the gain pot was gone and replaced with a 100k resistor, I put a new one in. The other 2 pots (Bias and Balance) both originally bolted to a mount. Since both amps were without the bracket for the balance pot, I fabricated a new one. I was lucky enough to find the original RCA wafer jacks that fairchild used, and installed them, replacing the isolated jacks the previous owner installed. I also used a shielded cable for the input, which Fairchild did not originally use in the 255, but they did in the 260.

I apologize some of these pictures are a little fuzzy.
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Diode replaced selenium rectifier for Bias supply
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I obtained a matched quad of black glass Brimar 6L6 tubes, which are fantastic, and some very early 1950's philips ec92 tubes. Currently rolling some Brimar 6067 (Yellow T 12AU7)
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ec92.jpg


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Overall I am very happy with how they turned out. They really sound fantastic, and are super quiet, absolutely no hum or noise at all. Enjoy!
 
Wow ! It was yours ! I wondered who inherited of these poor things.

I am very happy you are taking care of them.
I really like what you've done, keeping the original resistor type.

Very well done. Congratulations. :beerchug:

I am ATM redoing a pair of 255A. Very similar but with EL34 and GZ34, but mine were less modified.
I will do the same with new carbon comp from Japan. And same as you for the PS.

Just a few questions:
- What value for the first cap off the rectifier did you use? I modeled the PS with Duncan PSU and I see, in stock form quite a bit of ripple. Increasing the first cap and second help a lot.
- What model of Mundorf coupling caps did you use? There are many silver one: Supreme, supreme silver oil, EVO etc.. I am in the process to choose my coupling caps and was going to go with Musicap RTX film foil. Originally was bumblebees installed on those amps, and they all leak like mad.
- Did you think adding a DC power tube balancing pot on the bias line in yours. Mine have only the bias and AC pots. So no tube balancing on this one. :(

I was thinking not using the gain pot and going straight to the first tube grid via a 10k resistor + 470k to ground. The first cap is not needed as all modern preamp dont leak DC into the amp. I cap less in the signal path.

Anyway, fantastic job. Very happy to see an modified unit back close it its original state.

Cheers,
Brice.
 
You did a very nice job of cleaning things up! The CE multisection is your only weak component. Authenticaps are so much better in terms of quality. Not sure what the values used are. Brice, do you know?
 
Yes originally 40 40 30 30.

I am using Authenticap KTL25 80 40 30 20. I have yet to decide how but I think I am going for 50 then 120, and use 4.7uf film caps for the rest.
With PSUII, that gives me the best results, still been safe for the rectifier.
 
The CE can is a 40/40/30/30, as per original fairchild specs, I was going to order a custom one from Hayseed, but I had these laying around from a previous project, so i might very well have Hayseed make some, since they are usually what I use.

Brice, I used a 40uf 450v cap off the rectifier. Thanks for the info on the PSU, you should post a thread on your build, and keep me posted.

The mundorf caps are supreme silver oil I believe (i will double check) and they sound great. I did not realize the 255a does not have a balance pot, it would not be to difficult to add one. After getting the original fairchild 255 schematic, the bias voltage should be -32v, compared to -33v on the 255a.

So your going to use 50uf off the rectifier and then 120uf? And 4.7uf film caps in place of the .25uf?

I was torn between modifying the amp and keeping it bone stock, but I think it would be a good idea for me to increase the filter cap off the rectifier.
 
Yes I will, once I am done.
Yes no tube balancing, but an AC balance on the driver side which is not common, but nice to have.
And bias adjustment of course. I'll add a 1R on each side of the power tube to sense what's going on.
Yes, 50uF then 120uF, and 4.7uF film for B+2 and B+3.
No the coupling caps will remain as spec, 0.22uF 400V.

Yes I agree, it's a small modification IMO, and it reduces the ripple by 2/3: worth it.

Will keep you guys posted.
 
Much improvement underneath. What were the large rectangular black things, coupling caps?
 
They’re PS film caps to replace electrolytics. Definitely an electrical improvement, but not a clean implimentation/installation.
 
They’re PS film caps to replace electrolytics. Definitely an electrical improvement, but not a clean implimentation/installation.

Very true, In my other projects I try to use as many film caps as possible. The CE can is just temporary for now until I decide how I will implement increasing the PS capacitance as Brice pointed out.
 
Very true, In my other projects I try to use as many film caps as possible. The CE can is just temporary for now until I decide how I will implement increasing the PS capacitance as Brice pointed out.
It seems to me that the original modder, did not think the original components were to his ear's liking and he put in those other parts. But, i agree that the implementation was rather shoddy and definitely more thought could have gone into a cleaner layout.
 
Here is the PSU modelling, original on left, modified on right.
You can see the oscillation where everything settles.
Definitely better when caps are increased a bit.
PSU-255.jpg
 
Awesome, thank you for that!

I just checked the coupling caps, they are Mundorf Supreme Silver Oil.
 
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I finished up these amps today! The power supply was improved, after modeling in PSUD2, the caps were changed to 40uf/100uf/4uf/4uf at 550v. The PS was really a weak spot for these amps. Hayseed Hamfest made the can caps for me, they are a bit wider than the original cans (and the CE cans) so I had to slightly modify the capacitor mount in order to fit the bottom cover on.

-10 ohm resistors were added to the cathodes of the power tubes, so I can measure their current and bias is set to 42ma.
-Mills and Kiwame resistors were used in the power supply, and the bias supply, all other resistors are still NOS Allen Bradley.

-I measured the output power, and they put out 25w into an 8ohm load.
-Frequency response was also measured, I have attached a graph below.

As I said before, these amps truly sound fantastic!

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Fairchild_255_Frequency_Response.gif
 
Thanks! I must thank Brice for his extensive help working on these amps and teaching me how to use PSUD2.
 
Looks great. it appears the HH cap is the same diameter all the way up where the old style ones necked down right above the crimp. Guessing thats the fitment difference you had?
 
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