Postcard from Cali... Fairchild 670...

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SSLtech

Well-known member
Joined
Jun 3, 2004
Messages
5,447
Location
Florida (Previously UK)
I'm out of town at the moment... doing a 'service call' on the west coast.

One of the items which needs attention is a rack of BA-11s which is buzzing a little (after the pad but before the gain)


Anyone got a schematic that they can link to/post/email???

Yooz guys are awesome!

Keith
 
Here's a 1A. Has two 1620's like the 11. Might help, might not. Still searching.

BA-1A.JPG


For more RCA's: http://www.one-electron.com/FC_ProAudio.html
 
Most of those old tube preamps would be easy to clone if it weren't for one thing: the transformers. The transformers usually account for a good deal of the sonic character. You could duplicate the circuit exactly as shown on the schematic but if you didn't use the original transformer types, it might very well sound completely different.
 
Coolio...

I found the problem... it's in the PSU. I'm a re-cap this bad boy tomorrow.

I just heard some magic words...

" Keith, can you fix the Fairchild 670"

:shock: :grin: :shock: :grin: :shock:

Keef
 
What!!!!

Get the numbers of the small bias transformer. Check the pots for weird taps/ground lugs (not the Davens)
Look inside the can to see if the control input is Triad HS-14 or UTC Y series.
Pull off the A-33 shields to make sure thewy use A-26 backwards.
Did you bring your camera?
Get cap brand names.
Measure the fron panel, the black one, not the outter one.
Look for HS 52 signal output iron.
Steal the 6386's! :razz:
 
...say it's a complete write-off and needs fondling..er..restoring in the privacy of your hotel..er..workshop...and it may take a few months...

:wink:

Mark
 
Kato Sowter {UK} do some irons for that RCA BA1A I bought a set the OP tran is huge, IMHO my pre has a sweet sound I like it a lot, I also put a switch to turn on off the negative feedback for clean or dirty sound, ive never heard an original BA1A probably sounds different as NYD said the trans make a lot of difference, to add to that I don?t think the Sowters are meant to be exact copies anyway, but I do like its sound as it is & was easy to build,would love to hear an original tho.

Cheers Gary O.
 
yeah, measuring frontpanel! :green:
the black one costs 60€ @ schaeffer :wink: I think this time the frontpanel is the cheapest part of all :green:

fairchild670_big.jpg


I'd seen some spare knobs at leeds radio... but i think its the wrong diameter?

GR_KNSP-12.JPG


http://www.leedselect.com/parts-knobs.html "General Radio KNSP-12"
 
okay, I'll have a go at that stuff tomorrow if I have enough time.

The measurements will be as accurate as I can do with what I have here...

By the way, the RCA problem was the cable from the PSU to the preamps. (presumably electrostatic) noise was getting in to the heater AC feed and coupling to the power lines. the gain of the preamps was feeding the power line noise into the signal outputs. I put filters to ground everywhere and things improved, but it wasn't until I shortened the DC power cable that it all got really quiet. The AC heater feed was running parallel to everything else. I think an isolation tranny with a decent electrostatic shield would make things even better, though they're happy as it is.

Keith
 
Robb, slowly but surely getting the parts. Located some orig. Davens complete with the Gen Rad knobs ansd some original VU meters.

Transformer compliment is now known, UTC A26 w A-33 shields (have those) Triad HS 52 signal output, HS 14 control ionput, although I have talked to someone who owns a 670 and his has a UTC Y-xxx . I think you could actually use an A-10 in this spot also. And control output will be the Sowters, as the original Chicago PO series is said to have stability problems which Brian fixed.
I also have some Victor tube turrets, those funny thingys that the components get mounted to.

Also, that Sola constant voltage transformer runs the heaters, so we don't need that. It was for when power in New York fluctuated 30 percent back in the ol days.

Saving up for air fare to NYC to talk to Mr. Narma.
cj
 
CJ

I`ve pretty much got everything, although all my audio Iron is Sowter. I`ve bought the bits slowly over the last 4 years !! I wish I had the info you`ve just supplied at my disposal when I was buying them since the A26`s go for much less than the sowters (thinking with the exchange rate)

The only things I`m missing is the Davens & I`d like to find the tapped pots. I got some 600ohm Daven ladder attenuators mistakenly, & am still on the look out for the bridged T variety.

My PSU will be made up of ex miltary Parmeko oil filled C core transformers. The PSU will be slightly different in that I have separate HT & filament transformers. I built a regulated filament PSU for the 6386 using an LM388K regulator (I`ll have to tuck it away out of sight to not offend the purists out there).

I designed a chassis in Autocad, which I will need fabricating before I can start the build. I`ve got to get all the cutouts & holes for the valve bases done, because the thickness of the metal to take the weight of the transformers is too much for my hole punches !! One thing is for sure. Its going to weigh quite a bit if the 3 boxes of parts I have sitting here are anything to go by !

And control output will be the Sowters, as the original Chicago PO series is said to have stability problems which Brian fixed.

A few years back I was in touch with a very helpful Dutchman who had built one using Sowter Iron. He said that he had an instability problem in the control amp which he had to Zobel out.
 
Check this action!:

"Hi, Happy New Year
I am OK, expect to come to Bay area and to Orange County within the next month or two. Where do you live? We could chat then.
Regards,
Rein"


(Rein Narma, inventor of the Fairchild 670)


Robb, I have a handle on the Daven's but they are 100 bucks a pop.
cj
 
But of course. Thanks, didn't recognize the brand.

Thinking of the 670, anyone know of any other decent comp/limiters able to go vert/lat (or m/s)?
 

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