Brice
Super Member
Hi guys,
A quick post on a pair of Fairchild i just finished last week.
I didn't know Fairchild before, and never heard one of their product. To me it was a new adventure.
First, let's start of how they sound: fantastic !
Those little guys are extremely dynamic. They have a perfect integration top to bottom. Very clear and detailed sound, perfect for Jazz and vocals. They may lack some 3D, but the punch they have and the accuracy compensate. And they are loud. Small but loud.
I really enjoy listening to them since last week and I am sure they will make it often to the amp rolling addiction I am suffering.
I equipped the amps with Jan GE 6AB4, RCA clear top 12AU7, EH 6CA7EH, Sovtek GZ34.
What I have done to them:
1) Rebuild the bias supply and removed one selenium rectifier. I increased a bit the caps there.
2) Rebuild the PS with additional capacity. I used PSUD to find the best choice of caps. My findings was to use a KTL25 Authenticap as the big can, and additional film caps for the B+2 B+3 supplies. I used 50uF off the rectifier (30+20) and a 1uF film cap in parallel, then the rest of the KTL25 so 120uF (80+40), then 4.7uF film and finally also 4.7uF film. The original design had 40 40 30 30, and now I have 51, 120, 4.7 4.7. I also used high quality resistors for the supply. I added 2 diodes before the rectifier for added safety. I also added a fuse 250 mA sloblo on the B+ rail. The ripple has been lowered to less than a volt, as of before it was over 3 V.
3) I did not use the gain pot, so I re purposed it as a DC tube balancing pot, as on this model, there was only a bias and an AC balancing pots. So instead of the gain pot, I now have a 10K bias pot adjustable with a screw driver, feed by the bias supply, and on each side a 100K to ground, and with a 47K to each coupling caps. I had to tweak the last resistor of the bias supply to compensate the changes, so the 10K was boosted to 39K. The bias range is now -41V to -31V.
4) As mentioned, I don't use the gain pot. The RCA input goes straight to the grid of the driver tube via a 10K resistor and is grounded by a 470K. I removed the input cap as there is no need really as there is no DC present there (and of course my preamp does not leak DC
)
5) Nothing has been done to the amp itself, other than replacing some out of spec resistors. I tried to re-use CC resistors where I could.
6) I use the amp on a bucking box as the voltage were higher to specs. Both power transformers are getting hot after a 4 hours session, so I prefer to use a small USB fan on them. Works perfectly. Both amps consume 77 watts at idle.
So in conclusion I now understand why Fairchild has such a good reputation. These are top quality amps. Well build, solid, and simple. And I think all the magic is happening in the Fairchild output transformer.
Here are my pics. Enjoy.
Brice.
Original (almost)
Now:
THD full power test:
1K square:
10K square
A quick post on a pair of Fairchild i just finished last week.
I didn't know Fairchild before, and never heard one of their product. To me it was a new adventure.
First, let's start of how they sound: fantastic !
Those little guys are extremely dynamic. They have a perfect integration top to bottom. Very clear and detailed sound, perfect for Jazz and vocals. They may lack some 3D, but the punch they have and the accuracy compensate. And they are loud. Small but loud.
I really enjoy listening to them since last week and I am sure they will make it often to the amp rolling addiction I am suffering.
I equipped the amps with Jan GE 6AB4, RCA clear top 12AU7, EH 6CA7EH, Sovtek GZ34.
What I have done to them:
1) Rebuild the bias supply and removed one selenium rectifier. I increased a bit the caps there.
2) Rebuild the PS with additional capacity. I used PSUD to find the best choice of caps. My findings was to use a KTL25 Authenticap as the big can, and additional film caps for the B+2 B+3 supplies. I used 50uF off the rectifier (30+20) and a 1uF film cap in parallel, then the rest of the KTL25 so 120uF (80+40), then 4.7uF film and finally also 4.7uF film. The original design had 40 40 30 30, and now I have 51, 120, 4.7 4.7. I also used high quality resistors for the supply. I added 2 diodes before the rectifier for added safety. I also added a fuse 250 mA sloblo on the B+ rail. The ripple has been lowered to less than a volt, as of before it was over 3 V.
3) I did not use the gain pot, so I re purposed it as a DC tube balancing pot, as on this model, there was only a bias and an AC balancing pots. So instead of the gain pot, I now have a 10K bias pot adjustable with a screw driver, feed by the bias supply, and on each side a 100K to ground, and with a 47K to each coupling caps. I had to tweak the last resistor of the bias supply to compensate the changes, so the 10K was boosted to 39K. The bias range is now -41V to -31V.
4) As mentioned, I don't use the gain pot. The RCA input goes straight to the grid of the driver tube via a 10K resistor and is grounded by a 470K. I removed the input cap as there is no need really as there is no DC present there (and of course my preamp does not leak DC
)5) Nothing has been done to the amp itself, other than replacing some out of spec resistors. I tried to re-use CC resistors where I could.
6) I use the amp on a bucking box as the voltage were higher to specs. Both power transformers are getting hot after a 4 hours session, so I prefer to use a small USB fan on them. Works perfectly. Both amps consume 77 watts at idle.
So in conclusion I now understand why Fairchild has such a good reputation. These are top quality amps. Well build, solid, and simple. And I think all the magic is happening in the Fairchild output transformer.
Here are my pics. Enjoy.
Brice.
Original (almost)
Now:
THD full power test:
1K square:
10K square
Last edited:
and
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