Hi guys,
I've completed my beautiful pair of Fairchild 275 and wanted to share my experience with you.
It's the second pair I restored so I am starting to get to know them quite well.
But first, I wanted to thank Dave dearly for his remarkable work and sharing it with us.
Dave's post helped me tremendously and paved the road for my restorations.
So Dave, thank you very much.
I started to document the boards on my amps as they were different. One was the older model and one the newer. I had to make modifications in the wiring on both to accommodate my changes.
First I decided to have all components on the turret board, for both amps (versus some on the 6AB4). So I've made schematics of the original boards, then with the modifications needed for the components relocation.
And I managed to relocate the 2 bias capacitors on the board, as well as the bias rectifier. This makes things cleaner.
I changed the ground path as Dave's recommendation, and indeed both amps are very quiet.
I installed 2 100R 1/2W screen resistors directly on the socket.
I also added a fuse on the B+ rail going to the OT CT. I didn't want to drill holes on the chassis so I made 90 degree brackets to mount the fuse holder on the PT stud. I painted them as close as I could and they are not an eyesore, kind of look original.
The only modifications I've made from Dave's are:
- I changed the test jack circuitry with a stereo socket, and wired the cathodes through a 10R 0.1% Vishay resistor on each side to ground, the classic way really. This allowed me to sense the bias current and the balancing of the tube at once. Then I made a little gizmo bias and balance rig, connected to the amp via a guitar jack and monitor the balancing and bias at once. Helpful little thing I can reuse for other amps.
- I modified the power supply by adding SS rectifiers upstream the rectifiers. Nothing new here but I noticed that the original PS had a B+ ripple of almost 2.4VAC RMS.
I wanted to reduce this. Then I found a nice 3x100uF 550VDC in a small can format suitable for the F275. So with 100uF off the rectifiers, and 2 spare 100uF gave me the idea to use them as an extra decoupling stage before the rectifiers. So I connected the 2 sections to the SS rectifier cathodes. The end result is a VDC higher by 10V, not really an issue, but a ripple B+ now at 0.2VAC RMS.
I can't really tell the difference but I preferred to make the best cleaner PS I could with the real estate I had.
Here is the results:
Older version:
New version:
Both amps are stable, extremely detailed and beautiful sounding. I am very happy with the result.
I am burning in the GL KT88 and so far no issue: all stable.
On the frequency response, I get the same as Dave's.
These amps are simple, well designed but quite difficult to work on due to the way they were manufactured. The soldering is not McIntosh's approved, and so some had to be redone. Cleaning the turrets is also a pain.
But the end result is stunning and well worth every penny and hours put in these beauties, as Dave wrote.
Thank you for reading.
Brice.
