Been a long time...
This amp was owned by my brother in-law, who worked for Allied
Electronics in the early sixties. I came upon it in the late
sixties as part of the basement stereo in the duplex we lived in.
Sometime later, I rushed home with my first album - In-A-Gadda-Da_Vida
- and created enough racket with it to get a visit from "upstairs" in
about three minutes. "What the hell is that???"
As the amp was considered "old junk" by the early seventies, no one seemed to mind when I grabbed it to build my first stereo in my bedroom with a set of AR 2ax speakers and the Fisher tube preamp from the basement. (Wonder where that went? Wish I still had it.)
In about 1978, when I was working on my EE degree, I stripped the HF-87 down to nearly an empty chassis, then rebuilt it with a slightly modified circuit adding several updates:
As the amp was considered "old junk" by the early seventies, no one seemed to mind when I grabbed it to build my first stereo in my bedroom with a set of AR 2ax speakers and the Fisher tube preamp from the basement. (Wonder where that went? Wish I still had it.)
In about 1978, when I was working on my EE degree, I stripped the HF-87 down to nearly an empty chassis, then rebuilt it with a slightly modified circuit adding several updates:
- Film caps and metal film resistors in the signal path
- Some serious filter caps and a 1H choke for the B+ supply
- Differential phase splitter with a transistor constant current source (so much for tube purity...)
- Regulated bias supply with adjustable bias on each tube (another transistor polluting the tube purity)
- Cathode coupling of the output tubes through the output transformer secondary
Here's some sketches of the circuitry I did at the time:
The amp worked well for a lot of years. It didn't sound as good
as my buddy's Audio Resarch stuff - it was softer and the bass wasn't
as tight - but it was cheap and it beat the hell out of the solid state
receivers that everyone else was using at the time. I just found
some data I took with a scope at the time, and it was pretty flat from
well below 20hz to beyond 30Khz where it was down less than 3db.
Finally, here's a picture of a stock HF-87, including the chassis and a old Eico datasheet.
Finally, here's a picture of a stock HF-87, including the chassis and a old Eico datasheet.