Eico HFT-90 Heaters
- Thread starter scott2000
- Start date
Help Support GroupDIY Audio Forum:
What is it? NOT an audio amplifier. 60Hz hum won't get through circuits with 13-turn plate loads.
It is a HIGH gain amplifier. It pulls in MICRO-volts from thin air and makes Volts at V4.
The common supply (plate and heater) leads are a super-highway for big signals at the end sneaking-back to the sensitive input stage. It will oscillate!! If not that, the stray feedback will screw-up the carefully designed filters.
The Cs and Ls in the heater and plate leads do about the same as our R-C filtering between stages in a guitar amp, except optimized for 10MHz and 100MHz instead of audio. The HARD ground on one side also helps with sneakage.
It is a HIGH gain amplifier. It pulls in MICRO-volts from thin air and makes Volts at V4.
The common supply (plate and heater) leads are a super-highway for big signals at the end sneaking-back to the sensitive input stage. It will oscillate!! If not that, the stray feedback will screw-up the carefully designed filters.
The Cs and Ls in the heater and plate leads do about the same as our R-C filtering between stages in a guitar amp, except optimized for 10MHz and 100MHz instead of audio. The HARD ground on one side also helps with sneakage.
Arguably there could be hum in V6 cathode follower. But this is a high-level stage with total NFB. I'm sure EICO's dudes would have heard hum if it existed. If it hums now, you know what to do with old e-caps.
Next you will be noticing that 68k and 0.001uFd is not the US deemphasis corner. But there is also the 250k pot. I make it 53uS. Hmm, should be 75uS in the US. Once upon a time I might have cared. Considering how most of today's broadcasters pump-up the tone to cut-through crowded bands into crap radios, going a bit low might be less annoying.
Next you will be noticing that 68k and 0.001uFd is not the US deemphasis corner. But there is also the 250k pot. I make it 53uS. Hmm, should be 75uS in the US. Once upon a time I might have cared. Considering how most of today's broadcasters pump-up the tone to cut-through crowded bands into crap radios, going a bit low might be less annoying.
