ORIGINAL MESSAGE:
On Sat, 8 Dec 2012 13:27:00 -0500 (EST), mikflathead@aol.com wrote:
>I am getting ready to start construction of a 8877 amplifier. Any good
>designs? Any pointers before I start drilling.Thanks Mike
REPLY:
On my homebrew 8877 I decided to keep it as simple as possible and I'm quite
happy with the results.
1. I have no fancy protective circuits other than two fuses in the 240 VAC line
and one fuse in the 120 VAC line. Whenever I change bands or antennas I always
turn the drive all the way down and bring it up slowly. The 8877 has a pretty
rugged grid (25 watt dissipation) but I take no chances.
2. You do not need an input matching circuit. The 8877 input impedance is very
close to 50 ohms anyway. If you insist on a perfect match, the antenna tuner in
your transceiver can do the job.
3. You do however, need a parallel resonant tank in the input circuit to provide
the 'flywheel' effect. I used a Q of five. This requires a 9-position switch for
the input and a means of tuning each band. I used individual toroids and mica
compression trimmers. Do not try to 'tap' a toroid. Use 9 individual toroids.
4. One thing I didn't do and I wish I had: Install a variable capacitor
accessible from the front panel to fine tune the input.
5. You don't need a VHF parasitic suppressor if you keep all the leads in the
input circuit very short. This avoids parasitic resonances in the input. Do NOT
mount the input band switch on the front panel. Mount it right next to the tube
socket. My amp has no suppressor and is perfectly stable on all bands and
modes.
Good luck and keep us posted.
73, Bill W6WRT
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