At 11:09 AM 8/3/97 -0700, you wrote:
>...snip
>This issue hardly seems to be a matter of personal choice.
>- There is about 20A of tank RF circulating current in an amplifier
>whose anode current is 2A and whose tank Q is 10. The 58-series
>capacitor in the amplifier is rated at 1.4A at 1MHz. Since roughly half
>of the tank circulating current flows in this load-C padder at 1.9MHz, it
>appears that the amplifier's designer made an engineering error.
>Rich---
>
As I stated in earlier posts, 160m was truly an afterthought on the 77DX/SX's.
A pair of the same doorknobs (858S's) are used in the plate block. The current
through them on 10m is far in excess of their rating. Also an 80uh plate choke
with only 5000 pf of bypass capacitance is hardly enough on 160. Raising the
total bypass cap string to .01 mfd @ 10kv makes the stock plate choke
passable.
Also ungrounding one side of the filament circuit and adding a filament choke
keeps most of the 160m RF out of the power supply/a.c. mains.
On the QSK bias switching problem; later models added R208, a 2.5k ohm pot
to adjust out the garbage generated on ssb. The circuit works fine on CW as
is.
You mentioned earlier that the tuned input must be redesigned when adding a
second tube in the 77DX. All Alpha did was furnish a 2:1 unun transformer
with the QRO kit that installs in-line between the present tuned input tank
and the tube cathodes.
Someone asked in a post if there were differences in units of the same model.
I have seen several 77DX/SX manuals, and no two have been alike! They made
several changes on the PC boards over the years.
Perhaps the saddest thing is that it took them years to discover that the
gold paint they used on early models never dried! Later models were in a
brown case with a tan front panel. The gold painted units make dandy
fly paper to this day!
(((73)))
Phil, K5PC
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