[Heathkit] foldback

[Jim Brannigan]

Well I agree and disagree.
Heath was famous (notorious?) for using cheap components in their kits, but
all things considered they have stood up fairly well.
The paper coil forms are not my first choice and it would not hurt to
replace the capacitors with new ones.
But, I have never had a problem with the input impedance drifting from
prolonged use.  This includes many CW and SSB contests.  I also used the
input circuit components for the SB-200 in a Home-brew 572B amplifier and
never experienced the heat induced impedance drifting.
The SB-220 and 200 are still the best values in amplifiers.

BTW I really like the SRS KW amp. THAT is built like a battleship!!!!

Jim


> Actually,
> The problems encountered with the input circuit on "Consumer Amplifiers"
of
> the SB-220
> variety is space and heat, in a nut shell! Now before everyone goes "OFF"
I
> will try to
> explain.
>
> The alignment procedure in the manual is correct to a point, however the
> space inside
> the cabinet is NOT conducive to good air flow across the input circuit
> components, and
> they are physically small. When in operation the circuit components heat
up
> the coils
> migrate and change the tuned freq. This leads to the high reflected power
> back to the
> driver source. Paper tubes are NOT good for coil forms that handle any
> appreciable
> power, the recommended type is still ceramic forms with good point surface
> contact for
> heat-sink conduction away from the coil. The caps used in the circuit are
> mica caps or
> were, some have been replaced with higher voltage disk ceramics, in any
case
> the best
> replacement cap is the CRL 800 series of transmitting caps in the 1000
volt
> rating. This
> solution would correct the problem, however the owners of the SB-220 units
> don't like
> to modify the unit in this way.
>
> The "Bottom Line" is that the SB-220 configuration is a compromise due to
> the Table
> Top packaging of the unit. Bear in mind that the military and government
> agencies use
> the same configuration 2 X 3-500Z class "B" amplifiers, however they also
> spec that it
> WILL operate over a certain temperature range, not require depot level
> maintenance
> to maintain proper operation of the unit, and pretty much run 24-7 with no
> problems.
> When they say the source will see 50 ohms, that usually means whether it
is
> in the desert
> or the Artic, and they get this "Feature" by using large components with
> known temperature rating, adequate cooling arrangements (read air flow)
and
> over rated items.
>
> You can run the pants off an SB-220 in SSB mode but only for a little
while,
> after things
> heat up and move around, the signals get wide, and trashy, and spurious.
>
> For what it is worth, go here http://eshop1.chem.buffalo.edu/SR-120.html
> read the
> description and checkout the Eimac spec sheet for the 3-500Z then look at
> the components used in the circuitry and note that these rigs run 24/7 at
> full power output,
> most importantly note the fan and the air flow of the plenum!
>
> My two cents!
> Jim
> WB2FCN
> http://eshop1.chem.buffalo.edu