Carl brings up some VERY good points!
I'm not a fan of all this Harbach mod stuff.
Replacing the original diodes with better ones is darn good advice.
When I built my SB-201, I replaced the Heath supplied diodes VERY
shortly after building it.
There was too much voltage sag.
Tubes are relatively cheap when put up against the entire HV Section.
The SB-220/221 power transformer is going to be hard to come by today.
The only real solution would be to have it rewound. That isn't an
inexpensive path either.
I'll add an Amen to the parasitic oscillation voodoo also!
Bob - N0DGN
On 11/6/2011 4:48 PM, Carl wrote:
>
>
> I dont particularly care for the Harbach PS board for the SB-220
> family, Ive had too many arrive here in customer amps that have the
> bias and HV metering components blown out. Those parts are seriously
> undersized.
>
> It just as easy to rebuild the original board with 1N5408's, Ohmite HV
> resistors, and a string of 1N5408s's on perfboard for the bias. The
> filter caps can be replaced with 330uF/450 Snap In type that fit the
> original blocks perfectly. and 75K 3W MOX equalizers.
>
> Soft Key is a good way to preserve the On/Off switch, especially when
> going with higher value filter caps; otherwise it does nothing useful.
>
> The parasitic stuff is pure voodoo. Ive covered the 21st century
> version on here several times.
>
> Carl
> KM1H
>
>
>
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