This message should be of interest for GSB-201 owners running on a 50-Hz power
line.

At 50 Hz, during receive the high voltage stays at about 2,000V. During transmit
instead, the four-811As idling current is sufficient to bring voltage down to
the normal value (about 1,650V). So, most of the time voltage stays at 2,000V,
thus stressing the electrolytic capacitors and causing a higher dissipation in
the bleeder.

Such problem is due to the fact that the GSB-201 input-choke power supply was
designed for 60-Hz operation, and the choke inductance therefore results to be
too low, at 50 Hz, for keeping voltage down during receive. Replacing the choke
with one of higher value would be a crazy choice, and using a lower-value
bleeder would cause undesirable extra heat.

So I decided to try converting the GSB-201 input-choke power suppy into a
resonant-choke power supply, putting a high-voltage capacitor in parallel to the
choke. After cut and try, I determined that the precise capacitance value that
resonates the choke at 100 Hz (i.e. twice the mains frequency) is 110,000 pF. I
used a 3000Vn capacitor (not 3000 Vp) which should be safe enough.

Now voltage stays normal in all conditions.

I am not sure whether all the GSB-201 series utilize the same choke; anyway
trying different capacitor values is quite easy, as the choke leads are easily
accessible from the amplifier top, and the choke is on the negative side of the
supply (you should anyway be very careful because of the high voltage across the
capacitor. Moreover, capacitors should be discharged by shorting the leads every
time they are removed after a test).

73

Tony I0JX