To avoid having to build a separate power supply box, I am thinking of using an
SB-220 transformer. As I see it, it was meant to run a pair of 3-500's at
about 2700volts and 700mA. I am thinking of using it to run either three or
four 813's. My calcs show about 170 to 180mA per 813 in class B at 2700v,
which tells me I am probably pushing the transformers capability with four
An additional concern is the voltage drop on key down. I read an online
article by K0GKD regarding the voltage drop and impact on linearity with Heaths
stock 25uF of filter capacity. He increased the filter capacity to 56uF and
still noted a 10% voltage drop, but noted better regulation (??).
If I end up with a voltage drop on key down or voice peaks, that tells me the
plate current will rise and further tax the transformer.
So, my thought is the SB220 tranny may work well for three 813's but maybe not
with four 813s.
Is my rationale correct?
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