> From: D Conklin
>
> I got a SB-200 fitted with 2 811A tubes. I was assuming
> that this was a plug-and-play replacement for the 572A. A
> local ham who has an extensive but not detailed memory
> indicates that this was a common mod due to 811 tubes
> being cheaper, but that it involved a substantial amp
> mod and wasn't just a plug and play replacement.
If you're lucky the only changes made were to change the
power supply from a voltage doubler to a half-wave rectifier.
Without that change, the 811 (rated for operation at about
1250 volts) would arc!
With the voltage doubler the SB-200 ran about 2250 volts
no-load and 2000 volts (at 500 ma) "key down" for 550 - 600
watts out. With the voltage doubler changed to a half-wave
rectifier, I would expect to see 1150 volts no-load and 1100
volts (at 350 ma) key-down and maybe 250 watts out.
> I have searched the net for info on modding a SB-200 for 811
> tubes and found nothing. I also searched the QST index
> without luck. I need to know the mod so that i can check
> if the previous owner (SK) did it right, or if he completed
> it - this amp looks like it was in the process of being
> refurbed at the time of his passing - maybe he was in the
> middle of converting it to or from 811s.
Download the manual from http://bama.sbc.edu and compare the
plate circuit to the original. If the plate coil and or the
80 meter padding capacitors have been changed it will be
much more difficult to restore. However, if the only changes
are the power supply (rectifiers and filter caps), you should
be able to get everything you need to restore the amplifier
from Harbach Electronics (www.harbachelectronics.com).
You will also want to check for changes in the grid circuit
(bias).
73,
... Joe, W4TV
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