[Hammarlund] Bias supply zorching the audio driver transformer

[Todd, KA1KAQ]

On Wed, Jul 25, 2012 at 3:38 PM, Richard Knoppow <1oldlens1 at ix.netcom.com>wrote:

>
>   There is another change which might be of importance in getting old
> Super-Pro receivers to work. The power supply for those before the SP-400-X
> were arranged to power a loudspeaker field which was also used as a second
> filter choke. If the supply is used without a speaker field it should be
> replaced with a choke or resistor of the right value.  the SP-200 manual
> says that there was a "special" supply for use without the fieldcoil which
> contained a second choke.  The military power supplies all have two chokes.
> The SP-400 supply is of this type. The terminals for the speaker field
> should NOT just be linked since the lower voltage B+ will be too high. If
> it is open no voltage will get to anything but the power output stage.


That is certainly true of some SPs, Richard. But many omitted it from the
start. Can't recall the exact correlation, but some had 600 ohm
transformers (sealed type as I recall) while others had 8 ohm iron not
requiring a field coil speaker.

To make it more confusing, supplies tended to get mixed up along the way so
a receiver needing the extra choke might show up with a supply that omitted
these extra connections. There's a good shot of the back aprons on Henry's
page not quite halfway down, under power supplies:

http://www.radioblvd.com/hammarlund_super_pro.htm

And the earliest supplies (SP-10) had a supply with a slightly lower B+,
using a 1V tube instead of the type 80 used in later supplies. My SP-10
lacks this supply (though for some reason I have a bunch of spare 1V
tubes). The later versions can be adapted, but I'd still love to locate one
before I die.

Al's going to have a Super Pro overdose if we're not careful. (o;

~ Todd/KAQ