[Hammarlund] Bias supply zorching the audio driver transformer

[Todd, KA1KAQ]

On Tue, Jul 24, 2012 at 8:37 PM, Al Santangelo <ve3ajm at gmail.com> wrote:

> Hi Todd,
>
> Great to meet up with you here OM. Hope to hear/work you soon on 75 or
> 40m for sure. The group is still going strong.
>
> I know it was mentioned to be careful about the bias supply and
> damaging the audio driver transformer in the Super Pros.
>
> I was just looking at the schematic for the SP-400X and I see that the
> schematic shows the use of cathode bias on the PP 6F6s. ie. the centre
> tap of the secondary of the driver transformer is grounded.
> I don"t the SP-210 schematic here, so I'm thinking that this is one
> other difference between the 400 and the earlier SPs?
>

As others have covered Al, the circuit is basically the same across the
series. The 400, like the 129, were basically dressed up versions of the
original 200 and 120 pre-war models that allowed Hammarlund to have
something to offer quickly postwar.

I have never come across a set with the audio failure you describe. I would
be a few folks presented improper loads through speaker choice, though, not
knowing if their set was 600 or 8 ohm.  I do have a pre-war 200SX here that
someone started to gut the audio section from. Fortunately the iron is
still in place. Have seen a few with the audio section complete gone, no
way of knowing if a roached transformer was the reason or the typical
mod-of-the-day approach promoted in the 50s-60s.

To frequency coverage, there was speculation for a while that the low
frequency was offered but not produced until the 200 came along. This was
found to be false when W3JN landed a SP-100L at Timonium a few years back.
An amusing story behind it. The coverage differs slightly from some of the
SP-100 ads, but it does exist. Not sure if any SP-10Ls were ever built. The
-10 wasn't in production long before the updated 100 replacement came
along. Less than a year, I think.

It's interesting to watch the progression, from all glass 'big pin' tubes
in the SP-10 to a combination glass and metal octals in the 100, to all
metal octals in the 200. Also the change from R meter to S meter.

They are really neat rigs, especially for their time. You'll enjoy yours a
lot once they are online, Al. I have the SP-100X next to the bed where it
is usually tuned to AM740 near you. They pound in down here most nights.

~ Todd/KAQ