[Hammarlund] Bias supply zorching the audio driver transformer

[Richard Knoppow]

----- Original Message ----- 
From: "Todd, KA1KAQ" <ka1kaq at gmail.com>
To: "Al Santangelo" <ve3ajm at gmail.com>
Cc: "hammarlund" <hammarlund at mailman.qth.net>
Sent: Wednesday, July 25, 2012 10:08 AM
Subject: Re: [Hammarlund] Bias supply zorching the audio 
driver transformer


> 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

   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.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com