[Hammarlund] Bias supply zorching the audio driver transformer

[Richard Knoppow]

----- Original Message ----- 
From: "Al Santangelo" <ve3ajm at gmail.com>
To: "hammarlund" <hammarlund at mailman.qth.net>
Sent: Tuesday, July 24, 2012 5:37 PM
Subject: [Hammarlund] Bias supply zorching the audio driver 
transformer


> 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?
>
> Al Santangelo

    The amplifier is the same in all the Super-Pro series. 
Cathode bias 6F6's connected as triodes and running AB2. The 
transformer coupling to the grids can supply the small 
amount of grid current that they draw on peaks.
    There is no bias going through the audio transformer in 
an unmodified receiver.
The bias supply is used for the RF, and IF stage bias, it 
would be difficult to damage it.
    There are schematics for the SP-400 and SP-100 at BAMA 
and for these and earlier versions at Nostalgia Air.   The 
main differences over the models was the change from glass 
to metal tubes going from the 100 to 200 series and the 
addition of the new-type crystal filter in the 200 series. 
The optional crystal in the earlier sets was of a rather 
crude design.
    The BC-779 has different frequency coverage from the 
SP-200-LX. The LX has a single LF band covering 150 to 300 
khz while the BC-779 has two bands covering 100 to 200 and 
200 to 400 khz in place of the two lower frequency bands.
    Older RCA tube handbooks will have the data for the 6F6 
tubes as operated in the Super-Pro.


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