[Hammarlund] Super Pro bias supply

[djed1 at aol.com]

I do have one BC-779 with a replacement driver transformer, so maybe it happens.  On the other hand, maybe it's not a big problem, based on an experience with my "keeper" BC-779.  I turned it on one night and let it warm up for a few hours.  When I went to use it, there was no audio, and the power supply was REALLY hot.  In troubleshooting, I found the bathtub capacitor at the input to the 270V line had shorted.  The resistance of the two filter chokes apparently limited the current into the short to a manageable value, so that the fuse didn't blow, and the chokes didn't burn up.  I don't know what the survival current would be for the audio transformer, but it would be somewhat limited by the chokes.  (and I now have a 100mA fuse in the B+ line).



-----Original Message-----
From: Carl <km1h at jeremy.mv.com>
To: Brian Harris <cosmophone at yahoo.com>; wrcromwell <wrcromwell at gmail.com>; Al Santangelo <ve3ajm at gmail.com>
Cc: hammarlund <hammarlund at mailman.qth.net>
Sent: Wed, Jul 25, 2012 5:54 pm
Subject: Re: [Hammarlund] Super Pro bias supply


You could also use a combination of fixed and cathode bias so that if the 
supply failed the tube wouldnt exceed its ratings enough to take out the 
transformer.

Or switch to three 6V6's, drop the B+ to a safe level, change cathode 
resistor in the output as needed, and get the same 14W at half the 
distortion and at less cathode current. Id also want to try a 6K6 driver to 
further reduce the load on the driver transformer since the 6V6's would be 
biased for AB1.

Others then may wish to change the driver to a 6SL7 or 6SN7 as a phase 
inverter/preamp and bypass the driver transformer.

These are radios that beg for little changes that help a lot.

Carl
KM1H



----- Original Message ----- 
From: "Brian Harris" <cosmophone at yahoo.com>
To: <wrcromwell at gmail.com>; "Al Santangelo" <ve3ajm at gmail.com>
Cc: "hammarlund" <hammarlund at mailman.qth.net>
Sent: Wednesday, July 25, 2012 8:26 AM
Subject: Re: [Hammarlund] Super Pro bias supply


If one installed a relay in the power supply which is energized by the bias 
supply and whose contacts, when closed, allow B+ to flow, would solve the 
scorched driver transformer. Seems simple enough to me.


Brian Harris
cell 214-763-5977
email cosmophone at yahoo.com


________________________________
 From: Bill Cromwell <wrcromwell at gmail.com>
To: Al Santangelo <ve3ajm at gmail.com>
Cc: hammarlund <hammarlund at mailman.qth.net>
Sent: Wednesday, July 25, 2012 6:37 AM
Subject: Re: [Hammarlund] Super Pro bias supply

On Wed, 2012-07-25 at 07:16 -0400, Al Santangelo wrote:
> OK on the further information.
> So then, what is the connection between the failure of the
> -50v bias supply and the failure of the audio driver transformer that
> I've read about?
> This bias supply is not connected at all to the audio driver
> transformer ie.T7 in the SP-400X receiver. Thanks.
>
> Al Santangelo
> ______________________________________________________________

Hi Al,

I went to BAMA and snagged a copy of the manual. If the bias supply
fails the audio driver will turn full on (zero volts at the control
grid) and it gets it's plate voltage through that transformer. The
transformer will be ruinated. You can't stuff tens pounds of baloney in
a one pound sack.

73,

Bill KU8H

______________________________________________________________
Hammarlund mailing list
Home: http://mailman.qth.net/mailman/listinfo/hammarlund
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:Hammarlund at mailman.qth.net

List Administrator: Duane Fischer, W8DBF
** For Assistance: dfischer at usol.com **


This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
______________________________________________________________
Hammarlund mailing list
Home: http://mailman.qth.net/mailman/listinfo/hammarlund
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:Hammarlund at mailman.qth.net

List Administrator: Duane Fischer, W8DBF
** For Assistance: dfischer at usol.com **


This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html


-----
No virus found in this message.
Checked by AVG - www.avg.com
Version: 10.0.1424 / Virus Database: 2437/5152 - Release Date: 07/24/12


______________________________________________________________
Hammarlund mailing list
Home: http://mailman.qth.net/mailman/listinfo/hammarlund
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:Hammarlund at mailman.qth.net

List Administrator: Duane Fischer, W8DBF
** For Assistance: dfischer at usol.com ** 


This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html