Bottlehead Forum

Bottlehead Kits => S.E.X. Kit => Topic started by: xcortes on July 03, 2012, 02:23:57 PM

Title: Shorting OPT primaries in a SEX
Post by: xcortes on July 03, 2012, 02:23:57 PM

I have a Sex (previous version, with autoformers). I have the MQ upgrade in Ni. I'd like to install switches to short the primaries to avoid magnetizing of the core at turn on. Just want to confirm that this is OK with an autoformer.

Saludos

Title: Re: Shorting OPT primaries in a SEX
Post by: Paul Joppa on July 09, 2012, 06:57:50 PM

No problem with shorting the primaries. I'd short the whole thing, primary hot to ground, rather than just the primary - that startup transient current goes to ground from the parafeed cap. It will create less magnetization going through just the secondary (fewer turns) but not zero magnetization.

Title: Re: Shorting OPT primaries in a SEX
Post by: Grainger49 on July 10, 2012, 02:52:25 AM

I'm going to suggest a spring return switch.  The shorting position can be bought as a spring return to open.  This means you hold the switch for 30s after each startup but guarantees you won't short the amp after startup.

Title: Re: Shorting OPT primaries in a SEX
Post by: xcortes on July 10, 2012, 03:46:33 AM

Thanks Paul, Shorting it all is what I had in mind!

Thanks Grainger, I'll look for these.

Saludos

Title: Re: Shorting OPT primaries in a SEX
Post by: denti alligator on July 10, 2012, 04:16:57 AM

What would be the advantage of doing this?

Title: Re: Shorting OPT primaries in a SEX
Post by: Grainger49 on July 10, 2012, 04:29:56 AM

Sam,

Since you ask, and I got this idea from Xavier, when the amp turns on there is a little leakage through the Parafeed cap as the cap charges and blocks all the DC in a few seconds.  This is the turn on thump heard with some equipment.  It is why the output of expensive preamps is shorted by a relay on turn on. 

In an amplifier this leakage DC magnetizes the primary of the transformer.  Playing music demagnetizes it.  A tape demagnetizer is just an AC electromagnet.  So as a tape demagnetizer works the music removes the magnetization.  This is one of the reasons why the amp sounds better after 15 minutes of play.

Title: Re: Shorting OPT primaries in a SEX
Post by: Doc B. on July 10, 2012, 05:03:58 AM

Actually John Camille was the one who figured this out, back in the late 90's. I'm pretty sure the first example of it is in a VALVE article called Buddhafied Afterglow.

Title: Re: Shorting OPT primaries in a SEX
Post by: xcortes on July 10, 2012, 09:52:35 AM

For reference, the switches I'm ordering for my Paramounts are:

Mouser 611-7101-064

And for the Sex:

Mouser 611-7101-064

Title: Re: Shorting OPT primaries in a SEX
Post by: Grainger49 on July 10, 2012, 09:56:28 AM

Xavier, just put the two switches in the SEX close together so you can flip both with one thumb while turning the amp on with the other hand.

I thought of that shortly after my last post but hadn't come back.

Good to have the information out there.