Bottlehead Forum

Bottlehead Kits => Legacy Kit Products => Paramount => Topic started by: Jim R. on December 26, 2012, 10:43:57 AM

Title: Softstart and nickel core magnetization?
Post by: Jim R. on December 26, 2012, 10:43:57 AM

Paul,

Just curious if the soft-start board in the paramount (or if used in paramour II) will help or eliminate the nickel core turn on magnetization issue?

Thanks,

Jim

Title: Re: Softstart and nickelcore magnetization?
Post by: Paul Joppa on December 26, 2012, 11:22:45 AM

Good question, but I have no good answer. I have not done enough modelling to assess the relative importance of the various current surges at startup and shutdown. Then time constant of the output tube's cathode emission is also unknown quantitatively, as it depends on the thermal mass and radiation efficiency of the filament as well as its temperature/emission relationship. Tube makers are a little secretive about some of these variables...

Still, I suspect the fastest transition and hence peak magnetizing current is charging the parafeed capacitor through the plate choke. That would happen before any of the cathodes start to conduct. Interesting to speculate - it's conceivable that a relay coil in the cathode bias circuit, shorting the OPT primary until the cathode draws enough current to actuate the relay, would do a good part of the job.

Title: Re: Softstart and nickel core magnetization?
Post by: Jim R. on December 26, 2012, 12:11:58 PM

Hi Paul,

"Still, I suspect the fastest transition and hence peak magnetizing current is charging
the parafeed capacitor through the plate choke. That would happen before any of the
cathodes start to conduct."

Intuitively that makes a lot of sense to me too -- thanks for your thoughts on this.  I suppose I'll just have to listen once the amps are built and broken in.

-- Jim

Title: Relays like to be turned on and off quickly
Post by: VoltSecond on December 26, 2012, 12:21:59 PM

One of several app notes on this topic: 
      http://relays.te.com/appnotes/app_pdfs/13c3264.pdf (http://relays.te.com/appnotes/app_pdfs/13c3264.pdf)

I'd design something that makes the contacts open and close crisply. If the coil is not turned on and off abruptly, the contact life is shortened.  If the contacts are closed slowly, like when the coil is in series with a cathode bypass resistor, use a socket for the relay so it can be easily replaced.

Or you could just simply play some loud bass for a second after power up and let that degauss the core.

Title: Re: Softstart and nickel core magnetization?
Post by: xcortes on December 26, 2012, 01:52:23 PM

I have a combination of a thermistor in the iec and the soft start switch. I have found that the amps(full nickel) still need some time to sound their best (one lp side). Still the first side doesn't sound as congested as it did before the soft start upgrade.

I like the idea of a loud bass to demagnetize the core. Quick and easy. Although I usually listen to some music from the adjacent dining room before sitting in the so called sweet spot so by the time I'm ready to do serious listening it's always ready.