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RE: Another thing ...

Posted by mqracing (M) on September 11, 2009 at 19:55:40

In Reply to: RE: Another thing ... posted by DanK tubes on September 11, 2009 at 09:00:34:

Hi Dan:

you wrote;

:::That said, I've never heard anyone suggest that a resistor was needed across the BCP-16.:::

There are two pretty decent LC resonance calculators that you can run your numbers through... voltsec has one and gianluca wrote one as well.

Bottom line... with any rudimentary care your grid choke will work well without a damping resistor in most real world cases\circuits.

Actually... I've played with both Voltsec's and GL's programs at length... it takes a fair amount of "engineering" to get the wild twenty and twenty four db peaks... and it is relatively easy to just change component values which in most cases can move the PROGRAM CALCULATED resonance as much as a decade below the audio band.

Bear in mind that ALL of the programs and spice sims fail to model in or take account of core losses as just one example. None of the programs\sims recognize that the indutive reactance itself at two hertz is going to be very much smaller than say at fifty, sixty, or a hundred hertz where most of the L measurements are traditionally made.

None-the-less, in one such imperfect simulation that I did using voltsec's program I did find quite interesting results.... yep... I could engineer in a huge twenty db something peak at 1.85 hertz... but the peak was extremely narrow.... you basically had to hit just exactly the target value of 1.85 hz... move just a tenth of a db off in either direction and the wild peak was largely gone.

The other thing that would happen if you had some wild twenty some db peaks in your response in the sub-sonic hertz regions... IF you were even able to excite the frequency.... it would quickly (assuming your operating at some X amount of output power to begin with) overload many of the irons in the signal path... (as well as many of your other circuit components) which would then act as a helluva brake on passing the suggested massive peak signals at this extremely low frequency. The "offending inductances" themselves would quickly evaporate as you overloaded the iron components ... and hence deprive the errant resonance of one of it's two necessary components.

But first you have to find a way to excite the 1.85 hertz resonance.... what program material do you have access to? If your going to say that there's power line noise down here... then you probably need to clean up your power supply... assuming your on say fifty or sixty hertz mains.

So basically... your comment that not much trouble has ever, ever been reported in real world applications is just plain utterly true. The horror scenarios seems to be the fodder of the war games players as opposed to many (or any) real world experiences by end users.

And if one did find (in a real world documented example) an untamed resonance impinging on the real world performance of their amp... and a simple change of some of the key component values did not suffice in snuffing out the resonance there are then other alternatives (Voltsec's DrP as just one example) to putting a resistor across the grid choke.

msl

Builder of MagneQuest & Peerless transformers since 1989

Edits: 09/11/09 09/11/09 09/11/09 09/11/09 09/11/09 09/11/09 09/11/09 09/11/09 09/11/09 09/11/09 09/11/09

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