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Yikes! I stand corrected!

Posted by Kurt Strain on May 12, 2002 at 21:04:28

In Reply to: Re: The secret to using iron IT's (like Lundahl 1660) posted by Lynn Olson on May 11, 2002 at 20:17:53:

You're right, Lynn. It's a 1:1.125+1.125 splitter IT configuration, with the loading added being 47K in parallel with 220pF across all secondary windings. That 47K load appears as 9.3K on the primary as you correctly calculated, with squaring the turns ratio. That's a hefty load, and it's killing gain for sure. The gain of this stage is thus lowered from a calculation of 13.8 to 12.7, not all that bad because of the virtual ground on the 6H30 cathode, easy to lose sight of the difference there. Nice to have low rp tubes like this.

As for the capacitance, I assume the 220pF load added is also 5 times more on the primary, to 1100pF. This large capacitance was found to be good for flattening out the LL1660 FR in this configuration, because without it the darn thing rises a few dB from 20 KHz to 44 KHz at the output.

The LL1660 is a big chunk of iron compared to the LL1635. Both needs something different in every application and configuration for best results. Smaller iron cores need less ampere-turns into the primary and less power transferred to overcome the hysteresis.

I should not be too general in my statements like that, but this is the idea why I think I was not previously successful with the LL1660. The main problem I see with the LL1660 is that it was designed for higher rp tubes to be able to get full FR; i.e. more inductance. But that's a big penalty to pay to make a 27 or 26 work with it, barely. It doesn't transfer it cleanly enough because the core seems to demand more coercion than those tubes can provide to sound good overall. Dave Slagle said it's better to sacrifice a little tube quality for quantity when driving IT's, and now I've heard it sound better in doing that. But the part missing in that statement was "get the secondary optimally loaded down".

I believe the high power tubes will lean out their lushy sound when driven with a tougher load, at least through a big IT core.

Kurt

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