SET Asylum

Single Ended Triodes (SETs), the ultimate tube lovers dream.

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Original Message

RE: they're fairly fresh

Posted by tube wrangler on June 27, 2012 at 19:06:51:

Nothing crazy about it. The problem is that the best capacitor in the world is really very bad as a linear, wideband, frequency neutral device.

No matter how good, in capacitors, all you get is junk.

Fortunately, some junk is much better than other junk. Is there any one capacitor that is, in fact, wide-band in an actual musical circuit where all parameters are constantly changing, where a single cap can work? NO.

That one cap-- regardless of quality, wants to form a resonant circuit--that is, it wants to tune-in to favor certain frequencies over others, based on the cap value and inductances and resistances in the associated circuit.

In order to get around this unwanted "tuning" effect (defect!), we have resorted to parallelling very high quality small caps with larger caps, coupled with very careful use of R.F. inductors.

In a simple SET amp, these capacitors will cost more than all the other parts in the amp combined-- you will end up using the world's best caps, and that is cheap compared to any usable alternatives.

I should slip-in here that push-pull amps do not have anywhere near these stringent requirements-- they're more wideband naturally, and are much more forgiving.

One or two small caps in parallel with a larger one in a power supply, a cathode bias system, or in a driver plate power supply will cause the amp to sound different, but not necessarily better. Usually, bandwidth will be gained, but incoherencies and hot spots in the frequency range as a whole will appear, rendering the amp pleasant, but not accurate.

Of course, the amp certainly isn't anywhere near accurate with only one cap in each of these places, either.

What we end up with is a requirement for as many as 11, 12, or 13 different capacitors of cascading values, installed at each of these places in the amp. If the right brands and values, and the right lead lengths are employed, there will be no trace of incoherency, and usable musical bandwidth will be widened, as will power supply and cathode bias response to signal. Dynamics will be improved so much that the only thing you can attribute it to is that finally, you have built a fast capacitor-- one that is fast at all the usable audio frequencies.

All of these advantages can and will be negated if any lead on any part of this (stack-up?) is too long, too short, or uses too small a conductor.

Sorry this got longish, and it's quite expensive-- this is what it takes.

---Dennis---