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Re: valve rectification v solid state

Posted by Peter Qvortrup (M) on June 7, 2002 at 10:11:41

In Reply to: Re: valve rectification v solid state posted by Frank S. on June 7, 2002 at 03:31:31:

Dear Frank,

You have obviously been reading the wrong books and the lights from Uranus and the moon at night are too dim to see anything anyway, so try switching on the light!

Why go to all the bother of designing and building a tube amplifier, especially an S.E.T, only to feed it with a set of one cent solid state diodes. If efficiency and heat are an issue then use semiconductors everywhere, build a solid state amplifier, it will measure a lot better as well.

Based on most contemporary textbooks, a simple at-a-glance theoretical analysis of audio must bring one to the conclusion that a high feedback solid state amplifier is the ultimate audio device. Unfortunately as a result most of todays tube amplifiers are designed with a transistor mentality (mind?) to behave on the test bench as close as possibly to this ideal.

This results in the following being present in modern tube amplifiers:

1) Semiconductor diodes.
2) Filter capacitors the size of Coca Cola size cans.
3) Tubes with transistor like parameters.
4) Multiple paralleled devices.
5) Active current sources and other such illegal aliens.
6) An irrational fear of transformers.

And other things I can't think of at the moment.

Briefly looking at silicon diodes from an electronic point of view:

They switch on very hard. When a capacitor input filter is used (especially with a Coca Cola can size filter cap) this hammers the mains transformer as the capacitor draws it's charging pulse. That pulse is also fed into the mains supply itself. Large solid state power amplifiers can actually clip the supply waveform.
They switch off very hard. The primary and secondary windings are one second (or fraction thereof) carrying a large capacitor charging current then the next the diode switches shut this current off. The parasitic capacitances and inductances in the transformer then go into a shock induced oscillation, radiating noise everywhere. This effect is usually far greater in magnitude to the reverse recovery transient of the diode itself.

Vacuum tube rectifiers switch a lot more softly, not only due to their greater internal impedance but also to the fact that their characteristic is square law and not exponential. They also do not suffer from a reverse recovery transients unlike their solid state counterparts.

As a result tube rectifiers, if applied correctly in a light footed choke filtered power supply, always sound sweeter and more open and should be the first choice if designing and building a tube amplifier.

There are no choices but to use a vacuum tube rectifier in a high grade tube and especially SET amplifier, just as there no substitutes for Black Gate capacitors in these applications, why are you kidding yourself that there is? The end results speak so clearly for themselves that it is indisputable.

It is really already in the language, semi conductor, half the waveform, half the signal, half the music!

Sincerely,
Andy Grove,
On behalf of Peter Qvortrup

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