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>> >> Can anyone explain to me why vacuum tube-based rectifiers for the DC
>> >> supply in an amp would make any audible difference to
>> >> semiconductor-based rectifiers (i.e. as in the Mesa/Boogie "Recto"
>> >> series).
>> >
>> >Hi,
>> >
>> >It makes no audible difference.
>> >In fact, it's better to use the semiconductor (more relaible).
>> >
>> Sorry, ace...that ain't right. A rectifier tube reacts differently
>> than a solid state diode when heavily loaded...the tube will "sag"
>> under high current demand, and this is damnsure audible. Some folks
>> prefer the sound of a sagging rectifier tube, as it adds considerable
>> compression to the saturated output stage. The main reason amp
>> manufacturers use SS diodes is that you can buy a whole handful of them
>> for the price of one rectifier tube. Anyone who owns one of those Mesa
>> amps which lets the player select the type of rectifier he prefers will
>> tell you that he can hear the difference QUITE easily.
>> Fat Willie

Ton Schuwer wrote:

>You're right. But thats only when your power amp is asking more current
>out of your supply that the stabelise capacitor can deliver at that
>moment (you're amp is sounding very loud then). If your power supply is
>overrated then this will only happen at power peak levels.


Whether the amp is really loud then depends on your assumptions. My old 15 watt amps have a tube rectifier. I don't know if a tube rectifier would sag when powering only a 15 watt amp. Maybe this effect only happens above 50 watts. I hope designers make 5 watt all-tube amps including a tube recitifier -- but would such a low-power configuration cause tube rectifier sag?

Dave Kamalsky wrote:

>>I hear a lot of people talk about tube rectifier "sag". What exactly is it supposed to sound like, and how does it happen? I'm guessing this is a good thing (tonally)?

Jonathan Krogh wrote:

> The most apparent setup where sag will be noticed would be at or near full volume. A solid state diode rectifier would let through the current needed by the power tubes no problem, but the tube rectifiers have a limit to how much current they can conduct. When hitting big notes and chords at full volume, especially with overdrive happening, this is where the most current is demanded by the circuit, so with the note hit, theres this sudden stress placed on the rectifier to let through this extra current, also, at this point, the extra current demanded will lower the overall B+ voltage going to the circuit. What results is this effect that a hard note hit doesnt seem as loud as it should, and there seems to be a sort of delay feel to the note, what is happening is that as the note is picked, the rectifier stress is high, the voltage drops kind of slowly, and theres this smooth note coming through, as the guitar signal decays, the stress is lowered, and the voltage picks back up, sort of creating the effect that there is more sustain. Tube rectifier sag is basically a natural compression. Smooths out the peaks of the playing signal a bit more, and gives that nice 'spongy' attack on notes.

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