Re: Upgrading a Fender Blues Jr



By the way, is an amp *really* a tube amp if it has a solid state
rectifier? Also, what the hell is a solid state rectifier? It seems
the Blues Jr has one?

Yes, it is. The big advantages of tube technology for amplification are:
1. Input transients ( such as attacks ) and other brief excursions in volume that
might cause the input stage to near it's limit (highest volume it can reproduce
without distortion) are handled much more gracefully by tubes vice transistor
based amplification stages. This results in vocals, piano, and just about everything
else sounding better - often the phrase "added tube warmth" is used to describe
this, but of course this phrase, since it has never been actually defined by anyone,
is exactly meaningless beyond identifying the speaker as on the bad side of the
sloppy/precise continuum. (Actually there are contexts wherein "added tube
warmth" is a well defined and desired state, but these are
more intimate than public discussion will allow, and also pretty sloppy, but in a
good way...) In any event, this better sound is associated with tube front end for
many recording situations.

2. Added harmonic content when driven into the non-linear range - i.e. distortion.
If you drive a tube amplifying element into it's non-linear (distortion) range the
non-linearity is much less abrupt than when doing so with a solid state device
(transistor). This results in a less violent onset of higher frequencies (harmonics)
that are not present in the input. This is correctly explained as "tubes in overdrive
sound good, transistors when overdriven sound really really bad". Essentially all of
these miserable little stomp box distortion pedals are based on a diode in the
signal path to clip the signal, which in the ideal/theoretical case introduces frequencies
that are unrelated to fundamental. These are then cleaned up by filtering in some form
go get something that is at least plausible. In contrast at tube stage just on the
edge of overdrive offers a wonderful pallet for selecting a different timbre just by
playing a little louder - pick sensitivity and such - a very good thing.

3. Sustain. (finally the answer...) Amplifiers run on DC (direct current) as opposed to
the alternating current that is provided to homes for reasons of efficient distribution.
Converting AC to DC is doing with a rectifier, which is a diode, which can be made from
either a tube diode, or a solid state diode. Tube diodes have these properties:
- they start from a cold state pretty slowly thus easing the transition of the amp from
off to on when mains voltage is switched on.
- they don't behave the same under high load as they do under low load. As current
demand is increased they continue to rectify, but they provide a lower voltage than
when more lightly loaded. This is called sag.
If you hit a note hard you draw more current from the power supply (with reasonable
assumptions on the output stage) and the voltage that is provided for amplification
is reduced from which it was an instant ago. As the sound decays in the string the
current demanded is reduced, and the voltage the (tube) rectifier can provide is
increased thus increasing the voltage available for amplification. This looks and
sounds just like a volume knob that is increased as the note decays - i.e. sustain.


For common use in the Jazz world items 2 and 3 above are not as important as item
1, but they're still very nice to have.



Regards,
b
.

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