[sdiy] outlet as variac? tube rectifiers

[Eric]

At 07:49 PM 2/19/01 -0600, you wrote:
>Didn't all the old Fender stuff have tube rectifiers?  Doesn't the warm up
>time of the rectifier take care of this?

Yes and yes.

>is anyone still using tube rectifiers and would are
>to comment / compare them to solid state 
>(for use in other wise tubed equipment of course).

1) Vacuum rectifiers have higher impedances and provide
less regulation at higher currents (voltage sag). 
Unique characteristic, not effectively
simulated with semiconductor rects and series resistor.
Difficult to simulate. Usually done with a complex
limiter circuit (it's not a simple linear or an exponential
characteristic) or DSP algorithm with look up table.

2) Vacuum rects are very FAST. They pass the AC peak current
pulses to the filter cap very easily. Whereas cheap 1N400x 
diodes are very slow, and can produce sharp current pulses 
which can end up in the output circuit as RFI or hash. 
Avoiding this either means considerable shielding and
filtering, or the use of special high-speed diodes such
as "Hexfred" diodes (not Schottkys--they are for low voltage
service only). It's a common upgrade to old tube amps that
used silicon diodes, to replace them with Hexfreds. It can
make a surprisingly big difference, esp. with the many old
receivers and integrated amps that used voltage-doubler supplies.

3) Don't forget, in a big tube amp, a delay in the plate
supply is a good idea. The power tube cathodes should warm
up BEFORE the high voltage plate supply is applied. This delay
is inherent in tube rectifiers and not in any other type.

4) You gentlemen would be shocked at how many tube amps
made today use tube rectifiers. In guitar amps, Fender,
Marshall, Mesa, & Peavey all have deluxe models of one type
or another with tube rects. And virtually ALL "boutique" gtr
amps have tube rects. And hi-fi amps...most tube hi-fi not
made by Audio Research or CJ is using tube rects now. I think
every current Cary Audio product uses them. 
(This is a major issue with those OEMs--there are no really good
tube rects being made right now. Svetlana's problems mean
no more 5U4s for Mesa-Boogie's Dual Rectifier and Triple
Rectifier amps.)
Even Metasonix TS modules use them--tiny little 6AL5s.
They tend to fail open, rather than shorting out as
silicon diodes often do. And they provide cleaner DC,
and they provide inherent slow warmup. And they provide
inherent current limiting (the cathode won't pass more than
its available space charge, whereas a silicon diode can pass
hundreds of times its rated current for a short pulse).


Eric Barbour
METASONIX