Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!uwm.edu!csd4.csd.uwm.edu!info-high-audio-request
From: bill@vrdxhq.verdix.com (William Spencer)
Newsgroups: rec.audio.high-end
Subject: Re: Tubes or Transistors: The Matching Game
Message-ID: <8488@uwm.edu>
Date: 20 Dec 90 13:54:20 GMT
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Sure, I have $.02 to offer on this subject.

First, something should be pointed out: there are really only 3 kinds of
active devices used (typically) in audio electronics. They are tubes and
the two transistor families, FET and bipolar. That is all. When you realize
the choice is that limited you also may realize they all must be considered.
There are, of course, variations of the major device types. Transistors
can be made in positive and negative polarities -- one advantage. Tubes 
have different types according to number of terminals -- triodes, pentodes,
etc. And they all have countless variations of geometry and power handling.
FET's and bipolar transistors do deserve seperate consideration as major 
device types because their principles of operation are very different.

Tubes have a relatively linear voltage transfer characteristic. 
This comparison is slightly unfair however. With tubes the output current is
reduced when the output voltage swings (in a voltage amplifier configuration). 
This is essentially a negative 
feedback effect of sorts. All the devices have a nonlinear input voltage to
output current response. The tubes require less external negative feedback
for linear response. Remaining distortion will be a gradual nonlinearity spread 
over the full voltage swing. This translates to larger amounts of lower order
(frequency) distortion components and little higher order harmonics.
Transistors are easier to use in more complex circuits with more open loop 
gain and more negative feedback. It is intuitive that the more complex
circuit is more likely to muck up complex audio information although it 
is hard to explain exactly why this happens. With proper design, that is.

Bipolar transistors have one troublesome characteristic that niether 
FET's nor tubes have: signifigant and nonlinear input current in 
normal operation. In other words, the transistor can distort its own input.
Complex.

Tubes run at high voltage (typically) and produce high impedance designs.
Tube preamps have lots of headroom typically. Input capacitance, for example,
is very low with tubes, but so is output current, so comparison is not trivial.
Tube-transistor hybrids are tricky. It's hard to find a MOSFET with high
voltage handling but designed for low power (power MOSFET's are oriented
toward switchmode power supplies more than audio). Even a low power MOSFET
will have far more input capacitance than a tube (typically a few pF).
And tubes are quite rugged even in low power sizes while you have to be careful
with FET's. While the power to input capacitance ratio of MOSFET's is comparable
to tube triodes niether compares to that of pentodes. 

Not all the differences between amplifiers can be attributed to device 
type. The circuit configurations generally used are very different.
Due to high impedance and voltage transformer coupling is used (usually)when
tubes must drive low impedance loads (speakers). Since only one device polarity
is availiable a balanced transformer is used when push-pull operation is
desired. The transformer is a major disadvantage (especially for bass)
and expense. However, the balanced configuration can be much more 
precisely balanced than a push-pull transformerless design. Also, power supply
ripple cancels. A transformer is an ultimate DC blocking device so biasing
problems are much less critical. Servo loops and speaker protection are
not used. Also tube amps are generally biased closer to class A than 
the majority of transistor amps.

This only touches on some of the main differences. Technical differences
definitely exist and the comparison is not at all one sided. Part of
the choice depends on application.

All things equal though, you might prefer transistors to tubes due to 
reliabilty and maintainance. The lifetime of tubes is good, however.

BTW, some of the reports of trouble with VTL's are probably due to tubes
damaged during shipping. Tube sockets can also be as much trouble as the tubes
themselves.

bill S.