Valve Questions
"janneman" wrote in message
On Apr 17, 9:50 am,
(Adrian
Tuddenham) wrote:
janneman wrote:
On Apr 16, 5:14 am, TheOctavist
wrote:
Tube Rolling:-
Valves have certain specs, which all manufacturers of,
say, an ECC83 adhere to in terms of gain, bias
sensitivity, mutual conductance etc. All manufacturers
have production spreads, so that a valve may have low
gain, another of the same type higher gain, but still
within the acceptable spread.
Is there any objectove evidence that a say, Mullard,
ECC83 is any different in spreads and performance than
any other manufacturer's.
Is there any objective evidence that a say, Mullard,
valve will sound any different to another
manufacturer's valve.
Is there any objective evidence that valve circuits
are so sensitive to changing valves unless it's clear
that it's due to the valves being at opposite ends of
their production spreads.
Or is it yet another audiophile myth?
If by 'objective evidence' you include measurements,
you can generally say that performance differences can
be measured with different tubes. As tube equipment is
often designed with little or no feedback they are much
more sensitive to individual tube parameters than ss
equipment where the feedback makes sure that individual
active device performance differences do not lead to
measured or otherwise objective differences.
That tubed equipment has less loop feedback and often has less local
feedback than SS gear is a matter of scientific fact.
I'm old enough that my formal education started with tubes and SS was sort
of an add-on. The first thing we were told was that the transfer
characteristics of transistors were much like pentodes, and that transistors
had far less built in resistance and local feedback than tubes.
Triodes already include quite heavy internal feedback
because the anode
potential affects the potential gradient between the
cathode and grid.
Right and quality tubed audio gear used lots of triodes.
Furthermore, the classic tubed preamp used cascaded triodes with loop
feedback, either for RIAA equalization or tone controls.
The screening grid of the tetrode and pentode removes
that feedback to
allow much higher voltage gain*.
Tetrodes and pentodes usually ended up in power amplifiers where they were
typically enclosed in feedback loops.
Because of this, triode circuits give reasonably stable
gain without external feedback,
Nevertheless quality equipment often put its triodes inside feedback loops.
whereas pentode circuits are more dependent on
individual valve characteristics. A prudent audio
designer always arranges feedback around a circuit containing a pentode
(things are not as straightforward at R.F.).
RF amplfiers in tuners often had "neutralization capacitors" hooked from the
plate to the grid. This is inverse feedback.
*The original purpose of the screen grid was to reduce
the anode-grid
capacitance (and the Miller effect) which restricted the
H.F. amplifiaction of triodes.
Agreed.
Ahh yes, good points indeed. But wouldn't you agree that
in tube equipment, even in pentode/tetrode circuits, feedback
factors are generally much lower than in ss?
This is true, but open loop gain and open loop nonlinearity is often greater
in SS circuits.
I mean, the fact that over the limited life time of tubes you can
measure (some say hear) changes does point to equipment
parameter variation with device parameters.
Agreed.
And of course tube rolling would be senseless if device
parameters were hidden by the circuitry!
The renewed interest in tubed circuits with no loop feedback seems to be
pandering to tube rollers.
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