janneman wrote:
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?
and Hey Mr. Ian, you rock Sir
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.
Triodes already include quite heavy internal feedback because the anode
potential affects the potential gradient between the cathode and grid.
The screening grid of the tetrode and pentode removes that feedback to
allow much higher voltage gain*.
Because of this, triode circuits *give reasonably stable gain without
external feedback, 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.).
*The original purpose of the screen grid was to reduce the anode-grid
capacitance (and the Miller effect) which restricted the H.F.
amplification of triodes.
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?
Yes, larger numbers are used when describing external feedback in SS
circuits, but they don't always accurately reflect what is really going
on. Perhaps less external feedback is needed in a valve circuit
because it already has enough internal feedback - or perhaps the valve
circuit contains a transformer with phase shifts which limit the amount
of feedback which could be safely applied around that stage - or perhaps
the transistor circuit generates such a high distortion level that it
only becomes acceptable with masses of feedback. In most cases the
designer knows the answer (if he is doing his job properly), but the
salesman is never going to let you find out.
You have to be very careful how you define the level of feedback.
Consider an amplifier which, without feedback, has 10 dB more gain at 1
Kc/s than it does at 100 c/s and 10 Kc/s (very poor) and you decide to
flatten the frequency response by applying12 dB of feedback. The
feedback at 1Kc/s would indeed be 12db, but at 100 c/s and 10 Kc/s it
would only be 2 dB.
Take another case of a Class-B transistor output stage where the gain
varies according to the current in each of the devices. How much
feedback are you really using when the loop gain, bandwidth and
slew-rate all vary wildly throughout each cycle of the audio waveform?
In both these examples, the true feedback is greatest under the maximum
gain condition, which is not very helpful for counteracting the
shortcomings at the low-gain ends of the performance (which is what
feedback is often intended to do).
Furthermore, you need to consider how much feedback already exists
inside the device or its immediate circuitry, before you apply external
feedback. You would find it difficult indeed to design a worthwhile
one-transistor audio amplifier without any feedback at all, the
distortion would be horrible at anything other than the smallest signal
levels. In contrast, a single pentode without feedback gives quite
passable results for domestic equipment - and single triodes were used
in good quality professional equipment without external feedback for
many years.
One big advantage of external feedback is that it puts the overall
performance of the circuit under the designer's control. He is not tied
to a certain characteristic determined by the valve manufacturers' grid
wire geometry. Worse still, the tolerances on semiconductors are so
wide (and so temperature dependent) that it would be impossible to stay
within any kind of worthwhile specification without feedback 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.
If the circuit is well designed, you should not hear any change until
the valves degenerate to the point where they fall well outside their
design characteristics. Even a small amount of feedback will take care
of that.
For triodes, which are used without external feedback, the grid geometry
determines the amount of internal feedback (until the end of life) and
that is unlikely to have any significant wear-out mechanism.
You may well hear changes if pentodes are used without feedback, as in
some domestic tape recorders and cheap P.A. amplifiers, but that is
because the design was driven by the need for the minimum number of
valves, not for a good specification and reliable long-term operation.
And of course tube rolling would be senseless if device parameters
were hidden by the circuitry!
I'm not sure what you mean by 'tube rolling', but if you mean using the
uncorrected large-signal characteristics of a valve to create distortion
effects, then I agree with you. In that case you would definitely
expect to hear changes as the device aged (more-so for a pentode than a
triode). However, that is the exact opposite of what a good valve
amplifier designer strives to achieve.
Valves can be used for stable high quality audio amplifiers, they can
also be used for effects units. Some poorly-designed amplifiers have
unintentional effects units built into them by mistake and a few
specialist musical instrument amplifiers have them by design. None of
this says anything about the valve itself (as compared with the
transistor), it is just a matter of how it is used.
--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk