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Is there any real advantage to cathode
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#1

04-02-2012, 02:05 PM

Rod
Beauvex

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Is there any real advantage to cathode
bias?

Or is it just used for simplicity or
economy reasons?

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#2

04-02-2012, 02:09 PM

battradio

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It , is self limiting , the more
current the tube draws , the more bias it produces .In some ways it more
linear than fixed bias .

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#3

04-02-2012, 02:30 PM

for_p1

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It provides negative feedback either
at DC (when bypassed by large cap) or at operation frequencies. Thus it
either stabilizes operation point or reduces distortion.

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#4

04-02-2012, 03:24 PM

mhardy6647

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cheap and simple to implement, too.

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#5

04-02-2012, 04:04 PM

gadget73

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Its very reliable. No worries about
the bias supply failing and causing the tubes to self-destruct and no
worries about someone mis-adjusting it and causing damage.

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#6

04-02-2012, 04:42 PM

Celt

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Best way to do it, imho.

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#7

04-02-2012, 06:12 PM

for_p1

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Quote:

Originally Posted by Celt

Best way to do it,
imho.

No, the best way to do it is
fixed bias with servo. You can do a lot of things with it, like signal
dependent bias point.

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#8

04-02-2012, 11:31 PM

dcgillespie

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Would not a signal dependent bias
point not be just another way to describe cathode bias?

Dave

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#9

04-02-2012, 11:38 PM

BuzzK

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Dave,

What's your opinion on
cathode bias vs. fixed?

Some say the midrange is clearer with
cathode bias, but not the bass or top end. I know there are probably no
universals, but . . . . ?

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#10

04-03-2012, 12:02 AM

Shadowdog

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It also depends on the primary
impedance of the output transformers in a given circuit which type can be
used. Cathode bias favors a higher primary so if you converted some amps
to fixed it may not be a benefit sonic wise. Credit goes to Dave on his
many posts and paper on his EFB mod and answering my questions.

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#11

04-03-2012, 01:12 AM

audiodon

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When a cathode biased amp starts to
run out of steam, it asks too much of the power supply, limiting the
available low end and distorting the high end.
My favorite Fisher tube
integrated is cathode biased.
Some call a cathode biased amp a working
man's amp.
A Harman Kardon 250 is supposed to be cathode biased, but
I've never seen one up close.
I hadn't thought of it as a midrange amp,
but I do like my midrange, and I count on a powered sub for the lows, so
that makes sense to me.

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#12

04-03-2012, 02:04 AM

Shadowdog

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From what I've read if you use an
output transformer with greater primary impedance than you would for fixed
bias you should be able to minimize the negatives you mentioned. Eico did
that with the HF-87/89, and looks like most big big amps are and were
back in the day fixed bias

Many small amps (6V6/ EL84 etc.) with
cathode bias probably could benefit in the conversion to fixed bias as
they have output transformers with the right primary impedance I would
think.

Randy

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#13

04-03-2012, 09:04 AM

dcgillespie

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The original question posed is so
general that it invites a number of possible answers. So the general
answer to the general question is that no doubt the real advantage of
cathode bias is the economic one.

From a strict standpoint of
circuit theory however, while there are situations where cathode bias can
perform as well as fixed bias, it can never perform better than fixed
bias. But even that statement needs a qualifier.

If we confine the
definition of performance to just power output and distortion, then with
all else equal, cathode bias can never provide the same power output as
traditional fixed bias can, because of the voltage loss across the cathode
resistor, or whatever device is placed in the cathode circuit to raise its
voltage to be more positive than that of grid #1 (the control grid), all
relative to circuit ground. Any voltage lost across a cathode bias device
is voltage that cannot otherwise appear (or be dropped) across the OPT
primary winding, and therefore represents a loss of power output.

Regarding distortion, the only time cathode bias can produce the same low
distortion levels as fixed bias can, is with a true class A push-pull
amplifier. In such an amplifier, the voltage across a cathode bias
resistor remains as stable as if fixed bias were used, because the current
draw through the two output tubes is exactly equal and opposite at all
times through out the entire power output range of the amplifier.
Therefore, in this scenario, distortion will be equally low which ever
bias scheme is used.

None of this is to imply that cathode bias
designs deliver "bad" performance. While they still cannot ultimately
deliver the same performance as that of an equivalent fixed bias design,
they can deliver quite good performance when designed correctly. The words
"designed correctly" means that adjustments have to be made to account for
the fact that cathode bias is to be used, where as with fixed bias
designs, no such design corrections are necessary.

Virtually all
consumer oriented vacuum tube audio amplifiers of push-pull design operate
in class AB1. That means that after a very low power output level has been
achieved, the operation of the output stage changes from one of class A,
to that of class B, and does so automatically. When the switch to class B
operation commences, the current draw through the two output tubes becomes
anything but equal and opposite, while the total current draw for the two
tubes starts to rise dramatically. Therefore, if cathode bias is used, the
bias voltage across the bias resistor will start to rise and fluctuate
significantly as well. To keep the cathode bias voltage as stable as
possible -- very necessary to achieve the lowest possible distortion -- a
capacitor is placed across the resistor to minimize the fluctuations. In
practice, it works pretty well. But the capacitor hardly makes the output
stage operate exactly like that of a fixed biased output stage.

While the cathode bypass cap can work to limit bias fluctuations across
the bias resistor due to fluctuations in peak current flow as power output
increases (and therefore work to maximize available power output), it
cannot limit the increase due to the average increase in current flow.
That is why the cathode bias voltage will always increase in a cathode
biased amplifier as power output increases. This bias increase due to
increased average current flow through the output stage then shifts the
operating conditions of the stage, requiring the OPT primary impedance to
be adjusted to reflect best performance at the point the operating
conditions shift to under dynamic (i.e. power output producing)
conditions. Because the shift is to one of less maximum possible current
flow, the OPT primary impedance must be raised to accommodate this shift
to maintain reasonable distortion levels. Raising the primary impedance
also means an increased turns ratio in the OPT, meaning that on top of the
power output lost through the cathode bias resistor, even more power is
lost due to the raised OPT impedance required. But even though power
output is reduced, with proper adjustment to the load line, distortion
performance can actually be quite good.

There is an additional
downside to cathode bias however, and that is that typical quiescent
currents must be raised to account for the inevitable shift in operating
conditions that cathode biasing produces. Raising the quiescent current
helps to minimize the overall shift that occurs, maximizing overall
performance from this type of bias scheme. That is why so many cathode
biased amplifiers run the tubes right at the limits of practical
dissipation levels. By comparison, equivalent fixed bias designs can
usually operate at lower quiescent levels to achieve the same levels of
overall performance. While higher quiescent current levels will affect
tube life somewhat, it does not affect it nearly as much as might be
expected, since the tubes were ultimately designed to pass peak currents
far in excess of the quiescent currents drawn -- even in cathode biased
designs.

Ultimately then, power output is always reduced with
cathode bias, but distortion can still be made to be quite reasonable at
the further expense of increase quiescent current draw. However, as long
as enough usable low distortion power can be safely generated with cathode
bias, then the practical sides of cathode biasing come into play; i.e. the
auto quiescent current adjustment feature, economy, etc. Since the 6BQ5
family of tubes can safely develop a very respectable 13-14 watts RMS
power output with cathode bias, and because so many wonderfully efficient
speakers exist that work so well with this power level, it's no wonder
then that with all things considered, the cathode biased output stage
(particularly using 6BQ5 type tubes) was by far the most popular design of
all.

Hopefully, this will help explain the design differences and
the considerations required of the two approaches -- and even help to
dispel some myths along the way!

I hope this helps!

Dave

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#14

04-03-2012, 10:17 AM

BuzzK

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Posts: 1,386

Thanks, Dave. Great post. Any
chance it could become a sticky?

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#15

04-03-2012, 05:13 PM

jaymanaa

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Posts: 51,218

Quote:

Originally Posted by dcgillespie

The original question posed is so
general that it invites a number of possible answers. So the general
answer to the general question is that no doubt the real advantage
of cathode bias is the economic one.

From a strict
standpoint of circuit theory however, while there are situations
where cathode bias can perform as well as fixed bias, it can never
perform better than fixed bias. But even that statement needs a
qualifier.

If we confine the definition of performance to
just power output and distortion, then with all else equal, cathode
bias can never provide the same power output as traditional fixed
bias can, because of the voltage loss across the cathode resistor,
or whatever device is placed in the cathode circuit to raise its
voltage to be more positive than that of grid #1 (the control grid),
all relative to circuit ground. Any voltage lost across a cathode
bias device is voltage that cannot otherwise appear (or be dropped)
across the OPT primary winding, and therefore represents a loss of
power output.

Regarding distortion, the only time cathode
bias can produce the same low distortion levels as fixed bias can,
is with a true class A push-pull amplifier. In such an amplifier,
the voltage across a cathode bias resistor remains as stable as if
fixed bias were used, because the current draw through the two
output tubes is exactly equal and opposite at all times through out
the entire power output range of the amplifier. Therefore, in this
scenario, distortion will be equally low which ever bias scheme is
used.

None of this is to imply that cathode bias designs
deliver "bad" performance. While they still cannot ultimately
deliver the same performance as that of an equivalent fixed bias
design, they can deliver quite good performance when designed
correctly. The words "designed correctly" means that adjustments
have to be made to account for the fact that cathode bias is to be
used, where as with fixed bias designs, no such design corrections
are necessary.

Virtually all consumer oriented vacuum tube
audio amplifiers of push-pull design operate in class AB1. That
means that after a very low power output level has been achieved,
the operation of the output stage changes from one of class A, to
that of class B, and does so automatically. When the switch to class
B operation commences, the current draw through the two output tubes
becomes anything but equal and opposite, while the total current
draw for the two tubes starts to rise dramatically. Therefore, if
cathode bias is used, the bias voltage across the bias resistor will
start to rise and fluctuate significantly as well. To keep the
cathode bias voltage as stable as possible -- very necessary to
achieve the lowest possible distortion -- a capacitor is placed
across the resistor to minimize the fluctuations. In practice, it
works pretty well. But the capacitor hardly makes the output stage
operate exactly like that of a fixed biased output stage.

While the cathode bypass cap can work to limit bias fluctuations
across the bias resistor due to fluctuations in peak current flow as
power output increases (and therefore work to maximize available
power output), it cannot limit the increase due to the average
increase in current flow. That is why the cathode bias voltage will
always increase in a cathode biased amplifier as power output
increases. This bias increase due to increased average current flow
through the output stage then shifts the operating conditions of the
stage, requiring the OPT primary impedance to be adjusted to reflect
best performance at the point the operating conditions shift to
under dynamic (i.e. power output producing) conditions. Because the
shift is to one of less maximum possible current flow, the OPT
primary impedance must be raised to accommodate this shift to
maintain reasonable distortion levels. Raising the primary impedance
also means an increased turns ratio in the OPT, meaning that on top
of the power output lost through the cathode bias resistor, even
more power is lost due to the raised OPT impedance required. But
even though power output is reduced, with proper adjustment to the
load line, distortion performance can actually be quite
good.

There is an additional downside to cathode bias
however, and that is that typical quiescent currents must be raised
to account for the inevitable shift in operating conditions that
cathode biasing produces. Raising the quiescent current helps to
minimize the overall shift that occurs, maximizing overall
performance from this type of bias scheme. That is why so many
cathode biased amplifiers run the tubes right at the limits of
practical dissipation levels. By comparison, equivalent fixed bias
designs can usually operate at lower quiescent levels to achieve the
same levels of overall performance. While higher quiescent current
levels will affect tube life somewhat, it does not affect it nearly
as much as might be expected, since the tubes were ultimately
designed to pass peak currents far in excess of the quiescent
currents drawn -- even in cathode biased designs.

Ultimately
then, power output is always reduced with cathode bias, but
distortion can still be made to be quite reasonable at the further
expense of increase quiescent current draw. However, as long as
enough usable low distortion power can be safely generated with
cathode bias, then the practical sides of cathode biasing come into
play; i.e. the auto quiescent current adjustment feature, economy,
etc. Since the 6BQ5 family of tubes can safely develop a very
respectable 13-14 watts RMS power output with cathode bias, and
because so many wonderfully efficient speakers exist that work so
well with this power level, it's no wonder then that with all things
considered, the cathode biased output stage (particularly using 6BQ5
type tubes) was by far the most popular design of all.

Hopefully, this will help explain the design differences and the
considerations required of the two approaches -- and even help to
dispel some myths along the way!

I hope this helps!

Dave

And I was going to answer the
thread title with a simple "No".

I have to do some reading on your EFB circuit. It kept
popping up last night whilst I was studying up on regulated screen
supplies.

May favorite cathode bias circuit if you can call it
that, is the garter.

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