This is the equivalent to connecting the cathode to a low voltage positive power supply. I think it suffers from a number of drawbacks:Circlotron said:
The valve will be in greater danger from runnaway than with other methods. It might be difficult to get the bias just right, and there are still some charge / discharge effects of the parallel cap.
Cheers,
edit: now if this was class D....
There's no problem with using a diode for cathode bias. Certainly, it's not as safe as resistor bias in terms of thermal runaway, but when was the last time you saw a fixed bias stage run away? If you're particularly paranoid, use a CCS anode load and benefit from the reduced distortion as well as reduced fragility.
overload recovery delay
Björn,
I don't have any links to hand dealing with this issue. I'll look around.
It is a phenomenon which more than half of amplifier designers pretend does not exist. RC coupled amps are more prone than transformer or direct coupled types.
The problem is to do with the behaviour of the amplifier following an overload transient. It is possible that the DC conditions of the amplifier are changed by the overload, and it can take some milliseconds before they revert to their proper values. During that time the sound is not normal.
Cheers,
Björn,
I don't have any links to hand dealing with this issue. I'll look around.
It is a phenomenon which more than half of amplifier designers pretend does not exist. RC coupled amps are more prone than transformer or direct coupled types.
The problem is to do with the behaviour of the amplifier following an overload transient. It is possible that the DC conditions of the amplifier are changed by the overload, and it can take some milliseconds before they revert to their proper values. During that time the sound is not normal.
Cheers,
SENSORY OVERLOAD.
Hi,
When a strong transient sends an amplifier into momentary clipping it will take RC coupled amps much longer to recover from this condition than xformer coupled amps.
One way to make this very obvious is the use of oversized coupling caps, the overload recovery will be increased in time and the condition will be added to the continuing sinewave as a superimposition on the continued signal but distorted and lagging behind in phase.
PSUs will take some time to recover from this condition as well and yes this is certainly all very audible.
A quick Google on " overload recovery delay" will yield a ton of information about this phenomenon.
How amplifier designers can deny its importance is beyond me, then again there are a lot of those "details" that seem to escape them.
Cheers,
Hi,
When a strong transient sends an amplifier into momentary clipping it will take RC coupled amps much longer to recover from this condition than xformer coupled amps.
One way to make this very obvious is the use of oversized coupling caps, the overload recovery will be increased in time and the condition will be added to the continuing sinewave as a superimposition on the continued signal but distorted and lagging behind in phase.
PSUs will take some time to recover from this condition as well and yes this is certainly all very audible.
A quick Google on " overload recovery delay" will yield a ton of information about this phenomenon.
How amplifier designers can deny its importance is beyond me, then again there are a lot of those "details" that seem to escape them.
Cheers,
Alright, I'll explain it.
That's right. Kobayashi's US Patent 1,913,449 uses a transformer to couple the anode back to the grid, but the amplitude stabilisation element is an RC network. (If you very slightly redraw his circuit, it looks like every audio power amplifier with global negative feedback and explains motorboating.)
What happens with coupling and bypass capacitors is this:
Imagine that you have one stage with its anode at +100V, and you capacitor couple this to the grid of a second stage which has +10V of cathode bias. The first stage's anode then swings to +120V, a change of +20V. The grid of the second valve tries to swing from 0V to +20V, but when it reaches +10V, there is 0V between grid and cathode, so the grid conducts heavily, and is clamped to +10V. The voltage across the coupling capacitor instantaneously changes from +100V to +120V - +10V = +110V.
The first anode now returns to its rest position of +100V, but the coupling capacitor is still charged to +110V, so the grid of the second valve must be at -10V. The cathode of the second valve is held at +10V by its bypass capacitor, so Vgk is -20V, and the second valve is switched off.
The grid-leak resistor has -10V across it, so a current flows, and the coupling capacitor discharges back to +100V across it, at which point, normal operation resumes.
The key point is that we deliberately make the RC time constant of the coupling capacitor and the grid-leak resistor quite large (100n and 1M gives 100ms), and it takes 5RC for normality to return. Thus, an amplifier can take half a second to fully recover from a momentary overload that might otherwise have been imperceptible.
dhaen said:
That's right. Kobayashi's US Patent 1,913,449 uses a transformer to couple the anode back to the grid, but the amplitude stabilisation element is an RC network. (If you very slightly redraw his circuit, it looks like every audio power amplifier with global negative feedback and explains motorboating.)
What happens with coupling and bypass capacitors is this:
Imagine that you have one stage with its anode at +100V, and you capacitor couple this to the grid of a second stage which has +10V of cathode bias. The first stage's anode then swings to +120V, a change of +20V. The grid of the second valve tries to swing from 0V to +20V, but when it reaches +10V, there is 0V between grid and cathode, so the grid conducts heavily, and is clamped to +10V. The voltage across the coupling capacitor instantaneously changes from +100V to +120V - +10V = +110V.
The first anode now returns to its rest position of +100V, but the coupling capacitor is still charged to +110V, so the grid of the second valve must be at -10V. The cathode of the second valve is held at +10V by its bypass capacitor, so Vgk is -20V, and the second valve is switched off.
The grid-leak resistor has -10V across it, so a current flows, and the coupling capacitor discharges back to +100V across it, at which point, normal operation resumes.
The key point is that we deliberately make the RC time constant of the coupling capacitor and the grid-leak resistor quite large (100n and 1M gives 100ms), and it takes 5RC for normality to return. Thus, an amplifier can take half a second to fully recover from a momentary overload that might otherwise have been imperceptible.
Very interesting.....but...
Yes, that's roughly how I understood it. It's hard accept that a patent could cover such a fundamental circuit. I bet it had been used many times before (but not described). But that's like a lot of things, I think.
BTW Did you know that "Kobayashi" translated directly as "Littlewood"?
Cheers,
Yes, that's roughly how I understood it. It's hard accept that a patent could cover such a fundamental circuit. I bet it had been used many times before (but not described). But that's like a lot of things, I think.
BTW Did you know that "Kobayashi" translated directly as "Littlewood"?
Cheers,
I would think the cap voltage rises according the the time constant of the cap and the first stage anode resistor that is supplying current to it.
Hi,
Maybe this article will provide a little more insight even though I feel EC8010 did a very good job explaining the phenomenon:
NON IDEAL BEHAVIOUR.
Cheers,
Maybe this article will provide a little more insight even though I feel EC8010 did a very good job explaining the phenomenon:
NON IDEAL BEHAVIOUR.
Cheers,
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