There is a featured article in the Feb. 2006 AudioXpress about a "Tube Amp
Design Breakthrough" featuring a "Clamped Bias, AB2, RC-Coupled Amp."

The circuit is "different." In a nutshell, the designer uses a zener to
dynamically clamp the grid voltage. It definitely improved performance at
clipping according to the accompanying 'scope pics. The wiring "under the
hood" looked like a Fiultra project, a labrythine tangled mess of wires . .
.. .

The author modestly offered, "Anyone is welcome to use my
dynamic-bias-clamp, which might soon sweep the tube amp board, pretty much
as ultralinear did around 1954. If it's taken half a century for the "nexy
big tube thing," better late than never." (Wow! Approaching the parabolic
limits of Jutean hyperbole).

The author declined to make "fatuous comments on sound quality", but I
wonder about the concept vis a vis the sound.

1. Zeners are noisy. What would prevent this noise from being directly
injected into the grids and thus the speakers?

2. AFAIK, noise is related to temperature. Does the fact that grids don't
draw much current impact the noise generated?

3. Zeners have a non-linear conduction knee. Wouldn't that play hell with
the transient response?

I'm not convinced that this approach would be "musical."

Jon

Jon Yaeger wrote:

There is a featured article in the Feb. 2006 AudioXpress about a "Tube Amp
Design Breakthrough" featuring a "Clamped Bias, AB2, RC-Coupled Amp."

The circuit is "different." In a nutshell, the designer uses a zener to
dynamically clamp the grid voltage. It definitely improved performance at
clipping according to the accompanying 'scope pics. The wiring "under the
hood" looked like a Fiultra project, a labrythine tangled mess of wires . .
. .

The author modestly offered, "Anyone is welcome to use my
dynamic-bias-clamp, which might soon sweep the tube amp board, pretty much
as ultralinear did around 1954. If it's taken half a century for the "nexy
big tube thing," better late than never." (Wow! Approaching the parabolic
limits of Jutean hyperbole).

The author declined to make "fatuous comments on sound quality", but I
wonder about the concept vis a vis the sound.

Zener clamping of grid voltages was employed by ARC many years ago in an attempt
to
stop severe grid current,

But any use of zeners or diodes around grid circuits won't prevent the charge up
of coupling
caps in overdrive, not does it have any effect on the 2H currents in cathodes
due to
AB operation of the tubes.

But where AB2 output tubes are driven with direct connected CF stages to power
into grid current,
I fail to see where zeners would have a place.





1. Zeners are noisy. What would prevent this noise from being directly
injected into the grids and thus the speakers?

Zeners only are noisy while conducting,
especially at low currents between being just a little bit on and fully on.


2. AFAIK, noise is related to temperature. Does the fact that grids don't
draw much current impact the noise generated?

Depends on the topology.

Where is the schematic of the application of the zeners please??????



3. Zeners have a non-linear conduction knee. Wouldn't that play hell with
the transient response?

I'm not convinced that this approach would be "musical."

Jon

Depends on the app.

The is usually no need for zeners anywhere near grid circuits.
I'd be very surprised if the circuit app in Audio Express was not tried 20
years or more ago
since just about anything to cure bias overload behaviour has been tried, and
long ago.

Patrick Turner.

Two words: clamp tube.

Patrick Turner wrote:

Jon Yaeger wrote:


There is a featured article in the Feb. 2006 AudioXpress about a "Tube Amp
Design Breakthrough" featuring a "Clamped Bias, AB2, RC-Coupled Amp."

The circuit is "different." In a nutshell, the designer uses a zener to
dynamically clamp the grid voltage. It definitely improved performance at
clipping according to the accompanying 'scope pics. The wiring "under the
hood" looked like a Fiultra project, a labrythine tangled mess of wires . .
. .

The author modestly offered, "Anyone is welcome to use my
dynamic-bias-clamp, which might soon sweep the tube amp board, pretty much
as ultralinear did around 1954. If it's taken half a century for the "nexy
big tube thing," better late than never." (Wow! Approaching the parabolic
limits of Jutean hyperbole).

The author declined to make "fatuous comments on sound quality", but I
wonder about the concept vis a vis the sound.


Zener clamping of grid voltages was employed by ARC many years ago in an attempt
to
stop severe grid current,

But any use of zeners or diodes around grid circuits won't prevent the charge up
of coupling
caps in overdrive, not does it have any effect on the 2H currents in cathodes
due to
AB operation of the tubes.

I'll have to look more closely, but I am fairly certain that his circuit
does prevent charge up of the coupling cap. That was the whole point of
his design and the article.

Phil

But where AB2 output tubes are driven with direct connected CF stages to power
into grid current,
I fail to see where zeners would have a place.





1. Zeners are noisy. What would prevent this noise from being directly
injected into the grids and thus the speakers?


Zeners only are noisy while conducting,
especially at low currents between being just a little bit on and fully on.


2. AFAIK, noise is related to temperature. Does the fact that grids don't
draw much current impact the noise generated?


Depends on the topology.

Where is the schematic of the application of the zeners please??????



3. Zeners have a non-linear conduction knee. Wouldn't that play hell with
the transient response?

I'm not convinced that this approach would be "musical."

Jon


Depends on the app.

The is usually no need for zeners anywhere near grid circuits.
I'd be very surprised if the circuit app in Audio Express was not tried 20
years or more ago
since just about anything to cure bias overload behaviour has been tried, and
long ago.

Patrick Turner.