What are doing those zener diodes ?

[kagliostro]

You know, I often look to old schematics trying to understand how they work and hoping to find something of  interesting

I've find this '73 Fylon schematic

Initially I was attracted from two things, the Bias Supply & a bunch of zener diodes connected in a strange (to me) way

Looking better I noticed that the Bias Supply is simply a double pot Bias Circuit

But what about those zener ? I've seen diodes connected from plate of the power tubes and ground but zener connected at that position are new to me

Please someone can explay me what they are there to do



Many Thanks

Franco

[sluckey]

Those three zeners in series act like a single zener with a reverse voltage of 49.6v (22+22+5.6) and a forward voltage of about 1.8V (the forward voltage drop of three silicon diodes in series). So, looks like they just clip the positive portion to 49.6V and they clip the negative portion of the signal to about 1.8V. I don't know why.

[trobbins]

The zener-capacitor node would appear to likely sit at +49V for no signal condition, given a bit of coupling cap leakage.  Then that node would be pushed towards +26V as signal level increases to a symmetric clipping, given that the coupling caps would charge to new DC levels - sort of like blocking distortion.

I've not seen that use of a common secondary side fuse before, but can see that it may nicely disable the amp for a number of bad faults, including a fault on the output stage B+ (and not then allowing the screen voltage to remain).

[kagliostro]

Thanks Steve

To know a bit is better than to know nothing


Franco

[kagliostro]

Thanks Trobbins

.. sort of like blocking distortion

 

Bloking Distortion sounds like something we would like to avoid ....... please can you try to explain me it with other worlds

I've not seen that use of a common secondary side fuse before ....

I too, but you were so an attentive observer to notice it, thanks, interesting indeed

Thanks

Franco

[trobbins]

One form of blocking distortion is caused by the change in dc bias across the coupling capacitor between driver and output stage (but usually only happens when signal level is high and causes grid conduction of output stage valve).  I think your schematic would also function by the dc bias on the coupling caps changing, but it is somewhat different as the bias change would occur with any increase in signal level.

[kagliostro]

Many Thanks Trobbins

Franco

[kagliostro]

Just for knowledge

at an italian forum a friend told me that as the zener are connected to the circuit between two capacitors they are not DC coupled and due to that their behavior regarding the signal is symmetrical (they cut positive and negative signal equally)

Franco

[HotBluePlates]

The approach generally reminds me of Paul Ruby's zener diode method to prevent blocking distortion.

I've also seen tape recorders use diodes at a grid to clip the signal & prevent overdrive/grid-current/grid-blocking.  Yes, this trades one form of distortion for another, but likely recovers from overdrive faster.

[kagliostro]

Very Interesting HotBluePlates

I'll read with attention


Many Thanks

Franco

[trobbins]

as the zener are connected to the circuit between two capacitors they are not DC coupled and due to that their behavior regarding the signal is symmetrical (they cut positive and negative signal equally)

Yes, for a steady large signal level the clipping is symmetric, but any dynamic change in signal level causes the dc bias on those coupling caps to shift and that causes attack-decay and non-symmetric signal distortions.

[kagliostro]

Ciao Trobbins

Interesting, Many Thanks

it will be nice to hear a demo of that amp, I'll search for it

Franco