Diodes in output section . . . ?

[acheld]

Merlin Blencowe sells small PCBs -- one for each section of an amp. 

In reviewing a "suggested" design for a small amp, he is uses diodes off the plate, and a Zener diode off of the cathode.  I have not seen these before, and he provides no explanation on his website or his pure-amp books.

What are the reasons for using the plate diodes??   and the Zener?   

I do not have plans to use them (as of now), but would like to know what problem he is trying to solve.  His plans are usually spot on . . . so I'm sure there is a good reason for them.

[jjasilli]

A string of 1N4007's off the plate is for surge protection, sometimes considered part of "bullet-proofing" an amp - often for an amp used professionally on the road.  See: "How to Service Your Own Tube Amp, Tom Mitchel, p.228.  The concept was used in older Musicman Amps.  It's debated if it affects tone.

There were posts about this on this Forum, but I think a long time ago.

Not familiar with the Zener in the K side of the circuit.

[AmberB]

A zener diode could be considered a simple voltage regulator.  It dumps all voltage above the voltage rating of the diode, so it prevents the voltage from rising beyond that point.  Why it would be used in the cathode circuit of a tube is an interesting question...

[silverfox]

Amber: Why it would be used in the cathode circuit of a tube is an interesting question...


As a backup to the Anode protection. Surge voltages arising from the primary of the OT, perhaps from a sudden loss of load on the output. Like a car ignition coil.


silverfox.

[PRR]

That snip-image leaves a lot to my imagination. Here's the whole scheme for context.
https://d1f7geppf3ca7.cloudfront.net/origin/564390/1534359375060_el84_push_pull.jpg

[shooter]

Thank you, I was "imagining" an SE, that clamped the neg 1/2 cycle, I moved on to something else 

[pdf64]

...What are the reasons for using... the Zener?...

For AB1 amps, it would act to prevent the cathode voltage rising excessively at high signal levels. Aiken describes that characteristic as ‘squish’.