[diyfabtone]

No real downside. A Zener regulator does not keep voltage from dropping below the threshold but it does a good job dropping it down from a higher level and keeping it at that point.
Just string together the right amount of zeners (i.e. 6 x 50V to get 300V limit) with enough combined power rating to safely handle the total load current in a failure.

 

[Latole]

Zener diodes can be noisy: multiple series elements may be more noisier...I would decouple them to reduce the risk of noise generation.

Noisy ! I dont't know, I don't hear it.

 

[Paul Running]

Another alternative is operate the plates at full B+ and just reduce the screen-voltage to the output tubes; for most power tetrodes and pentodes, you can operate the plates at the design maximum value+, with low screen-voltages, with low-risk of failure. Note that the tubes will overload (break-up) sooner, as the screen-voltage is reduced.

 

[Latole]

Carbon resistors are noisy too . Can you hear it !

 

[Paul Running]

Noisy ! I dont't know, I don't hear it.

Google: noise generator or noise from Zener diodes.

 

[Paul Running]

Carbon resistors are noisy too . Can you hear it !

Most noise is inaudible however, it can cause issues with electronic gear.

 

[Latole]

Google: noise generator or noise from Zener diodes.

I did not say they are not noisy, I say I can't hear it.

 

[Latole]

Most noise is inaudible however, it can cause issues with electronic gear.

I own the amp and 20 years later, it working fine.

I don't make money in zener market , use or don't use I don't care.

 

[Alan Small]

Any down side to this? It seems really simple to calculate and implement.
What are the limitations?

I would like to lower the voltage running into my 5E3 just a bit and this looks like a simple candidate.

It is quick and inexpensive to try it out😊 solo stocks 1000v piv 5 watt zeners in 16, 33 and 100 volt drop for a couple $ each

 

[Moosehead]

I went into an exam once with a B+, brought it down to a D+.

Possibly I got a variac wrong but Im pretty sure it was the zener diode. Or a combination of both.. Big choke for sure.