About the Plate Suppressor Diodes Parameter

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The new "Cygnus X-2" amp modeling introduced a new parameter called "Plate Suppressor Diodes". Plate suppressor diodes, also known as "snubber" or "flyback" diodes are diodes connected between the power tube plates and ground.

The purpose of these diodes is to clamp the inductive "kick" caused by the reactive load seen by the plates. A speaker is a reactive load and has a positive reactive component at high frequencies. This means it looks inductive. Inductors resist change in current. When the plate voltage drops rapidly to zero the inductance of the load causes the voltage to undershoot.

A suppressor diode clamps the voltage and prevents undershoot. This does two things:
1. It prevents excessive voltage at the plates and transformer primary. This protects the transformer from damage due to dielectric breakdown.
2. It also reduces "fizz". The undershoot manifests as increased high frequency content in the range where the load is inductive, typically from 1kHz and up.

Most amps don't have flyback diodes but there are a handful of notable exceptions: Trainwreck Express, Fender Blues Jr., Peavey 5150, and several others.

Flyback diodes were originally added to amps to protect the output transformer but most, if not all, techs don't realize that they also change the clipping behavior.

Consider the graph below:

snubber_diodes.PNG

The blue trace is a plot of a tube power amp output with no flyback diodes. The green trace is the same power amp with flyback diodes. Those spikes in the voltage are what can damage a transformer and also what cause "fizz".

In a real tube amp these diodes are under intense stress and may fail. In our virtual world these diodes are indestructible.

So if you like your tones as fizz-free as possible experiment with the Plate Suppressor Diodes parameter. Note that turning them on will reduce the "chime" at edge-of-breakup and may not be suitable for vintage tones.
 
I'm so glad to have a new Tech Note after six years! This is awesome. The detailed explanation is so helpful, as someone whose been using Master Bias Excursion to reduce fizz recently; I'm really excited to try Plate Suppressor Diodes instead, or in addition. This is the information that allows a player to have a much deeper relationship with the amp. You are awesome for sending out this info and for giving us the means to shape tones in these ways.
 
Very cool to see a new tech note as they are always very informative...and definitely a worthwhile, and welcome, addition to the modeling.

That graph looks like it's from LTSpice...cool...it's my go-to SPICE sim.
 
Agree with the comments above - simple explanation for the masses.

It had been a long time since we saw any Tech Note posts from the man himself and hopefully.
 
Everything that reduces fizz is welcome. Maybe that's why I never use a Fuzz and I don't like sparkling water, soda, champagne or a SM57?? :D

Thank you for the feature and for the clear explanation!
 
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Love this addition. Amp fizz is a complicated relationship. Without it, the lower strings lose presence and clarity. With it, the higher strings sound like a dying mosquito.
 
Love this addition. Amp fizz is a complicated relationship. Without it, the lower strings lose presence and clarity. With it, the higher strings sound like a dying mosquito.

sounds like a modifier on the suppressor parameter would fix you up
 
sounds like a modifier on the suppressor parameter would fix you up
I haven't looked... Does that parameter support a modifier? What are you thinking he should use? Pitch tracking?

I've actually had the same problem. When I play on the low strings, I find myself wishing I could blend in a little more high-end fizz. But if I do that, I find I am unable to get the rounded violin tone that I want to hear when playing higher up the neck on the thin strings.

I have experimented before now with tracking the pitch to control a modifier on an EQ block both before and after the amp. But I've never been quite satisfied....
 
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