Cathode Squish

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What is it? Why would I want to adjust it? Where the heck do I find it?

Answering the last question first, in the Advanced tab of the Amp block are two parameters called Cathode Squish and Squish Time. These control the amount and time constant of the virtual power amp's "cathode squish". Cathode squish is a phenomenon that occurs in cathode-biased power amps. A cathode-biased power amp is one where a resistor is placed between the power tube cathodes and ground. This self-biases the power tubes. This method is in contrast to the more common fixed-bias technique where a DC voltage is generated and applied to the grids.

This method is commonly used in so-called "Class-A" amps with notable examples being Vox, Matchless, Budda et. al. Some Fender (5E3 Deluxe) and Marshall (18W Bluesbreaker) amps use this method as well.

In parallel with the bias resistor is a capacitor which bypasses the resistor giving the power tubes more AC gain.

Since so-called "Class-A" amps aren't really Class-A but Class-AB amps biased very hot a unique phenomenon occurs. The cathode capacitor charges up when the amp is driven hard since the net current flow through the tubes is positive. As the capacitor charges up this effectively reduces the bias of the amp pushing the amp towards Class-B operation. When this occurs the amp generates more crossover distortion. The gain also decreases. The net effect is that the amp gets more fizzy or sizzly and more "open" sounding.

A moderate amount of squish keeps the amp from sounding stuffy and flubby. Too much and the amp can sound excessively fizzy.

The Cathode Squish parameter controls the amount of squish in the virtual power amp. The Squish Time parameter controls the rate at which the cathode charges and discharges. Experiment with the Cathode Squish first to get a feel for how this parameter affects the tone. Turn it all the way up and you'll hear the amp get more open and fizzy. Turn it down and you'll hear the amp get more focused and compressed.

A related phenomenon that occurs in both cathode-biased and fixed-bias amps is Bias Excursion. Bias excursion has a similar result in that it shifts the bias of the amp towards Class-B but the mechanism is different. I'll cover bias excursion in a future article.
 
This stuff from Cliff and Paco is awesome.

I honestly have no brains to add anything useful other than I hope it makes it to the Wiki! :)
 
Informative write up and interesting parameters to adjust for sure. Cathode biasing is a part of why I like my Orange Rockerverb 30 (30W Class AB using 2x EL34) so much, especially when playing thru the clean channel (only a volume control and flat/no EQ). I generally prefer the sound/feel of Class AB/cathode biased amps over fixed bias.

It's also cool the AFX it doesn't blow up when the cathode bypass cap shorts and takes out the tube/output transformer as can happen on a real amp...
 
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I am by no means an expert on the inner workings of a guitar amplifier so excuse the naivety of the following question:

Can you explain how this differs from the sag and bloom produced by a tube rectifier?
 
Squish: refer to power tubes. Sag: refer to the amp (both pre & power). Squish is faster, sag is slower. IMHO. :D
 
I'm asking because I'm not a big fan of rectifier sag. I'll have to play with a few presets and see how I like it.

I really appreciate Cliff and the guys at Fractal taking the time to explain how some of the more obscure parameters work!
 
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