Simulating dying pre/power tubes?

Sean Ashe

Sean Ashe

Power User
Okay, so I have a bit of a weird request here which is: what is the best approach for simulating an amp that has dying tubes and is incorrectly biased?

In short, I used a friends Carvin Legacy for a recent single of mine and it is not exactly in ‘ good working condition’. The amp would have to be played through for a couple of minutes before the volume would normalize and also a couple of the tubes in the preamp were mismatched and not biased correctly.

As a result, the amp had a crazy amount of sludgy low end but with also some extremely sweet high end harmonics.

I’m trying to dial it in the best I can in my Axe Fx, but it’s been awhile since I’ve messed with the advanced parameters to this degree so any direction would be appreciated. Thank you!
 
I’ve tried to replicate mismatched power tubes by adjusting the Power Tube Mismatch. Not sure what’s the difference between negative and positive values, but guessing negative values are pushing half of the power tubes less and vice versa. I’ve tried to replicate an amp running half of the power tubes too hot and half too cold by cranking power tube grid bias to 80-90% and setting the mismatch to -0.5, which is quite a lot since the suggested values on these forums are -0.3 to 0.3. No idea if my settings are set in the right direction.

Also no idea about preamp tube mismatch.
 
Try the variac and turn it down under the power amp advanced page to simulate not enough voltage in the tube ?

Also turn up the power amp bias and supply sag parameters for your use case

Just my 2 c
 
Please keep us posted on your progress with this. One thing I love about my FM3 is that in addition to "slick" sounds (that I don't use much), one can dial in sounds with a lot of personality and wildness, even in the "is this amp OK?" territory that I love. And some of the design-flaw awesomeness of classic amps, namely the weird "spitty" high end that occurs with classic marshall distortions sometimes. Love it!
 
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