Most important tube for 'tone'? Phase inverter

Silent Sound

My take is anyone who says "this" or "that" tube is the most important for tone, doesn't understand how tube amps work. They're all important. Any one can make or break an amp's tone if you're not careful. A tube amp is like a symphony. You have a bunch of individual pieces working together to make a greater whole. If just one of those pieces decides not to play nice with all of the others, you'll get crap. So any tube (short of a tremolo tube or something not being used in the direct signal path), can make an amp sound terrible, and they all have to work well to make an amp sound great.

That being said, I generally wouldn't consider the phase inverter to be the "most important" when it comes to tone. The reason being is that it often isn't used to amplify the signal, but rather just to duplicate and invert it. Usually your first stage and power tubes are where most of the heavy lifting (amplification) occurs. Stuff like phase inverters and cathode followers play supporting roles to make other parts of the circuit work better, such as output tubes and tone stacks respectively. So while they're certainly important to tone, I would have a hard time arguing their status as the "most important".

Also, in that link you referenced, the author neglected to discuss types of phase inverters. He just went off about types of tubes used in phase inverters, which is far less important and impactfull on tone and far less interesting that all of the different circuit variations. For instance, the difference between a Long Tailed Pair AC or DC, Cathodyne, and Paraphase is a lot larger than the difference between a 12AT7 and a 12AX7. That says to me the author probably shouldn't be taken very seriously as an authority on the subject. Well, that's my opinion anyway. You're free to have your own, as always.
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Guinness Lad

Rather than worry about tube type instead look at how good the amplification factor is and if it's reasonably balanced.


I use a Baldwin Slyvania in my v4 Marshall because it warms the amp up nicely. PI does make a nice difference!


The more efficient a Phase Invertor Tube is
The more solid the output of the amp - due to
the voltage being consistent to the power supply.

So in that way it's important - but if you pulled the rectifier
you wouldn't have any sound - so wouldn't that be considered
the 'most important'??


I'd agree that changing any tube in the signal path will change the sound of an amp. One more thing, though: When a non-master-volume amp is driven into clipping, the primary means by which clipping occurs is that the grid conduction of the output tubes clamps the signal from the phase inverter. So, in this type of amp, the phase inverter tube shapes the overdrive/distortion tone more than the other preamp tubes.

Mark Bartel
Tone King Amplifiers
Baltimore MD


For a monstrosity like the Mesa Heartbreaker with 7 preamp tubes rectifier and 4 output tubes. Yeah the PI is the only tube that can't be removed for one to be able to play a guitar through the amp. Since each channel has an input tube and you can bypass a channel, bypass the reverb and the effect loop. You can pull 2 of the output tubes and use the solid state rectifier, but you cannot remove the PI.

Maybe it is not the most important for tone but if you got no sound, you have no tone.


The way I look at preamp tubes is two-fold :
- amplification factor
- imagine it being a filter, accentuating certain frequencies, rejecting others
obviously this will influence anything that happens downstream, so it makes a big difference where you put such a filter in your signal path
- for those of you familiar with compression, EQ post comp reacts as expected, EQ pre comp gives you frequency dependent compression instead, very much the same in tube amps !

let's take a basic Plexi circuit as example :
- 3 Mullard i61/63s will give you a really mid-focused, classic Marshall OD, Tungsrams will give more bite depending on position, RFTs tend to be gritty and dark, JJ12AX7s smoother and very dark
- if you're going for clean, Hendrix- or Johnny A style tones, it's well worth experimenting with other, less mid focused tubes
I happen to like i63 > i63 > Telefunken (smooth long plates) a lot in a 1959 tweaked for cleaner stuff, ymmv
- a good 5751 or 12AT7 in V2 will give you better control over the clean spectrum, at the expense of max overdrive

other circuits follow similar basic recipes, does not mean they react identical though !
For a monstrosity like the Mesa Heartbreaker with 7 preamp tubes ...
in fact, the HB is pretty pure by Mesa standards. :dunno
the signal flow is V1 > V2 > V3/PI * , exactly like in a Plexi or tweed Bassman, but :
- each of the two fully independent channels has its own dedicated V1 & V2, the signal only ever passes through one of those pairs, safe to ignore the other, inactive channel and focus on the currently active one
- there are two additional "utility" tubes which are not doing much for the actual tone, they are used for reverb and FX loop.
- most modern high gain amps have more tonally relevant tubes in the path, for better or worse

all a bit simplified of course, but it gives me a certain "roadmap" when trying out different tubes
in the end, theory can help a lot, but what counts is what you hear !


* refer to the manual for actual, physical position

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