PP with Unbalanced concertina = 2nd H?

[DrewP]

NOTE Not a discussion for purists, and not talking about an amp with an overall global feedback loop.

Some time back I was pondering people deliberately "sweetening" amplifiers by operating tubes in a manner that would generate a degree (not lots) of second harmonic distortion. The first method that I pondered was picking a load line where the response was not symmetrical. Given that this requires either significantly altering the B+ or the bias current it's not terribly easy to do on the fly (say via a switch).

It occurred to me the other day, that an unbalanced concertina might achieve the same thing (and because it only requires changing the impedance to ground of the cathode connected resistors, would be simple to switch in and out).

Has anyone played around with this type of thing? (I realise that it is anathema to those who want a linear amplifier)

[SemperFi]

Yes indeed. I've used unbalanced long tail pairs this way. I've also used unbalanced current mirrors to drive the PP output stage with up to 50%H2.
I do this sometimes to add some spescial character to the tone of my guitar amps. I also do it to test how much H2 I can have before it is noticable, and I'm a bit shocked that I don't hear it till its too high for me to even admit it here
This experiment has led me to almost ignor H2 in measurements of my amps, unless they are skyscraping of course. I only worry about H3 and above.

The first thing I notice with high H2 is a fuller (warmer) bass. I was told by a friend who's an expert in acoustics that a H2 does just that, makes bass seem fuller and even deeper in frequency. The higher frequencies dont seem as affected, though I guess some smearing occurs at some point.

I certainly don't mind the sound of H2...

Please note that I worked on fighter jets in my young age, have many children and a wife, so my hearing isn't very 'golden' anymore

[SemperFi]

Another way of unbalancing PP pentodes is using different value screen resistors.

[SpreadSpectrum]

That seems like a perfectly valid way to unbalance the output. I have a Fisher amplifier that has an 'AC balance' adjustment(a pot in one side of the concertina) that does just that, only the intention here is to generate just enough imbalance to compensate for mismatch in the output tubes.

[leadbelly]

Pretty common stuff. Note that the pot that unbalances the driver stage is called the "body" control in the guitar amp world.

[SemperFi]

Originally Posted by leadbelly

Pretty common stuff. Note that the pot that unbalances the driver stage is called the "body" control in the guitar amp world.

Hehe. It is called all kinds. I've called it 'the mystery control' just to get guitarists to use it. Normally I call it 'color'.

There is absolutely no standard nomenclature for anything on guitar amps. Master can be 'growl', gain can be 'exhaust',etc,etc...

[leadbelly]

Originally Posted by SemperFi

Hehe. It is called all kinds. I've called it 'the mystery control' just to get guitarists to use it. Normally I call it 'color'.

There is absolutely no standard nomenclature for anything on guitar amps. Master can be 'growl', gain can be 'exhaust',etc,etc...

Well I guess when you start building amps on a commercial scale somebody will let you name them. I call it the "body" control because that is what it is called on the few commercial amps that have used it on a big production scale.

Also, your example sort of contadicts your point. You pointed out that what is overwhelmingly called the "master" is called "growl" by a boutique amp maker. This would not be notable EXCEPT for the fact that "master" is the most common and therefore standard term.

[SemperFi]

Originally Posted by leadbelly

Well I guess when you start building amps on a commercial scale somebody will let you name them. I call it the "body" control because that is what it is called on the few commercial amps that have used it on a big production scale.

Also, your example sort of contadicts your point. You pointed out that what is overwhelmingly called the "master" is called "growl" by a boutique amp maker. This would not be notable EXCEPT for the fact that "master" is the most common and therefore standard term.

Actually I aint contradicting anyone. Just saying you can call it what you want, and there is no industry rule regarding that issue, wether it is a mass produced commercial brand or a one-a-month garage product. The 'industry standard' is 'gain', 'master', 'bass', 'treble', etc. But there is no rule and no regulation about it. Some use the more correct term 'volume' and they are by no means setting the standard or doing anything wrong.

The H2 tweak is not very common at all, some do it, the idea is old, but it is not common, and there is certainly no standard term for that feature.

but hey, at least we all know how to get some 'body' to our tone now

[Wavebourn]

I used a gain control pot in class A guitar amp, to control drive of one of output tubes.

[tubelab.com]

There is absolutely no standard nomenclature for anything on guitar amps.

I have used some creative names for some of the knobs on some of my amps, like "bend" and "break" that controlled the balance between positive and negative feedback. I also had a "meltown" knob that did just what it says. The master gain was called "crank". Bass, Treble, and mid....Thump, Scream and Honk.

However, no one can beat Eric Barbours amp and names. Eric used to be an applications engineer for Svetlana and has authored several magazine articles. He now designs and sells vacuum tube based analog music synthesizers and effects boxes. He designed a guitar amp that uses 23 tubes and unveiled it at NAMM a few years back. Lets just say it has some rather creative names for the amp itself and many of the knobs.

The unveiling can be found on you tube and the amp is on his web site.

WARNING !!!!! don't look there if strong language offends you, or small children are looking over your shoulder. WARNING !!!!!

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