Pentode Overdrive, what makes it sound so good?

[will]

Hi,

It seems that a triode overdriven produce lots of 2nd harmonic distortion, when the peak of the bottom half of the wave is rounded. In other words, a natural compression of ½ of the waveform occurs when the stage is overdriven. (This is what the Vulcan is doing with the biased diode in the signal path). When a triode clips, a lot of the high frequency harmonics are rolled off due to the limited bandwidth in the typical guitar amp circuits.

Pentode power tubes operate differently especially coupled into transformers and (non linear, inductive-resistive loads) speakers.  It seems that the Pentodes are even more bandwidth limited than the triodes and are both current and voltage limited by the surrounding circuitry. They also seem to have less power supply regulation (especially with tube rectifiers) so they have a natural compression due to the power supply sags during the peaks. Since most power amps are configured class AB for output, there will be some crossover distortion due to biasing differences between tubes. Also some asymmetric gain differences between the top and bottom part of the wave due to differences in the pentode tubes.  Different power tubes types have different sound, 6V6, 6L6, EL34, KT88 etc...

I’m interested in trying to emulate / reproduce the natural tube distortions using low cost available transistors. Just wondering what other differences need to accounted for in the design. I’m wondering if any available circuits seem to do a good job emulating an overdriven power tube.

I’m interested in your ideas and comments!

Regards,
Will

[petemoore]

I can't elaborate so good on this one.
 Jfets seem to earning a reputation that coincides with the theory that they can in some ways mimic the way a tube works and sounds.

[Eric H]

Well, just to make things more interesting, the 6v6, 6l6, kt88, and kt66 are tetrodes

-Eric

[petemoore]

Has all that...7199ers and a GZ34...it Would be cool to be able to get that kind of response out of a SB.
 I think there's really something to be said for the soft attack that is produced with the use of a rectifier tube.
 I read in guitar player magazine sometime ago that there are Transistor Rectifiers which emulate tube rectifier sag very well. they plug in the tube socket. Possibly something in the Si rectifier could be looked at. Emulating a tube rectifier sag might neat.

[Eric H]

 I read in guitar player magazine sometime ago that there are Transistor Rectifiers which emulate tube rectifier sag very well. they plug in the tube socket. Possibly something in the Si rectifier could be looked at. Emulating a tube rectifier sag might neat.

Here you go Pete:
http://www.webervst.com/ccap.html
It's primarily some  resistance in series with the diodes.

To the original poster: There are people that have spent years on this, and are still working on it. It's easier to build a small tube amp.
Personally I'm looking for new sounds out of solid-state --rather than trying to emulate a sound that's already, simply available. YMMV
You should try searching patents --there's a lot of information on tube-emulation on the internet.

-Eric

[Doug H]

IMO, tube rectifiers and sag are kind of overrated. There are other ways of getting compression in an amp, even clean compression, without using a tube rec. (Speaking of the subject line, a pentode preamp can do wonders for that.) Besides, I like the fast transient response of a SS rec. Only problem with diodes is that they can get noisy, but that's what snubbers and FREDs are for.

Doug