creating asymetrical waveforms, and its effects

[black_labb]

im planning a simple 100w amp to be used for bass, and possibly guitar as well. the concept is to have only a volume control in the signal path, and change the tone through manipulating the workings of the gain stages. this would include changing cathode bypass caps (a rotary control), having a 1k potwired as a variable resistor in series with an 820ohm resistor for the cathode resistors, as well as a presence control.

i am contemplating putting in a post pi gain stage (either a 12au7 or a 12ax7), and was thinking that it might be a reasonable idea to be able to change the coupling caps, as well as the cathode resistance, which got me thinking of what the effect would be if i made it so that i could adjust the cathode caps independantly, and add a pot that could add some extra resistance to one of the cathode resistors. this would add/remove some gain from the triode at different frequencies, making one side of the push-pull pair push a bit more power than the other.

what kind of sound would you think this would make? i can visualise the waveform on paper, but that doesnt help much.

anyone ever mismatched the coupling caps after a PI? any comments on the sound?

on another note, how do you feel about a bass amp without equalisation? i build my own basses and like to incorperate as many options for the sound (passive, but incorperate series/paralell switches and such). i like quite a middy sound, but would it be too middy? i plan on using 4 el34's.

thanks for any input/thoughts

[Steve Conner]

what kind of sound would you think this would make? i can visualise the waveform on paper, but that doesnt help much.

A loss of headroom and more cheesy farting distortion. That's OK if you only ever play distorted bass, but I'd rather start with the maximum possible clean power from my PI and power tubes, and make dirt in the preamp. Then I can make the dirt go away (by turning the master volume full and the gain down) when I don't want it.

on another note, how do you feel about a bass amp without equalisation?

It probably won't have enough bass. Remember that the classic guitar amp tone stack is flat with the bass and treble at 0 and the mid (if fitted) at 10. So if you leave the tonestack out, you're permanently stuck on that setting, and what bass player ever used that? Why go to the trouble of lugging big amps and cabinets if you can't goose the bass and shake peoples' fillings loose?

So I'd fit a tone stack with bass and treble knobs at least. If you have a good bass and you like the tone of its wood and pickups, that might be all you need. Otherwise I guess you could add an EQ pedal.

I'd only leave the tone stack out in the case of an active bass with its own EQ on board. I've seen people use these essentially plugged straight into a solid-state PA amp.

[black_labb]

thanks for that. i knew about the tone stack, but wanted to hear someones opinion, as i like the idea of a very simple circuit path.

the uneven gain will be able to be adjusted from even to a certain difference ( a difference in resistance at the cathode bypass resistor), so its just a feature i would like to try. im interested what one aspect of distortion sounds like without all the other effects of overdriving a tube amp, or using pedals.

thanks for the input, i may use a simple fender/marshall tone stack, and probably tend a bit towards the marshall values to retain the mids.

[cbarrow7625]

I have an amp set up with independent volume controls feeding each half of the push-pull output stage. I can send uneven drive levels to each tube if I want. I find that it really only sounds good & is useful in one of two ways:

1.) Using it to exactly match the drive level to each half of the PP outputs (the typical guitar PI has uneven drive levels. Even if resistors are used to try to even them out it is still subject to the exact gain of each half of the PI tube). Having the drive levels matched exactly really tightens up the sound (for overdrive/hi gain playing). I like it alot.

2.) To completely turn off the drive to one of the output tubes when it is running in class A (or at least close to it). Basically end up with a single-ended type of tone. Nice & rich, full of even harmonics. Just the opposite effect of option #1 described above.

I, too, thought it would be cool to mismatch the drive just s a little going to each half of the PP pair so that I could get "the best of both worlds" and just a little more even harmonics out of it. In reality, the in between settings (with asymmetric drive) are kind of dull and not very interesting sounding compared to the two uses I described above. Everything else is just an uninspiring version of those two nice sounds.

Additionally, I found that with a class AB output stage, the asymmetrical drive really didn't work so well. Since each half of the PP is in cutoff for a portion of the cycle, you could just hear the nasty sound of the cutoff coming through. To get happy with the setup I had to bias the outputs into class A. That way, each half of the asymmetrical drive gets through the power amp "unaltered".

Also, please note that when changing cathode resistors with a pot you are also shifting the DC levels of that stage. Variable DC will often be very noisy (like a scratching sound). Please also note that connecting a pot directly to the tube can be a safety hazard. If the ground opens up, the full B+ DC will be present on the pot. If the pot has a fault, you have the full B+ DC on your amp chassis and/or your fingers. That can kill you.

I have done all of the things you describe (changing cathode caps, resistors, inter-stage coupling, etc.) also in the quest to get any tone I wanted. In reality, you almost always end up with one or two sounds you like & all of the effort that went into putting all of these variables into your amp are really wasted. You will be much better off finding those one or two settings that really work rather than making everything variable.

The other thing you end up doing when making everything variable is you run wires EVERYWHERE through the amp. Not only does it make for a rats nest, it also gives noise, crosstalk, leakage capacitance, etc. more places to enter your signal flow. If it doesn't make you amp oscillate (don't try this type of thing in a hi gain amp) it typically will dull & deaden the sound. Running a wire from your cathode to the front panel for adjustment can not only be dangerous but it can make your amp sound awful. Remember that the cathode is also an input to the tube. Anything picked up on that wire (or by the pot) gets fed back into the tube and amplified (like a grounded grid stage). Best practice is to get your cathode resistors from the tube to ground with as short a path as physically possible.

Anyway, I know where you're coming from. You have to try variations to find a good tone. I have learned (the hard way) that trying things out in the design phase pays off a lot more than actually building it all into one amp.

Good Luck.

Chris

[black_labb]

not quite the answer i was hoping for, but very helpful. ill have to rethink the design a bit. thanks for the help the 2 of you.