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previous: ken gilbert ok. i'm starting to pursue doing m... -- 5/19/2000 4:21 PM

Re: positive feedback musings...

5/19/2000 9:04 PM
MBSetzer	Re: positive feedback musings... Well Ken, thought I would add my 2cents to this, don't expect to get your money's worth I like that article about describing triodes as pentodes encumbered by NFB. I used to be a brilliant scientist (until I got all my brain damage), so I follow the math on these things to an extent, but I do stop short before it comes to actually pulling out my slide rule. I tend to accept the equations when they are as well developed and logical as we saw presented. I know you go futher on the calculations than me, and when you present equations of your own I have a great tendency to accept yours also without having to independently derive something similar. I guess I like to have a basic mathematical understanding to help uncover or understand interesting phenomena, but I primarily focus on how it will influence my soldering. I want to hear the varying time constants, phase relationships, frequency response, etc. so I can gain an intuitive idea how an equation sounds. This time I was not motivated to solder, but play my guitar instead. At high gain settings, my circuits are usually already tweaked to the edge of stability with unstable tubes, but I experimented a lot more with acoustic PFB by playing closer to the speaker(s). If we accept the behavior of any triode (in my case just the preamp 12AX7s), even fully cathode bypassed, as being like a pentode whose efficiency has been attenuated by inescapable NFB, then it could rightfully be looked at like having a NFB reserve. This could mean that there is a possibility to inject limited Positive FB in a way that would somewhat offset but not overcome the inherent NFB without making for an unstable amp. Seems to me that might have inspired you toward this thread, especially since you have indicated preference for unbypassed preamp triodes, you would have more theoretical NFB reserve to tap into. IIRC you prefer not to have any NFB around the power tubes, letting UL alone do whatever it may. I don't have NFB or UL, whether I would have less overall NFB reserve in the whole amp than you would only be a guess, but with my bypassed preamp cathodes, likely I have less reserve. Anyway, like some ampagers enjoy a variable NFB control for different moods or guitars, or speakers, etc. I would logically want to also try varying any positive FB in order to dial in an interesting tone. When it comes down to it though, what I have been doing all along is trying to get as much control of tone into my fingertips on the strings, so I can do without a tone stack and still get decent variation, within limits of course. Well my positive FB control is physical like my tone control. How I vary the picking (and pickups) gives me tone control, how close to the speaker and the orientation of the guitar control the positive FB. This is nothing new. But I think a PFB circuit, if it were variable, might be able to compensate for various distances from the guitar to the speaker. You might be able to get some phenomenal acoustic feedback tones without having to be very close to the speaker. Unfortunately, it could also create a force-field around the speaker where you couldn't get any closer without reducing PFB. Seems like it would be a good feature to have controllable by a volume pedal. I'll be interested to find out any audible results you get if you start experimenting, and your opinions about how the electrical PFB might compare in tone to the wide variation of good & bad acoustic PFB that we are familiar with. Told you it wouldn't be quite 2cents worth . . . Mike

5/19/2000 9:04 PM

MBSetzer

Re: positive feedback musings...

Well Ken, thought I would add my 2cents to this, don't expect to get your money's worth ;-)

I like that article about describing triodes as pentodes encumbered by NFB. I used to be a brilliant scientist (until I got all my brain damage), so I follow the math on these things to an extent, but I do stop short before it comes to actually pulling out my slide rule. I tend to accept the equations when they are as well developed and logical as we saw presented.

I know you go futher on the calculations than me, and when you present equations of your own I have a great tendency to accept yours also without having to independently derive something similar.

I guess I like to have a basic mathematical understanding to help uncover or understand interesting phenomena, but I primarily focus on how it will influence my soldering. I want to hear the varying time constants, phase relationships, frequency response, etc. so I can gain an intuitive idea how an equation sounds.

This time I was not motivated to solder, but play my guitar instead. At high gain settings, my circuits are usually already tweaked to the edge of stability with *unstable* tubes, but I experimented a lot more with acoustic PFB by playing closer to the speaker(s).

If we accept the behavior of any triode (in my case just the preamp 12AX7s), even fully cathode bypassed, as being like a pentode whose efficiency has been attenuated by inescapable NFB, then it could rightfully be looked at like having a *NFB reserve*. This could mean that there is a possibility to inject limited Positive FB in a way that would somewhat offset but not overcome the inherent NFB without making for an unstable amp.

Seems to me that might have inspired you toward this thread, especially since you have indicated preference for unbypassed preamp triodes, you would have more theoretical NFB reserve to tap into.

IIRC you prefer not to have any NFB around the power tubes, letting UL alone do whatever it may. I don't have NFB or UL, whether I would have less overall NFB reserve in the whole amp than you would only be a guess, but with my bypassed preamp cathodes, likely I have less reserve.

Anyway, like some ampagers enjoy a variable NFB control for different moods or guitars, or speakers, etc. I would logically want to also try varying any positive FB in order to dial in an interesting tone.

When it comes down to it though, what I have been doing all along is trying to get as much control of tone into my fingertips on the strings, so I can do without a tone stack and still get decent variation, within limits of course.

Well my positive FB control is physical like my tone control. How I vary the picking (and pickups) gives me tone control, how close to the speaker and the orientation of the guitar control the positive FB.
This is nothing new.

But I think a PFB circuit, if it were variable, might be able to compensate for various distances from the guitar to the speaker. You might be able to get some phenomenal acoustic feedback tones without having to be very close to the speaker. Unfortunately, it could also create a *force-field* around the speaker where you couldn't get any closer without reducing PFB. Seems like it would be a good feature to have controllable by a volume pedal.

I'll be interested to find out any audible results you get if you start experimenting, and your opinions about how the electrical PFB might compare in tone to the wide variation of good & bad acoustic PFB that we are familiar with.

Told you it wouldn't be quite 2cents worth . . .

Mike

Replies:

Chris Harden Regarding triode circuits having in... -- 5/21/2000 10:15 PM