fender 212r

[J M Fahey]

1) To say the truth, U6 is not a DC servo, although it might contribute a little as a side effect.
I didn't even mention it because there was already a lot of contradictory info going around and I didn't want to pollute the game even more.
2) The Vbe multiplier is not a servo but a thermally tracking bias voltage source.
3) You must first understand what electro-mechanical damping is in a loudspeaker.
Understanding that, you'll automatically understand what effect does the generator impedance driving it have on the "Q" of that resonant system, a.k.a. speaker
4) Usually Tube amps have voltage negative feedback, which lowers their internal impedance and causes higher damping.
A classical example of a no feedback amplifier is the VOX AC30
Classic Fenders have a lot, and Marshalls and Tweeds use somewhat less.
All 3 kinds of amplifiers have quite different sound, in part because of this.

[lowell]

Yes I know the sonic effect of global negative feedback in a tube amp, and am familiar with the fact that more negative feedback=more damping.

As Zout of an amp goes down, damping goes up and vice versa. And I believe this is because the frequencies at which a speaker has less impedance are produced better due to a low z out. Or in different words, a speaker is more current hungry at specific frequencies, and a pa that has a low z can feed enough current to keep things linear. Not sure if my explanation or thought behind this is any good, but that's what I make of it thus far.

[J M Fahey]

a speaker is more current hungry at specific frequencies, and a pa that has a low z can feed enough current to keep things linear.

Yes, that's about it.
That's why negative feedback usually is thought as something good; more linear, higher damping,etc.
There's only that small group of nutty guitar players which just can't stand a linear and flat world.

[lowell]

And I'm one of them when it comes to that saggy power tube distortion sound. When it comes to a preamp overdrive, I prefer a pa with negative feedback to smooth the frequency response out. Neil Young Hey Hey My My is one of my favorite sounds, as well as Warren Haynes with the Allmans. One with negative feedback, one without.

[lowell]

Jm,
you using press 'n peel blue for those pcb's? Or another approach?

[J M Fahey]

Hi lowell.
No.
I *do* use laser transfer a little for prototype work, to be able to correct errors , or when I need a small PCB, say a guitar preamp or an add-on protection, XLR out, etc., say 1"x 1" boards up to 2" x 2", where I fill an A4 or at least half a sheet with them.
For production work it's very slow.
I iron for up to 15 or 20 minutes to be certain to transfer even the thinnest tracks.
For regular, production work, I silkscreen which is very quick (40 boards in 5/10 minutes) and the acid protection offered by etch-resist ink is the real deal.
I buy the full 3 x 4 Ft copper clad board; and given my "close to home-made" level I prefer phenolic single face; epoxy glass absolutely destroys regular drills and high speed Widia ones are very brittle and can crack 3 per board if even slightly careless.
Phenolic is easy to cut and drill; I design with *wide* tracks and pads to avoid stripping.
Well, that's about it.

[lowell]

How do you silkscreen pcb's? Can you direct me to another site on this maybe?

[J M Fahey]

Next time I print some, will take some pictures.
just to get your feet wet, look at this page:
Circuit Bridge : Make Your Own Printed Circuit Board
This is *not* the regular process, they sell you a pre-sensitized "cloth" which does not use a frame to keep it stretched but in general lines serves as an example.
The normal process is to have a very thin nylon ("silk") cloth stretched tightly on a wooden frame, which you soak into a sensitized emulsion and where you transfer your design.
Also:
How to Silk Screen
Also YouTube how to silkscreen.

[lowell]

Oh yeah! I've used that ez print screen stuff for making logos. Didn't know that they make etch resist paint. Cool! thanks!