qs about phase, cathode followers and feedback...

[swt]

Hi guys!. I have a diy tube amp. Afaik, one of the best design to get feedback at low volumes is the trainwreck, because output is in phase with guitar, so it gets a nice feedback at no so high pitches. Mine is kind of a d*mble/mark 3, etc, and it will be out of phase, considering 2 stages clean, and 4 distorted, and driver, of course. So here comes the question...it has a built in fx loop, two stages, a buffer, and a recovery. Will a buffer invert my signal??.
The loop is fully bypassable. when off... Can a invert the cable to the speaker to get it in phase with guitar?.
Also...will jfets invert the signal ( and miniboosters, etc), and what about cathode followers?. Thanks a lot for your replies!

[R.G.]

Will a buffer invert my signal??.

There are two kinds of buffers - inverting and non inverting. The noninverting kind is more common, but not guaranteed. Can't tell if all I know is "buffer".

Can a invert the cable to the speaker to get it in phase with guitar?.

Yes. This is the method I recommend. Very simple.

Also...will jfets invert the signal

JFETs may invert or non-invert, depends on the circuit. Any JFET, bipolar, CMOS transistor may be set up as inverting or non inverting. No way to tell without knowing the circuit.

( and miniboosters, etc),

The "minibooster" mu-amp is inverting.

and what about cathode followers?.

Anything called a "follower" is noninverting. That's why it's called a follower. True for cathode followers, source followers, and emitter followers. "Follower" is a name for a type of circuit that does not invert.

[swt]

Hey Thanks R.G!!. REally helpfull.
By buffer, i mean jfet buffer, or bipolar buffer like ts input, or rat input, or in my case a d*mbleator buffer in the send path. I guess these are non inverting right?.
Is the bsiab in phase??. Because it's two miniboosters, and a buffer at the output...and the bsiab II has another stage at output, which means is out of phase? :? .  
Are the stages in rog simulators, inverting or non inverting??. (odie, matchbox,etc). Is it the same with mosfets?. Is the mosfet booster inverting also...and rangemaster?. And what about the blackfire stage??.
Sorry for too many questions, but this will make me decide what kind of circuits will perfomr the best with my type of amp, and if i'll need two invert the speaker or not. Thanks again for your kind reply!!

[lovekraft0]

...Afaik, one of the best design to get feedback at low volumes is the trainwreck, because output is in phase with guitar, so it gets a nice feedback at no so high pitches...

Does absolute phase actually make a difference? I always thought the 'Wreck's great feedback came from the compression/saturation in the preamp. Is this Ken Fischer's secret?

[swt]

i haven't tried a real one, but it's a lively really expressive type of amp, and most of the distortion comes from power amp, ( also compression), mainly because of a great design, that includes low voltage on power tubes, and high reflected load. If you try just the preamp, you'll notice it's pretty clean and transparent with lots of highs, but the secret ( to me at least) is in the power amp, and that the whole amp is in phase with the output. There are 3 stages and the driver. The output transformer may be the cause of that tone also. ( it's big, afaik). I didn't try to clone a trainwreck, mainly because part of the magic is in the layout. This type of amp is pretty unstable because of the extreme highs it produces. I wish i bought one in the old times when he was making and selling them at normal prices. Hope the guy is ok, and i wish he could build more, or teach someone (Like me :twisted: ) some stuf to continue his work. I live in patagonia, argentina, but sure the hell i'll go there if something like that happens... :lol:  :wink:  :!:

[Elektrojänis]

I'm not an amp expert but generally the absolute phase or polarity of sound does not matter much.

Part of these next paragraphs is my assumptions and stuff I have forgotten where I learned. So correct me if I'm wrong.

The phase difference of the input and the output of most amps is usually at least a bit frequency dependent... In hifi amps the phase response might be quite flat.  In most guitar amps it is probaly less flat. If nothing else wrecks the phase responce then at least the tonestack will. This will affect the high end of the frequency spectrum more than the low end.

I'm not sure how much that will affect the acoustic feedback... However... It is probably irrelevant too.

Why? Because the distance of the speaker and the guitar changes phase too. If you move the guitar to half of the wavelength further (or closer) to the speaker then you have "inverted the phase". Wavelenght is of cource frequency dependent. On the low end of the guitar spectrum... let's say 100Hz (ok... its more like 82Hz for standard tuning but 100Hz is close enough for this) the wavelength is about 3.4 meters (so the half of that is 1.7m, 11.2 feet and 5.6 feet if that's easier for someone). On the high end of the spectrum, lets say 1000Hz the wavelength is about 34 centimeters (about 13.4 inches). Hmmm... Everyone can probably calculate the half of that.

For more wavelength calculations just use google and its calculator function. Try searching this with google:

speed of sound / 500Hz
or
speed of sound / 500Hz in inches (if you want it in inches... It can also do a lot of different measurement units too like attoparsecs so it's nice for converting between metric/imperial or whatever)

I could also make a counter argument to myself that usually when people play with the feedback stuff they stand in front of the amp. Then the phase of the amp will probably be more important.

Just thought to bring out some viewpoints... feel free to ignore them.

Anyway... The little experince I've had with controlled feedback with low volumes I think the gain/distortion/compression matters much more. With enough gain I have gotten controlled feedback with quite low volumes. The sound was quite seriously distorted then.. and very noisy too.

[swt]

i guess you are right about distance. Your reply has lots of good points to dig. But this d*mble type of amp just want to sing...it gets feedback really easy. I don't remember who was the guru of amps i've heard telling this, but i do know that Ken fisher made his design that way on purpose. Let's keep trying and learning in the process!! Thanks!