Umble with tube power amp type sag (compression)

aquataur · Post by **aquataur** » 14 Jul 2013, 12:11

Hi,

fuelled by all the hype about xUmble amps (or might the $ sign like $Umble be more appropriate as a place holder?) I built a R.O.G. Umble just to see what it is all about.

As usual, I made several subjectively worthwhile improvements, amongst which the IMHO most noteworthy are the addition of a compression circuit that works exactly like the sag effect a tube power amp would exhibit and a boosted and regulated PSU.

Ob course, both principles may be applied to other similar circuits.

Have a look over at my web pages and enjoy some sound files.

-helmut

atreidesheir · Post by **atreidesheir** » 14 Jul 2013, 14:05

I read your page. Interesting stuff. Did you add the sag circuit as a a seperate pcb, or did you make a new umble layout with the compression included?
great sounding demos.

aquataur · Post by **aquataur** » 14 Jul 2013, 17:09

There is so few extra components that it is ridiculous. The gate bleeder resistor that is there is replaced by a bridge of two resistors, the cap is soldered across the one to ground.
The pot is mounted to the case, so there is only three more components left which I wired directly with heat shrink tubing over them and wires.

Maybe I was not clear about that: the j-fet stage shown there is part of the original R.O.G. umble circuit. The compression circuit just consists of the few extra components wired around that.

This mod can be applied to any finished umble project that adheres to the R.O.G. schematic with ease.
It can indeed be applied to any other j-fet driven or maybe even mosfet-driven project as well that use the same method of producing a negative gate-voltage, provided it is located in a place where there is enough control voltage available.

Note however that heavy-biased or even class-A biased amps don´t necessarily exhibit sag, so if you install this, it will be super-realistic

have fun,

-helmut

aquataur · Post by **aquataur** » 18 Jul 2013, 18:33

aquataur wrote:---(or might the $ sign like $Umble be more appropriate as a place holder?)...

Darn. I should have taken the $ and run:
http://www.kvraudio.com/forum/viewtopic.php?p=4875831
They call it the Overprice Special

have fun,

-helmut

chicago_mike · Post by **chicago_mike** » 19 Jul 2013, 16:04

That bastard charges you to talk to him, and he doesn't build for any poor slob, you have to be "seen" to be good enough for his sorry ass to build you an amp.

Asymmetric clipping. That's it. We did that mod to a few fenders in our shop. As a test. Put a pot in the phase inverter circuit to make one side push harder...the upswing or downswing...Big damn deal...

Although..I wonder how often asymmetric clipping is done with pedals...I know..you can use uneven amounts of diodes..

I just wonder how far some people have gone.

aquataur · Post by **aquataur** » 19 Jul 2013, 18:30

Mike,

I have read this on one of the russian j-fet freaks pages. The way I understand, those guys want to sell their devices so they keep a low profile hereabouts.

But they have been ranting about how important asymmetric clipping is.

For the diode clipper pedals, they slowly rise to the demand. Now and again you see antiparallel diodes with either different series resistors (= tube like compliance) or a different number of diodes.

IMHO the compliance makes for the biggest tube tone. The K.O.T. derivate I made can do much more than the original and I eliminated some of the thoughtless flaws they built into it. This uses compliance, although not unsymmetric in the stock version. Compared to the umble, it sounds really similar in dynamic behavior apart from some different voicing, but the compression would kill the Queen stone dead if there were not the queen´s versatility.

have fun,

-helmut

chicago_mike · Post by **chicago_mike** » 23 Jul 2013, 17:35

Dumbleass got his fame from SRV. And ran with it. The guy and his practices just leave a bad taste in my mouth.

We did mods to a friends set of amps, bandmasters, concerts, a victoria bassman clone, and a few others ( he has money to burn..lol )

And after all the mods and tips and tricks to try out...

He went and bought a red plate amp.

I took video and notes of some things we did. But to me none of it was really that different than the stock amp breaking up. Sometimes it broke up too much, but its part of trial and error. I do like to experiment though..I love that stuff.

aquataur · Post by **aquataur** » 10 Aug 2013, 10:16

chicago_mike wrote:Asymmetric clipping. (...)Put a pot in the phase inverter circuit to make one side push harder...the upswing or downswing...

Mike, thought about that again. That is not what the Russians were talking about. You are talking about mutilating the symmetry in a push-pull circuit, the Russians talked about a preamp stage where the gate is fed with positive grid current on one of the half waves. Might sound different.

There is something ambiguous about the whole claim: a preamp stage was per definition not destined to be overdiven, it only appeared with master volumes that people overdrive the preamp heavily. "Early" distortion tones we know are power amp distortion, which does not exhibit the before mentioned symptom. Power amp distortion might sound differnt. So what are the Russians talking about?

It is very hard to tell because the online translations are very crude and the Russians have no interest to clarify (because they want to sell their own units). But what happened? Everybody repeats this prase like as sermon. Where is the proof? Any inputs welcome.

Yes a few diode approaches appeared recently, uneven diode voltage in both directions or different series resistance. I have dedicated an article to that on my pages. You´ll find those arrangements in virtually all stomp boxes that use clipper diodes. There are only a few ones that chose a different direction,amongst them Peavey and Baldringer.

-helmut