Wave shape of breakup in DC-coupled cathode follower

[jamesmafyew]

I'm clueless when it come to metal, so excuse the question, but why juuust on edge? Since the signal is already "prep'd" by the preamp, wouldn't clipping the PA just adds another layer of distortion on top? Or does that just become too mushy?

That's something I've been investigating for a while. It's not my "ears distorting" since I wear -25dB flat-response earplugs all the time. The simple answer is that the amp is still in the "sweet spot" but you're feeding it with a heavy high-gain signal instead of a mostly clean one. Undoubtedly it's a lot more complicated than that. Crank it too hard into power amp clipping and you're back to a basic square wave that swamps all the preamp's-tone shaping. Sounds too raw and the EQ becomes ineffectual -- and I think the late-EQ is enormously important to a high-gain preamp's sound. Could have something to do with moving in and out of NFB breakdown, losing control of the speakers. People are still debating the finer points of power amp overdrive with regard to vintage/clean amps, and I'm certain that much of the same applies in this case.

I should note that I'm not a shredder, and I don't think that playing this way would appeal to speed-metal types. Those are the folks who want a totally clean power amp, and often do fine with solid state. I'm more concerned with texture, and it's fine if the sound is loose and gnarly.

That's just bloody awesome, all that fire-power! So the guitar amps are the PA for everything but the vocal and drums! What speakers do you run with them? Wall of 4x12s?! Very interesting, must try and attend one of these shows... look out grandpa coming through

I use a heavily-modified Marshall TSL with a 4x12 w/ Eminence Governors and C-Rexes in it. I use an Ampeg V4 for a slave head (out from the Marshall's FX send), which has a delay line on it and drives another 4x12 on the opposite side of the stage, and that cab has two Governors and two Weber Thames in it. So the signal is true stereo, and I can engage a looper that will playback my recorded parts on the slave amp only, so it sounds like two guitar players (but I'm the only one). I use a rack EQ to send sub-70Hz frequencies from the Marshall FX send to a rackmount 300W power amp, which drives a 1x15 ported cabinet. I tune low, so this helps a lot. It retains clarity and punch even when the guitar amps are in power stage clipping... no lost bass response. The bass player uses an 800W Eden head to compete. It's the best of everything, basically.

[jazbo8]

Not only are you a guitar player, you are truly a sonic artist! Judging from your description, you are trying to fill up the the full spectrum (as far as the guitar can produce) using carefully EQ'd preamp distortion + some PA breakup + bi-amp + mixture of tube and solid state amps + various speaker cabinets + different speaker sizes + stereo + delay loops. Must took you awhile to figure it out and tune it, very cool! Love to hear some live recordings if you have them...

[Merlinb]

The output of the CF has this triangular shape on the leading-edge of the negative-going half, which looks like slew-rate limiting, but I would assume isn't really. What causes this, and what is it called? Why would the CF stage, with unity gain, "break up" before the preceding gain stage?

The CF is simply being driven to cut-off, which causes the bottom to clip.

Now, when the cathode voltage is falling the treble cap in the tone stack will release its stored energy and keep the cathode voltage artifically high, causing the CF to hit cut-off sooner than we might expect. As the stored energy depletes, the cathode voltage cannot be kept artificially high any more, so it ramps down to the 'expected' level.
Then the CF comes out of cut-off when the signal swings positive again.

If you try disconnecting the treble cap you should see the ramp disappear, mostly anyway.

I guess you could call this a slew-limiting effect, although it's not what we usually mean by slew limiting (I show an example of ordinary slew limiting in the new edition of my book).

[jamesmafyew]

Love to hear some live recordings if you have them...

Most of the live video recordings I've heard are pretty terrible quality, because the SPLs are too high for most camera mics. A YouTube search will bring up both studio recordings and live videos of a lot of the songs (warning: the vocals are not for everyone) for comparison. I record in the studio with the same setup, basically, though the looper parts are overdubbed instead.
rosetta red in tooth and claw - YouTube
rosetta lift - YouTube

[jamesmafyew]

Now, when the cathode voltage is falling the treble cap in the tone stack will release its stored energy and keep the cathode voltage artifically high, causing the CF to hit cut-off sooner than we might expect. As the stored energy depletes, the cathode voltage cannot be kept artificially high any more, so it ramps down to the 'expected' level.
Then the CF comes out of cut-off when the signal swings positive again.

If you try disconnecting the treble cap you should see the ramp disappear, mostly anyway.

I guess you could call this a slew-limiting effect, although it's not what we usually mean by slew limiting (I show an example of ordinary slew limiting in the new edition of my book).

Just wanted to follow up on this. Several people mentioned that the effect is mitigated when you reduce the value of the CF load resistor, say to 47k or lower. As you point out in your book, Merlin, that also increases the asymmetric compression of the DCCF.

I noticed that the particularly "crackly" sound created by the ramps on the negative-going peaks was NOT affected by turning down the treble. The treble control turned down the "good" treble that I wanted, but not the crackle. What I found though was that turning down the MID control got rid of the crackle. I realized later that turning down the mid control was causing the 33k slope resistor to appear as a shunt in parallel with the 100k CF load resistor for the 470pf treble cap in the tone stack, when the CF was in cutoff.

I added a 500pf cap between the wiper of the mid pot and ground, and now the crackly sound is reduced as though the mid control were at zero, regardless of where the mid control is set, but perceptually it doesn't seem to have rolled off the treble response of the preamp like a shunt cap across the MV would. I suppose the 33k resistor and 500pf cap form a shelving filter? Anyway it seems to have the desirable effect of a lower CF load resistor when the CF is in cutoff, but without changing the amount of asymmetric compression that you get with the 100k resistor.

EDIT: Except that now the mid control doesn't work properly, because of the loading of the 500p cap when the control is turned up high? Still working on this......

EDIT AGAIN: I notice the Sovtek Mig-60 has this exact cap in the schematic. It simulates well, I don't know why it appears that my mid control has been rendered ineffective.

3rd EDIT: Actually it does work, quite nicely. My mid pot is flakey.

[J M Fahey]

At the moment I'm just experimenting with high-gain preamps. Trying to understand what produces this or that type of sound. I think it would probably be easier to just attenuate before the gainstage/DCCF so that the previous stages' maximum output can't overdrive the CF.

You *may* , but you *shouldn't
CF > Tone stack > power amp is a combination that works because *after* the power amp clips, if you push it further, there still is "some change" in the perceived sound.
In this case, you can't "square the square" any more, after a certain point, because by definition you are slamming against either +V or ground, but the unsymmetrical CF creates ... well ... unsymmetrical clipped waves, with different duty cycle .
That's one of the reasons why "a Marshall sounds better the more you push it".
Mind you, I'm talking about no MV models (or with MV turned up to 7 or above).
If you use a MV at the lower settings it may become way too buzy and harsh.
Not exactly news of course.

In this case, I just don't think I noticed this behavior before, because in a cranked non-master-volume amp, it's totally swamped by the output stage and phase inverter clipping.
But with a master added to that same amp right after the tone stack, its influence is huge, particularly if the input is boosted or if an extra gain stage is added.

Well, in a classic non MV Marshall (Plexi and such) it's not *that* swamped at all.
Which is good.
"Symmetrizing" it would look good on paper or the scope screen but it's duller, literally.
We're lucky God didn't create "PNP tubes"

[Steve Conner]

I first read about this effect on KMG's solid-state modelling website.
Some nuances of using mosfets as followers in tube guitar amplifiers

PNP tubes would be interesting as they would be made out of antimatter

[J M Fahey]

Ok, whiz-kid, let's see if you can answer *this*:
If PNP tubes don't exist, what are Isaac Asimov's Positronic Robot brains made with?
Huh?

Next time R. Daneel Olivaw comes to get oiled, I'll open his brainbox and check the microtubes used

[jamesmafyew]

Robot brain microtubes use a cryogenically-treated unobtainium cathode material that appears to violate conservation of energy by teleporting in electrons from another dimension. In audio circuits they reportedly sound very warm but with authoritative bass and excellent rejection of digital glare. However this performance is nullified if the positronic nanotubule interface cabling is not made of similarly cryo-treated oxygen free strands of golden fleece from Mount Olympus.

[Steve Conner]

Word.

[GainFreak]

I first read about this effect on KMG's solid-state modelling website.
Some nuances of using mosfets as followers in tube guitar amplifiers

According to some such MOSFET CF sounds almost identical to a tube CF. Has anyone tried this in A/B tests?

[Steve Conner]

Well, the tube CF also draws a lot of grid current that rounds off the positive peaks of the waveform. KMG's MOSFET CF doesn't model this.

However, I would bet a positronic brain to a potato that it doesn't make a huge difference to the tone. Maybe detectable in a careful AB test.