Swirling phasing sound at hi output

[billyz]

What causes an amp to get that phasing swirling sound at very hi gain settings? Is it the Power supply node shared by the first 2 gain stages? Or, Too large a coupling cap? I kind of like it if it's not too dominate. Like old JTM 45's and Fender Tweed amps sometimes do.

[doctord02]

Lots of debate and discussion over at the Gear Pages. Some of the more knowledgeable folks say it's an artifact coming from the phase inverter imbalances...

If you frequent the Gear Pages, search for swirl or 'zhoosh' I believe - a phrase coined by producer/guitarist David Torn.

Like most things on GP, expect a bit of drama and loud opinions.

EDIT:

Here ya go-

http://www.thegearpage.net/board/showth ... ght=zhoosh

[Structo]

Oh for the love of.......
I love the Gear Page!

Never a dull moment.......

[Wayne]

I've seen vendors claims that a particular tube (brand or type) has nice "swirling content". That's usually right about the time I do one of these -

>>>

Not that I don't know the effect you mean, but I don't believe it comes from a specific "brand x or y" power tube.

Maybe that's what mojo sounds like (at least the part that's in the audible spectrum)

W

[billyz]

I doubt it comes from a brand of tube at least not in my experience. I have heard it in an amp needing new power supply caps. When the cap job was done it went away.

But, I find it in some of the amps mentioned even thought the power supply caps are replaced or new.

I looked at the Gear page link, hmmmmm not really much technical insight.

except Jim's comments about the phase invertor and his tech knowing what causes it.

I did find the open back speaker idea original and interesting. But, still no definitive answer, maybe there isn't one.

[paulster]

Here's another (long) thread about what may be the same thing:
http://forum.metroamp.com/viewtopic.php ... &sk=t&sd=a

At least in this one there's some technical input from Randall Aiken with his usual in-depth explanation and a solution. The solution sounds remarkably similar to Jim's 4 diode trick, although in this case two of them need to be Zeners.

Without having Matte's clip available from TGP to see if that's what 'zhooshing' is then it's difficult to say whether or not it's the same thing, but I think it's fairly likely.

[Alexo]

This sounds a lot like the "buzz" problem that comes up on 18watt.com.

I've had it in a few cathode-biased amps that push the output tubes really hard, and the zener diode trick can work wonders to cure it. You can also mitigate it in fixed bias amps by biasing hotter. It's related to crossover distortion and blocking distortion.

Crossover distortion gives you a buzzing sound, and basically introduces another frequency that rides your signal. The blocking distortion changes the wave length of the buzz frequency in proportion to how much voltage you're putting on the grids, this results in a shifting buzz, which sounds a little like "pppeeeoooowww," or as I read somewhere, "squishy mosquitoes" as the coupling caps to the power stage discharge.

An overdriven long-tailed pair phase inverter can also introduce some swirl in a similar manner, but it's usually not quite so extreme, and it's often masked by the overdriven power tubes at that point.

..My experience.

[billyz]

Thanks Alexo.

I am going to experiment with the Zener Clipping circuit. Randall Aiken gave a very detailed explanation of the theory behind this phenomenon.

If that is indeed what I am hearing. Sometimes I kind of like it so I may make it a switchable in and out.

[Kregg]

I never knew this "phenomenon" existed within the circuit. Perhaps it's the transformer ohm wire, or a loose component.

[Scumback Speakers]

My old amp tech fixed a Blockhead 18 watt that did this real badly with the diode mod.

On other amps I've heard with this, I swapped the phase inverter tube with a matched triode (balanced) 12AX7 and that fixed it on that amp.

[Voodoo_Man]

What causes an amp to get that phasing swirling sound at very hi gain settings? Is it the Power supply node shared by the first 2 gain stages? Or, Too large a coupling cap? I kind of like it if it's not too dominate. Like old JTM 45's and Fender Tweed amps sometimes do.

The ceiling fan in my bedroom causes my amps to produce a phasing, swirly sound. More pronounced at higher volumes.

God bless,
john

[paulster]

My old amp tech fixed a Blockhead 18 watt that did this real badly with the diode mod.

On other amps I've heard with this, I swapped the phase inverter tube with a matched triode (balanced) 12AX7 and that fixed it on that amp.

As you were partaking in the TGP thread back when Matte posted a clip (since removed) of zhooshing, is this the same thing as referred to on the Metroamp site and that Randall posted the fix for?

I'm just trying to figure whether there is one single phenomenon at play here, or whether swirling, zhooshing and mosquitos are all different!

[Scumback Speakers]

paulster, the tone may be similar in all, depending on the circuit. The 18w had this phasing tone going on anywhere over 5 on the volume, at all times. It was really annoying. Sounded like an old Phase 90 set about 1/2 way up for intensity and a quick phase effect. The higher the gain was, the worse (and more pronounced) it sounded.

Other amps I have do it, too. All amps with PI tubes, namely my Gibson Super Goldtone GA-30 RVH (quad el84 cathode biased) and my old Risson ETA-100 (dual 6550).

The Risson got fixed by testing five PI tubes (12AT7 IIRC for this amp) until we found one with almost perfectly matched triodes. That's a PITA to dial in, of course.

The Goldtone was fixed by a tube swap as well. Go figure.

My single ended Class A amps don't do it at all since there's no phase inverter tube. So that's where I think the problem is.

[Wayne]

I wonder if this is a subtle, just-on-the-verge AF oscillation - like motorboating, but with the boat moving really fast

W

[Alexo]

My single ended Class A amps don't do it at all since there's no phase inverter tube. So that's where I think the problem is.

They also don't have any crossover distortion in the power stage, which is also a significant source of peeeooowwing. A long-tailed pi is basically like a self-split push-pull stage, in a way, so those phase inverters can produce a similar shifting crossover notch just like the power tubes, I think. I've put a lot of time in to tracking this down and getting it out of some amps. The pi and the output tubes can both zhoosh/squish, these zhooshings have slightly different characters, but IME, both parts of the circuit can do contribute.