Power Scaling is Flawed?

[Alexander]

LOL! I hear ya, Bruce!

isn't the other name of 'that tone' the "Guitar Center House Sound"???

I'll consider power scaling when somebody builds a speaker that sounds the same from say 50 watts down to 1/10th watt. Until then, I'll keep associating 'scaling' & 'modeling' with 'fish' & 'glue'. In that order.

Solder on,
Alexander
Retrodyne Amplification

[tbryanh]

Well, it's a nice sounding theory, except I've never seen any evidence that speaker distortion starts to increase again at low levels. AFAIK, they only get more linear as the volume is decreased.

This might be referring to the distortion that occurs when the power to the speaker drops below about a watt or so.

Under this condition, the amp apparently has a hard time getting the cone to move. This apparently causes a different type of distortion that is apparently undesirable.

[mooreamps]

I'll consider power scaling when

Well, the guys who use my equipment playing Jam Night at the Quarter Note Club in Sunnyvale like that fact they can over-drive the power tubes at a lower volume, and still be playing plenty loud.

-g

[Arthur B.]

DIing to a mixer solves many of these problems, because you can crank it through headphones and/or EQ it. But it also removes the need for power scaling in the first place: you can just hook your amp up to a big 50 watt wirewound resistor and DI box, and dime it.

Of course, that renders the entire amp into a preamp. This begs the question: why not simply make a preamp that models a large poweramp using small signal pentodes like EF86s and then run the output of into a large clean tube poweramp?

[Steve Conner]

Hey, I used to like that bees-in-a-paper-bag tone, too

tbryanh: I don't believe such a form of distortion exists, I think the perception of it is explained by the Fletcher-Munson curve stuff I mentioned above.

Arthur B: I can't see any reason not to do that, except maybe that it's really just a master volume amp in another disguise. You just added a push-pull stage and transformer to generate more odd harmonics before the MV. So I guess I would expect it to behave like a post-PI MV.

I have an upcoming research project that will consist of a modelling preamp like you described, using ECL86s as the power stage or whatever, driving a 400W Class-D power amp. I just have to decide what to base the preamp on: I guess it'll be either a 5F6-A Bassman, or a SVT.

I don't see EF86s having enough balls to drive any available output transformer

[marcus_cps]

Hi there.

Its not that easy to find a schemo.
I sent an email do Kevin and he told me to buy TUT5...
But it a little hard to me... I'm from Brazil and it costs 60 US$ + ship... around 80 US$ (doesnt seems that much when your salary is in US$... but here think in REAIS... its like it is around 150$ or something like that to me).
Considering its just a hobby, it started to cost too much!

Kevin is a way cool dude. Put yourself in his position and it only makes sense that certain people would be ungrateful but the reality is he has helped very many more than he gets credit for and builds a damn nice amp to boot. If you don't find what you want email me and I'll shoot you a schemo.

As I told up here... I couldnt get it.
Can you help me with a schemo?
Thank you very much.

[Steve Conner]

Considering its just a hobby, it started to cost too much!

Well, presumably Kevin O'Connor has to make a living somehow. His power scaling circuit is his intellectual property, and as such, it's his right to control who can use it under what conditions.

There is a schematic of a power scaling unit developed by Tiago earlier in this thread: why don't you try that one?

[Ptron]

There's one on the Soultone site that you can link to over at powerscaling.com. It's a nice, simple circuit. It doesn't scale bias voltage so you'd have to make your amp cathode biased. I'm guessing that would put the kibosh on getting 50watts out of it but are you sure you need that much? Remember if you build it that you need a pretty heavy duty pot as it will have a lot more voltage across it than standard pots are rated for.

Tiago's circuit scales bias voltage for fixed bias amps and I don't think it needs a special pot. It is however more complicated and I don't think it's tested either. I don't know your electronics skill/level but unless your pretty advanced it my not be a good one to try.

[Ptron]

There's one on the Soultone site that you can link to over at powerscaling.com. It's a nice, simple circuit.

Oh, I should probably also mention... that circuit was designed for an 18W amp. I don't know what kind of power those components can handle. That could be important.

[Mouthurst]

KOC goes out of his way several times in TUT4 to point out that there is really no way to get around the lack of speaker overdrive/low volume issue and that you have to essentially change your expectations when using power scaling and not expect that it is just a volume difference but the same tone as when the amp is loud.

I was thinking of the KOC kit too but decided to go a different route.


What Im hearing Makes Sense to me. I found the same thing, with a different set up.
I sometimes use homebrewed 20watt 4x6v6's tweed twin 5f8a circuit which sounded terrible with 80 watts worth of speakers but got really heavenly when I switched it to 20 watts worth of alnicos.

Couldnt the same thing be done with a power scaling system, that is, use higher efficiency speakers which are rated around closer to the scaled output power?

[Amp Kat]

Another thing is that Pentodes in the front end are pretty damn noisey and hard to get out but Authur you are right on with the idea and that's basically what I do in several ways and Steve's analogy is correct as it basically a post master volume but this works extremely well with awesome tone. Use the preamp out of whatever device,preamp,stereo out or whatever and go right straight into the amps input and turn the preamp gain almost all the way down. Use the preamp gain of the first preamp out or volume for your gain and the amps volume for loudness. You can attenuate your volume and get as much drive as you want at any volume with effects or whatever you need. For some reason IMO the ultimate tone is a Marshall preamp out into the Fender just kicks. You can use processors also but they are much quieter going straight into the return bypassing the preamp alltogether but most have the dreaded digital sound. Amps work much better.

[Steve Conner]

Well, OK, why not keep two speaker cabinets? A big cabinet with hefty speakers for playing out at gigs, and a little one with a single low-powered Alnico 12" for playing quietly at home.

The other approach would be to use a big, linear speaker like the EVM12L that (apparently: I've never tried an EVM12L for myself) holds much the same tone at low volumes as at high volumes. Then design your amp to build plenty of overdrive and a good breakup with the preamp alone, and use a master volume and a big, clean power section. You have now made the setup that Amp Kat described, just in a single box.

Add extra tone controls just before the master volume to let you boost bass and treble, or scoop some mids, when you're playing quietly. This (IMO) counteracts the tendency to sound dull and nasal at low volumes because of the Fletcher-Munson thing. But don't scoop too much in case you let Bruce's bag of bumble bees loose!

Now, when you take it to a gig, I think it's very important to take off whatever EQ you were using, and let the midrange really rip out. If you're playing lead, you need to blast through the mix like a trumpet or sax would. If you got fancy boutique speakers, they probably have resonances in the mids that give them their character, and you need to get these singing. But you don't want to blast through your neighbours' TV show the same way!

This concept won't necessarily give you the gnarly, out-of-control feeling of a low-powered speaker driven to the edge of total cone rupture, but I bet it will scale well. This is the approach I stumbled on through messing around over the years. My Ninja Toaster might not be to everybody's taste, but it has proved to get some tones that I really like, both for practice at home and for gigs, without any form of power scaling. Most of the tone comes from the first two preamp tubes, and the second tube in particular. It's currently a half-worn out Telefunken 12AX7 that I found in a scrapped Grass Polygraph

[dai h.]

would post-speaker out tone controls be effective at compensation? Like say, an inductor and rheostat in parallel for the high end.

[drewl]

You people are insane (just kidding).
The ditzy broads I play to every weekend can't tell what amp I have and don't care!
Anyway, I power scale all the time, I use big amps for some gigs and smaller amps for others.
Sometimes I use a master, sometimes an attenuator...still a heckofalot easier than installing a power scaling kit into an amp.

[marcus_cps]

I will have a look at that. Thank you very much.
TiagoŽs circuit seems to be nice, but as he said, its not tested...
So, I think its not a nice try by now.
I can understand most of the circuits... but I dont have much experience in it.
That why IŽm seeking for something already tested in certain conditions (like a 50W amp)...
IŽll have to wait a little to start my project... When I have time for it, IŽll be back here.

Thanks