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Old 14th March 2019, 08:15 AM   #141
snaarman is offline snaarman  United Kingdom
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I have to say the one valve amp is the nearest I have come to my perfect amp. I was building it for a colleague but I may just give him another small amp and keep it for myself.
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Old 14th March 2019, 06:12 PM   #142
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by PRR View Post
JFETs are not vacuum triodes.
No disagreement from me there. Certainly the behaviour of the two devices under heavy overdrive is extremely different.

But for smallish signals, well away from clipping / saturation / cutoff, the sprinkling of mostly second harmonic distortion from a JFET does seem to sound acceptably like a vacuum triode operated similarly away from its limits.

As a bonus, some back-of-the-envelope calculations I did suggest that (some) JFETs are probably the lowest-noise amplifying devices we have for interfacing directly with a traditional high-impedance guitar pickup. Just pick one with a low corner frequency for 1/f noise, and reasonably good noise specs above that. BJTs aren't really in the running at source impedances this high, and vacuum triodes are handicapped by the high cathode temperature (which raises thermal noise) as well as flicker noise.

To be fair, guitar-amp hiss hasn't been much of an issue for me personally, probably because my tastes run to relatively low SPL and relatively low gain.


-Gnobuddy
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Old 14th March 2019, 07:26 PM   #143
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by snaarman View Post
I have to say the one valve amp is the nearest I have come to my perfect amp.
Now, there's a great result. Congratulations!


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Old 15th March 2019, 08:25 AM   #144
snaarman is offline snaarman  United Kingdom
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Originally Posted by Gnobuddy View Post
Now, there's a great result. Congratulations!


-Gnobuddy
However it still needs to be finished. So far I bought a cheap 12 inch speaker that has a coil rub, and an amp box that was smaller than it looked in the advert. Grrr.
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Old 15th March 2019, 12:27 PM   #145
Kay Pirinha is offline Kay Pirinha  Germany
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Remove the gasket, then carefully remove the dust cap, shim the voice coil (to center and to arrest it in it's correct vertical position), unglue the surround and the spider from the basket/frame, using MEK. The voice coil still sits firmly in it's position? Then glue first the spider and then the surround to the basket again.
If the speaker' values justifies your effort .
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Old 15th March 2019, 07:31 PM   #146
Gnobuddy is offline Gnobuddy  Canada
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So far I bought a cheap 12 inch speaker that has a coil rub, and an amp box that was smaller than it looked in the advert. Grrr.
You have my commiserations. That stinks.

Since much of this thread has been about exploring the road not usually taken, I'll mention that I've been on the hunt for affordable speakers for small guitar amps for a while. After a lot of searching and at least one dead-end, I may have located another good candidate for experimentation.

A little background: in the early years of the moving coil loudspeaker, amplifier power was expensive and meagre, so the focus was on maximizing efficiency. That led to big cones (10" - 15") made of the thinnest possible paper for lightness. Voice coils were similarly light. There was no separate surround - just corrugate the edge of the paper cone, that'll do just fine. Magnet materials of the time were weak and expensive by today's standards, so the speaker had a relatively weak magnet.

Unfortunately, it turns out that approach has many sonic limitations. In the world of good-quality audio reproduction (Hi-Fi, for lack of a better term), the speaker quickly evolved into something different, and capable of producing much more accurate sound, at the expense of sheer efficiency. Surrounds and spiders got softer. Cones got smaller, stiffer and heavier. "Whizzer" cones were added to the centre to extend treble response. Later, separate tweeters and crossover networks became usual (maybe mid-ranges too.) Magnets and magnetic structures (pole-pieces) became massive. Voice coils became longer and heavier. The materials used in construction improved. All this added cost and complexity.

But there were a few shadowed areas of the loudspeaker world in which the early crude speaker barely evolved at all, for one reason or another. One of those areas was guitar speakers. They are still, for the most part, made the same way as they were in 1940: a single big light floppy cone with a stiff corrugated surround, and a singular focus on efficiency, at the expense of almost everything else. Electric guitars don't produce bass below 83 Hz, or treble above maybe 6 kHz, so the simple, single large speaker worked tolerably well.

(Some of today's guitar speakers do use better magnets, glues, and materials than their 1940s forebears. But the changes are slight compared to the evolution of Hi-Fi speakers in the same period.)

Because of all this, today's guitar speakers ought to be inexpensive: they are still made with very few of the improvements that gradually raised the cost of Hi-Fi drivers and speaker systems. But as all electric guitarists know, guitar speakers today are actually very expensive. The cost is completely unreasonable for what they actually are. Instead, the cost is driven by "image". If you believe the narcissists, image is everything, so you will pay dearly for it.

The next question is, were there perhaps other dim corners of the speaker manufacturing world in which ancient, primitive speaker designs continued unevolved? Maybe an unfashionable area, where pricing was driven by reality, not by a manufactured "image"?

It turns out there is at least one such area. The ceiling-mounted paging / public address speakers used in factories, warehouses, machine shops, dispatching stations, and other similarly unglamorous work places, are still built a lot like speakers from WW II, i.e, like electric guitar speakers. Big floppy paper cone, check. Stiff paper surround, check. Focus on efficiency, check. Budget prices reflecting product reality and not advertised image, check. (We're talking $10 or $20 instead of $100 or $200 for the famous-brand guitar speakers.)

So I began hunting for mass-production paging speakers like this. While they're built very much like vintage guitar speakers, there have been two changes: they are now made to an industry-standard 8" size, and many have a smaller "whizzer cone" mounted in the centre, to extend treble frequency response.

This means two things: One, if you want sheer ear-bleeding SPL, you probably need the big 12" $300 big-name guitar speaker. Two, if we use an 8" paging speaker for e-guitar, we might need to do something to limit the extended treble response, which we probably won't want for e-guitar.

This post is already too long, so I'll continue it later today.

-Gnobuddy
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Old 16th March 2019, 02:15 AM   #147
PRR is offline PRR  United States
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> sheer ear-bleeding SPL, you probably need the big 12"

No. 16 Eights in array will whup most single Twelves.

Arrays have real problems of balance and coherency. But they are LOUD.

Look for Marshall Full Stack. 8 Tens in two stacked boxes. It really is the same idea: Jim had access to inexpensive PA speakers.
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Old 16th March 2019, 02:16 AM   #148
PRR is offline PRR  United States
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> sheer ear-bleeding SPL, you probably need the big 12"

No. 16 Eights in array will whup most single Twelves.

Arrays have real problems of balance and coherency. But they are LOUD.

Look for Marshall Full Stack. 8 Tens in two stacked boxes. It really is the same idea: Jim had access to inexpensive PA speakers.
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Old 16th March 2019, 09:36 AM   #149
snaarman is offline snaarman  United Kingdom
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I had my moment of discovery when I connected my 5 watt Champ to a 4x12 Trace Elliott array. It sounded astonishing, and incredibly loud. Nothing improves an amp more than a better speaker
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Old 16th March 2019, 09:40 AM   #150
Kay Pirinha is offline Kay Pirinha  Germany
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As far as I know, Jim Marshall used 12's by Celestion. It wasn't his own idea, but that of Pete Townshend and/or John Entwistle, wo wanted it as loud as possible, to get eight speakers into one cabinet for a total capability of 200 watts. Soon they found out that these were too heavy and a PITA to haul. So they have cut them in half and got the now famous 4 x 12 stack.
8 x 10's were/are sold as well. The first ones were introduced by Ampeg in 1969 and sold with their 300 watt SVT heads. These featured proprietary 32 ohms drivers, which are roumored to be made by Eminence especially for Ampeg.
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