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Topic: The Lux Amp: from stuff that was piling up in the shop |
Bryan Martin
From:
Quebec, Canada
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Posted 26 Jan 2018 8:40 pm |
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The design and build of the Lux Amplifier came about because of two reasons:
1. I had various clutter from stuff clients had abandoned, and it was piling up in my shop, and
2. Q: “How many amps do you need?†A: “One moreâ€.
The castaways in question were the failed attempt by someone to ‘rewire’ a 50s Gibson GA40, and the cabinet of a Peavey Classic 30. As luck would have it, the top-mount Gibson chassis fit (with minor modification) neatly into the Peavey cab.
The idea of a small combo with reverb and tremolo began to take root inside my head. And I don’t have a small combo with these attributes . . . and it would be the perfect amp to cart along on those lazy summer vacations. The Gibson iron was all fine, but the inside was a failed rat’s nest. So I set about figuring out just what type of circuit to put in there. It had 4 preamp tube sockets, two 6V6 output sockets, and a tube rectifier.
I only really needed one input channel, and I do have a fondness for Fender circuits. So after a bit of study I came upon the concept: the input, reverb and tremolo circuits were lifted from a Princeton Reverb A1164, combined with the output section from a Deluxe AB763, (The schematic is attached). To pull this off, I had to replace the 3rd octal preamp socket with a 9-pin, and mount the reverb transformer, the reverb in and out RCA sockets, and the speaker jack on the back of the chassis, (see pic).
I knew the circuit would be a tight fit inside the chassis, especially with the filter capacitors inside. But JJ makes a 4-section cap that really saved some space (it is the big black can just above and left of the power tubes). I have to thank Santa Claus for bringing the Eminence Legend, reverb transformer and Mod spring tank (the man certainly has connections).
I will call your attention to the far left of the chassis. A bolt can be seen sticking out of the side-wall of the chassis with a bunch of wires connected to it. This is the star ground connection for the power and output sections (all power supply and power circuits have a dedicated ground-wire that meet here). There is another at the right side (but not visible in the pic) for the preamp circuits. Building amplifiers with a star ground scheme takes a bit more time and trouble, but if all circuit have a dedicated ground wire that are connected at only one (or in the case of this amplifier two) point the noise floor is greatly minimized. In most amps, there are multiple grounds dotting the chassis. This can create eddy currents running through the chassis which contribute to the noise floor, usually heard as ‘hiss’. A star-grounded amp is eerily quiet . . . and you know were all your electrons are going. I personally believe the star ground contributes to improved sound quality as well.
I love the sound of the tremolo circuits that modulate the bias on the power tubes. It sounds much more liquid than the opto circuit: not so on-and-off (like a switch), but more like waves on an ocean of chimey goodness.
If you look to the right front of the control panel, you can see what seems to be two volume controls. But one is volume, and the other a stepped selector that changes resistors in the feedback circuit. I always tune the feedback circuit by ear. This amp sounded best to my ears with no feedback, but I added this step control so that the player can contour the feedback circuit with four discrete resistors.
The amp does not sound like a Fender. Not too sparkly and bright, but more even and throaty. It seems to prefer P90s over Fender single coils; must be that Gibson heritage and iron. The reverb is mighty, much more present than on most Fenders. It is very compact, but a bit on the heavy side.
The Lux was finished about 10:30PM January 31. Good way to ring out the old year: turning off the soldering iron, and shutting the lights on 2017.
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J Fletcher
From:
London,Ont,Canada
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Posted 27 Jan 2018 8:15 am
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| Very resourceful. I too am using up accumulated clutter in my workroom. Just built an old Thomas Organ Leslie amplifier into a toolbox. |
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Paul Arntson
From:
Washington, USA
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Posted 29 Jan 2018 10:24 pm
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Tune by ear is exactly right for the feedback resistor.
_________________
Excel D10 8&4, Supro 8, Regal resonator, Peavey Powerslide, homemade lap 12(a work in progress) |
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Jerry Erickson
From:
Atlanta,IL 61723
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Posted 1 Feb 2018 9:19 am
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| Very nice! That power xformer is huge! |
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Bryan Martin
From:
Quebec, Canada
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Posted 2 Feb 2018 7:31 pm
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Paul,
On all the amps I have built, I have always ended up using litte or no feedback. If you are a manufacturer, using a large amount of negative feedback is a failsafe to make sure the amp is, and stays stable, and guards against variations in huge parts shipments. It is not there to ensure great tone.
I have found that every amplifier sounds more open and alive with little or no feedback. If it is designed right you don't need the feedback to fix bad sound.
Jerry,
That is a big old Hammond. A thing of Canadian beauty.
Cheers, Bryan |
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Paul Arntson
From:
Washington, USA
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Posted 4 Feb 2018 9:22 pm
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Good call, Bryan. I modified a fender champ with a pot for the feedback resistor, and I agree with you. The higher the value, the more alive the amp felt.
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Excel D10 8&4, Supro 8, Regal resonator, Peavey Powerslide, homemade lap 12(a work in progress) |
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