NAD--Univox 1235

Discussion in 'Amp Central Station' started by marshman, Sep 30, 2013.

  1. marshman

    marshman Friend of Leo's

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    Well, it's amazingly clean for 40-ish years old. 3 12AU7s (Mullards) and a pair of 6L6GC (RCA Blackplates), SS rectification. Hums pretty vigorously (while the chassis is out, exposed, on my workbench under a 48" flourescent lamp), but seems to function properly otherwise. Rotating knobs yields some sort of tonal changes (and a fair bit of static). I plan to keep it as a bass amp for the time being, and start looking for a vintage Univox 1x15 or, heaven help me, a 6x10! Only thing that appears to be missing is the stock Univox badge. Not bad for $125.

    Still has an apparently original 2-prong plug and looks nearly all original inside based on my limited experience staring at the guts of my similar vintage 1050/1051 bass head. Schematic from univox.org is different than my amp, but which has been tampered with is anyone's guess...for instance, the chassis is stamped with "12AU7" over each 9-pin tube socket, though the schemo online has "12AX7" at V2, and there are 2 very small value caps that appear to have been added to the circuitboard after the fact.

    As it's probably 40 years old, my first order of business will be to order fresh electrolytics. One of the installed caps is only rated to 150VDC where 350VDC is called for, but since US voltages have risen the way they have (and that I measured 430VDC on the plates when I opened it up) I'll probably replace all with 450~ish units, and a 3-prong plug. It also looks like something tasted the sheathing on a few wires in the tone stack, I can see glints of wire ina few spots, I wonder if that should be replaced. Once I get it acting right, I might try some 12AY7s in V1 to see what happens.

    schematic: http://www.univox.org/pics/schematics/1236.gif
     

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  2. Octave Doctor

    Octave Doctor Tele-Afflicted

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    I got one of those w/Cabinet at the flea market a few years ago. Worked fine for a while, then demonstrated one of David Lindley's axioms--"An amp always sounds it's best in the five minutes before it blows up."
     
  3. BobbyZ

    BobbyZ Doctor of Teleocity

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    Looks like a printed board ? Can you get at the back of it to solder the new E caps in?
     
  4. Delta Blues

    Delta Blues Tele-Afflicted

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    Nice score on the black plates.
     
  5. muchxs

    muchxs Doctor of Teleocity

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    I can tell you what happens. The circuit is designed with low gain. Replacing V1 with a 12AX7 bumps the gain up a little. Replacing V1 and V2 with 12AX7s bumps it up a little more. It starts to grow a pair with 12AX7s in all three slots.

    A 12AY7 isn't a big jump from a 12AU7. Throw a 12AX7 at it and see if it sticks. :lol:

    The downside is the layout is a little noisier than it could be. The nose isn't a problem with three 12AU7s but the volume is way low for a pair of 6L6s. More gain brings th enoise floor up with the volume.

    Didja get the 1x15" cabinet that goes with it? I have an orphan Univox 1x15" cabinet if you didn't.
     
  6. marshman

    marshman Friend of Leo's

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    It is a printed board, and there is a cut-out in the chassis that leaves about 2/3 or the board accessible from the top side of the chassis...but the OT blocks about 1/3 of it from access. I notice that the PT and OT windings are parallel, wonder if that contributes to the hum...or that big-a$$ed chunk of missing chassis.

    Thanks, muchxs. I did NOT get the 1x15 cab that came with it. The same shop had one for about a year and a half a couple years back but it didn't have any specs, so I was worried that anything I own would overdo it, so I didn't get it. I'd probably be interested in one if you've got one.

    I know that there are a few considerations to be made going from 12A?7s along the scale, but for keeping bass usefulness, I'm going to stick with the low-gain options to start. I'm sure I'll eventually get wild with it and stack it full of 12AX7s and see how it goes, but baby steps. One of the guys I jam with uses a line6 rig which he can play very quietly, and with our gentle drummer, it might just be worth trying out. Unfortunately, my primary bandmates use HRDxs at combat volumes, so the Eden will not be out of work.

    I notice that my schematic says the PT is good for 4 amps@6.3V (the heaters are AC, is that contributing to the noise?), which means I might be able to run some KT66s in the circuit, dunnit?

    And yeah, I got a pair of lightly used RCA BPs, and an amp thrown in for $25! All the tubes tested at just under the hypothetical 100% in my friends tester.
     
  7. muchxs

    muchxs Doctor of Teleocity

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    Isn't that cool?! :rolleyes: I fabricated a plate to cover the hole in the last one I had. I could just see someone's toddler stickin' their fingers into the high voltage.

    I figure it's a low buck amp and a waste of KT66s. Those 6L6GBs you have will work fine.

    I forget what I paid for my last one. I even got a cabinet. Nyahh, nyahh! :D I sold the head to a harmonica player from Atlanta. I saw dollar signs when I saw the Mullards. You know how that goes. Some audiophile who sells tubes to other audiophiles can get $100 each for them. Mine got tossed into The Permanent Collection.
     
  8. marshman

    marshman Friend of Leo's

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    I've got a local muchacho who's an audiophile that deals to other audiophiles and even he has a box of 200+ NOS testing 12AU7s from all over the world. And I have been wanting to try KT66s in a bass circuit for my own edification for quite some time.

    I'm contemplating removing all the parts and giving the chassis a good cleaning, as it's got a fair few small corrosion marks on it.

    The amp has a 100u/350V and a 50u/350V cap in it, any reason I can't replace them with a single can, and if so, would a dual 100u be too much? I'll probably go @450V, just to be safe.
     
  9. muchxs

    muchxs Doctor of Teleocity

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    The amp has doesn't have a tube rectifier so 100uf is reasonable. I think I used 68uf for the big cap in mine. 68uf, 47uf, 47uf all at 450v works fine

    The only reason not to use a sinlge multisecton is cost. If you're payin' $10 each for your caps there's no reason not to use a multisection.
     
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