NAD (Almost)! 1960s Univox U45B

[JMAnz]

So, I'm trading my 1970s Lab Series L3 solid state for this rare and unique tube amp. I've been looking for a good sounding clean amp to run A/B/Y with my SIBLY, and this one delivers in spades. Only about 10 watts, but it stays clean (at least what I call clean) up to about 3/4 on the volume before distortion hits in. It's just as loud as where I've been running the solid state on clean, so it should keep up with the band real well. Sparkly and dynamic. Delivers what I've been looking for in spades.

Uses 6MB5 tubes for pre and output in a push-pull config (unique), and a 12AX7 for the tremolo. Used to have a tube rectifier, but was converted at some point to a SS rectifier (the original rectifier tubes are no longer in production, so at least there's a good reason). Also has the original Jensen Special Design 12" speaker in it.

I work at a music shop, and I'm trading my L3 to my boss for this one. I took her home today to try everything out before we seal the deal, but so far I'm QUITE happy with this. After everything is official, I'm gonna have the local tech convert the 2 prong cord to a 3 prong so I'll feel a bit safer.

[singlecut]

nice
i own 1 and hippofutura is also a fan

[JMAnz]

Good to know there's others out there with these amps. Mine's a little rattly, so I'm gonna take it all apart, clean everything nice, and put a new gasket on the speaker. It really blew me away. I love these vintage small amps. Such great tones to be had. I read about plugging into the accordian input and bridging the other two inputs for a bigger overdrive tone. Might be good sounding for recording.

[Cygnus X1]

Cool!

[JMAnz]

I've spent the rest of the evening playing on her with a buddy... I can't get a bad tone out of her. Even if she's not loud enough with the drummer, the tones are EXACTLY what I'm looking for (at a FRACTION of the cost, seeing as it's a trade), I'll mic it through my PA. Really falling in love here. Definately gotta tame the rattles, but it looks as though the chasis was put in backwards, which would account for the cage rattling. The handle rattles a bit too, but she's so light I may just remove it all together...

As I said, LOVING this amp. The perfect compliment to my SIBLY. I've been experimenting with pushing the distortion, and dimed she sounds GREAT. Easily nails that Zep I & II tone. Tweaked my wah (a custom job based on the Clyde McCoy i built), and found some great tones in there. If you can find one of these amps, they go for cheap. Pick one up! Definately worth it. They can also do the big distortion at low volume thing, which is great for you studio cats.

Thursday morning I'm walking through the door with my L3 and finishing the deal.

[JMAnz]

Today I went down to the hardware store and picked up a 3 prong cable for the amp. I checked cap voltages before starting, and suprisingly they were sitting at 0. I also looked up a schematic on this amp, and I'm finding a few discrepancies. There's two PT's in this one, and one OT. The first PT just sends power to where the tube rectifier was. The other wires for it are cut off and sealed. The tube rectifier socket (no tube installed) just has 3 diodes (2 in parallel, then 1 in series)running from it, back to the board. The second PT powers the rest of the tubes. The "death cap" was already removed, and the negative on the power cable ran to two wires (one from each PT). The other side went to the switch, then the fuse.

I swapped the cable out and grounded to a lug under one of the PT legs.

It sounds great, but I wonder how true it is to original design at this point. Looks like all the caps minus the filters have been swapped out for Sprague Orange Drops of varrying values. Filter caps are stock (from looking at pictures I've seen), but look good. The tubes it came with are all old (maybe not the 12AX7, there's no branding on it, but the stamp could've worn off). The two 6MB8's are a GE and a Realistic. Aren't these supposed to be matched? I know you can have a matched pair of varrying brands, but I don't run across that very often.

[Cygnus X1]

Since these don't get much in the vintage market it probably doesn't matter about originality.
As long as it sounds good to you, well that's the prize!


Highly detailed gut shots might help.
Hard to tell but I think it's cathode biased.
Odd tubes!
Someone may have already compensated for differences, look for split bias resistors.
Here is a link to the U45 schematic:

http://www.freeinfosociety.com/elect.../univox45b.pdf

[JMAnz]

I have that schematic. I did a lot of electronics in the Air Force, but after that I didn't mess with anything for nearly 10 years, so I'm pretty rusty with following a schematic, but I'm getting better at it. Building lots of pedals (and troubleshooting them when they don't work the first time) has helped.

Here's some pics I shot. Sorry for the blur on some, my camera's been acting up lately.

[JMAnz]

And yes, very odd tubes. Pisses me off, because about a year or two ago I ended up with all the old parts from my grandfather's TV repair biz from the 70s. I had a HUGE case full of tubes. There were some 6MB8's in there, but I had no use. GE, Raytheon, and Sylvania brands. I got rid of what I had no use for. That, at the time, included those. ::kicks self::

[Cygnus X1]

Holy flying diodes!

They did the recap job bass-ackwards.
Changed out all of the tone caps but forgot to do the filters.
Those will likely have to be done soon.

I would unbolt everything on the front panel and gently lift the board
to get to the filters.
Double check but it looks like it has bleeders, so it should discharge quickly.

[JMAnz]

Thanks, Cyg! I thought it strange that they recapped the preamp but left this filters out of the equation. Maybe they changed the voicing a tad when they did that? I really don't know as we've had this amp in the shop for a couple years now, and no one remembers where it originally came from. It sounds great, but I do plan on changing out the caps. Question: how can I tell if the caps are leaking? I have a DMM that measures capacitance. Wouldn't that be a clue in if they were below where they should be?

And I think it has bleeders as well, as it was 100% discharged when I worked on it, and it had been turned on and used HEAVILY most of the day before (possibly off for 10 hours when I measured for DC).

Used it with the band last night. I'm finding such great uses for it. Not just for cleans, but also when I want a low volume distortion on song intros, it works great there as well.

[JMAnz]

Did some research, Cyg. It's grid-leak biased. I guess that's similar to cathode in that I don't have to rebias with tube changes (from what I understand). Just ordered a matched set of NOS 70's winged-C 6BM8 tubes from the bay. Now if I could just figure out where the last bit of rattle is coming from in the cab...

[hipofutura]

Great amp, JM! I love the old Univox amps. My first electric guitar was a Univox Hi-Flyer and first amp was a Univox combo.

[JMAnz]

Better than what I started with... A late 70s Harmony Strat (with a BADLY warped neck), and a Lab Series L3 (which was just now traded for the Univox).

[Cygnus X1]

I generally (OK, I never) don't test old filter caps.
I just replace them and don't worry about it.

[JMAnz]

Thanks again, Cyg! Any suggestion on cap brands for filters, or will just about anything do that's rated for proper voltage?

[Cygnus X1]

F&T's come highly recommended.

Most builders are using Rubicon, Nikichon, Tung etc. available at AES, Weber, Mojo, and many other places.

Spragues have become nothing more than a high priced paint job but the performance is good.
Just don't overpay for them.

[JMAnz]

Cool. My local electronics store has F&T I think. They may have a few of the other brands as well. I'll look into it before I order, otherwise I'll probably go through Mojo, as I've come to love those guys...