Univox U-45B

[jrfrond]

Several years ago, I donated a boatload of old Univox schematics to www.univox.org to scan for their website. Here's another link:

http://www.univox.org/pics/schematics/u45b.jpg

BTW- there is MUCH room for improvement at the first input stage of these amps. It uses a "contact bias" or "grid leak" bias system, whereas there is no cathode resistor or cap, similar to something like an old 5C3 Deluxe. The bias voltage is developed across the series input cap, which charges via the input signal, going to the grid and the 4.7M is there to provide a high time constant for the cap while providing a load for the grid. If you remove and jump the series .01 cap, open the tube's cathode connection to ground and add a 1.5K resistor and 22/25 cap in series with the cathode to ground, then the 4.7M resistor to 1M, the amp will be able to handle hotter signals and stay cleaner. It becomes your typical BF preamp stage.

[capnjuan]

In addition to what's available there, on the Hoffman Amps site there's a pdf that rolls up 14 or 15 Univox schematics
http://www.el34world.com/charts/Diagrams.htm

Scroll to Univox.pdf,

Other Univox schematics also available there. CJ

[Shamus]

To update this thread - time flies when you have a baby girl!

I am ready to tackle this amp. I have fresh glass ready to drop in, and a brighter speaker to try as well.

What I need help with is identifying the correct capacitors to purchase:

1. Trem is not working - want to replace trem caps
2. Electrolytic caps

I will purchase a 3 prong chord. I have a buddy who works on amps that will help me do the replacement. Any other caps I should replace? - all currently original.

Thanks in advance!!
Shamus

[Dave Curtis, dB AudioTech]

Replace all the caps, and it'll sound as good as (or better than) new. There's not that many, and you'll be glad you did. Just be careful with that ancient circuit board; if you leave the heat on it too long, you'll separate the thin copper foil traces from the board. Hot iron, minimum time on the solder pad. I've tried to attach a pic of the caps I changed in mine...

[sepulchre]

Hello,

know this is a rather old thread but I just saw a video of this amp: https://www.youtube.com/watch?v=H27QxL8A4Qc and recognized it as the first amp I ever owned when I was 15. Cost me $50 because it was a year old. I had it for decades and it finally bit the dust in a fire at my storage. I always thought it sounded pretty good. These days I build amps of the hand wired custom sort. Seeing that old friend, well I think it would be a good project just for fun. Plus, from hearing that video I was right, it does sound pretty darn good.

Lots of info in this thread thanks to you guys. I'm curious if there's a layout floating around (doubtful) or a good photo of the circuit board. I can make my own, that's what I usually do, but I'd like to get close to the original. That said I might try the Gibson-esque mod for kicks just to see how it sounds, but that would be later.

Also, I'm looking around and don't see a 16K primary OT off the shelf anywhere, though I haven't dug very deep yet. In a pinch Heyboer will wind one for me . . . for a price.

Anyway, I'd like to hear how the Shamus' amp came out. And again, if anybody has a picture of the board that would be nice, something with a little better angle than that Lafayette 10 posted above.

All the best,
Ken

[ron vogel]

Having restored/owned one at one time...I can say with confidence the Jensen in it is pretty bad. The amp goes into saturation above 5-7...but has a nice touch sensitive feel to it. Not a jack of all trades amp, but it does it's own thing well.

http://www.gearslutz.com/board/geeks...ivox-u45b.html

[loudthud]

I'm about 99% sure this board is from a U45B. Sorry for the low resolution.

[sepulchre]

Here are two schematics.

one is an original u45b, the other is a great modified version.
the mods make it closer to a Gibson GA8-T that Univox copied.

Bob

I noticed that the reservoir cap has been upped to 33uF. While that's fine for the 5Y4 rect that the Gibson amp used, the little 6X4 data sheet cites a maximum reservoir cap of 10uF. So to make this closer to the Gibson version either the rect tube should be changed or the reservoir cap should be left at 10uF.

[sepulchre]

I opted for an Edcor model GXPP15-4-8K which, as the number indicates, is designed for a 4 ohm secondary (load) and provides an 8K primary impedance. So if loaded with an 8 ohm speaker it gives you a primary impedance of 16K, just what the doctor ordered. The techs at Edcor assured me that this will work fine - no problem for the transformer.

I'm also using an Edcor PT, number XPWR164-120. It provides 460V (230-230 center tapped) at 60mA and has taps for 6.3V center tapped at 3 amps and another 6.3V at 1 amp. That second 6.3V will be handy for powering relays for use with a foot switch for tremolo and boost.

So now I've got everything ready to go. Unfortunately, I'm currently busy with another project, but it should be finished soon and I can get this little gem together and see how she sounds. Here are a few pics of the progress. (BTW, I replaced the 22uF reservoir cap with a 10uF. I just haven't taken pictures since.)



As you can see I'm calling it "Harlow", one of my favorite old movies stars, the original platinum blonde. So that's what I'm calling the boost switch.



I'll post more pics when she's done along with some clips.

[Bill Greene]

Univox U-45B fuse

The fuse that was in my Univox U-45B is blown. I don't know whether the bad fuse is original equipment or not. The markings on it are λ1A. The U-45B schematics don't specify fuse values.

Would the 1 amp fuse at Antique Electronic Supply (https://www.tubesandmore.com/products/F-ZS), 250V, be a suitable replacement?

[The Dude]

Welcome to the place Bill!

I would say that should be just fine.

[Jazz P Bass]

Not too sure that I like the term "bad fuse".

It seems to imply that the fuse itself failed for no reason.

At todays high line voltages, the amp may be pulling more than one amp. (fully cranked)

[Bill Greene]

Not too sure that I like the term "bad fuse".

It seems to imply that the fuse itself failed for no reason.

Blown fuse, then.

[Bill Greene]

todays high line voltages, the amp may be pulling more than one amp. (fully cranked)

I'm afraid I don't understand this statement. Have U.S. line voltages changed since the mid-1960's?

That the Univox was pulling more than one amp would certainly explain the blowing of the 1A fuse.

What I seek to know is a suitable value for a replacement fuse for the U-45B.

[Jazz P Bass]

Yes, the mains voltage has increased.

In the 70's, 117Vac was the norm.

Today, the mains can & does go as high as 126Vac.

I would try the another 1 amp Slo Blo fuse.

If that blows again, then you either have an amp problem or you need to find a 1.5A Slo Blo fuse.