Univox U45-B..... Alchemy! Sh&t to Gold

 Ah.... the Univox U45-B. I like to call these the poor man's AC10.

Stock these SUCK. But with a little bit of modifying they are gold. I've worked on maybe half a dozen over the years.... curious little amps. Japanese made but always with an American old Jensen speaker loaded into them. 

So what's up with these. Why so crappy? They hum, badly. In fact you can remove all of the tubes, turn it on and you'll hear a hum coming through the speaker. 

So step one....they are under filtered. They have a 10uf well cap, 10 uf screen cap and a 3uf cap for the preamp stages. Be bold.... go with a 40uf well, 20uf screen and an 8uf preamp cap. Why not? I like a quiet amp that is punchy and clear.....

You will also want to replace EVERY signal cap. Japanese amps tend to use these sad paper in oil caps. They are no Sprague Vitamin Q caps. These are long dried out and dead. In fact every Japanese guitar amp or hi fi amp I've ever worked on had these and every single one of them was bad. (Maybe I'll make an article on the glorious Lux integrated amp I rebuilt recently?)

Change the load resistors, they are usually outta tolerance. That should do it right? NO! More to be done.

Here's where it get's interesting. That tube free hum. The power and output transformer are mounted right next to one another and not 90 degrees off like they should be. There isn't enough room inside to turn the output transformer so it's best to remove it and put it on the other side of the chassis. You'll need to replace the wires for the extra length. Typical of Japanese and German tube gear they have the high voltage exposed on the terminals. I don't understand why. It's quite dangerous! So make sure you heat shrink those connections!

Next....... These amps have a silly ground scheme. If you turn the amp to zero it hums. Midway up the hum is reduced..... further up it gets louder. Good ol' nasty ass ground loop.

I remedied this by cutting the solder trace between the preamp filter cap and the other two. I removed the ground wire coming from the board by the filter caps that connects to the power transformer HV center tap, filament center tap and output transformer ground. Sheesh! Stock all of this is grounded through the PC board to the input jacks!!! 

Trace cut, ugly but pre clean up!

I then added a wire from the heater center tap to the 250 ohm cathode resistor to float 10VDC on those heaters. 

I added another wire to the Hi Voltage center tap to the chassis ground (where I grounded the amp when I added a 3 prong cable.)

I added a wire to the well cap, screen cap and cathode resistor and soldered that to the chassis where the output transformer stood:

The preamp filter is grounded to the input jack along with all of the audio grounds. This is okay. 

Next.... I've read that there is a ground loop created by the foot switch jack on the front panel. You can disconnect it and tape it off to see. I tried this to no effect. I did however cut the ground wire between the tone pot and the tremolo pot. I then put that ground wire between the tremolo pot and the ground lug on the foot switch jack. 

But here's where the front panel gets funny. They use these fiber washers on all of the jacks and pots. They appear to look like they isolate the components from ground. Nope. All they do is make a comically poor ground connection. I replaced all of them with steel tooth washers. I could literally touch the jacks and hear hum come and go! Now they are tight like they should be. 

Oh yeah.... ground the whole thing and get rid of that stupid death cap. And make sure you have the power going to the 115v input rather than the Japan use 100V.

Anyway, now this amp is quiet. I mean, dead quiet. Turned up loud there it your typical gain noise you get from any amp. But now this amp is ready for your studio. And it is a killer. If you like a Vox amp but can't afford one this is your ticket to Liverpool baby. I would take one of these over any of the current Vox amps being made today. They can be found cheap. Once you pay a fella like me to get them straightened out they are no longer cheap but..... soooooo worth it. You'll have a very nice quality amp for your low powered gigs or studio use. Oh, and these do kick ass too. 12 watt's o' beautiful sound.

I'm of the opinion that the Japanese used very high quality transformers. I mean the hi fi iron made over there is probably the best in the world. Great iron equals great potential. I have yet to meet a Japanese amp I didn't like once rebuilt. Lux, Royal, Sansui...... Fantastic and undervalued. 

JB