Univox U-45B

[Shamus]

I've seen some info on these amps. I have one and was going to sell it but am considering keeping it now. It still has a 2 prong power cable. I peered inside at the guts and all the caps and transformers appear original.

Listening to the amp - it's tonally a darker amp. With my tele I find I like the tone knob nearly dimed. Also, when cranked the overdrive is a little too thick and without clarity.

I found this great website through doing a google search that landed me on a thread that talked about a simple mod.

What I would like to do is this:

1. retube the amp (does this require a rebias?)
2. get the trem working
3. do the mod to get the most out of the amp's natural overdrive.

I have a soldering iron and can get the tubes and such. Is there any easy way to learn how to do these mods myself? Of course, I know that this can be very dangerous.

That being said, I would like to learn the steps. I don't care to learn the electrical theory, just the steps which I can follow to do this safely and precisely.

Thanks!
Shamus

[PRNDL]

The first step is to find a schematic and then start tracing out the circuit inside the amp and identify what is what.

The first mod is to install a three-prong power cord and properly ground the amp, and remove the "death cap".

As for the tone being dark, that could be several things.

I'd begin by measuring all the resistors and replacing any that are way off. Next is to measure all the circuit voltages, which will tell you if any of the coupling caps and tubes are bad. A "cap job" may also be needed, along with bias, which may be set too high.

Depending on how old the amps and coupling caps are, replacing them with new ones often brightens an amp considerably, although some argue that it changes its unique tone that comes from really old caps.

Here's an interesting web page with photos
http://www.stevenfenner.com/Music/Univox/Univox.htm

[BobB]

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

[Dave Curtis, dB AudioTech]

The first step is to find a schematic and then start tracing out the circuit inside the amp and identify what is what.

The first mod is to install a three-prong power cord and properly ground the amp, and remove the "death cap".

As for the tone being dark, that could be several things.

These are voiced on the dark side, and tend to break up early.

I'd begin by measuring all the resistors and replacing any that are way off. Next is to measure all the circuit voltages, which will tell you if any of the coupling caps and tubes are bad. A "cap job" may also be needed, along with bias, which may be set too high.

I'd definitely suggest replacing all the electrolytics if they're original. I'd consider changing the power supply dropping resistors to flameproof metal, too. Actually, I changed quite a few resistors in mine. The originals weren't so good...

Depending on how old the amps and coupling caps are, replacing them with new ones often brightens an amp considerably, although some argue that it changes its unique tone that comes from really old caps.

Good post. In my amp, the old paper and oil Atlas caps were bad in the trem ckt, so I decided to replace all of 'em. Sounds pretty damn good. Mine is the later model with forward-facing controls; same ckt (chassis only).

On another note, about 10 years ago I picked up an older U45b cabinet that someone (guy I knew back in high school) had turned into a head by taking out the speaker and putting a homebrew PP 5881 power amp in the bottom and using the chassis as a preamp (totally rewired). Eventually, I used the chassis to build a PP 6GW8 using a Bogen CHB 20 power tranny, a very old Voice of the Music output tranny and one of the speakers from my Pro Reverb. It's a sleeper ;o) I had originally thought of restoring the cab with the original parts from the chassis, but I already slapped together a box for the chassis... [/ramble]

Here's an interesting web page with photos
http://www.stevenfenner.com/Music/Univox/Univox.htm

Cool!

[Dave Curtis, dB AudioTech]

This is the ckt in mine.

[Bruce / Mission Amps]

Seems to me that the Atlas oil coupling caps in these amps are kinda cheap/junky and anytime I have replaced them with inexpensive plastic caps such as Mallory 150s, the amps would come alive.

[capnjuan]

This is a Univox U305R; except for two oil-filled capacitors of unusual values, all of them have been replaced with either Mallory 150s or current make/model electrolytics. When the can cap was replaced, as soon as it was powered up, one of the three PI oil-filled caps let go ... phhhttt ... and small cloud of white smoke. No way to know how many more are nearly gone, so ...... Changed the bias rectifier from selenium to silicon and changed the reverb out/in connection from hard-wired to audio jacks; ridiculously difficult to get the chassis and reverb can out/in the cabinet. John

[tubessence]

Lafayette looks like Univox

Hi, I'm new to this site. It's quite informative, since I just got a Lafayette amp that looks like a Univox U-60A but seems to have the circuit of a U-45B! I haven't fired it up yet but I'll keep you guys posted. JB

[capnjuan]

Hi JB; nice get ... post some pics of the circuit board when you get a chance. CJ

[tubessence]

guts

Here's a shot of the circuit board

[tubessence]

Death Cap?

Where is the death cap that was mentioned earlier, is that the one coming off the trans. to gnd.? Or actually off the power in? JB

[capnjuan]

Hi; you may already know that some of the audio caps have been changed out from oil-filled to those orange drops and the polyesters(?). The three filter caps framed by the two large resistors can be easily replaced with F&Ts that will fit there pretty well. Remember to drain the old ones with a 5w resistor before fooling with them. If you snip the leads close to the body, you'll have vertical leads to tie the new ones to. Good luck with your project. CJ

[capnjuan]

Where is the death cap that was mentioned earlier, is that the one coming off the trans. to gnd.? Or actually off the power in? JB

If there is one connected from one of the 'power in' lines - either from the fuse post or the power switch - to ground, that's it. If there is no such cap, then it's the one on the lower left of your pic near the jack; if one end of it is tied to a lug that says 110V or leads back to the primary service and the other end to ground, then that would be it. I don't think there would be a need for both but don't forget that you'll also need a 3-wire cord for it ... sooner is better than later. CJ

[tubessence]

Site Found

Well, this amp is a Univox U-60A. I found a site with pics. http//tubeampinfo.com/univox
Thanks for the "death cap" info. JB

[capnjuan]

Done good; you're on your way. CJ