Need Help with Old MIJ Tube Amp (Univox?)

[STLMatt]

I have an unlabeled, late 60/70's mij tube amp that I got when I started playing guitar 25 years ago (I think it's a univox design). I never really used it though because it's always sounded pretty bad. I want to restore it to like new condition because I've heard good things about the old univox tube amps, and I'd like to know more about it and source the schematics if possible. I'm a mechanic and I'm comfortable with soldering and using a multimeter and know basic electrical theory like ohms volts amps and circuits, but I've never really gotten into electronics. I could really use some advice in terms of what the best procedures are in terms of replacing the caps or other components and if there are any other things I ought to do now that I've got everything apart. For instance, I don't really have a handle on the difference between foil and electrolytic caps or specifically what kinds are used for amps. So, to start with, what kind of caps should I use to replace the old ones with and what are the guidelines if I can't find new ones that match the values of the originals? Should I just replace all the caps and resistors entirely? I don't really care about perfectly replicating the original sound or anything like that. I just want to turn this thing into a safe, usable, reliable amp without spending too much money.

I've got some pictures and these are the features and what I know about it so far:

-It has no labels at all except for "muscal amplifier" model H printed on the pcb board. Muscal is a typo that seems to show up on a lot of these old mij univoxes, lafayettes and some other brands. It looks almost exactly like a Realistic model 212, and sort of like the univox u-320 head unit. I also saw a Lafayette amp that had the same pcb board layout on google.

-2x12 vertically mounted speakers.

-2 channels with hi and lo jacks for each channel for a total of 4 guitar inputs; tone and volume knobs for each channel

-reverb and tremolo with speed and intensity controls

-6 tubes: 2x 7591 ; 3x 12ax7a ; 1x 7247 The 12ax7a are mullard and the other 3 are GE

-Badly frayed 2 prong cord which will have to be replaced with a 3 prong

Thanks! (And Happy Thanksgiving!)

[52 Bill]

Welcome to the place.

I agree that it is a typical Japanese made amp, could be a Univox or a Lafayette, etc. There probably was a name on the original grill that has been replaced. The outer shell also looks like it has been recovered. The speakers appear to be Radio Shack replacements, probably made by Utah.

What is wrong with the amp now?

As an answer to your question regarding caps, typically film or signal caps will be nonpolarized and have fairly small values under 1 uF (micro farad). Electrolytics will have positive and negative terminals and have larger values like 22 uF or 47 uF.

I don't recommend replacing all caps or any parts until you know what the problem with the amp is and whether or not it is worthwhile to invest time and money into the project.

[STLMatt]

Thanks Bill, well, the reason it's apart right now is because the power cord is badly frayed and needs to be replaced. It's loud and everything works on it, but the sound from it has always been very poor. It's very staticy and it drifts. It's kind of like trying to tune in music being broadcast from across the ocean on an old shortwave radio. Also, it's a big powerful amp and the lack of grounding and the "deathcap" issue has always made me a little leery of the thing. I'd like to finally correct all these things now. As far as the time and money aspect goes, the tubes and electrical components seem to be all readily available at reasonable prices and I've been wanting to dig into a project like this for a while now. But that doesn't mean I want to spend time or money that I don't need to either. If I can rejuvenate the thing by doing some tests and replacing a tube or a couple caps that'd be great. My thinking though was that this is an almost 50 year old amp and that the atlas oil and nippon chemi-con caps were likely shot or that they soon would be with any regular playing regardless, and that replacing the electronic components would be cheaper than a new set of tubes. I'm really sort of hoping that the old vintage tubes still have some life. What do you and the forum think though and what to start with from here?

And as for the electronic components, based on the polarity, the values, visual appearance and so on it should be fairly obvious what I need if I have to order this stuff from digikey or wherever?

It's funny you mention the radio shack speakers because I've got the ad for the Realistic version of the amp and they make a big deal out of the fact that it's got Jensens. It'd be nice if someone can link it to a more popular model from univox or another manufacturer that has a schematic available.

[MarkusBass]

The amp looks like this one: https://reverb.com/item/29415-univox...-tube-amp-rare
A good demo of similar amp is here:

[Jazz P Bass]

Did not have any luck searching for "Univox/ Realistic 212 schematic".

The closest that I can figure is the Univox 255R: univox255r.zip

[nevetslab]

Well, you're in luck in that the 7291 Power tubes are readily available, as is the 7247/12DW7 dual triode. And, in luck that the bottom of the chassis appears to be open enough to have access to all of the solder connections of the PCB components.

Replacing the power cord is a fairly simple task. The hardest part is opening the plastic strain relief that anchors the power cord within the chassis. I'm lucky enough to have a Heyco strain relief tool, so releasing them is simple. Going at it with common pliers is a task for sure. You'll want to disconnect the AC mains wires to the Ground Switch, so it is taken out of circuit permanently. Be forewarned that it may be used as connection points in the AC mains wiring, so eliminating it may take some re-wiring, or perhaps involve using insulated in-line wire splices to bypass it. If the strain relief is similar to the Heyco parts, which fit into a double-D hole, you'll have to select a new power cord that will fit the strain relief. Once that has been acquired, you'll want to attach the Ground wire of the new 3-wire power cord to a su9itable ground lug. This should go directly to the metal chassis, using an attachment point that is NOT being used by other items. I'm guessing there is some rust or oxidation on the chassis, so the ground connection point should be clean.

With new power tubes, preamp tubes installed, you're at a good starting point to see what the amp sounds like now.

I'd advise tracking down the schematic for this amp. I notice there is an adjustment pot on the rear panel. I'm assuming this is for adjusting the bias of the power tubes, though it could be a hum balance control. If it's wired to the heater wires, it's a hum balance pot. If it's wired to a negative voltage circuit, it's the bias pot. I'd have to look up what typical plate currents are for the 7291 power tubes.

The front panel controls will all need cleaning. I normally take controls apart at that age, clean the resistive wafer, then re-lube the shaft and apply a good contact cleaner. You could try a spray like Caig DeOxit 5 for starters, and exercise the controls a lot until they become quiet. If they remain staticy-sounding, they could either need replacing, or there may be some DC leakage voltage present on them. DC on audio controls makes noise when you turn them.

A full cap replacement job is probably in order, as you're thinking, but since it does work, I'd begin with replacing the tubes and cleaning the pots. That will tell you a lot.

[Jazz P Bass]

I would hold off on purchasing 'new' 7291 tubes.

They are NOT the same as the originals.
(they sound like ass)

[g1]

Also, as well as cleaning the controls, clean the tube sockets. Those 2 items alone could be responsible for your static type noise.

[sgelectric]

I believe jrfrond was a service rep for Univox at one time.
He has not been active in a long time but is a wealth of info re these amps
He helped me with a U-45 once.
I hope he is OK
Might try PMing him.

John R. Frondelli
dBm Pro Audio Services, New York, NY

[nevetslab]

I would hold off on purchasing 'new' 7291 tubes.

They are NOT the same as the originals.
(they sound like ass)

That's good to know. I've never come across an amp that was using them, so had no first hand knowledge of them. When I briefly looked for them on line, and found a J/J data sheet, the schematic for the tube looked wrong. Pins 1 & 4 are shown in parallel, while Pin 1 is labeled g3, Pin 4 is labeled pin 2. Then, Pins 5 and 8 are shown in parallel, with Pin 5 labeled as g1 and Pin 8 as K, but connected to the middle grid where Pins 1 & 4 are connected! Pin 6 is also labeled as g1, and shown as the input grid on the symbol. Pins 2 & 7 are heater connections. Very strange to see something so conflicting on a J/J spec sheet! Also, this was shown as a 7591 power tube. My typo in my other post.

[Jazz P Bass]

Looks as if I 'copied' the typo.

Yeah, JJ 7591's suck eggs.