Univox Amp Question

[Sparky664]

I have an old Univox tube amp I got at a yard sale. After some cleaning and a half a can of tuner cleaner, the amp works (kinda). It has no model number on it although there is a tag inside that says made in Japan. Four tubes, a 6X4, two 6BM8, and one 2AX7 (12AX7A I assume). On the front a volume, tone and tremolo knob. Three inputs for guitar, acc and aux. A footswitch jack and on/off toggle switch. The speaker is a 12" Jenson. Does anybody know the model or name of this amp and does anyone have schematics for it? I think it will sound pretty good after I replace the tubes. Any help would be greatly appreciated.

[Enzo]

Try the U-45b, Schematic Heaven has it.

http://www.schematicheaven.com/barga...nivox_u45b.pdf

[Brad1]

Does it look like this?





http://www.univox.org/amps/uniamp.html

I have one. Crank it up to patent-pending and dig that early-60's Stones crunch!

Brad1

[cbarrow7625]

Sparky,

I have the same amp. I love it. It's pretty basic but has a nice sound to it. That Jensen "special design" speaker in it is the best sounding speaker I have ever played through. It is only rated for about 15 Watts of power. It is far sweeter sounding than the new alnica 25W Jensens. I had to find out the hard way after I accidentally blew out the voice coil on mine. (I still have it if anybody knows someone who can fix it for me I'd be grateful - I've had no luck).

I found that all of the small oil-or-whatever-they-are-filled caps in the circuit were horribly dried out & leaking DC voltage like crazy between stages. I replaced them all and now it works great. I used 715 orange drops but I wouldn't recommned them. Not nearly as sweet sounding as whatever was in there before. Maybe this is a case where the mallory's would do better if there is space.

If you wnat to replace the speaker in it, I'll happily buy the old one from you.

Chris

[jrfrond]

These are great amps! I am a BIG Univox tube amp fan.

The ONLY mod I would make to this amp (aside from recapping it, which is not a mod, it's maintenance) is right at the input. There is a .01uF series cap and 4.7M resistor to ground on the first stage, providing a "contact bias" scheme. This scheme is fine until you hit it hard with either powerful humbuckers or booster of some sort. The first stage then becomes prone to "blocking distortion", which is a condition where the capacitor cannot discharge fast enough and the grid of the tube builds up an increasing DC sharge as a result. Eventually, the tube gets driven into cutoff, and the signal gets "blacked", hence the name. It's a real nasty, raspy sound that results.

It would be best to remove the cap, add a jumper in it's place, then open up the cathode connection to ground and add a 1.5K resistor paralleled with a 22uF cap between the cathode and ground. This more ubiquitous setup makes the amp more user friendly, and just sounds better.

[Sparky664]

Wow! Thanks guys, didn't expect replies this quick. Yep, thats the amp.
The schematic is a big help already. Thanks for the mod tip, I did notice that when I ran my acoustic through it last night that it cut out when I had the volume cranked up on the guitar. Now for some tubes and caps. Thanks again for the help. When I get this done I've got a Super Twin I'm gonna need some help with before I trade it for something a little less plaster crackin' LOUD.

[jrfrond]

I made one mistake by omission with the bias mod: R4 (4.7M), stays where it is (from grid to ground), but get changed to 1M.

[Sparky664]

Thanks John, I'll make a note of that. If this tropical storm pushes on up into SC, I'll have some time to play with it this weekend.

[sidoog]

I have an old Univox tube amp I got at a yard sale. After some cleaning and a half a can of tuner cleaner, the amp works (kinda). It has no model number on it although there is a tag inside that says made in Japan. Four tubes, a 6X4, two 6BM8, and one 2AX7 (12AX7A I assume). On the front a volume, tone and tremolo knob. Three inputs for guitar, acc and aux. A footswitch jack and on/off toggle switch. The speaker is a 12" Jenson. Does anybody know the model or name of this amp and does anyone have schematics for it? I think it will sound pretty good after I replace the tubes. Any help would be greatly appreciated.

The amp is...UNIVOX U45-B

Schematics are available on this site & On-Line
TUBES
6x4, power...
12AX7,(1/2 inputs & 1/2 Tremolo)
(2) 6BM8...Mixer & Power out

[Boogyman]

These are great amps! I am a BIG Univox tube amp fan.

The ONLY mod I would make to this amp (aside from recapping it, which is not a mod, it's maintenance) is right at the input. There is a .01uF series cap and 4.7M resistor to ground on the first stage, providing a "contact bias" scheme. This scheme is fine until you hit it hard with either powerful humbuckers or booster of some sort. The first stage then becomes prone to "blocking distortion", which is a condition where the capacitor cannot discharge fast enough and the grid of the tube builds up an increasing DC sharge as a result. Eventually, the tube gets driven into cutoff, and the signal gets "blacked", hence the name. It's a real nasty, raspy sound that results.

It would be best to remove the cap, add a jumper in it's place, then open up the cathode connection to ground and add a 1.5K resistor paralleled with a 22uF cap between the cathode and ground. This more ubiquitous setup makes the amp more user friendly, and just sounds better.

I know this is an old thread but I'm a n00b (1st post) and just purchased a U255R 2x12 on eBay. Just wondering if the input on mine is the same as the U45B and this mod would apply? Thanks for any help!

[tboy]

Hi Boog', and welcome to the site. I just looked at the schematic for the U255R, and it uses conventional cathode bias on the first stage rather than the contact bias method in the U45B, so the mod in question wouldn't apply to your amp.

I would think about changing the input scheme though, because the input impedance is rather low for guitar, and there's a voltage divider for the MIC and GUITAR inputs that throws away half (or more) of your signal before the first preamp stage. There's also a bit of a design flaw in that if an AC coupled device (like a pedal, or a guitar with active electronics) is plugged into the ACC input, the grid of that first gain stage is left floating.

As an experiment, try plugging your guitar straight into the ACC input, with no pedals or anything else in between. That way the first stage will get it's ground reference through the guitar wiring, and you can hear what it sounds like without the loading and padding of the GUITAR or MIC inputs.

[Boogyman]

Thanks tboy. Yes I notoced on the schematic the (impedance?) value is higher on the guitar input. Also the ACC input is switched, correct? So what if I removed the (resistor?) on the guitar circuit?
I have to confess I'm not a real electronics guy, I'm just a tinkerer trying to learn as much as I can. I was an auto mechanic by trade before retiring, but I've played guitar for 45 years and picked up a few basics messing with my own gear. I understand what impedance is, how to parellel/series speakers, and use a multimeter to find faults and repair basic stuff, but when it gets deep into the circuits I'm lost.
Also the amp hasn't arrived yet, so I'm not able to perform the test you suggested yet. I'm anticipating this amp is going to need the capacitors replaced and a good going over from what I've learned through research, so I'm trying to get ahead of the game for when I'll probably have to take it to my 'guy'. He's actually a TV repairman about my age (50-something) who knows a few things about the old tube technology having worked with it most of his life.
Maybe I should start a new thread on the U255R when it arrives, as a running forum project for anyone else who might want to mod this amp as I intend to do. I've read that it's actual output is only around 15 watts even though it was rated at 40 by Univox, so I'm pretty sure I'll want to hotrod this thing a bit. I hope to learn a lot in the process thanks to the gurus on this forum.

[tubessence]

U-45b inputs

The inputs on my u45b all look the same to me, am I missing something? jb

[tubessence]

Never mind I thought you were still talking about the u45b. jb

[J M Fahey]

If you have a U45B as depicted, yes, do the mod.
You should also lift the grounding contacts on every input jack except on the guitar one.