Univox Amp Question

Sparky664 · 08-20-2008, 04:06 AM

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 · 08-20-2008, 07:03 AM

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

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

Brad1 · 08-20-2008, 02:18 PM

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 · 08-20-2008, 03:30 PM

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 · 08-20-2008, 03:37 PM

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 · 08-20-2008, 11:21 PM

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 · 08-21-2008, 04:11 AM

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 · 08-21-2008, 11:15 PM

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 · 02-14-2009, 07:23 PM

Quote:

Originally Posted by 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.

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

08-20-2008, 04:06 AM	#1
Sparky664 Junior Member Join Date: Aug 2008 Location: South Carolina, USA Posts: 5	Univox Amp Question 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.

08-20-2008, 03:37 PM	#5
jrfrond Senior Member Join Date: Apr 2008 Location: Oceanside, NY Posts: 609	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. __________________ *John R. Frondelli* dBm Pro Audio Services, New York, NY

08-21-2008, 04:11 AM	#7
jrfrond Senior Member Join Date: Apr 2008 Location: Oceanside, NY Posts: 609	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. __________________ *John R. Frondelli* dBm Pro Audio Services, New York, NY

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08-20-2008, 07:03 AM	#2
Enzo Senior Member Join Date: May 2006 Location: Lansing, Michigan, USA Posts: 9,266	Try the U-45b, Schematic Heaven has it. http://www.schematicheaven.com/barga...nivox_u45b.pdf

08-20-2008, 02:18 PM	#3
Brad1 Senior Member Join Date: Jun 2006 Posts: 204	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

08-20-2008, 03:30 PM	#4
cbarrow7625 Senior Member Join Date: Mar 2007 Location: Philadelphia suburbs Posts: 364	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

08-20-2008, 11:21 PM	#6
Sparky664 Junior Member Join Date: Aug 2008 Location: South Carolina, USA Posts: 5	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.

08-21-2008, 11:15 PM	#8
Sparky664 Junior Member Join Date: Aug 2008 Location: South Carolina, USA Posts: 5	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.

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