Assistance with output transformer in U-45B?

[DocGeorge]

I realized that this post really belongs here- I originally posted it in Guitar Amps- I actually once took a course in tube circuits and have worked as a wirer-and-assembler, and I know that the output transformer is critical to the sound of a tube amp..............


I bought a Univox U-45B a while ago- I have wanted one for years, ever since I played through one- BUT when I got it (via ebay) the output transformer was shot. Does ANYONE know what make and model of transformer this is? (It is totally unmarked!) According to the schematic, the input impedence is 17k. I was going to rewind it myself, BUT I have never rewound a transformer, and IF I can find a real replacement, I would rather do so. Various people have recommended using a generic replacement, saying that it wouldn't make any difference, bit I know that that isn't so. IF any of you have played through a U-45B, you know why I want to get this right. Thanks for any help...talk to ya....

Doc George

[Bruce / Mission Amps]

I'd stick a universal Hammond 125E OT in there and wire the thing so your 8 ohm speaker will reflect up around 13K-18K.

http://www.hammondmfg.com/125a.htm

the impedance chart:
http://www.hammondmfg.com/125.htm

[Synaptic Amps]

Not sure that they have any exact replacement but you might send it down to the folks at Mercury Magnetics. They offer a free service to test failed transformers and ID the cause of failure. They do vintage restorations as well which might be worthwhile for you cause I'm guessing that might be a somewhat rare OT. Here's another schematic I found on the forum, it might be a little different then the one you've got already.

[sportster4eva]

That's the right schem. I have the same amp. If you get an all nylon pot you can mod the amp to have a tremolo intensity control. put it in place of R21 and connect it's wiper to the junction of the 220k grid resistors. I used the 3rd input hole as the spot for the pot. You have to cut a trace on the PCB to do this. If you don't use an all nylon pot it will hum. I tried it with a regular 500k pot, and thought it was fine until I bolted it into the chassis.

[jrfrond]

On the cheap, you will get close with an OT from a Blues Jr.

[J Martin]

Those transformers were all made in Japan in the 60's. Its unlikely you will find an original. The two suggestions above are good, but if you like the amp as much as you say, you will want to have a copy built.

[Synaptic Amps]

Here's the link to Mercury Magnetics vintage restoration page - I've never dealt used this service of theirs so I have no idea what it would cost but I expect it would be the best way to retore your OT to new working order preserving your original iron.

http://www.mercurymagnetics.com/page...ercury_Vintage

[Brad1]

I've got a U45B, and it's a nasty little critter (in an early-Stones sounding good way) cranked up.
Bruce did an easy tweak on the tone circuit to get a hair more highs, but other than that, it's stock. Definitely worth fixing, if not costing TOO much.

I'll have to check out that tremolo mod.

Brad1

[unclebubba]

I had a problem with my U 45 amp and found that one of the wires going to the winding of the output transformer had become very brittle and had snapped but the break was barely visible without a magnifying glass. A quick touch with my soldering iron and a small amount of heatshrink to cover the solder joint did the trick. I had the exact same problem with a larger univox amp.
Hope this helps. These cheap transformers become very brittle over time.

Larry

[capnjuan]

I'd stick a universal Hammond 125E OT in there and wire the thing so your 8 ohm speaker will reflect up around 13K-18K.

http://www.hammondmfg.com/125a.htm

the impedance chart:
http://www.hammondmfg.com/125.htm

+ 1 on the Hammond ... or could be a Weber W022913. I've used both; too good and too inexpensive to consider anything else. J

[MarkC]

I had a problem with my U 45 amp and found that one of the wires going to the winding of the output transformer had become very brittle and had snapped but the break was barely visible without a magnifying glass. A quick touch with my soldering iron and a small amount of heatshrink to cover the solder joint did the trick. I had the exact same problem with a larger univox amp.
Hope this helps. These cheap transformers become very brittle over time.

Larry

The exact same thing happened to my u45b. Twice. The first time I had an amp tech fix the problem. The second time I fixed it myself after a drunk bandmate caused it to take a fall. . . .

MarkC

[TPM]

I've got a U45B, and it's a nasty little critter (in an early-Stones sounding good way) cranked up.
Bruce did an easy tweak on the tone circuit to get a hair more highs, but other than that, it's stock. Definitely worth fixing, if not costing TOO much.

I'll have to check out that tremolo mod.

Brad1

I know this thread is ancient (4 years old), but do you have any more info the "easy tweak" to increase the high's? I just bought a U45B, and everything works great, but it's pretty dark...I'd love to mod it to increase the high's. Brad1, I PM'd you as well...thanks all.