Univox U-45

[sepulchre]

I just found this video: https://www.youtube.com/watch?v=H27QxL8A4Qc

That's the first amp I ever owned. I thought it sounded pretty good then and this video makes me think I was right. So I'm considering building one. I found a schematic (attached) but, though it doesn't show the HT, it does show the PT secondary at 250-250 at 50 mA. I'm unfamiliar with 6BM8 tubes but I'd bet I could get NOS examples for cheap. The 12AX7 is only used for the tremolo! And the 6X4 is common enough, I think. I even found a modded schematic that allegedly gets it closer to the Gibson GA8-T it's supposed to be patterned after.

There's a U-45 and a U-45b. I don't know the difference - some say the 45 is single ended and the 45b is p/p. The schematic here is the 45b. The one in the video looks just like the one I got when I was 15 (used - a year old) so I'm thinking that mine was the 45b.

Anyway, just wondering if anybody has any experience with these or thoughts about them.

[joshdfrazier]

hmm 6bm8, i'm pretty sure those are 1/2 pentode, 1/2 triode. preamp and output in a single package! I'm a huge fan (maybe this is obvious lol) of univox amps. I have played through a PP U45, and they are really great sounding amps. However, I would try to snag an original on ebay. I see these go for no more than $300 from time to time. You can't build "mojo" haha.

[sepulchre]

Well I can't afford $300 for an amp that will mostly be something just to play around with. I've got amps and couldn't get past the wife factor for that kind of cash - too close to retirement.

So I'm going to give it a go. I bought a matched pair of NOS 1970 Svetlana =C= 6BM8s (6F3P), and an NOS American 6X4, 60s I think. Coming up with a chassis shouldn't be too hard and there's lots of parts in the cabinet. So now I'm looking at iron - maybe Edcor or Classictone. I think I can get it to sound decent enough with a little TLC.

[statorvane]

Guys over on PPwatt are doing some interesting stuff with 6BM8s.

FYI.

[sepulchre]

Thanks! Maybe they can point at some appropriate iron.

This needs a 16K primary 15 watt pp OT. I'm not too swift with iron so, if I were to use an 8K primary with a 4R secondary but hook it to an 8R speaker to get 16K at the primary would that half the wattage rating?

[jazbo8]

Well I can't afford $300 for an amp that will mostly be something just to play around with. I've got amps and couldn't get past the wife factor for that kind of cash - too close to retirement.

So I'm going to give it a go. I bought a matched pair of NOS 1970 Svetlana =C= 6BM8s (6F3P), and an NOS American 6X4, 60s I think. Coming up with a chassis shouldn't be too hard and there's lots of parts in the cabinet. So now I'm looking at iron - maybe Edcor or Classictone. I think I can get it to sound decent enough with a little TLC.

I found it is hard to build an amp that looks and sounds comparable to the original - certainly not for $300. Yes, I could build one for less money (not counting the time invloved) when the circuit is relatively simple, but by the time I factored in the cosmetic then it is usually a borderline case - and as mentioned there is no mojo factor - it's just another DIY amp. So I would put the cut-off point at perhaps $500-600, below that it just does not seem to "add up"... Just my 2c.

[sepulchre]

Really? Where would all that money go?

After checking around I've found iron for under $100, both PT & OT. The tubes were $38, NOS no less (not the 12AX7, I've got a few of those to choose from). I already have the pots, resistors and most if not all of the caps. Scrounging up a decent chassis will be cheap or free. Again, I have stuff around here.

I'm not concerned with the look. I suppose an elaborate combo cab could take a few bucks. But I can make a nice head cab for it with the scrap in my shop and plug it into existing speakers - got a perfect 1X12 box with a Greenback ready to go.

So I'll give it a go. If it works out well I'll post a clip. If not, well it'll be fun trying. But think it will sound just fine.

Btw, I think you can build mojo. Ohm's law works the same on my bench as it does on HAD's or KF's.

[Stevem]

Check that U45B schematic again, that first section of the AX7 is the first gain stage, the second section is the trem oscillator

[jazbo8]

I'm not concerned with the look.

Btw, I think you can build mojo. Ohm's law works the same on my bench as it does on HAD's or KF's.

The Look - that's the hardest thing for me - I can't build chassis or cabinets worth a sh*t, they just always look amerturish, so I really had to buy them already made which add greatly to the build cost.

By "mojo" I did not mean the electronics part, but rather the overall package, so again for me, if I can get a great looking/sounding vintage amp for $300, I think I'm actually saving money, obviously YMMV.

[sepulchre]

Check that U45B schematic again, that first section of the AX7 is the first gain stage, the second section is the trem oscillator

Yes, I saw that. But I think a good modern AX7 should do okay. If I had an NOS one laying around I'd throw it in but that's not the case.

@Jazz: Okay, gotcha. I love my wood shop and I feel sure I could do a combo box justice but for this project I think a nice looking head cab will do.

Thanks, guys! I'll post results as they occur. Might be a little while though. Gotta get through the current "To-do" list.

[sepulchre]

One question: the schematic shows the OT primary at 16K, kind of hard to find. So would 10K work alright? I see them used with EL84s which I believe have similar requirements to the 6BM8.

[joshdfrazier]

I believe the el84 has a much higher plate resistance than the pentode inside of the 6bm8. In my experience, not using the right OT for the job has resulted in bad/weird/useless tone. With that said, if you have the 10k OT on hand, it never hurts to try...

[sepulchre]

Hmm. I missed that on the data sheets. No, I don't have one, but they're easy to find. So I'll keep searching. Thanks!

[Stevem]

The rule of thumb on OT impedance is to take the tubes plate resistance spec and take 1/5 to 1/10 of that and things will work and not sounds like crap.

[Default]

I have the Realistic version of that amp, but with a 12" Jensen. It's a killer amp for low volume grind and I paid $175 for it. Check Fleabay and craigslist for Realistic and Lafayette guitar amps. They are Univoxes but sell for even less.