Univox U-45B

[tubessence]

Sweet!!

Man, I plugged her in, and Holy C&@p what a sweet amp! Totally quiet and has that sound that just makes jaws drop! The trem doesn't work at all, as everybody said! So I guess I'll try another 12AX7, wait a minute, it looks like you have to have a footswitch for it to work! Maybe i'll get even luckier! I would recommend one of these to anybody! Easy to work on too, everything is accessable with just two bolts. Sweet!! jb

[capnjuan]

any single button SPST (on off) footswitch will do.

[tubessence]

Thanks

Thanks, that's what I thought. I was looking at the schematic as I was typing the last letter! jb

[tubessence]

U-60A schematic?

As it turns out, the U-60A circuit is actually slightly different than the U-45B in a few minor details. Does anyone have any idea where to get the correct circuit? If not I guess I'll have to figure it out. When I do I'll post it. jb

[Dave Curtis, dB AudioTech]

link corrected:
http://tubeampinfo.com/Univox/U60.htm

[Dave Curtis, dB AudioTech]

As it turns out, the U-60A circuit is actually slightly different than the U-45B in a few minor details. Does anyone have any idea where to get the correct circuit? If not I guess I'll have to figure it out. When I do I'll post it. jb

Compare your circuit to the one I posted earlier; this is a "compared & corrected" U45b schematic I did when servicing mine, which has forward-facing controls and is likely the same amp as yours.

[tubessence]

Where did you post that? I can't seem to find it. Thanks jb
BTW on the backside of my board it does say U45

[tubessence]

OK! I found it! Are those the values and circuit that came in your amp or was that the circuit as it came with some values changed by you? That's definitly the same amp as mine. jb

[Dave Curtis, dB AudioTech]

Those are the values as recieved, with the exception of the 3-wire AC supply. I'm pretty sure mine has U45 on the board as well; it's the same board, the circuit is just a little different. The NFB loop being the biggest difference. There was a newer resistor in one spot, so I don't know if that one is an original value.

[tubessence]

Dave, Yours seems to be slightly more evolved than mine. I've got the same circiut but the values in some spots are more like the original U-45, mainly in the power supply mine still has 10/10/3. One thing I wasn't sure of on your drawing is the resistor in the NFB that couples to the moved bias/filter, you have it labeled 100, is that just 100 or 100K? Also did yours have a grounding jack for the trem? Thanks, I hope yours sounds as good as mine, I'm reluctant to change anything but the signal path caps right now. JB

[Dave Curtis, dB AudioTech]

Dave, Yours seems to be slightly more evolved than mine. I've got the same circiut but the values in some spots are more like the original U-45, mainly in the power supply mine still has 10/10/3.

Oops. I changed those. They were originally like yours.

One thing I wasn't sure of on your drawing is the resistor in the NFB that couples to the moved bias/filter, you have it labeled 100, is that just 100 or 100K?

It's not really moved; I just didn't have room under the triode for the 100 ohm resistor (and I couldn't find the white-out). It's possible that I may have misdrawn that; the NFB loop may well tie in at the juction of the 100R and R6/C5. I'll have to check (it doesn't look right).

Also did yours have a grounding jack for the trem? Thanks, I hope yours sounds as good as mine, I'm reluctant to change anything but the signal path caps right now. JB

The trem switching is correct; there's a switch on the pot and the switched jack.

[Dave Curtis, dB AudioTech]

Dave, Yours seems to be slightly more evolved than mine. I've got the same circiut but the values in some spots are more like the original U-45, mainly in the power supply mine still has 10/10/3.

Oops. I changed those. They were originally like yours.

One thing I wasn't sure of on your drawing is the resistor in the NFB that couples to the moved bias/filter, you have it labeled 100, is that just 100 or 100K?

It's not really moved; I just didn't have room under the triode for the 100 ohm resistor (and I couldn't find the white-out). It's possible that I may have misdrawn that; the NFB loop may well tie in at the juction of the 100R and R6/C5. I'll have to check (it doesn't look right).

Also did yours have a grounding jack for the trem? Thanks, I hope yours sounds as good as mine, I'm reluctant to change anything but the signal path caps right now. JB

The trem switching is correct; there's a switch on the pot and the switched jack.

[tubessence]

I'm going to change the trem jack to a switched one just for conveiniance. I was playing it today and it had alot of scratchy sounds even with the vol down, so I guess I'll have to change out the power supply caps. Plus I might as well change out all those oil caps in the signal path as long as I'm in there. I'll keep you posted. Thanks for all the help. jb

[jonyguitar]

death caps

Having exchanged many different views from many techs, all with mod, bias and board experience, altering the electronics of anything "vintage" needs consideration.Having performed on many stage sets and worked with many stage managers and Marshall valve amps, I have never encountered a voltage problem that working polarity switches could not cure in the 100 watt amps with 2 prong cords.

The opinions are split down the middle. Just keep in mind it is not the voltage that kills , it is the completed circuit! Work with form fitted rubber gloves around the valve amps, devoid of watches,jewelry,rings etc. and only one hand inside at a time for good measure, a static foot strap is good for any chips that may be on board newer valve state configurations. It is not that I like telling it as it is...but caution is key here.

If you step to a mike with a guitar and valve amp use a wind screen if you want to play it safe! Older vinatge amps were not configured to electrocute players....the liability would have been enormous ...some techs will work inside the 2prong amps and some won't! This issue has been way over blown! Just be smart and remember "vintage "means as it was and still is! Origional tubes were IEC (actually mullard tubes ) Mullard tubes if you could find them, are very expensive but gave the amp the intended unique tone, also preamp tubes wane and take fat tone with them until you replace them!...nature of the beast!

[Dave Curtis, dB AudioTech]

Having exchanged many different views from many techs, all with mod, bias and board experience, altering the electronics of anything "vintage" needs consideration.Having performed on many stage sets and worked with many stage managers and Marshall valve amps, I have never encountered a voltage problem that working polarity switches could not cure in the 100 watt amps with 2 prong cords.

So you're saying that a life is worth a 20 cent capacitor that might just short when it fails. Excuse me, but keeping the customer alive needs consideration. Snip the cap and install a $10 cord. I'll remind you that there is no polarity switch on a U45b. Updating the line cord and switching on an old death trap needs no consideration; it's a life safety issue. If the customer refuses the 3-wire cord, they sign a waiver. So far, none of my customers have signed one. I don't think you'll find many techs on this board that advocate keeping the 2-wire cord on a death trap. If you're gonna use the amp, get it updated.

The opinions are split down the middle. Just keep in mind it is not the voltage that kills , it is the completed circuit! Work with form fitted rubber gloves around the valve amps, devoid of watches,jewelry,rings etc. and only one hand inside at a time for good measure, a static foot strap is good for any chips that may be on board newer valve state configurations. It is not that I like telling it as it is...but caution is key here.

I don't care about opinions, the fact is that 2-wire cords on equipment with exposed metal have killed people; that's why they're not allowed these days unless the unit is double-insulated (check the code book); they're hazardous. BTW, I just completed a circuit through both hands, but since there was no voltage, I didn't die. It's the current that kills, but you need high enough voltage to push it through the body. Be careful using that ground strap when working on a tube amp...
Any amp with "chips" in it are also going to have a grounded cord.

If you step to a mike with a guitar and valve amp use a wind screen if you want to play it safe!

Yeah. And make sure you're wearing HV gloves or don't be grabbing your strings when you go to adjust that mike stand... If you want to play it safe, put a 3-wire cord on the damn amp.

Older vinatge amps were not configured to electrocute players....the liability would have been enormous ...some techs will work inside the 2prong amps and some won't! This issue has been way over blown! Just be smart and remember "vintage "means as it was and still is! Origional tubes were IEC (actually mullard tubes ) Mullard tubes if you could find them, are very expensive but gave the amp the intended unique tone, also preamp tubes wane and take fat tone with them until you replace them!...nature of the beast!

Old amps were made with 2-wire cords because they were allowed. They're not allowed anymore, and haven't been for a long time. Remember why. And no, that's not what "vintage" means. What you mean is "all stock". To me, the term "vintage" means nothing without a year. It's overused to death, and just means "old" to most people, it seems.

Vintage:

1 a (1): a season's yield of grapes or wine from a vineyard (2): wine ; especially : a usually superior wine all or most of which comes from a single year b: a collection of contemporaneous and similar persons or things : crop
2: the act or time of harvesting grapes or making wine
3 a: a period of origin or manufacture <a piano of 1845 vintage> b: length of existence : age

The tubes that came in the U45b were Matsushita; made on Mullard equipment in Japan. BTW, the 2-wire cord was made by Kawasaki...

-Dave
1960 vintage
;o)