Univox U-1061

[Jazz P Bass]

I am putting out a request for some help on a modded Univox U-1061 bass amplifier.
When received the amp had a blown output transformer (original OT )
The 6550 big bottle tubes test good.
The diode array on one side was shorted.
The ot is bad.
I believe the amp was operated on the 8 ohm tap with a four ohm load.
Now onto the question:
This output section on this amp has been modded.
1: 100 ohm/ 2 watt plate sharing resistors: removed
2: 1.5K screen sharing resistors: removed
Items added:
1: four 50K grid resistors
2: four 47 ohm/ 5 watt screen resistors
3: a triple string of diodes (plate to ground)(251D 1000v/ 2.5)
4: .001/ 2000v cap across diode string
My question is : are these good mods?
Why did the OT blow?
Why is the diode string shorted?
What the he## is the .001 cap for?
I have attached the original & modded schematics.

[Enzo]

Why did the OT burn up? Your flyback diodes were shorted, grounding B+ through the primary until it burnt up.

Did they also modify the mains fuse to something larger?

WHy did the flyback diodes fail? WHo knows? Parts fail. They also might have been trying to do their job when. someone cranked the amp into no speaker load. That could also have lead to arcing in the OT and aubsequent failure.

What is the cap for? beats me. You sometimes see caps across rectifier diodes in power supplies, someone may have added them here emulating that. Or perhaps they were trying to kill some parasitic oscillations.

Good or not? Adding flyback diodes seems like a good idea to me, not sure about the cap. The resistor between tubes at both plate and control grid, that was to reduce intereaction of the tubes, PV used to do that on some amps. These guys made it more conventional, screen resistor per tube, grid stoppers. If it works it works. You now have 6550s instead of 6L6. No comment.

[Jazz P Bass]

The resistor between tubes at both plate and control grid, that was to reduce intereaction of the tubes, PV used to do that on some amps.

Thanks for the reply Enzo.
Would it be in the best interest to put the plate resistors back in?

[Enzo]

If it works it works. I'd have no reason to put them there, nor would I be motivated to remove them if they were there.

[Jazz P Bass]

It kinda sucks that the diodes that where put there to 'protect' the OT took it out.
I ended up buying a Hammond 1760W OT.
Took out the diodes & the .001 caps until & see what the amp is doing.
Test time!

[Enzo]

That was only a hypothesis, we do not know the true sequence of events. An unloaded transformer could have arced, destrying both diodes and transformer at the same time. we just do not know.

Flyback diodes are now on many if not most tube amps to protect against no load conditions. In my view they are like air bags or seat belts. Any problems they cause are mostly going to be far outweighed by their benfits.

[Bruce / Mission Amps]

It kinda sucks that the diodes that where put there to 'protect' the OT took it out.
I ended up buying a Hammond 1760W OT.
Took out the diodes & the .001 caps until & see what the amp is doing.
Test time!

Hard to imagine all three diodes in one string failed but I guess that is quite possible.
IMHO, the diode thing actually works fine... "IF" the center tap of the OT is also fed through a 500ma fuse link.
It will blow the fuse and protect the OT from catching fire most of the time.
Otherwise, no fuse, those shorted diodes are a Hi-V DC dead short to ground and the thin wire of the OT will be aching.

[Jazz P Bass]

Good idea on the 500ma fuse, Bruce.
I think the EH6550 big bottles where a bad idea.
They do not fit the socket spacing. The glass touches.
I found one that arcs internally when cranked.
It came from the side with the shorted diode.
The 6550's are powerfull, I'll give them that. 120 watts clean. 4 amps mains draw.
Which exceeds the rated 105W output of the amp.
The schematic also shows the heaters being rated at 4A's. Which the 6550's surely exceeded.
I am putting in a quad of Sovtek 6L6WXT tubes, leaving the mods (sans .001 cap) & adding an OT center tap fuse.

[chad1655]

I have the same amp and noted a broke 470 ohm 10 watt cement resistor. There are two of them tied together. I am not seeing them on the schematic. Anyone have any idea why they would be there? Any help would be GREATLY appreciated.

http://i102.photobucket.com/albums/m...655/photo3.jpg

http://i102.photobucket.com/albums/m...655/photo2.jpg

http://i102.photobucket.com/albums/m...partneeded.jpg

[Enzo]

What do the two ends connect to?

I'd wager someone wanted a 220 or 250 ohm power resistor and didn;t have one, so they paralleled a pair of 470 to make 235 ohms - close enough. I don't see one on the wschematic, but someone might have replaced a bad power supply choke with it. hence the question, what is it wired to?

[chad1655]

What do the two ends connect to?

I'd wager someone wanted a 220 or 250 ohm power resistor and didn;t have one, so they paralleled a pair of 470 to make 235 ohms - close enough. I don't see one on the wschematic, but someone might have replaced a bad power supply choke with it. hence the question, what is it wired to?

The one end connects to a capictor (Pict 2 below). The other end connects to a board (pict 1) where it appears to be connected to a chain of resistors then back to the transformer and/or power switch.

http://i102.photobucket.com/albums/m...a9197d0cd8.jpg

http://i102.photobucket.com/albums/m...655/photo6.jpg


3rd Pict is just for a different view of this side of amp. (Note, power cord replced with three wire vs. orginal two conductor). This was bought used so I have no idea what other mods were made and/or why. I plugged it in, tubes light, sounded fine, but weak (assume tubes). Upon opening up to clean pots I noted the broke cap.
http://i102.photobucket.com/albums/m...655/photo1.jpg

[Enzo]

AH, then as a guess I'd say it was a sag resistor added. The four green things the blue arrow points to look like rectifiers from here, so I'd guess them to be your high voltage rectifiers, and the resistor is wired from them to the main filter cap. It is not on the schematic. If I am right, it was added to emulate the dynamics of a tube rectifier. Or possibly to act as a current limiter for the rectifiers.

[chad1655]

AH, then as a guess I'd say it was a sag resistor added. The four green things the blue arrow points to look like rectifiers from here, so I'd guess them to be your high voltage rectifiers, and the resistor is wired from them to the main filter cap. It is not on the schematic. If I am right, it was added to emulate the dynamics of a tube rectifier. Or possibly to act as a current limiter for the rectifiers.

So should I replace with a 235 ohm, another 470 (x2) or take out completly?

[Enzo]

It may be cracked, but that could just be the cement. Does teh pair measure roughly 230-250 ohms? If so, leave it and see how well everything else works.

[chad1655]

It's split in half. the outer cement was off when I opened the amp and as soon as I touched it it broke in half. The bottom one seems to be fine. Curious if these are not on the schematic what would it hurt to take them both off. I could replace the broken one for a few dollars, but curious if the weak power (lack of volume) I got the 1st time I plugged it in was part of this problem. Gave virtually nothing for this amp and it did orginally sound good so have no problem sinking a "little" money in it.

P.S. I really appreciate your help.