univox u1511 restoration

[zelenko]

Hello, i have picked up an univox u1511 all tube amplifier from 70's. so im restoring it to the original state and want to make it usable for gigs and etc.
Ive found schematics on the internet http://www.4tubes.com/SCHEMATICS/Music- ... /u1011.gif and they says everything is the same as the u1011 model except the 6550 tubes which u1511 uses.
there is one strange issue the plate voltage is 750 volts is it save for 6550 tube to work like this ?
and there is one strange resistor that used to connect one of the filaments with pin 8 on other power tube ( look at the pictures ) and the other fillament was grounded. can someone give me any clues what to do with this? and what is it for?
im plannig to change everything, tube sockets capactitors rewire and do a full restoration because chassis is in very bad shape, rusted, and need to remove everything so i can clean it up, and everything seems ok and as on schematic except for these thing i mentinoned.

Thanks in advance

[sliberty]

I can't answer your questions, but I have to wonder...
Have you heard one of these amps?
Generally speaking, the blue series of Univox amps didn't sound like much. I don't know the 1511 in particular, but the 1011, much like others in the line, were rather blah. Loud and clean, but without much character. They are better as bass amps than as guitar amps in my opinion.
If it were me, I would turn it into something different. Since you are gutting it anyway, you could do whatever you want to it.

[Stevem]

That schematic appears to be screwed up as it is only showing the 1.5k pin 5 grid resistors for the second of each pair of output tubes.

That upper right hand tube socket seems to be wired wrong and there are no screen resistors in the schematic which may be ok for 6L6 type tubes, but not for 6550s.
Your voltage reading of 750 is likely due to the having the output stage up and running in which case its current draw will lower that voltage down.
The max plate voltage a 6550 is rated for is 660 and its screen voltage is rated at 440.
I would start by removing all the wires from all 4 sockets and look very close for signs of acring, next I would do the low ac voltage test out on the OT that you can find on line and confirm that the OT is sound.

If that checks out then replace any iffy output tube sockets, rewire and then confirm that you have at least a minus 60 volts on each and every pin 5 before you load in the output tubes and flip her on!

If you want to stay on the safe side and not burn out the OT while you get things up and running then place a in line 1/2 amp Fast blow type fuse in the OTs power supply center tap.

[zelenko]

ok im going to test PT outputs and let you know. Is there any way to modify the power supply to drop the voltage?

ive removed everything and cleaned the repainted the chassis as it was. i will start rebuilding everything 2nt and let you know of the progress.

Found the pictures on the net. This is a bass amp but it has the 6550 tubes in the output and the wiring is very similar to mine. Same resistor is wired between filament and 8pin

[martin manning]

That schematic appears to be screwed up as it is only showing the 1.5k pin 5 grid resistors for the second of each pair of output tubes.

But that is exactly the way it is wired in the photo.

That upper right hand tube socket seems to be wired wrong and there are no screen resistors in the schematic which may be ok for 6L6 type tubes, but not for 6550s.

I think the wiring is ok on that upper right socket, except that the plate lug is completely burned off. 6550 is a beam power tube just like a 6L6; screen resistors are a good idea with either one.

[Stevem]

If you mean drop the voltage down enough to use 6l6 tubes and not 6550s than I think you will be better off to do that by means of a different pt.

[zelenko]

well yes the pin 3 is completely disconnected. and the wiring inside the amp is like on th schematic. but what about this 10w resistor? and grounding one of the filaments?

[martin manning]

Re the 10W resistor, do you mean the big green one? That is a replacement for one of the 100R 2W going from plate-to-plate. The 47R 5W in the later picture going from pin 7 to pin 8 on the adjacent tube is a ground reference for the power tube filament circuit. In the schematic it shows a separate winding for the power tubes which does not have a CT. Since the cathodes are grounded it's a convenient place to connect it. Considering the high plate voltage, perhaps keeping the power tube filaments separate and giving them a robust path to ground is a safer arrangement in the event of an arc-over.

I'd leave the voltages as they are, but put a 1k5 grid stopper and a small screen resistor (100R-470R) on each power tube.

[zelenko]

OK thanks very much for your help! and for fast answer too. I have cleaned everything, replaced sockets, replaced everything in the power sectiong, rectifier circuit was for nothing so ive build the other one.

the voltages are the same and during the build i discovered that if i remove the OT center tap i get at he B1 point 610volts and when i connect the OT ct the voltage jumps to 746v is there any explanation?

i measured the PT outputs they are all fine and within +-10% range.
can i put the two 100R resistors from output tube fillament to ground? ?

and can u show me how to put these 1k5 grid stopper and a small screen resistor (100R-470R) on each power tube?

Thanks in advance and sorry for bothering

[martin manning]

Definitely keep the PT center tap connected. It is balancing the voltage on the two 100uF reservoir caps and setting the screen voltage at ~half of the plate voltage.

Screen resistors are probably best placed on the power tube sockets from pin 6 to pin 4, then connect the screen supply lead to all pin 6's.

You have two grid stoppers supported on pin 6's now, so you'll have to move them. I would connect them to pin 5's, right on the pin, and leave them sticking out and up at a 45 degree angle (Marshall-style). The grid leads will be soldered close on the other end. Bridge the two tubes on each side of the OT, and run the leads to the board directly down to and along the chassis.

Yes you could put two 100R resistors to ground from the filament circuit, one from any pin 7 and one from any pin 2. Be sure that there is no other ground or connection to the other filament circuit.

[zelenko]

Thanks for explanation! its not the PT center tap, its the OT center tap

[martin manning]

That is odd. When you observed this were the power tubes installed?

[zelenko]

i dindnt do it with the power tubes. im dont want to blow them so i havnt install them.

the wiring u mentioned before is like this?
http://www.thevintagesound.com/ffg/sche ... layout.gif

[martin manning]

I'm thinking like this 2203 Marshall (only one side of the output is shown). Screen R's on pins 6 and 4, screen lead (yellow) to pin 6, grid stopper on pin 5 sticking out at an angle, grid lead (orange) soldered to the free end.

[zelenko]

OK and wha about this high plate voltage?