Univox U-1246 Bass amp with 700 volts at plates

[Latole]

Hello ,

I'm working on a Univox U-1246 Bass amp.
The two 6L6GC power tubes need to be replaced.
One of the tubes is short-circuited and burns the fuse.
My problem is that the plate voltage on these tubes is close to 700 volts; see the schematic. This is the amp's normal voltage.
I refuse to run modern 6L6GCs at these voltages.
I won't accept more than 500 volts.

I could probably add resistors.
Or a zeners diodes stack to reduce B+ . I do that with success on fews amps

The power transformer is rated for 117 ac primary.
Modern voltage is 120 to 125 volts.
The filament voltage is already 6.8 volts ac.
Any suggestions?
Thanks

 

[Dave]

It is possible to put resistors, say 1/2 ohm in series with the heaters to drop the voltage.


It is also possible to use zeners on the B+ although 200 volts sounds like a lot to me.


Have you thought about a bucking transformer, or just using a variac while using the amp?


Dave

[Latole]

Thank's for the answer Dave.

 When I receive amp with blowing fuse in my shop, I always use a bulb current limiter.
 I do a very very short test with amp plug in the wall and with meter on new 6L6GC 's plates . I put amp Off when I read 700 volts

  I'll look if I modify the voltage double on power supply is not the thing to do

[Williamblake]

I would avoid normal-screen-high-plate-voltage amps like the plague. But if you think you need it use vintage tubes? yolo.

[Latole]

I would avoid normal-screen-high-plate-voltage amps like the plague. But if you think you need it use vintage tubes? yolo.

I will not use vintage tubes

[sluckey]

700 volts at plates sounds like a lot until you realize there is only 335V on the screens. I don't see a problem.

[Latole]

700 volts at plates sounds like a lot until you realize there is only 335V on the screens. I don't see a problem.

I understand I can use 6L6GC safely .

[Latole]

I will also add a 470 ohms (or more)  resistor on screen pin 4 on each 6L6GC

[astronomicum]

It is also possible to use zeners on the B+ although 200 volts sounds like a lot to me.

I have used zeners on the AC feeding the doubler but only enough to bring the voltage back down to design. Keeping the screen voltage in check is important. A couple 6.8V 5A on each AC line has done the trick for me.

[Latole]

It is also possible to use zeners on the B+ although 200 volts sounds like a lot to me.

I have used zeners on the AC feeding the doubler but only enough to bring the voltage back down to design. Keeping the screen voltage in check is important. A couple 6.8V 5A on each AC line has done the trick for me.

Thank's

[glass54]

Hi Latole,
Just for interest, Dynacord Eminent II has B+ at 750Volts and Screens at +410Volts BUT they used EL34 Tubes (Good German ones  )
I repaired one of these many years ago, good for 100Watts if I remember right. Initially freaked me out with B+ at 750V!! But I felt better after reading the EL34 tube data. Any chance this amp may have had EL34's fitted originally?
Regards
Mirek

[Latole]

Hi Latole,
Just for interest, Dynacord Eminent II has B+ at 750Volts and Screens at +410Volts BUT they used EL34 Tubes (Good German ones  )
I repaired one of these many years ago, good for 100Watts if I remember right. Initially freaked me out with B+ at 750V!! But I felt better after reading the EL34 tube data. Any chance this amp may have had EL34's fitted originally?
Regards
Mirek

6L6GC are the tubes show on the schematic and tubes 's model are engraved on the steel chassis.

Schematic ; https://el34world.com/charts/Schematics/files/Univox/Univox1246.pdf


Thanks for the information it reassures me and with what said sluckey, who is very competent in this field, I will advance the work.
I wouldn't want the amp to come back to me in a few weeks, the 6L6GCs burned out at the price they cost.

I'm going to try to put some pictures of this amp and the circuit, they don't exist on the web.
This forum is really great, thank you all.

[Latole]

Picture;
Click to enlarge

[Latole]

Another one. The cab from the rear

[Latole]

Another one . Preamp circuit

[Latole]

Another picture ; power supply and power section.

[tdvt]

An interesting PS design.

Is it born out of economy, & just bumps up that first B+ node for some headroom?

[Latole]

An interesting PS design.

Is it born out of economy, & just bumps up that first B+ node for some headroom?

Yes a voltage doubler, no need high voltage PT. No need center tap on high voltage ; low cost building

[mresistor]

I think JJ 6L6 can handle the higher PV of this amp, also is it possible to use KT88s?  never mind that the PT might not be able to handle the higher current demand of KT88s.

[Latole]

I think JJ 6L6 can handle the higher PV of this amp, also is it possible to use KT88s?  never mind that the PT might not be able to handle the higher current demand of KT88s.

KT 88 are too large.

Thank's, I'll stay with 6L6GC . it should work

[clyde]

7581A's would work. 

[PRR]

KT 88 are too large.

6550. 110mm off the deck.
https://www.jj-electronic.com/images/stories/product/power_tubes/pdf/6550.pdf

[Latole]

KT 88 are too large.

6550. 110mm off the deck.
https://www.jj-electronic.com/images/stories/product/power_tubes/pdf/6550.pdf

On my Univox, with 43mm diameter each, the glass of the tubes will touch each other.
Anyway , JJ's 6L6GC are ordered.
 

[Latole]

I finished work on the deouis amp a few days ago, and testing it over several hours confirms that everything works fine.
The bias was done at 63% of the 6L6GC's power.

The problem with this amp is that the inverted circuit board makes parts difficult to access. You have to take the pots apart.
The volume, which sounds scratchy when turned, receives DC voltage. A coupling capacitor must be to blame. My customer prefers to leave it as is, not wanting to pay for the extra work.



Here are a few photos that will also be useful to those who do a Google search.
I've never been able to find them on a Univox amp.

[Latole]

Another one

[Latole]

And a last one;

[Latole]

Never say "last"....