Univox U-1011 question

[57nomad57]

I just finished a fender 5f4 super build that turned out really well. So that gave me the courage to rebuild a Univox u1011 amplifier- 105w.

The univox amp was modded several times and only one channel was usable with no tremolo and no reverb. I was able to get a spare circuit board and rebuilt the spare circuit board with new caps and resistors, I installed it but I’m having trouble with the 2nd stage of gain. the first stage of gain is working properly, then the signal goes to the volume pots.After which the signal comes from the volume pots into the grids of V2. There are 2 channels ( from the 2 volume posts) that are going into V2.Channel one has 2 grid stopper resistors in series 100K and 330K . The other channel has the grid stopper resistor at 470K. The plate resistor is 100K , the cathode has a resistor that is 1.5k and has also 10uf 16v cap. The b+ voltage to the plate resistor is about 220v ,but after the 100k resistor I can only get 7.32 volts. I need 145v at this point. I've tries several variations on the plate a grid resistors but no luck. Any suggestions I’m stumped.

[57nomad57]

u-1011 schemeatic

[TomCarlos]

And what is the voltage on the 1.5K resistor? Is it 1.2 vdc? If yes, then you have .8ma current?

If you have 240V at B5 and 145v at the Plate, that means you have 100 V dropping across the 100K resistor - thus ~1 ma. And that 1 ma feeds both sides of the 12ax7?

You checked to make sure the heater voltages are correct?

Checked the .02 cap to make sure it is ok?

And the voltages on the Grids?

[57nomad57]

I'll check all this tomorrow.

[g1]

That tubes heaters are lit up properly?

[Jazz P Bass]

I seriously doubt that you have a 200 volt drop across that 100K resistor.

If you do:
1: the resistor is not terminated properly.
2: the termination point is not correct.
3: the tube is bad.

As TC stated: what is the voltage on the cathode?

[Enzo]

What they said. are you sure that side of the tube has a running heater? The plate load resistor is what it ought to be?

Those are not really grid stoppers, those are mixing resistors, so the two channels can be wired together without fighting. They split the ch.1 one in half to tame the tremolo effect.

[57nomad57]

Ok thanks for all your responses, I had a weak tube in that spot. I checked all the tubes on my tester before I started, But this one must have slipped by.

After changing out the 12ax7 in the 2nd stage of gain, I'm still only getting around 60 volts dc on the plate. (short of the 145v I should have) , MA reading was 120 ( the b+ before the 100K plate resistor is around 200vdc)
I took the resistor out and checked the value again.
the cathode votage is 1.63v
and the voltage at the grid is .85v

the .02 cap is good, I measured it with a capacitance meter and it reads right at .02. It was the same on in that spot originally , a green mylar cap.

the heater current to this tube is 12v.

[g1]

You should not have any DC on the grid. Not sure which .02 you are speaking of, but the cap tester won't tell you if it can handle the high voltage. Only substitution with a good one will tell you.
Find out where that grid voltage is coming from (there are 4 paths to the grid).

[The Dude]

I would try temporarily grounding that grid and see what happens to your plate voltage.

[57nomad57]

I adjusted the b+ voltages to get the proper 240vdc so I would have a good reference point to deal with this issue.
this bumped my plate voltage to around 80 vdc.

I replaced the resistors before the grid with higher values to see if I could get the .85v ( at the Grid) down.

for instance on channel 2 there is a 470k reisistor before the grid - Replaced this with a 510k, then then 560k, then 620k then 680K .
this was increasingly getting the voltage down some on the grid (at 680k-- .7vdc) and at the same time there was a gain at the plate
to 90vdc. so there is some progress. Should I just keep increasing resistor value until it squeezes off the DC at the grid?

or should I be looking elsewhere to get the voltages down? I haven't measured the pots , they look original but may have been replaced
with a different value other than the 1 meg they are supposed to be.

[g1]

What happens if you ground the grid like Dude suggested in post #10?
You are adjusting resistors when your likely suspect is a leaky cap. Unless your cap meter is measuring them at high voltage, it can not tell you if they are bad.

[The Dude]

^^^^^^ What he said. There's no reason to redesign the amp. It worked once- now it doesn't. It's broken. Fix it. Changing component values will get you nowhere. Again, I would try temporarily grounding the grid and see if your plate voltage becomes normal. If it does, you know the problem is something at the grid. If plate voltage is still low, you need to look elsewhere. Alternately, you can temporarily lift the coupling cap and see what happens to both grid and plate voltage. I agree with g1. The likely cause is a leaky coupling cap, but you need to verify by troubleshooting.

[The Dude]

Oh, and by the way.....Welcome to the place!

[57nomad57]

first thing-- I grounded the grid and got a boost of 7vdc, So I measured the coupling caps at the first stage of gain. There's two coupling caps , .02uf caps, one for each channel. The .02 cap on channel 1 is leaking and that is where I was getting the voltage on the grid. It goes from 130v on one side to about 3-4 v on the other. Channel 2 is 130 on one side and .004 v on the other side. I would imagine this is acceptable. I tried more .02 caps that I had , a used orange drop cap, a blue molded cap from a fender and an yellow boxy .022 cap. they all leaked, especially those yellow boxy caps.



So Now I restored my orginal resistor values before the grid, 470k channel two and 100k &330K in series on channel one, and now I have 73vdc at the plate on the 2nd stage of gain, (V3) and the following coupling cap (.02) is leaky. I do have a few .02 somewhere but again they are used.I'll order new caps tonight.