Univox U1011 Hum

[tubemonster]

PS - I've prowled this forum a few times in the past. There are certainly some knowledgeable people here. Much more knowledgeable with electronics than I.

[tubemonster]

Ok, I hooked up the scope to check the filter nodes. Probes on 1x on Channel 1 AC coupled.

I figured I'd post the pictures of the three 20uf nodes. I must say, trying to get a decent picture was fun. Even though things were adjusted so the wave was completely across the oscilloscope screen my smart phone camera couldn't keep up. But, here they are.

1st node (just after the choke). Settings are .1 volt/div and 5ms time/div



2nd node. Settings are .5mv volt/div and 5ms time/div



3nd node. Settings are .5mv volt/div and 5ms time/div


That's where I'm at with the ripple measurements.

[tubemonster]

Assuming you have a DMM, it should be able to measure smallish AC noise like 60 or 120 Hz and ignore the DC component. Set the voltage to AC volts, probe the B2, B3, etc. voltages relative to ground. With no input there should be no AC voltage (ideally). It gets a little weird because sometimes by probing your meter leads can act as antennas and couple noise onto the node you are probing. If there is AC on the preamp plates it is going to end up going out to the speaker because it is in the audio frequency range.

I am gong to remember this for future reference, but for this u1011 there was no ac with no single input. I did it two ways. One with the meter simply in AC mode, to which there was no AC for it chew on at any of the positions. And, I also used a capacitor to block the DC while the meter was in AC mode - this produced very low readings at each point - essentially zero.

[Helmholtz]

Audio Cyclopedia by Howard M. Tremaine

Ripple May Be Calculated:...

We usually don't discuss ripple as an AC to DC or dB ratio.

I did this under idle conditions, because I'm getting hum at idle and didn't think it was necessary to bring it up to max current load (I could be wrong though).

That's fine. Ripple voltages look ok.
What's the ripple on B1?
Check power tube cathode currents for balance.

Please post a scope pic of hum at the speaker output.

BTW, a good DMM shouldn't require the 0.1µ DC blocking cap as it separates DC from AC in AC mode.
But some cheap ones do need it.

[glebert]

That's fine. Ripple voltages look ok.
What's the ripple on B1?
Check power tube cathode currents for balance.
.

Wouldn't hum from power tube current imbalance happen no matter what the preamp settings or preamp tubes pulled?

[tubemonster]

We usually don't discuss ripple as an AC to DC or dB ratio.




That's fine. Ripple voltages look ok.
What's the ripple on B1?
Check power tube cathode currents for balance.

Please post a scope pic of hum at the speaker output.

BTW, a good DMM shouldn't require the 0.1µ DC blocking cap as it separates DC from AC in AC mode.
But some cheap ones do need it.

Hello Helmholtz, and thank you for the input. I will do what you suggest, but it may be a day or two. For today/tonight I'm done. Time to eat supper, watch some x-files/sci-fi, and play guitar a bit.

My old Beckman multi-meter gave out a while back and I replaced it with an Amazon Commercial 90DM610. It's supposed to be 'true rms'. I had another bench type multi-meter, but got rid of it somewhere along the ways - it was pretty big. I also picked up up a Klien Tools MM400 I still have. I've got a handful of other DMM's that are certainly in the cheapo range. One of these days I'll have to break down and get a nice Fluke or something. Any suggestions for a buy it once DMM for tube & solid state amps?

===========================================

But, before shutting things down I checked for sensitive wires and check to see if I could find any signal wires that when shunted to ground would cause the hum to disappear. Here's what I found.

Sensitive wire wise, the only one I found ran from the board to V3 pin 7 (12ax7 tube).

Shunting signal to ground at the following areas cause the hum to disappear.
- V3 P7
- Bass pot lugs 2&3. (the center #2 lug of the bass pot goes to V6 pin 7)
- and, V6 pin 7 that goes to the center of the bass pot
* note that the wire from the bass pot center lug IS NOT sensitive at all...

[Helmholtz]

Wouldn't hum from power tube current imbalance happen no matter what the preamp settings or preamp tubes pulled?

You're right.
I admit not having read the whole thread.

[g1]

This is a little difficult as the schematic does not have any pin numbers or tube numbers. Any extra descriptions of connections are helpful.
If V3 is the paralleled 12AX7 after the volume controls, and grounding the grids kills the noise, I think the hum must be coming in before those grids.
Remind us again if the volume controls can kill the hum?
And verify that pulling the 6AN8 and the tremolo 12AU7 makes no difference?

[tubemonster]

This is a little difficult as the schematic does not have any pin numbers or tube numbers. Any extra descriptions of connections are helpful.
If V3 is the paralleled 12AX7 after the volume controls, and grounding the grids kills the noise, I think the hum must be coming in before those grids.
Remind us again if the volume controls can kill the hum?
And verify that pulling the 6AN8 and the tremolo 12AU7 makes no difference?

V3 is the paralleled 12ax7 tube.
It sure seems like it should be that area.
Maybe I'll try shielded cable on either side of the board for that connection, albeit the leads are pretty short and moving them around has no effect.
I tried grounding out signal traces around/connected to the trace leading to V3 - no effect.
It appears to be located around lugs 2&3 (that has the .001 cap strapped across them) of the Bass pot and the trace/wires connected to it.
Lug 3 does connect to the Treble pot and you can shunt that treble pot lug to ground and the hum goes away, but the other two lugs on the treble pot have no effect when shunted to ground.

Volume/controls wise - all is still pretty much the same:
So, the hum.
1. With both volumes down turning the bass up brings in the hum.
2. With the bass turned down turning up the channel one and/or two volumes produces no hum.
3. With the bass turn up, and the hum humming, turning the channel one volume knob from 0-10 changes the hum from a lower pitched hum to a higher pitched hum. Note that the channel two volume is at zero.
4. With the bass turn up, and the hum humming, turning the channel two volume knob from 0-10 does nothing to the hum until you hit 10 and then the hum goes away.
5. With the bass up, channel two volume at 10, turning the channel one volume brings hum back slowly and at 10 gets pretty loud again. Funny thing. To me, this hum sounds likes it’s in between the lower and higher hum spoken of earlier (#3).
6. With the bass up, channel one volume at 10, turning the channel two volume has the same effect as in #4, only with the volume control switched.

Pulling V1 12ax7, the 6an8, and/or the tremolo 12au7 does not effect the hum.

I'll try some shielding and rewire and double check the base pot - recheck the grounds again - try to verify and double check things - again - lol.
The board in this amp has the components sandwiched between the chassis and board, so the board traces are point up with easy access.
I'm in no hurry, it's my amp, gives me something to do
Something tells me I'm missing something simple and I'm going to do the Homer Simpson DOH when I find it.

I really do appreciate the help and assists.
Oh, and output tube wise - neither set of quad 6l6gc's I have on hand are matched real well (I even tried mixed up the tubes from each set from), and I'll end up having to buy a matched set, so at the very least they are probably contributing to 'some' hum - just not at the bass pot centered hum, at least I wouldn't think so, but... then again I'm a hobbyist and could be wrong.
I enjoy everything being stated on this thread, and if the hum gets fixed, well, someone may have a good read in the future, like the reads I've enjoyed here on more than one occasion

[tubemonster]

P.S. how do I change my picture? I can't seem to find it in the settings.

[tubemonster]

So, before heading out the door today I took some time to run a couple shielded cables from the bass pot to the board, from the base pot to V6 pin 7 (phase inverter), and from the board to V3 pin 7 (paralleled 12ax7). Made no difference in the hum, but the shielded cable running from the board to V3 pin 7 is no longer sensitive.

Short of the hum, the amp works fine, but that bass pot brings in a LOT of hum as far as I am concerned.

[Helmholtz]

I don't trust that schematic.
A 500µ cap across the PI plate resistor would be total nonsense (might be 500pF, though).
Also I doubt that the NFB actually connects at the grid of 12AU7.

[tubemonster]

I don't trust that schematic.
A 500µ cap across the PI plate resistor would be total nonsense (might be 500pF, though).
Also I doubt that the NFB actually connects at the grid of 12AU7.

Right on both accounts.
The 500uF is a typo on the schematic (at least in comparison to the amp I have).
And the NFB crossing the grid of the 12au7 on the schematic, with the schematic expanded, looks to be more of a bad printout/blob than a dot/connection, either way the NFB doesn't connect to the grid of the tube.

[Helmholtz]

And the NFB crossing the grid of the 12au7 on the schematic, with the schematic expanded, looks to be more of a bad printout/blob than a dot/connection, either way the NFB doesn't connect to the grid of the tube.

It should connect to the cathode.

[tubemonster]

It connects to the cathode.
Connecting and disconnecting it does the norm and functions as it should.