I feel like the wheels are coming off this build. Now 1 side of Tube voltages way low

[SoulFetish]

It seems every time I try to fix one thing, some other shit happens. So much of this all began with the original tube sockets I bought from Partsconnexion. First, I had to replace the PI socket. Replacing a tube socket, while keeping all existing heater/terminal wiring and components is problematic. The the socket pin tension on the output tubes was failing and those needed to be replaced. This was just to get it back up running to test it for ohter things. So, I plugged it back in with the output tubes pulled to set the driver cathode voltages (which are the output tube grid bias voltages), and one of the triode sections in the 12AU7 is biased in cuttoff at -30V, while the other section is operating to its design at -4V. At first I though it was maybe the tube, but I tried three different 12AU7s! Hold on a sec, I'll show you on a schematic so it makes a little more sense...

[SoulFetish]

Here are some voltages:



It's weird, looking at these voltages on paper like this, it seems like half of the driver and PI aren't conducting..?
did I F- up my heater wiring? could that cause this type of behavior? If I remembered correctly and assigned the voltage readings to the correct plate, then it leads me to think something with the heaters because I wired them out of phase. But I've never seen this before.
What do you thinks going on?
Dude, I'm thinking of riping out the whole back end of this amp (in the red border) and starting from scratch. Heaters and all.

[g1]

It seems to be conducting, the voltages across the 18K resistors imply one side of triode is approx. 5mA and the other half 7mA, not a huge difference.
Try another tube in there and see if the differences remain similar.
Edit: sorry I see you tried that.
All resistances and connections in grid and cathode circuits check out?

[xtian]

Hail, OCD-man! Wow, what awesome lead dress you have, there. I want to see more photos!

Those big black boxes are filter caps?

And what about those blue-handled tools at left in photo?

--

Don't mind me, man. I looked up your other threads about this amp, and found lots of good photos. Your graphic design chops are great as well. Cheers!

[SoulFetish]

It seems to be conducting, the voltages across the 18K resistors imply one side of triode is approx. 5mA and the other half 7mA, not a huge difference.
Try another tube in there and see if the differences remain similar.
Edit: sorry I see you tried that.
All resistances and connections in grid and cathode circuits check out?

Yeah, the resistances checked out. Thats the weird part.

[eschertron]

So how does adjusting those 500R bias pots affect each side? If the offending side makes no change, then you're probably right about it not conducting. You have three resistors and trimmer there to look at. I have faith. Don't tear it apart just yet.

[SoulFetish]

So how does adjusting those 500R bias pots affect each side? If the offending side makes no change, then you're probably right about it not conducting. You have three resistors and trimmer there to look at. I have faith. Don't tear it apart just yet.

Yes it asjust it a bit more negative by maybe 2 or so volts? The highest it will go is -30V.

[g1]

The three tubes you tried are all different brands?

[SoulFetish]

The three tubes you tried are all different brands?

Yup RCA, GE, sylvania(?)
I tested them all for emissions

[SoulFetish]

Only happened after replacing output tubes

[The Dude]

If you remove the output tubes, what happens to the voltages?

[SoulFetish]

If you remove the output tubes, what happens to the voltages?

The output tubes ARE removed.

But, I wanted to update the schematic voltages. These are actual voltage readings, much better to follow, and help isolate the problem. Check it out:



if you look at the grid curves, the tube is doing exactly what it should. The problem is that the resistance between the grid and cathode (as it is cathode biased) is higher that I'm reading and higher that what it is designed for.
Also, in the image above, when I say "should be the grid -3.4V in reference to the grid/cathode bias voltage, I mean that's what I should actually be reading according to other measurements (as you can see). Ideally, the grid/cathode bias voltage should be around -2.4/2.5V (as shown below).

[g1]

Can you verify that 7.8V between cathode and grid, one probe of meter at cathode and one on grid.
Depending on the setting of the trimmer, you have somewhere in the 5 to 10mA range of current. That seems fairly high for 7.8V bias.
Also, can you include voltages at same points for other half of tube?

[Dave H]

Hanging meter probes on the lower 12AU7 could be causing it to oscillate giving false readings. Remove the probes from the 12AU7 and plug in the output tubes. What do you measure across the 1 ohm resistors for their bias currents?

[Dave H]

what awesome lead dress you have, there. I want to see more photos!

And what about those blue-handled tools at left in photo?

Anything I make needs to be in a black box but this one should be housed in plexiglass. It would be a crime to hide such beautiful work. And what are those blue tools?