SOLVED-new Build voltage issues

[studiodunn]

I just got around to firing up a new build today and instantly had the bias voltage climb out of control and popped the 1ohm resistors on my bias test points. As I went back over all the connections and components in the bias section I couldn’t identify an obvious problem.

I thought this was at the bias tap, but Martin helped me eliminate that scenario.

The screen voltages are dropping and the bias voltage is raising.

18094.PNG

[martin manning]

With no tubes installed, disconnect the Red/Green wire (bias tap) and measure voltage from it to the CT (ground). If you don't see 50 VAC or so, something is wrong inside the PT.

[studiodunn]

Thanks Martin - 49VAC at the lead.
Screen voltage drops and bias voltages rises as thing heat up. Not sure what I’m missing here, but I’m sure it’s obvious....just not to me

[pompeiisneaks]

So even with power tubes out this happens? I'd definitely keep them out until you can rule out the bias circuit itself. Since you get 49VAC at that input, I'd just keep power tubes out for now and step through the different points on the path and ensure you've got negative voltage at each spot and the like. If it gets through the 221k resistors as a good negative bias, then something's amiss with the power tubes. etc.

~Phil

[studiodunn]

So even with power tubes out this happens? I'd definitely keep them out until you can rule out the bias circuit itself. Since you get 49VAC at that input, I'd just keep power tubes out for now and step through the different points on the path and ensure you've got negative voltage at each spot and the like. If it gets through the 221k resistors as a good negative bias, then something's amiss with the power tubes. etc.

~Phil

Thanks Phil.
The amp is stable with power tubes out. All my voltages match closely to the voltages Jason gave me for the same set of iron. As soon as the tubes go in the bias sky rockets. I have used 3 sets of tubes that I know to be good.
I only have -9.63vdc at the grid, so there's a start. I can lower R44, but I'm not sure that tells me why I am so far off.

Here is the schematic I used.

schematic_bias.PNG

[sluckey]

R44... 120K

120K is meant to be connected to one side of the HT secondary, not that red/green bias tap. Just tape off the red/green.

Or, if you prefer to use the red/green bias tap change that 120K resistor to something much lower like 470Ω or 1K.

[studiodunn]

R44... 120K

120K is meant to be connected to one side of the HT secondary, not that red/green bias tap. Just tape off the red/green.

Or, if you prefer to use the red/green bias tap change that 120K resistor to something much lower like 470Ω or 1K.

I am an idiot! Jason even mentioned R44 in a conversation and I didn’t connect the dots. 5k got me biased in the middle of the trimmer.

Thank you so much!