Dumble ODS #124 Low Phase Inverter Voltages

[TimDude]

I completed a Dumble ODS #124 build yesterday. It sounds great! However, my phase inverter is putting out somewhat low voltages. The schematic calls for 290V on pin 1 and 280V on pin 6. However, I'm getting right around 260V on both pins 1 and 6. I tried adjusting the "bias" pot from 10:00 to 2:00 and it only made a difference of about 2 volts, up from 258. Should I just try maxing out the bias pot and seeing where that takes me?

Thanks for any replies.

[wpaulvogel]

The bias pot doesn’t directly affect the voltage of the phase inverter. It’s the power supply dropping resistors. You’d need to put a smaller value dropping resistor between the choke and the phase inverter supply.

[martin manning]

Should I just try maxing out the bias pot and seeing where that takes me?

That would probably take you to the parts store for a new set of output tubes. The bias pot is used to set the power tube idle current and nothing else.

[johnnyreece]

Have you tried different tubes in the PI position to see what you get?

[TimDude]

The bias pot is used to set the power tube idle current and nothing else.

Martin, are you referring to the regular bias pot or the PI Trimmer? I was referring to the PI Trimmer. Hopefully we're on the same page. Sorry for any confusion.

Have you tried different tubes in the PI position to see what you get?

Johnny, yes, I have tried other tubes in that position. They all give me the same results.

The bias pot doesn’t directly affect the voltage of the phase inverter. It’s the power supply dropping resistors. You’d need to put a smaller value dropping resistor between the choke and the phase inverter supply.

I'll look into this. I'm curious, is this somewhat normal for a fresh build for voltages to be off by 10%?

Hmmm... I was just looking back at my biasing voltages from pin 3 of my output tubes. They too are about 30V too low... I'm surprised I didn't notice that sooner. They're down around 410V to 415V. Could the problem be something else? Or may there not be a problem at all?

[sluckey]

Hmmm... I was just looking back at my biasing voltages from pin 3 of my output tubes. They too are about 30V too low... Could the problem be something else? Or may there not be a problem at all?

Adjust the real bias pot to set your output tube pin 3 voltages to the point you think is correct. This will cause all B+ voltages throughout the amp to increase. Now recheck the PI voltages and see if they make you happy.

[TimDude]

Adjust the real bias pot to set your output tube pin 3 voltages to the point you think is correct. This will cause all B+ voltages throughout the amp to increase. Now recheck the PI voltages and see if they make you happy.

One thing I would like to note here, I had gone through Rob Robinette's bias calculator, and I calculated that I'm already at about 65% to 70% dissipation. Should I sorta disregard that and crank up the bias anyway and only pay attention to the voltages? Thanks!

EDIT: I can't get the voltages to go above 411V on pin 3.

[sluckey]

If you prefer to set the bias for 65-70% idle dissipation, then do so. But you will have to accept the voltages as they are under that condition. Just know that the hotter you bias the output tubes, the greater load (current) you place on the power supply and the lower the B+ voltages will be. Conversely, the cooler you bias the output tubes, the lower the load (current) you place on the power supply and the higher the B+ voltages will be.

You really don't have a problem. There are many factors that can cause your voltages to be a little different from voltages you may see on a schematic. Biasing is one. Using a different power transformer is another factor. Did you use the exact PT as the one you are referencing voltages to? Even differences in line voltage can explain the voltage differences you have.

I'd say if the amp sounds good and you want it biased at 70% then you are good to go. Write down all the voltages in YOUR amp and keep for future reference.

[TimDude]

If you prefer to set the bias for 65-70% idle dissipation, then do so. But you will have to accept the voltages as they are under that condition. Just know that the hotter you bias the output tubes, the greater load (current) you place on the power supply and the lower the B+ voltages will be. Conversely, the cooler you bias the output tubes, the lower the load (current) you place on the power supply and the higher the B+ voltages will be.

You really don't have a problem. There are many factors that can cause your voltages to be a little different from voltages you may see on a schematic. Biasing is one. Using a different power transformer is another factor. Did you use the exact PT as the one you are referencing voltages to? Even differences in line voltage can explain the voltage differences you have.

I'd say if the amp sounds good and you want it biased at 70% then you are good to go. Write down all the voltages in YOUR amp and keep for future reference.

That's a wrap folks! I got the answer I was hoping to hear! Thanks! I may sound like I'm being a smartass, but I'm serious as a heart attack. I'm closing this thing up and letting this bad boy rip!

[martin manning]

I'm closing this thing up and letting this bad boy rip!

What plate voltages do you have on the preamp tubes? You might want to adjust the first dropping resistor to dial those in so you get the full flavor of the circuit, and, you have to spend some time obsessing over the PI trimmer setting (see long thread in files section) and the FET bias. ;^)