mwl168

Nothing seems out of ordinary to me based on your description. If the offset trim pots are responding but you cannot get the offset to 0V, most likely you just need to trim the value of the resistors in series with the trim pots. I am going by memory without seeing the schematic but this information is in this thread somewhere.

This what I did exactly with my Megatron.

Did you measure the PZTA42 Vce between the two channels before making the mod?

Did you take another measurement after the amp warmed up? Channel 2 PZTA42 Vce don’t look right. It’s also odd that it happens to both PZTA42 and yet the voltage drop across the 20K is correct?

I first came across this group here...

So I did indeed install 150K and 200K chain for the bias resistors of the C2M1000170D when I first built my Carbon. Today I replaced the 200K with a 150K, I had also dialed the bias back to 18mA and disengaged the servo. Then took a few measurements after letting the amp warmed up. Now the PZTA42 Vce is around 13.7V, voltage drop of the 20K R is 25.4V and the Vgs of the C2M1000170D is around 3.3V. And I did not blow up the amp.

Got it. Thanks. I got work to so then.

Thanks MLA. That was what I was thinking too. Then I don't understand why Simmconn was worried about exceeding the C2M1000170D's max Vgs rating? Here is an interesting thing - I went back to my PM's and could not find anything that discussed this mod to the 175K+175K bias resistor chain. However, I came across an early correspondent with Kevin about my plan to use 200K+150K instead (values I had on hand at the time). Now I wonder if the one that looks like a 100K may be in fact a 200K. It's hard to tell visually. I can desolder the resistor in question and measure it. But part of me, the lazy part, is thinking if all is well why not leave it along? Maybe between the +/- 406VDC rails and the 20mA bias current somehow puts the PZTA42 Vce in the desired region?

Today I decided to make this mod to my Carbon I built in 2015 and parallel a resistor to one of the 175K to raise the Vce of the PZTA42. My Carbon is running on +/-406VDC rails and 20mA bias. To my surprise, instead of the 175K + 175K bias resistor chain, I had already replaced them with 150K + 100K at some point. As such, I measured Vce = 10V for PZTA42 and 21.8V drop across the 20K resistor. According to the graph above, this should put the PZTA42 in its linear operating region. Some wise man must have suggested this mod to me (I suspect SorenB) very early on. However, I measured Vgs of the C2M1000170D to be only 3.2V. Based on how I understand simmconn's posts, I was expecting 20V or higher. What gives?

Happy Birthday Day Nate 🎂

IIRC, I encountered a similar “issue” when I was adjusting my Blue Hawaii first time I powered it up. As Kevin already said, it turned out my offset pot was way off range. Once I got the offset adjusted to within reasonable range, the balance started to fall in place as well. I need to find the schematic but sometimes it’s necessary to change the resistor value that’s in series with the offset pot to be able to adjust the offset to 0V. Good luck!

@spritzer: You are correct - re: the two hand-drawn schematic; first one says “3ES pre-amp section”, second one says “3ES electrostatic headphone amp section” on top and “same for 4 channels” at bottom, the script in the rectangle in the schematic says “3ES amp module”. According to the OP, the pre-amp section is “ tube-input and tube-output”, headphone amp section is “transistors”.

I would be very wary of buying hard to get parts from aliexpress. In these high voltage applications, a fake part that breaks down can easily cause catastrophic and expensive failure.

I use M5 and find it good for that job.