5E3 Deluxe P.I. voltage confusion.

[Mark]

I have a 5E3 in front of me and I’m a little perplexed at what I’m seeing.

I have 212VDC on the plate, I have 14VDV on the grid and 47VDC on the cathode. I have two sources that show that this is quite normal, but it’s not making sense to me, I would think that the voltage on the junction of the 1.5K(45VDC) and the 56K should be present on the grid. Thus the bias current would/should to other triodes in the circuit.

Any thoughts on this?

https://www.thetubestore.com/lib/thetub ... Sn0m16xx3X

https://ceriatone.com/wp-content/upload ... 9/5E3.xlsx

IMG_2804.png

[sluckey]

I have 212VDC on the plate, I have 14VDV on the grid and 47VDC on the cathode..... I would think that the voltage on the junction of the 1.5K(45VDC) and the 56K should be present on the grid.

Those voltages are fine. This PI circuit uses bootstrapped biasing which is high resistance. Your meter actually puts a significant load on the high resistance grid circuit and causes the grid voltage to read much lower than it really is. Your thinking is correct. The actual voltage at the grid is really the same as the voltage at the junction of the 1.5K, 56K, and 1M. I don't even bother measuring the grid voltage on any bootstrapped biased circuit, but I do measure the voltage at the bottom end of the grid resistor and annotate the schematic (see my attached schematic).

The three most common bootstrapped biased circuits are this AC coupled PI, some AC coupled cathode followers, and most LTP PI circuits. Measuring voltage directly on the grid pin of any of these circuits will always give unbelievably low voltages. If you want to measure the real bias voltage on any bootstrapped circuit, just put one probe directly on the grid pin and the other probe directly on the cathode, or just measure the voltage at the bottom of the grid resistor.

[Mark]

Thank you for helping me again. I really appreciate it.

I would have thought the resistance/impedance of the meter would be quite high, at least high enough not to affect the voltage reading. The meter is an early 80’s Fluke 77, it still works so I wouldn’t throw it away.

The input impedance of the Fluke 77 series 1 is 10M, I estimate that resistor would measure 909K with the meter across it. I’m surprised that it made a difference and embarrassed that this is the second time I fell for it and had you explain the error of my way. If I fall for it again I may have to start posting on the gear page.