Peavey Windsor Doug Hammond Mod Phase Inverter

[bobloblaws]

If the PV B+ is 120v lower than the MArshall, that right there might exoplain why the voltages are lower than expected.



Your B++ is only 270v to start with and you measure 268 at pin 1. Pin 6 is not "so much lower", pin 1 is so much higher. There is no voltage drop across R45, which means that triode is not conducting. No conducting, means high plate voltage and low cathode voltage and no amplification.

That is weird, I even tried swapping in another tube last night. I'll try that again and compare the voltage on pin 1.

[Enzo]

Are BOTH heaters glowing on that PI tube?

[bobloblaws]

Are BOTH heaters glowing on that PI tube?

Yes, definitely. Both tubes I tried test fine on my tube tester as well. I verified the heater circuit and that both elements were glowing. I have 6.3 VAC across pins 4/5 and 9. I checked that the tube pins are all connected and I re-tensioned the clip for pin 1. Connection between pin 3 and 8 is sound. Got me stumped.

[Enzo]

OK, the following is VERY unusual...but it happens. The pins in a socket have two ends. Up top is the part that grips the pin on a tube base. And underneath is the part that solders to the circuit board. The pins can break in the middle. This results in a gripper part that grips real well, and a bottom part that is soldered in real well, but the two are not connected. I am assuming when you take voltage readings that you are probing the "bottom" end of the pins. Pull the tube and probe the top gripper part of pins 1 and 6. With no tube, you should read pretty much full B+, like you already do on the bottom end of pin 1. If my very rare condition exists, you would read full B+ on the underside, and zero or very low voltage at the gripper.

Same can happen at pin 3, only no voltage with tube gone. Pins 3 and 8 might be connected below, but measure resistance to ground from each pin, 2 and 8. Both should read the same.


I don't recall, did we verify pin 2 is NOT shorted to ground?

[bobloblaws]

I don't recall, did we verify pin 2 is NOT shorted to ground?

You called it. I'm an idiot. You see in the lower right of the diagram where it says "5 point terminal strip - mount to existing PCB standoff? Well I did exactly that and the middle lug is connected to where it mounts and the standoff I used is connected to the chassis. Thanks for patiently working though this with me, much appreciated!

[Enzo]

So now your voltages are restored to more reasonable levels.

[bobloblaws]

So now your voltages are restored to more reasonable levels.

Yes, please see attached.

I'm measuring 91 watts at the output, not bad.

But I'm seeing some crossover distortion so I will attend to the power tube bias.

Thanks again!