Help with measuring tube voltages in a Rocket.......

[Paultergeist]

Greetings,

I could use some clarification regarding the measuring of tube voltages. The current build project is a Rocket-inspired amp, but I would think that these matters would apply just as much to any tube amp:

Power tubes: My understanding is that I measure the plate voltages with respect to the cathodes. Is the same true for the screens -- i.e., screen voltages measured with respect to the cathodes?

V1, 2, V3 (Pre-amp / Phase Invertor):
Am I correct that the 12AX7 plate voltages are also measured with respect to their cathodes (of the same internal plate-triode combination, etc)?

Is it also correct that the pre-amp PI (12AX7) grid and cathode voltages are obtained (measured) with respect to chassis ground?

thanks for any clarification.

[Phil_S]

For DC volts, plate, screen, grid, or cathode, red probe on the tube pin and black probe on the chassis ground. I like to use clips to assure a stable reading. If you have some tube math to do for any reason, like plate minus cathode, you have all you need to know. If you are posting voltages to a chart or the forum, no math is involved.

[gui_tarzan]

Clips on your test leads will help prevent accidental slippage that could end your life (or at least some components) too. I use 'em every time.

[rp]

pay attention when there's a cathode follower.

[xtian]

pay attention when there's a cathode follower.

OK, I'll bite. Pay attention to what?

[Paultergeist]

Also......let me again say "thank you" to all for the helpful replies.

I am also pretty certain that I have encountered the notion of measuring tube plate voltage with respect to the cathode in some context -- I am wondering if that is used for calcualting the bias of the power tubes?

[billc]

The difference between plate to ground and cathode to ground, etc., is what you want, same deal across resistors,etc (i.e if plate to ground is 350V, cathode to ground is 20V, then plate to cathode = 330V, etc.)....that way one probe is clipped to ground and you are only handling one at a time so your other hand stays in your pocket for safety...of course powering down and connecting a clip is safer still. I like using tube extenders myself, a relatively safe way to check voltages...

[img:600:800]https://lh6.googleusercontent.com/-yuR4 ... 154911.jpg[/img]