"actual" plate voltage ?

[worth]

Is there a difference between "actual" plate voltage , ( DC voltage between pin 3 and 8 on a 6L6 ) and the formula for plate voltage... HV secondary X rectifier = plate voltage ? I ask because the "actual" plate v on my 5E3 w/6L6s' and GZ34 rectifier is 366v... but using the formula the plate v is 418v. For instance , the "formula" for this amp is 322v X 1.3 = 418.6 plate voltage.

[HotBluePlates]

Well, yes... as you found out.

The only formula you can be confident in is that the peak voltage of a sine wave is 1.414*volts RMS. But that doesn't tell you the whole story.

1. The rectifier may drop some amount of voltage, and the drop could vary with the current passing through it.
2. The voltage may be reduced by voltage drop across resistance in the power transformer, as well as any series resistance in the B+. If you have a choke ahead of the OT, it might also result in voltage drop.
3. There will probably be some small voltage drop across the OT primary.
4. The voltage may be less than expected due to smallish filter caps. Small-value filter caps store less charge (not less voltage), which is then more easily drained by a heavy load.
5. If you draw more current from a transformer than it is rated to supply, it won't burn or stop working (unless you go overboard), but the output voltage will sag. You might not be able to predict the sag, because it depends on many factors of the design of the specific transformer.

But here's the real rub: you have measured the voltage the tube "feels," from the plate to cathode. So you have subtracted the voltage across the cathode resistor. That probably accounts for much of your discrepancy.

[worth]

So .. you're saying that the "actual" plate voltage" + voltage across the cathode resistor = approximate plate voltage... more reliable than the formula ?

[sluckey]

and the formula for plate voltage... HV secondary X rectifier = plate voltage ? I ask because the "actual" plate v on my 5E3 w/6L6s' and GZ34 rectifier is 366v... but using the formula the plate v is 418v. For instance , the "formula" for this amp is 322v X 1.3 = 418.6 plate voltage.

That formula is not meant to be used for plate voltage. It's meant to be used as a quick calculation/guesstimation of B+ at the output of the rectifier. And the formula is flawed because of all the reasons HBP listed above. Some time ago, someone put together a table of different rectifier tubes and assigned a number factor to each type of tube that's handy at guessing B+ based on HT AC voltage. IE, 1.4xAC_RMS for solid state, or 1.3xAC_RMS for GZ34, or 1.2xAC_RMS for NOS 5Y3, etc. These numbers are good for showing the relative B+ obtained for different rectifiers in a specific amplifier. But you cannot use it as an absolute. IE, using 1.3xAC_RMS to determine B+ from a GZ34 'may' work in one amp and fail miserably in another. Why??? There are just too many factors that influence the actual B+ obtained from a rectifier. Things like load current, filter caps, PT internal resistance (power rating), etc.