How to measure plate voltage?

I have a bias probe with a simple multimeter.
I have a PRRI with a chassis hole for access to the bias pot.
Is there a way to measure plate voltage without getting into the chassis?

I need step by step details, as I have no clue.

By the way, bias measures 19.0 with the original 6V6GT's. I'm looking to install JJ6V6S's in thier place.

Thanks in advance. I don't want a shocking experience.

Take the amp to a good tech, and hire him (or her) show you. This is something that should be shown; since you don't have a clue, it would be better done in person. I don't think there is any way to see plate voltage other than measuring it at the pin, which is well inside the chassis. It can be done safely, but there is a great oppurtunity for doing something silly and causing damage, injury, or indeed, death.

BTW, since you can measure and adjust bias, why do you want to know the plate voltage?

6V6's have their plates on pin 3... what kind of bias probe do you have?

one of the probes purchased from eurotubes with a multimeter.

don't I need to know plate voltage to properly adjust bias?

Without a baisrite or simular tool, one has to go inside to measure that voltage, and as SCooter notes....it is dangerous in there. IF one knows how to be as safe as possible, it is a simple but still dangerous thing to do. That is a big 'IF', and that is why SCooter suggests getting some training in person. The 'how' is simple. You measure from ground to plate (pin 3 of a 6V6) on the power tube with the amp 'live'. Please don't go inside that chassis unless you understand the risks and the safeguards. Your phrase...not mine..."I have no clue" is why SCooter and I are saying...don't go there without some training. There is a good reason why there is a warning on the back panel of these amps. Even if you don't do harm to yourself, you may do harm to the components...like a transformer.
Why would one want to measure plate voltage? Plate voltage meassurement is necessary
in order to figure the plate dissipation factor, which is the number that some people find of greater interest than even the current draw number. Even if one sets the bias by ear
(for the sonics) and eye (for redplating), which results in the sonics that one prefers; it is interesting and perhaps necessary to know that plate dissipation factor number. Go here to the tech info section for a better explanation.....
http://www.aikenamps.com/

bio, you can set the bias by ear and eye if you want. Install your tubes and watch them as you bring them online with your meter reading the current draw. IF they don't redplate, play a bit. IF you like the sound, leave the bias alone. IF you want to see if you would like another setting, unplug the guitar and using your meter adjust that bias while watching the tubes. AS you increase the current draw...that is, see the number on your meter go higher....watch the plates..the long dark elements of the tube....to see if they start to glow orange/red. IF..and they will at some point if the pot and resistors allow the adjsutment to take the tubes into excessive current draw.... they do, immediately back the adjustment back a bit until the orange/red glow in the plates disappears. At this setting, the tubes are drawing as much current as they can without burning down in a matter of minutes perhaps. IF you like the sound, leave it there. This is the 'hottest' sound those tubes will yield in that amp....that is, they will distort sooner and more aggressively adn thickly. They also will live a slightly shorter life because they are physicallly runnning hotter.
IF you would want to go to the other end of the range of bias, you will hear the amp get cleaner and crisper. At the extreme end of the 'cool' range..if the pot will take it there....the amp will start sounding too thin, cold and harsh. That is the unacceptable end of the cold end of the range of usable bias. Anywhere in between those two extremes is acceptable, and you may choose the sonics you like for your style of playing....cool, medium or hot.
IF one chooses to install and bias tubes in this manner, it is still desirable to understand the current draw, plate voltage and consequently the plate dissipation factor......but it isn't totally necessary. This 'ear and eye' method can get you through a gig if you have to replace the power tubes during the job and want or have to quickly get going again with some assurance that the amp and tubes are safe to operate and that you can get acceptable sonics, right?
Curiousity is a good thing, but get some understanding of the dangers inside that amp before diving in. IME, even if you take precautions, there will be times when the snake will bite. I don't know of anyone who works on these things who hasn't felt the bite.
Maybe a biasrite or something simular is in your future???

Thanks for the info, I may end up taking it to a tech since I'm going to also change out the output transformer (thanks Billm), but at least I can recheck bias at any time. I may very well tweak the bias pot to look/listen to results. I guess I've read bits that ballpark the bias around 22mv (close to the 70% magic number) for this amp assuming that my plate voltage is running average spec.

In the end I'm looking for more clean headroom and not having a brittle sounding amp.

have you watched the Eurotube bias video? With their bias probe and a multimeter it's super easy to bias your amp.

yep, but they have a weber probe that can also measure plate voltage simultaniously, nice, but I don't have one, just a plain old cheap probe. So really, I'm guessing that the plate is running spec as I make adjustments, but looking for red plating and listening, plus what others have been getting should put me in the ballpark. I may end up taking it in for a one time 85.00 fee anyways, who knows.