Speaker cutting out intermittently

Yup.  Checked and wiggled.  All clean intact and no corrosion.  Even considered crossover in speaker. Recapped @8 yrs ago.  Still think it would work or not, even after rest period.  Voodoo at this point.  Being retired, I can sit on a little stool in front of the speaker.  It’s like taking the car in for a fix, when it’s running ok.  Just have to wait for repeat performance 

Yup.  Checked and wiggled.  All clean intact and no corrosion.  Even considered crossover in speaker. Recapped @8 yrs ago.  Still think it would work or not, even after rest period.  Voodoo at this point.  Being retired, I can sit on a little stool in front of the speaker.  It’s like taking the car in for a fix, when it’s running ok.  Just have to wait for repeat performance 

check the connections at the speaker ends to be sure

It is possible that some very fine wires in the speaker cables have come loose on the + and - terminals causing them to touch each other and partiall "short out" the signal to the speaker. If your close vision is not top notch, use a bright lignt and a magnifying glass to check. I actually solved an intermittant problem in a church PA system that had gone on for years cutting in and out as the heat protection circuits in the power amp was triggered and then, when the amp cooled a bit, the amp came back on. (With a tube amp, that probably isn't happening.) The problem was a sloppy soldering job in a phone plug carrying sound to one of the speakers. I hope you find the problem.

Thanks all for the feedback.  I bought the amp new last summer.  Those connections on the back seem bulletproof.  Cable connectors are new. Sometimes though, you ignore the simple obvious and default to more complex possibilities.  

If it only happened once I would not give it a second thought. I would not think it caused by tubes… if something happens to them, they don’t tend to work again. 
 

Loose connection most likely. I guess it is possible a tube is not fully seated. When off, with a glove or cotton cloth you might verify the tubes are fully seated. 

One engaging quirk with my now vintage Primaluna Prologue One are the thoroughly old school control knobs. Yeah, perhaps it’s partly due to my humid, tropical locale but both knobs semi-regularly get noisy and glitchy. Sometimes a channel will totally become silent. I fix the problem by setting the amp onto the kitchen table upside down, taking the bottom off of the amp’s case and spraying Deoxit onto the knobs’ contact surfaces. I enthusiastically twist each knob several.times and then re-install the bottom plate. It never fails to cure the amp’s noise and glitchiness. It never fails to improve the amp’s sound quality.

george53

I reversed speaker connections on the back of the amp, and powered up.  Everything good.

Great! Maybe you had a loose connection on the back of the amplifier. I wouldn't worry about it unless the problem returns.