Not so different from being offered a 50 year-old english sports car for 500 bucks. That's the cheapest part of the thousands you'll spend before it's a daily driver.
As a learning experience and a project? See if you can knock him down on the price even more.
Just kidding. If it really is "working" the market value of the tubes is probably several times the seller's asking price.
This is the dilemma I point out when someone asks me to put one of these back on the air. So long as the customer does the dirty work of obtaining sufficient tubes, we can talk. And that includes the tubes that flash brightly blue and go "SNAP!" the third or tenth or thirtieth time you key it.
Remember that the more tubes you tie together in one parallel "stage", the better they have to balance. The strongest tube of four, or six will pull more than it's intended share of the load current, cherry up and fail.
Two tubes in parallel can only get out of balance by a factor of two-to-one. If one of them is a bit stronger, you can just dial it down a bit to keep the stronger tube at a safe temperature.
Six tubes in parallel must means that the balance between tubes is three times as important. Dialing down enough to keep one strong tube safe out of six? Probably would hold you to about half power, more or less, depending on how much stronger the 'new' tube is compared to the other five.
Sure don't want to count on that discipline being maintained. But hey, seeing this kind of dynamic play out first-hand would be a learning experience.
Go for it!
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