Where do you guys buy replacement tubes?

"the amp blows up":tech jargon for "it broke down", which could be anything (blown resistor, capacitor) and might be
something simple or serious. It's really not possible to
try and diagnose stuff online without more details than you
are giving (and possibly some details you might not be able
to give without some experience working on tube amps).

BTW, you can buy brand spanking new (they still make them)
Cetron 6336A or 6336B from Richardson Electronics, but they're not cheap...$141.00 list. A better idea for this
tube type (or other weird industrial tubes) is try
Fair Radio, www.fairradio.com , they've got new 6336B's listed for $30 each, which is a heck of a deal.
Yes, you can replace 6336A with 6336B.

Oh, yeah, Kev, you might talk to your Fedex rep, I don't
know if they can do it where you are at, but we managed
to get our Fedex Express pickup pushed back to 7PM, which saves a lot of last-minute hustling...if you ship enough
Fedex Express they ought to be giving you a discount, too.

Uncle Ned

Yeah, it helps that we're not far away from O'Hare airport, which of course is one of Fedex Express' major dispatch points. We switched to Fedex Ground from UPS because the Fedex Ground guy picks up here about 3 hours later than UPS guy will.

Yes,Richardson really does/did make Cetron 6336's..like you said, there's plenty of other stuff they don't make that they relabel and sell as Amperex & National, but
not in this case. I'm not endorsing Richardson, mind you. Besides, it's kinda silly for him to be paying $141 for 6336's when he can get Tungsols from you for $40 or Cetrons from Fair Radio for $30!