Where is the best place to buy vacuum tubes?

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Will,

A "scope" is an oscilloscope. You can diagnose and repair many problems with them.
I think, at this point, you needn't worry about one.
Let's see.
You've already done an excellent job so far, so you're gonna get it solved. It helps that you have a good piece of equipment in front of you. Pretty, too.
I will look around and see what I can find in English on tubes for you, but for starters, try googling "valves" instead of tubes. In German and English, we talk about Röhre/Tubes. In Britspeak, they call them valves. There is an awful lot of really good stuff written by antique collectors in British English.
In order to recap. you really just need to be able to follow directions, work safely, use a VoltOhmMeter and soldier cleanly. As long as you remember to never work on a live circuit and always check your work, you can't go wrong.
My experience is that a tube is shorted or it is not. If yours tests both way, something is weird. Cleaning contacts is always a good idea, but I still would still replace the tetrodes.
Good Luck - I'll see if we have anything on them in English in the technical library this morning.
 
I found a source for 7591 audio power tubes-try the link.Its a muscian oriented site-they carry tubes for equipment.Look under "Other" and you will get a link to tubes.And they have 7591.I remember these in other hi-fi and Muscical instrument amps-found some in a Lowerey organ-in the audio output stages for the speakers.And I have seen them in receivers made by Scott,Fisher,and Sansui.Its a good compact power tube that can provide up to 40watts push-pull.I have a McIntosh amp that uses them-#225.I conveerted the amp to use El-34 since at the time I acquired the amp 7591 wasn't available anywhere.Now there is a source!I could reconvert the Mac amp back to the orig.

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New tubes! Cool Rex, thanks!

Kevin, Thanks for the help and moral support!

Now I just have to face those fears with the caps. I seem to vaguely recall a 'how to' type site for cap replacement posted here a few months back. I thought I saved it, but I can't seem to find it. Anyone still have it?
 
here's the link to recap

I think this is what you wanted. There's not too much work involved, it is just a case of working carefully and safely.
If you can get Sam's photofacts, that would help. Try your library, first. But if you can't, you can still do it - just be very sure both channels get the exact same specs.

When you do the crossovers, remember that newer caps are physically much smaller, so be sure they can't vibrate and rattle when those 101 strings get going. I like to tie things together, not too tightly, rather than anchor with glue. But a drop of epoxy here or there can sometimes make your life easier.
Since you are going to have some RF circuits to do once this amp is done, I suggest you practice getting things clean and connections short and stress free. Never install a component under strain.
Here's where some folks will disagree with me. I work a lot on static discharge sensitive equipment, so use a soldiering station. In your case, a simple, dual heat 15/40 watt iron from Radio Shack will do you just fine. Radio grade soldier is a must, the kind with the rosin core. Try to get the thinner stuff which comes in a plastic "tube" - you pull it out of a hole in the cap.
Always make sure the metal bond is tight first, then heat the bond and melt the soldier onto it, not the tip of the iron. Let it cool for a few seconds before moving it. It should be glossy and slightly overfilled.
I would like to hear from the experts if he should add some protection to the cathodes of the 7591s? See this link: http://www.antiqueradio.org/fish02.htm
Mention it cause of the "short".

 

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