i hit a tube jackpot, but need some advice

[zeppman]

Lastnight I went over to a friend of a friend of a friend's house and his dad happen to be big into vacuum tube radios, stereos...anything. So I got several mullard, rft 12ax7s and one 7025 pre-amp tube, they all tested new, and he only charged me $40. Then he offered a quad of 6l6s (he claims he has little use for them in his applications) for free. The only problem was that they were different manufacturers.... I think I got 2 rca blackplates, an old GE and something else that I can't remember. He claims they all match, and I"ll have no problem running them in my amp. Does this make sense? I watched him test them, and sure, they all basically read the same number (whatever that number is) on his tester, but I'm worried that being different brands, I'm gonna screw up my amp. Anyways, does anyone have any advice?

Timbre Wolf, I especially want your $.02

 

[908ssp]

No you won't screw up your amp. But his numbers may not mean they "match" for your amp. You need to check the bias in your amp. You might be lucky and they are within specs. To cold won't hurt the amp to hot and they will burn up quickly. When I took my MkI to my tech I had put some good GE Blackplates in the outside sockets and since I generally only run it on the 60 watt setting I had him change the bias resistor to warm them up. I can check and change the bias if there is a pot if not I am chicken.

 

[zeppman]

Well these are going in a mark iv which obviously has fixed bias. Is there a way to tell if they are too cold without using a tube tester?

 

[Timbre Wolf]

What a lucky score! I'm curious which Mullard 12AX7 types you've got - you can tell by the etch codes on the side of the tube, where you'll see something like "I63" "mC1" or "f91". Check this ( http://www.tube-classics.de/TC/Tubes/Valvo/Phlipscode.htm ) to see what I mean. RFT ECC83 is cool too: it easily breaks up, and it has more rounded treble frequencies, but is much less articulate than any Mullard. Good for leads.

As for the power tubes - yes you could run a set that's composed of different manufacturer's tubes, as long as they're matched to each other. Listen to 908ssp's advice about matching them to your amp, though, it is very important. You've got some of the most highly-esteemed 6L6GCs there, and I recommend you don't try them in the amp until you're sure they'll work there. You might have to pay a nominal fee to a reputable tube dealer, who knows the specs for your amp, and have them re-tested. This would mean sending them off in the mail, but it'd be worth it for those tubes. Free RCA black plate 6L6GCs!!!

I'd recommend Jim McShane ( http://pages.prodigy.net/jimmcshane/ ), Mike at KCA NOS Tubes ( http://kcanostubes.com/content/Default.asp ), or Lord Valve at NBS Electronics ( [email protected] ) as candidates for requesting testing on your tubes.

I'd recommend strongly considering an adjustable bias mod on your Mark IV. It isn't that big of a deal to get done, and it really opens up the possibilities of tube choice and lets you adjust your power section to taste.

Cheers!

 

[zeppman]

Does anyone know the what specs the power tubes should fall into for a mark iv? I know someone with a tube tester, I just don't know what they should test too.

 

[zeppman]

Timbre- The mullards are I63s. In all I got two mullards, a yugo (hot rod or something), rft, and a 7025 (don't know the brand but it looks old?)

 

[Timbre Wolf]

Timbre- The mullards are I63s. In all I got two mullards, a yugo (hot rod or something), rft, and a 7025 (don't know the brand but it looks old?)

The Mullard "I63" is a short-plate version of their 12AX7. My favorites are the long-plates, but those short-plates are very respectable. They can be kind of bright sometimes, but they're a nice tube.

If you can post (or send) a photo of the 7025, I may be able to help identify it. Are there any numbers on it (like "274" or "312")? Are the plates gray or black?