6CA7 or EL34 vs. KT77; Need Help!

Marc Jenkins · From: Victoria, British Columbia, Canada

Well folks, I bought an old Garnet Session Man. It sounds great! The tubes are original (mid 70's) 6CA7's, and I'd like to buy some backups. I've heard great things about KT77 tubes being a more 6L6-like replacement. The tubestore has a bunch to say about the tonal characteristics of the KT77 in comparison to the EL34's.

The one potential problem, however, is that the Garnet doesn't have an adjustable bias. Is this going to be a necessary precursor to installing a pair of KT77's? I know it's held to be best to do it no matter what tubes I choose, but Gar Gilles, the designer, stood by his decision until he died last year to NOT include a bias adjustment...

By the way, this thing is 60 watts, so if I lose a bit of headroom, that's ok!

Here's links to the spec sheets for the JJ KT77 and the JJ EL34 for comparison.

Thanks!

J Fletcher · From: London,Ont,Canada

Why not use 6CA7's? They are (were) an american answer to an EL34, probably more like a 6L6 than an EL34 or KT77. You can buy "large bottle" 6CA7's still. You'll probably have to have the bias adjusted when you replace the output tubes. Can't help you out on KT77's, never used them...Jerry

Jim Sliff · From: Lawndale California, USA

Different terms for the same tube.

In the 50's these were al slightly different - now it's all marketing. Heck, Sovtek's 6V6's used to be their 6L6's that didn't pass some testing!

Basically all three mentioned are interchangeable, but WILL...not may...require at minimum a bias CHECK. Most techs can check it for you in 5 minutes for ten bucks.

And I guarantee you will want it modified for adjustable bias to get optimum tone. When Garnets were designed tubes were consistent, and we bought them out of racks under the testing machine that was in the corner drugstore.

But now, with the vast array of manufacturing plant locations and wide QC variances, you cannot have ANY idea if the tubes you plug in will be biased safely...or for the best tone.

Ratings some companies use (Groove Tubes, Mesa etc) are a way around it - but not all that effective, and you leave the safety of your output transformer - the "heart" of the amp - in the tube manufacturer's hands.

I would NEVER retube a fixed-bias (a confusing term to many, since it actually means "not self-biasing" in layman's terms, but means it CAN be adjusted - sometimes with minor modifications) amp without at least checking the bias. I'd equate it to putting 4 tires on your car - without any regard to size other than they fit the rim. The car will run - but you might or might not be able to drive straight, and might or might not destroy the drive train.

Hope that helps.
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Dave Mudgett · From: Central Pennsylvania and Gallatin, Tennessee

Duplicate post.

Dave Mudgett · From: Central Pennsylvania and Gallatin, Tennessee

Unless it's cathode-biased, one needs to be prepared to check and adjust the bias if needed. I don't see how you get 60 watts out of 2-6CA7 cathode-biased.

If it has old American-style 6CA7 tubes in it and you move to a modern imported version like JJ - it is a different tube, IMHO. For example, the 70s-80s Phillips/Sylvania 6CA7 could take a 700 VDC plate voltage on a Music Man amp with relative impunity. Although the pin-out and usage is the same, the internal construction is quite different.

Unfortunately, a lot of the desirable NOS 6CA7 tubes are priced in the stratosphere anymore:

http://www.tubedepot.com/nos-6ca7-holland.html

http://www.audiotubes.com/el34.htm

http://www.vacuumtube.com/guitar.htm

A couple of semi-reasonable-looking alternatives are here:

http://www.vacuumtubes.com/6ca7.html - look at the "fat-boy" American style 6CA7's. Not exactly cheap, though - $125 ea NOS, $65 used,

and my old standby Angela Instruments, for late-80s Siemens 6CA7/EL-34 tubes for $90 the matched pair

http://www.angela.com/

I imagine these are not as clean as the old American-style 6CA7's, but probably sound great. Just do a search for "6CA7", it's all he shows right now.

The plate voltage matters a lot - I don't know what they run on this amp. If it's not too high - in the 400-450 VDC range, any good-quality tube should work fine. But when the voltage starts getting up to 500 VDC and beyond, I'd want NOS. Just my opinion.

Marc Jenkins · From: Victoria, British Columbia, Canada

Thanks, guys. Forum=info! Much appreciated.

I checked out the Garnet's tubes last night. All the 12AX7's and the single 12AU7 are Japanese-made GE's, and all but one seemed ok. The power tubes are Phillips 6CA7 big-bottle. Unfortunately, while they sound nice, they are REALLY microphonic. It looks like I'll be buying tubes sooner than I thought. I can't really afford mods right now, so I might just go with the EH 6CA7, which is SUPPOSED to meet the same specs as the Phillips, and get the bias checked and hope it works out, and look to a mod in a few months.

NOT NOS tubes, though! Wow! I'm broke enough just from buying the amp...

Keith Cordell · From: San Diego

http://search.ebay.com/search/search.dll?from=R40&_trksid=m37&satitle=6CA7&category0=

The tube prices at the stores that specialize are far too high. Ebay is a great source for these, as many of the sellers are not guitar guys, they are electron-pushers and buy and resell this stuff as parts. You can get old tubes fairly inexpensively, if you watch. I have 24 blackplate EL84's I bought off Ebay last year for $3 each...

Michael Brebes · From: Northridge CA

The Garnet is very similar to the Traynors. Came with Sylvania 6CA7's but I'm running Winged C EL34's and actually like the detail much better. I do have an old set of 6CA7's and have checked bias against the Winged C EL34's in a JTM45 and they are very close with no real need for rebiasing.
Winged C is the original Svetlana company that got swindled out of the use of the Svetlana name in the US.
www.tubesandmore.com
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