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Old 2nd March 2007, 09:31 PM   #1
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Default 6L6GT vs. EL34/6CA7 and WHY?

OK, so in an attempt to better educate myself, I thought I'd ask the experts for opinions on this. I did search, but got no real answer....

It has been recomended to put 6CA7's in place of the 6L6GT tubes and re-bias of course in the Ming-Da MC34-B amp.

Ok, I can accept that, but WHY? What is the reasoning behind this, besides more output power? I'm trying to better understand this.
As it is, the amp sounds pretty good to me. So I have been reading reviews etc. on tubes. This tube is more defined, bass is tighter, bla bla.... Man, this is confusing stuff. If there is this ultimate output tube, then why are there so many others??

"It just sounds better" is not enough for me. I need some electonics theory to back this up. Anyone want to take some of the Voodoo out of this for me?

My brain hurts....

Marc
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Old 2nd March 2007, 09:47 PM   #2
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Default Re: 6L6GT vs. EL34/6CA7 and WHY?

Quote:
Originally posted by Runco990
"It just sounds better" is not enough for me. I need some electonics theory to back this up. Anyone want to take some of the Voodoo out of this for me?
And you got into tubes instead of SS because?
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Old 2nd March 2007, 09:51 PM   #3
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I am no expert, but...

Well, say you change the output tubes for the EL34's ... there are still plenty of different brands of 6CA7/EL34 around, which will, you will read, all give subtle differences in sound. And so it goes on and on and on.

The short answer for
Quote:
If there is this ultimate output tube, then why are there so many others??
is: there is no ultimate tube. Just like there is no ultimate sport car, no ultimate blabla... I once read, here at diyaudio, a comment by someone: 'the fact that there are so many CD players around indicate there is no ultimate - there are lots of different tastes' (or something like that). Engineering is about compromises...

Sit back, listen to the music and relax - and do not read to much threads: you will dispair among the different tubes used through the people around here. But if you insist, I can recommend Morgan Jones Valve amplifiers, thirf edition: reading in it you will understand the amplification step by step... and then, you will get more dispaired, as you will soon note that it is not just the output tubes that influence sound, but also the driver tubes, trafo's, power supplies, wiring, capacitors... you have been warned

Have a nice weekend!

Erik
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Old 2nd March 2007, 10:11 PM   #4
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Default Re: 6L6GT vs. EL34/6CA7 and WHY?

Quote:
Originally posted by Runco990
"It just sounds better" is not enough for me. I need some electonics theory to back this up. Anyone want to take some of the Voodoo out of this for me?
There are a lot of answers that you might get. Take a look at the data sheets for the two tubes, pay close attention to any differences, then pick any that you like.

Obviously, I'm not being very serious. The problem is, any theoretical explanation as to why one tube sounds better than another (to any particular person) is useless unless it is predictive. It's not enough to be merely descriptive.

In other words, suppose you pick the fact that 6CA7's have higher transconductance (sensitivity) than 6L6's as the reason they sound better in an amp. In order for it to be a useful theory you would need to show that you can pick another tube with even higher transconductance, and that that tube will necessarily sound even better. If you can't do that, then the theory is not really a theory at all, it's just a guess. Some might call it a hypothesis, I call it a guess.

Unfortunately, I expect that all you're going to get are guesses. Maybe I'm wrong. Don't misunderstand me, I think it would be great if I'm wrong, but I don't think I am. After spending decades experimenting with these things I've come to accept that there is a certain amount of black art involved. I don't believe in magic, but black art ...

-- Dave
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Old 3rd March 2007, 12:17 AM   #5
Salas is offline Salas  Greece
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I have used both EL34 Svet, 6L6GC Svet and Sylvania NOS 6L6GB in my amp. Now I experiment with KT88 EH.
My experience is that the higher gm tubes give more output, are easier on the phase splitter and create more open loop gain so that the global feedback rises more and lowers Zout, noise and THD. Now there are tubes and tubes. For instance I have listened in a guitar amp RCA black plate NOS 6L6GCs killing Groove Tubes EL34. In general when your transformer reflects 6K to the primary the EL34 is going to give 40% more wattage than an 6L6GC with less Zout. EL34 sounds more rounded and 6L6GC flatter. It depends very much how your driver stage, B+, mains trans VA and global feedback are optimized. If you have say 450V B+ and 0.6A stereo (4Xtubes) capability you can push 6L6GC to 55ma and make it sound dynamic yet not rounded. For 40-45mA I would bet on a good EL34 over a good 6L6GC. But one tube that may please you very much, that retains the gain and output in an EL34 amp but sounds bigger and less distorted is KT90. Sadly EI Serbia stopped it and its NOS now, but the new KT90EH is almost as good but far more reliable than post raid produced EIs.
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Old 3rd March 2007, 12:25 AM   #6
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Here's someone who did what you are suggesting:

http://audiokarma.org/forums/archive...p/t-35775.html

Jeff
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Old 3rd March 2007, 02:01 AM   #7
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Quote:
Originally posted by vinylkid58
Here's someone who did what you are suggesting:

http://audiokarma.org/forums/archive...p/t-35775.html

Jeff
Interesting.... I'm intrigued by the battery bias mod. Hmmm...

As always, thanks for the replies to a newbie. But I'm learning fast!

Marc
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Old 3rd March 2007, 02:33 AM   #8
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If you replace the 6L6 with EL34's I hope the power transformer can handle the extra 1.2 amps of filament current...

Sal Brisindi
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Old 3rd March 2007, 05:00 PM   #9
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Quote:
Originally posted by Sal Brisindi
If you replace the 6L6 with EL34's I hope the power transformer can handle the extra 1.2 amps of filament current...

Sal Brisindi
After much reading, apparently it can. My guess is they just used the same transformer from the MC34-A, which takes EL34's as well.
While they MAY use different OPT's, I doubt they'd make a different main transformer for a product so similar.

Guess I'll find out. I decided to pick up a set of winged =C= tubes and listen for myself. I would have replaced the chinese tubes anyway, so why not. I'll also pick up a set of =C= 6L6GC's just to do some listening tests. I guess it really boils down to that.

Marc
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Old 3rd March 2007, 05:51 PM   #10
anatech is offline anatech  Canada
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Hi Marc,
Sal got to the heater current issue before I did. Worth mentioning for sure.

Now, 6L6GC has a max. plate dissipation of 30W, EL34 and 6CA7 are around 25W. I think the design of the circuit determines the best output tube for it. I would tend to stick with 6L6's and try the 6L6EH. If you go for EL34 types, I have found a 6CA7 tends to sound better. At this time I'm using 6CA7EH's and I'm very happy.

-Chris
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