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Old 9th April 2010, 03:38 PM   #1
rknize is offline rknize  United States
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Default Morgan Jones and the ECC88/6922/6DJ8

One thing about MJ's book (at least the 2nd ed. I have) is that he seems to gravitate to the 6DJ8 and friends just about wherever possible. It always struck me as a little odd because I would think he'd want to expose the reader to other possibilities. Was this tube in style at the time (late 90s) or is it just a matter of it being his personal favorite tube? I ask here because I seem to recall that one or two folks here seem know him personally.

What brought it up was that I finally found my "Bevois Valley" amp chassis. It never worked due to stability issues, but I know a great deal more now about tubes than I did a decade ago and so I want to fix it. I'm think of converting it over to use 12AT7s instead of 6DJ8s as I've had more experience with the former. In the past year or so that I've been on this forum, I really haven't seen anyone use the 6DJ8 in any new design. Any particular reason for that?

The amp is on a compact chassis meant for a bookshelf and our office needs a real amp, but that is another thread....
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Old 9th April 2010, 04:21 PM   #2
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The ECC88 just about fits for every application with good performance. It is also one of my favourite types. I get great results from it in whatever configuration and it's variants are plentiful and fair priced.
This tube was produced en mass in the 1950's for UHF TV tuners and early FM and finds itself ideal for amp front ends.
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Old 9th April 2010, 04:22 PM   #3
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Originally Posted by rknize View Post
I really haven't seen anyone use the 6DJ8 in any new design. Any particular reason for that?

The reason it's popular with MG and commercial designs is obviously the availability. I wouldn't normally use it simply because so many other tubes sound better. 12AT7 is not one of them IMO.
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Old 9th April 2010, 04:31 PM   #4
SY is offline SY  United States
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Originally Posted by rknize View Post
What brought it up was that I finally found my "Bevois Valley" amp chassis. It never worked due to stability issues, but I know a great deal more now about tubes than I did a decade ago and so I want to fix it. I'm think of converting it over to use 12AT7s instead of 6DJ8s as I've had more experience with the former. In the past year or so that I've been on this forum, I really haven't seen anyone use the 6DJ8 in any new design. Any particular reason for that?
He talks about several other tubes in the 3rd edition (worthwhile to get!). But the ECC88 is indeed very easy to get, performs well, and is pretty inexpensive. An example of a 12AT7 used in a circuit with the same ancestry as the Bevois can be found in my Red Light District article. I haven't heard the two amps side-by-side, but I'd suspect that the differences between them at levels lower than clipping would be pretty minor. The RLD would, I believe, have better clipping and overload recovery behavior.
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Old 9th April 2010, 04:33 PM   #5
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The reason it's popular with MG and commercial designs is obviously the availability. I wouldn't normally use it simply because so many other tubes sound better. 12AT7 is not one of them IMO.
Since I am about to embark on an amp with a 12AT7 driver, what would you suggest as a similarly parametered alternative that sounds better? My application is a direct input 12AT7 LPT with CCS tail driving PP EL34s.
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Old 9th April 2010, 04:39 PM   #6
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Quote:
Originally Posted by SY View Post
He talks about several other tubes in the 3rd edition (worthwhile to get!). But the ECC88 is indeed very easy to get, performs well, and is pretty inexpensive. An example of a 12AT7 used in a circuit with the same ancestry as the Bevois can be found in my Red Light District article. I haven't heard the two amps side-by-side, but I'd suspect that the differences between them at levels lower than clipping would be pretty minor. The RLD would, I believe, have better clipping and overload recovery behavior.
Indeed, I have looked at your RLD design in the past. I've been poking around with George's Simple PP as well and have learned a few things in the process.
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Old 9th April 2010, 04:49 PM   #7
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Quote:
Originally Posted by analog_sa View Post
The reason it's popular with MG and commercial designs is obviously the availability. I wouldn't normally use it simply because so many other tubes sound better. 12AT7 is not one of them IMO.
OK, well obviously that is a matter of personal taste.

As another example of where I am coming from, the overall consensus here and elsewhere is that the 12AU7 is not a good tube to use in a gain stage because of its non-linearity. I built a preamp back in the same era using one...mainly because I had a pile of them but also because they look like a good candidate on paper. I used it in a "mu-follower" config (using MJ's design process) and didn't find the linearity to be all that horrible. Maybe because there is some local feedback in that design?

I also used it as a cathode follower which worked very well, but obviously that has no gain to speak of. My other "Thomas" amp uses one as a phase splitter, but again...no gain.

So yeah...I was just trying to make sure I wasn't missing something.
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Old 9th April 2010, 04:54 PM   #8
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what would you suggest as a similarly parametered alternative that sounds better? My application is a direct input 12AT7 LPT with CCS tail driving PP EL34s.

I have successfully used E280F and D3a triode wired in a similar application. Obviously two tubes will be needed to replace one 12AT7 which means more space, heating power and expense.
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Old 9th April 2010, 05:00 PM   #9
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didn't find the linearity to be all that horrible.

I wouldn't use such a strong term to desribe any tube. After all the ARC SP10 is by many considered the best preamp ARC ever made and it's chock full of ECC88s. And Jadis use 12AU7 in top flight amps. So, yes, it's largely a personal preference.

Last edited by analog_sa; 9th April 2010 at 05:29 PM.
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Old 9th April 2010, 06:43 PM   #10
tomchr is offline tomchr  United States
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One thing to keep in mind about Morgan Jones is that he is very opinionated. This shines through quite clearly in his book. For power amps it's 6SN7 and friends, push-pull, global feedback or go home. None of this crappy, inefficient, good-for-nothing, single-ended directly heated triode sh....tuff. That said, I do find that I frequently return to the 3rd edition of his book simply because it's such an easy read and covers some approaches that aren't strictly tube-only (SS CCS, LED bias, etc). The 3rd edition of the book has a fairly comprehensive comparison of a bunch of different tube types with respect to THD, and I find his reasoning behind picking a particular tube over another to be quite reasonable.

Reading Morgan Jones reminds me a lot of reading Douglas Self. With opposite sign, of course (Self hates tubes with a passion).

~Tom
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