12DW7, 7247, ECC832 -- A Cool Tube!

[SlidePicker]

This is a very useful tube for a lot of applications, with one half of a 12AX7 and one half of a 12AU7 in the same envelope. JJ is currently making a version as the ECC832, and also offers a version with the sections reversed. It has no officially recognized international designation, but JJ calls it an "ECC823." I think it might be VERY interesting to harpers who want a lower gain tube in the first stage of an amp but don't need to drop two stages to such low gain as a 12AU7's. I'm sure you can think of many other situations where this combo would be very useful, such as a reverb drive and recovery tube.

Here is the JJ spec sheet for their version(s) of this tube: http://www.jj-electronic.sk/pdf/ECC%20832.pdf

[Bruce / Mission Amps]

I've been using this tube for years and the only complaint is that up until JJ started making them again, you had to find NOS ones.
Also, for best resuslts, you can't just build a 12AX7 amp around the low gain triode of this tube, you really need to use lower value plate, cathode biasing resistors, bigger bypass and bigger coupling caps with lots of idle current through the triode for best fat greasy tone.

Bruce

[Ray Ivers]

I contacted JJ about ECC 823 samples, and was directed to Bob at Eurotubes; I'm waiting on a reply from him now.

There are so many cool tube types, many of which are still easily available or in current production. Looking at the vast majority of guitar-amp schematics, though, you'd think only a dozen (or less!) each of octal/9-pin tubes were ever made. IMO it's most excellent to see a thread on a tube like the 12DW7.

I remember when I first started messing around with amps and mods, and the only thing that mattered in a tube was GAIN! - had to have GAIN! - and I just couldn't understand why the tube manuals were filled to overflowing with all these useless tubes. But there's plenty of amp circuits in which "gain" is the just about the least important aspect required from the tube in that socket, but what do you inevitably find there? A 12AX7. Yeah yeah, I know - cost, availability, selection, marketing, etc.

OK, no more tube ranting.

Ray

[SlidePicker]

Look at all the gain reduction networks we end up putting between all those 12AX7 stages to make them perform like low-mu triodes. What a waste.

[Rick Erickson]

I'm confused here - why would you need the gain stages reversed? Can't you just hook the wires up to the other side of the socket? I've never cared much for the 12DW7/6247 tubes. Nothing but trouble. Microphonics, noise etc. Perhaps the new JJ's will be better, but I've had the same experience with their 12AX7's and tend to avoid them too. As for 12AT7's & 12AU7's: these tubes weren't designed with high gain circuits in mind and tend to be rather microphonic compared to most 12AX7's. ymmv

RE

RE

[Ray Ivers]

Rick,

I was thinking that it would be kind of a bummer to rewire the socket only to find out that the 12DW7 swap did nothing for you, especially in an older Marshall or something similar; the reversed-12DW7 was mainly to make the plug 'n play option available for more amp types.

The EI 12DW7's have worked well for me so far, but I haven't tried the JJ's yet. I've definitely found some microphonic US-made 12DW7's in dual-chassis SVT's, although that arrangment always seemed kinda microphonics-prone to me anyway.

Ray

[andrew4566]

I am thinking of trying a 12dw7 in a plexi/18watt using the 12au7 section to drive the cathode follower heading into Marshall 1974 powersection. Any opinions on setting up that section? I was going to use the standard 12ax7 circuit but perhaps there is something better?

-Andy

[Ray Ivers]

Andrew,

If you set up the 12AX7 half of a 12DW7 using a 100K Rp and 820 ohm Rk, I would use an Rk value between 33K and 47K on the 12AU7-half CF; lower Rk values bias the CF hotter, and make the 'magic' of the gain stage/CF combination happen a little quicker.

Ray

[soundmasterg]

Ray, can you offer up any good suggestions for plate resistor values to use with a 12AY7 in a CF/gain stage arrangement? I am using a 75k tail resistor in the CF on my brother's modded Silvertone, and I used that because it sounded better than a 100k, but I have no idea if it is an optimal value to use or not?

I like the 12DW7 a lot mysef too, and plan to use one in a project soon if I can get some others out of the way first!

greg

[Ray Ivers]

Greg,

Do you know what the CF grid voltage is? Within quite a large range, the CF Rk primarily sets the idle tube current (and also max Zout on negative signal swings), and as this tube is 10mA max I'd shoot for maybe 2-3mA idle current; measure what you have and see if it's close.

The 12AX7, 6SL7, and 12AY7 all have the same Gm, but decreasing Rp (66K/44K/25K respectively), so scaling load resistances accordingly will reult in the same load line for each. I don't know if this is germane to this thread or not, but it popped out of the tube manual at me so I thought I'd share it.

Ray

[Steve A.]

I'm confused here - why would you need the gain stages reversed? Can't you just hook the wires up to the other side of the socket? ...

Well, it's a dirty word around here but some people play amps with pcb-mounted tube sockets, and they may not want to go to a lot of trouble just for an experiment.

I agree with you about a lot of tubes not being very musical...

Steve Ahola

[soundmasterg]

Thanks for the info Ray. I'll have to do some checking and report back. The amp is in my brother's possession right now, but it has a crackling problem that doesn't manifest itself on the bench, but he still wants me to fix it. So I will be getting it back sooner or later to mess with again.

Greg

[Alexander]

another use for 12DW7's

hey gents-
I used the excellent JJ12DW7 in my SE project amp- check the schemo page:

http://www.retrodyne1.com/BlackcatSE.html

It's also the ideal tube for concertina /split-load PI's for driving EL84's or 6V6's/EL34's i.e. anything that 30 or so VAC swing will overdrive...I used the 12DW7 as the PI in my "Postman" amp, though you can use a 12AY7 there too and it sounds sweet...but the clean sounds especially seem 'greasy' as Bruce opines... I've used the 2M2/1M 'positive grid biased' resistor scheme between the AX and AU triodes with splendid results. Try it if you appreciate a little eye-opener now & then!

http://www.retrodyne1.com/postman.html

I'm right in the middle of two more application tests for this tube since I find it a very tuneful device & haven't had any microphonic issues at all. I have a few NOS USA versions that sound very similar, not the night/day difference some of the 'reissue' power tubes suffer from. From CE Distribution I get them for 6 or 7 dollars apiece, a bargain!

Rock On,
Alexander

[markbul]

Alexander

I can't load your pages.

[Alexander]

seem to load okay here...

sorry, don't know what the problem is. I checked the hosting, all's fine with that...I cleared cache memory, they still open in new windows...

-Alexander