12dw7 cascode to approximate EF86

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jhawk
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12dw7 cascode to approximate EF86

Post by jhawk »

I've been more enamored with the early Vox sound recently, especially the thick yet chimey sound of the EF86-equipped normal channel. I've heard nearly everyone say that the EF86 goes microphonic in a combo, however. Since I plan to build a combo, I want to skip the EF86 altogether and use a cascode.

I've heard that cascodes do a generally good job of subbing for a pentode. The 65 Amps Marquee and the Carr Artemus both employ cascodes in place of the microphonic EF86. From another thread, someone figured the cascode in the Artemus looked something like this, using a 12ax7:
IMG_6701.JPG
I've heard, however, that the 12dw7 (one triode from a 12au7, the other from a 12ax7) is a better fit for a cascode than a 12ax7. Could someone help me with a cascode schematic using a 12dw7? Preferably in a vintage AC15 setting?
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Kagliostro
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Re: 12dw7 cascode to approximate EF86

Post by Kagliostro »

Give a look here and see if there is something of interest for you (about the use of the 12DW7)

https://www.tubecad.com/2014/05/blog0291.htm

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jhawk
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Re: 12dw7 cascode to approximate EF86

Post by jhawk »

This looks interesting, although he uses the 12dw7 in a cascade arrangement, not a cascode:
IMG_6704.PNG
He does have cascodes outlined on other parts of his site, however. https://www.tubecad.com/2012/10/blog0248.htm
IMG_6705.PNG
https://www.tubecad.com/2012/10/blog0247.htm
IMG_6703.PNG
In the SE amp example, I'm slightly confused with the capacitor in parallel with the resistor attached to the cathode of the input triode. The Cascode standalone example doesn't have this capacitor. What is its purpose?

Also in the SE amp there's a resistor right off the grid of the second triode which is absent from the standalone example. What is the purpose of this resistor?

Sorry for my [probably] dumb questions!
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jhawk
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Re: 12dw7 cascode to approximate EF86

Post by jhawk »

The resistor is just a plain old grid resistor, isn't it? :roll:
strelok
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Re: 12dw7 cascode to approximate EF86

Post by strelok »

If you use a 12DW7 I'd recommend putting the AU7 half on the bottom (input) and the AX7 half on top. This should give you optimal gain and performance out of the circuit. Though that's coming strictly from a Hi-Fi perspective. Which way around sounds better you'd have to experiment with to find out. I'd also recommend biasing the top grid with a voltage divider rather than off the cathode, its a bit easier and more stable. You could also replace the bottom resistor of the divider with a zener stack to get the correct voltage but this may be a little too stiff for a guitar amp.
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TUBEDUDE
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Re: 12dw7 cascode to approximate EF86

Post by TUBEDUDE »

The cascode doesn't really simulate an EF86. Have you tried a 5879? It's a real pentode and has less noise issues. If the Voxy chime is what you're after, that may be a stronger option. Try an isolated socket also. Have you heard the Artemus or Marquee?
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jhawk
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Re: 12dw7 cascode to approximate EF86

Post by jhawk »

How does this look? Merlin B uses a self-bias arrangement with a 12au7 cascode near the bottom of this page: http://www.valvewizard.co.uk/cascode.html. My schematic draws from his and the Artemus schematic in my first post.
9B428D10-4362-4E71-83E7-2A30311FB07C.jpeg
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jhawk
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Re: 12dw7 cascode to approximate EF86

Post by jhawk »

TUBEDUDE wrote: Tue Oct 17, 2017 11:08 pm Have you heard the Artemus or Marquee?
Yes. See my original post where I mention both these amps (which both employ a cascode in place of an EF86).
tubeswell
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Re: 12dw7 cascode to approximate EF86

Post by tubeswell »

FWIW, here's some other variants of cascode circuits using dual triodes
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jhawk
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Re: 12dw7 cascode to approximate EF86

Post by jhawk »

Thanks tubeswell! I’ve seen you pop up in cascode discussions before. How many of these have you tried before? Is there anything special about using a 12dw7, or would a 12ax7 or 12au7 be just as good or better? (Better in this case meaning closer in sound/response to an EF86.)
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Kagliostro
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Re: 12dw7 cascode to approximate EF86

Post by Kagliostro »

EF86 .... BTW, have you ever seen this ?

Image

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tubeswell
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Re: 12dw7 cascode to approximate EF86

Post by tubeswell »

As you know, the 12DW7 is a 2-types-in-one triode. The input triode controls the input sensitivity and transconductance, so its best to use a high gm triode (the AU7) for this, and the AX7 as the grounded grid amplifier (output triode) for more gain, a reasonable amount of clean output signal swing, and good bandwidth.

With a 12AU7 input triode, you ought to be able to get the 'screen'* voltage down to around 60V and still have reasonable performance (although around 80V would be 'optimal'). The choice of 'screen'* voltage is a balancing act, because if it gets too high, output signal swing is reduced. If its too low, gain is reduced.

* the g1 voltage of the upper triode

JJ do a 12DW7 http://www.jj-electronic.com/en/ecc832-12dw7 They used to do a version called the 'ecc823', where the pins are on the opposite sides, but I can't see it on their website these days.
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gingertube
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Re: 12dw7 cascode to approximate EF86

Post by gingertube »

A couple of "off the cuff" commects.
The gain of a cascode is approx. equal to the gm of the lower triode x the upper triode load resistor - so use the higher gm 12AU7 half of the 12DW7 as the lower triode.
Just finished a Gibson GA40 Les Paul Clone - didn't have any 5879 pentodes so I used 6BR7 since they are closer in specs to 5879 than an EF86 is - very happy with it, no microphonics problems.
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TUBEDUDE
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Re: 12dw7 cascode to approximate EF86

Post by TUBEDUDE »

jhawk wrote: Wed Oct 18, 2017 10:01 pm
TUBEDUDE wrote: Tue Oct 17, 2017 11:08 pm Have you heard the Artemus or Marquee?
Yes. See my original post where I mention both these amps (which both employ a cascode in place of an EF86).
Sorry, i saw the mention of these amps, and what other sources revealed about the circuitry, but it was not clear if you had heard them.
If they have that chime, i am interested also. That cascode circuit looks a lot simpler than the mu-follower i tried in my last amp. Looking forward to progress, and hopefully sound samples down the road.
One mention, biasing up the heater voltage can insure the heater to cathode voltage is not exceeded.
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jhawk
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Re: 12dw7 cascode to approximate EF86

Post by jhawk »

Tubedude, my apologies! I read “Have you heard of the Artemus or Marquee,” which is obviously not what you said. I have not heard either amp in person, only YouTube demos.

There’s a fellow who did a build using the Artemus cascode with decent sounding results:
https://youtu.be/nIoAKSxsXwE
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