cascode designs

[blaise]

Hello again everyone, I've been gone, training, learning, mastering the art, of the mystical tube
Yeah right !!! So as it probably happens/happened, to everyone at some point,
the more I learn the less I know.
So i've built a few basic designs now to some success, feeling a bit more confident.
The other day I received a set of 6sl7's by accident, the vendor said " keep em"
Fair enough. I thought about a "modified cascode" for bass pre. hoping the
harmonic overtones From the crkt design paired with this tube will give me a rich warm
Chorusee type sound.
Plan:
6sl7 cascode front end, moderate gain, 1/2-12ax7 slight growl stage, with bypass switch,
5f6 tone stack, 1/2-12ax7 cathode follower out. Sounds easy, looks easy, on paper.
here's where I realize I don't know s#*t.
How the hell do I plot loadline for cascode's. Why o why didn't I buy that book "duh"
Well I am waiting for one book to arrive.
In the meantime, anyone ever try this design for bass ?
Blaise.

I added pic of my original scriblings just a base idea, numbers probably don't work,
Since then idea has changed a bit, but basically would like to stick close to this theme
Once I can plot loadline of first stage, I think I'll be off and runnin.

[Prattacaster]

The Valve Wizard
All you should need on cascode design hopefully.

[kg]

the problem is, that's not a cascode front end.

[mooreamps]

the problem is, that's not a cascode front end.

Doesn't matter... All of you should be looking at how this is set up.... Just a couple of redlines on your print...

1. Missing plate coupling cap on the second gain stage.
2. On the third stage, the cathode follower circuit.. D1 in the wrong place. Remove D1 and tie R7 directly to R6. Then , insert D1 between R6 to the cathode pin 6 of the cathode follower in the forward bias direction..
3. on the first gain stage, remove the jumper from pin 4 to pin 6 and replace with 900K 1/4 watt.
4. Plate coupling cap on the first gain stage, use either 2.2 to 4.7 nF.

good luck....

-g

[blaise]

Thanks everyone, wow only 4 redlines, I don't feel too bad about that !!
The valve wizard site is most helpful too.
I'll redraw and calculate values.
If I do get this thing realized I'll be sure to post it.
Back to school !!!
Thank you,
Blaise.

[redelephant]

Doesn't matter... All of you should be looking at how this is set up.... Just a couple of redlines on your print...

good luck....

-g

Nah. He asked about cascode, and that's all I quickly had a peek at. Completely ignored the rest. Sometimes I do thorough analysis, and sometimes only look at what is in question.

As kg said, it's not a cascode. It's close to a SRPP, but more like the 'Aikido' type stage found at 'tubecad.com'.

In a cascode you can approximate gain by assuming the plate voltage of the bottom triode is constant. So modulating the grid will modulate the plate current close to the tube's Gm. So if Gm is 2mA/V and the load resistor (the one on top of the top triode) is 50kohms, the gain is 100x.
Unfortunately the voltage does swing a little on the bottom plate creating some negative feedback, so the real gain is a little less. There is of course a proper formula, but not in my head, but get it close enuff using the mentioned reasoning.

Edit: forgot to mention: The circuit you first drew, with the top triode acting as load for the bottom works very well, you might want to try that as well as cascode.

Here's a cascode:

[blaise]

see I learn more here than anywhere else, that's why I drop in from time to time
I'm not trying to ride anyone's coat tails, it's just nice to have someone to ask a question.
Maybe one of these days I'll be able to give something back.
Thanks,

[redelephant]

Oh, another thing with this type of circuits. Remember to watch the tube's max cathode to heater potential. I've ignored it many times and been lucky so far, but you should have it in the back of your mind.

Yeah, I learn a lot here too. That's why I keep bugging in and often talk more than I walk, but how else will I learn?

[blaise]

I just noticed when I originally posted this, I edited posting to add the
Image, but it started new thread, I don't see a delete thread option
To get rid of the other one.
I checked out that tubecad site, some interesting designs there to chew on.
I think I gotta slow down and stick with some more basic designs for now.

[kg]

Doesn't matter...

the title of the thread disagrees with you.