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.

[surfsup]

Did you ever to the cascode? I just designed a 6sn7 cascode input stage but I would put the CF before the TS perhaps.

CCS(6sn7)->CF->TS->GS->CF->SE PA

I plan to use a mosfet for the first CF before the TS and then a tube CF as the last one using a lower R to get some current draw and 2nd order compression.

I realize the 6sl7 is not the same tube, just curious what you think of the CCS input stage.

[Gaz]

I just built an amp with a cascode input - a 12AU7 one, in fact. It's low gain compared to other cascode circuits, and sounds just a little different than a triode. I think the magic happens when you have a high gain cascode(or pentode) hitting the PI hard with little tone shaping circuitry inbetween. Also, actual pentodes go into cutoff earlier than a cascode, so you can actually get a little character out of the pentode with just your guitar signal. I cascode may be better placed later so it can actually be overdriven.

[Gingertube]

I've thought about it but never done it (yet).
The beauty of a cascode is high gain and low noise. The ideal place for it is right at the input (1st gain stage). It gives you clean signal level boost allowing more effective overdrive of latter stages. The ideal tube for cascode is ECC88 (6922, 6DJ8).
There is at least one small commercial amp manufacturer doing it but he introduces a new "wrinkle" in that the cascode has a control he calls TULP (Triode Ultralinear Pentode). Do a search on "TULP" or on "ReVintage".
Note that the Cascode has an output impedance approximately equal to the top tubes anode load resistor (since there is a VERY high impedance loking into the top triodes anode) and its gain is approximately equal to the lower triodes gm x the top triodes anode load resistor (so keep tube current up to maximise gm). It also has next to BA (Ozzie Vernacular for "bugger all" meaning not much) power supply rejection and therefore requires a clean power rail.
Cheers,
Ian

[surfsup]

Well I promised to post my results so here they are:

I got to play this about two hours fairly cranked with just one retired couple in the building. I am digging the amp at this point. Very touch sensitive. Holding the pick lightly and strumming gets me nice clean chords. Firm grip and jamming the strings gets me a nicely distorted chord with pleasing tone. Same with single strings or double stops, etc. The MV move to before the PA seems like it was what it needed so the signal is preserved as much as possible through the TS into the 12ax7 GS.

Modding today, I paralleled a 1k2 with the 1k5 on the CC input stage Rk. I think that's a bit too dirty as even single strings have a little distortion. I am trying to decide whether to leave it 1k5 or parallel a 2k7 with it (removing the 1k5 is gonna prob be too much trouble) to get me in the 1k0 ballpark. They both sound good, 1k5 being really clean with the 12ax7 cap switched out, but just not quite enough dirt I think if I bypass the R. Maybe I will try and adjust the gain on that tube a touch with a different Rk on the GS or put a 47k on the CF Rk instead of the 56k. Still no grid stop on the EL34.

Still needs a bit of tweaking and cleaning up (the amp has been modded several times now and getting a tad messy inside) but sounds good already.

[vibrovox7reverb]

I'm not trying to steal your thread, but I have a question that may be related. Since cascodes generally have lots of headroom and high gain would they be suitable as drivers for single ended amplifiers? They sound like they might be, but I've read that it's actually better to drive some power tubes with a high current tube instead of high gain.

[Alf]

I just built an amp with a cascode input - a 12AU7 one, in fact. It's low gain compared to other cascode circuits, and sounds just a little different than a triode.

I would very much like to see the schematic for this one. I have had this in mind for quite some time now. Too little time unfortunately.

Maybe you would be willing to share it here ?

Regards, Alf