Heh. I don't even know where it's going! Basically I was looking for a new tone character in my 18W Normal channel and this seems to be doing the trick for me, as in doing something new. However now I am in the process of converting "new" to "good". There are definitely some good things going on with the cascode but I'm not sure it's entirely mated to the rest of the 18W just right yet.
More testing to ensue
I'll be back in the studio tomorrow so I'll bring it along and record some with it. I always forget to check this stuff with my strat so I'll try and do that too. It's hard to crank it at home, and I don't think I can record here without a different mic pre because a fully-cranked 18W will cause the pre to clip.
FWIW I really love this cascode running clean with my Tele!
Noel Grassy wrote:This has been a fun raed. I like where it's apparently going. Dehughs,sir, what's the Latin in your signature translate as? Everytime I've read one of your posts I feel it says something about me.[guilt?] I never really studied up on some Latin befores. cheers
If memory serves, it is translated roughly as "Time, the devourer (or eater...) of all things." I believe it is from one of Ovid's works, but I can't remember exactly...
I like your Spinoza signature...I enjoyed reading his work, but haven't done so in a while.
I think I settled on a 820 ohm cathode res. and 28K plate.
I reworked the tone control and need to work on it some more since it goes from "barely not bright" to "insanely bright" through the range right now. But the gain range is right. The amp freakin' screams!
BTW according to a calculator I downloaded, of course ignoring biasing of the cascode, I may have in the range of gain = 300+ in this circuit. That's pretty hot! It's definitely in a whole different league of crunch compared to a standard 18W.
krash wrote:I think I settled on a 820 ohm cathode res. and 28K plate.
I reworked the tone control and need to work on it some more since it goes from "barely not bright" to "insanely bright" through the range right now. But the gain range is right. The amp freakin' screams!
BTW according to a calculator I downloaded, of course ignoring biasing of the cascode, I may have in the range of gain = 300+ in this circuit. That's pretty hot! It's definitely in a whole different league of crunch compared to a standard 18W.
You left out the cathode bias resistor for the top tube in your schematic. Self-biased cascode requires it to properly bias the upper tube. Use the same value as you have for the lower tube. You can bypass it with a cap if you like for a bit more gain, but it isn't necessary.
I have been considering using the self-biased SRPP (Totem Pole, Cathode based Cascode) shown in the Glassware Tubecad software. Each triode has an identical Rk. B+ goes straight on the upper triode and Ra is on the lower triode, right below the upper Rk. The output comes off the cathode of the upper triode. With a 12AT7 at 400V B+, Ra 11K, Rk 1k2, current 2 mA, gain is 57, Vo max is a huge +/- 141V, and Z out is 586 ohms.
Given Mr. Aiken's preference for the 12AT7, how does this look as either a first stage or final peamp stage?
The trem guitar track is the trem channel of the 18W. The rhythm-crunch guitar that comes in on the chorus is the stratocaster into the 6DJ8 cascode channel. That's with the vol on about 5 IIRC.
It's a big, fat, gritty, aggressive kind of tone. Not smooth. More like "mean". I like it a lot, especially since it's a whole different flavor from any of my other amps.
This is my 1st post on the Amp garage, so why not dig up an old topic simply because I presently am doing similar things however using local feedback loops along with a nice twist on the Ultra-linear cascode topology.
the measured voltages are from a working design using several fresh samples of each listed tube type and the Avg. published.
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Hi, If I wanted to use the Valve Wizard self biased cascode 12au7 in a Rocket would I still use the 500pf / 500k pot coming off the anode? Thanks for any advice.
andrew wrote:Hi, If I wanted to use the Valve Wizard self biased cascode 12au7 in a Rocket would I still use the 500pf / 500k pot coming off the anode? Thanks for any advice.
Yes- that's you're coupling cap and volume control- you need them!
FWIW, I am a huge fan of this cascode 6DJ8 design, and I am putting it in many of my production amps. I have been meaning to try it with a 12AU7. I think this is an unfortunate neglected gain stage design that has a lot to offer.
cheers
