Rump Roast - submini tube version

[jubal81]

I posted this on Madbean's forum, too:

I've been in love with this pedal since I put it together - don't play without it turned on.
I was thinking it would be really cool to design it using submini tubes to use as a pedal or possibly as a preamp with a poweramp stage in a tiny amp.

I can follow others' recipes pretty well, but I know very little about designing with tubes.
Here's a VERY basic mockup blending diagrams from Madbean and Frequency Central. ALL credit goes to them.

[frequencycentral]

Nice! Soundclips? 

[jubal81]

Nice! Soundclips? 

LOL. I wish. Just photoshopped this together a little bit ago. Actually hoping you might take a quick look to see if anything is wildly out of place.

[jubal81]

Here's a quick demo of the pedal with an iPhone.

[frequencycentral]

Ah, sorry, misunderstood, thought you'd done it.

Grid #2 of the 5672 (pin 2) does not correspond to source of the J201, though I see what you're doing - but the source of a FET actually corresponds to the cathode of a tube, and the cathode of the 5672 is not accessible as it is directly coupled internally to the heater's 1.25v. So you're going to have to make some changes there. Discard what you have there and go back to the Pentadriver and see how that's set up with respect to grid #2.

Secondly, the trimmers on the drains of the J201's in the Rump Roast are there because (biasing) FETs vary so wildly, this is not the case with tubes. No need for a trimmer, a fixed resistor is fine. Again, go back to the Pentadriver. The Pentadriver values with respect to tha anode and grid #2 may (not) be optimum - you might find a better set of values.

Other than that, good to go. You gonna breadboard it?

[iccaros]

FC
not to step on the conversation but for my education, if you do not mind.

I understand mixing the pentaboost in this design (more gain) , but would not a single 6111 or 6021 be a better match, you could replace the fets one for one and just drop the bias adjustment.

In this case you would configure the pentode in a triode configuration and tie pin 2 to the anode?  
The resistors and caps on the cathode help with the bias, so you may have to adjust the anode resistor when you have not access to the cathode. you can calculate the roll off and try placing the tone shaping after the ouput of each tube (off the anode)

my understanding is that the rump roast is a take on a DR. Z amp preamp section --> http://www.flickr.com/photos/49108115@N04/4501456659/

[jubal81]

Thanks for the advice guys.
It's for sure based on a Dr. Z - the Carmen Ghia.
I know the audio on that demo is awful, but I was really trying to get across the clarity and subtle harmonics the pedal adds to my rig.
Another idea I think might be cool is if a power stage is added to make it a tiny amp, to also add output switching so it could also be used as a pedal, with just the preamp stage going out.

Here's an updated draft using two halves of a 6111 to replace the transistors.



And here's a copy of Madbean's schematic:

[iccaros]

good, except the heaters, 6111 takes 6.3 volts not 1.2

do you have any 6111 to breadboard with?

[jubal81]

good, except the heaters, 6111 takes 6.3 volts not 1.2

do you have any 6111 to breadboard with?

Unfortunately, no. And I'm moving soon, so I don't want to place any orders. It'll probably be a few weeks before I can play with this.

[bean]

I've got a couple of 6111s. I'll give this a try next week and report back.