1J24B part II: Tube Based Audio Timbre Trasher and Experimenters' Board

From this previous post: audio bench experiments using the interesting, inexpensive, small, low voltage 1J24B pentode continue. 

What else can we use this miniature valve for, in our rack, our bench, and so on?

How about a timbre modifier? While breadboarding, I noticed that putting a capacitor between the tube's screen and ground seemed to distort the waveform found at the anode (the tube's "output").  As the screen voltage changed, differing frequencies were driven through the cap, causing odd waveform distortions you could see on a scope (to use US slang--we are "trashing" the signal--a "timbre trasher"?  Why/why not?).  

Breadboarding a 1J24B, with its tiny exposed wires, proved difficult, so last time I designed a 100mil breakout board for the tube and its power (post here, project notes with gerber is here).  

I could probe the buffered and cap coupled output of the single 1J24, and auditioned the prototype through my bench preamp/amp/speakers. 

To my ears, with all the buffers set up correctly, the design sounded a bit "filter like", maybe reminiscent of a 12db/octave VCF without resonance.  Cool!

But even with the 1J24B breakout board on my bench things got unwieldy. 

I figured a dedicated "tube trasher" PCB for further experimentation would help a lot. So I drew one up....

The PCB looks like this:

I got this fabbed by the blog's sponsor, PCBWAY, and here's the shameless plug: please help out the audiodiWHY blog and check them out.

The design pretty simple; the 1J24B is the center of the design, with a 30V cathode/plate voltage differential (an idea borrowed from Ken Stone's tube VCA, read more about that here--clever!)  Also a few op amps, since tube circuits like this require bias offsets and buffering, as well as 4 pots set up as voltage dividers between the V+ and V- rails.

 

The PCB is relatively large for the number of components used, making trace cuts, customizations, and/or offloading parts of the design to a breadboard, relatively easy and quick.

For fabrication I elevated the passive components somewhat, to make it easy to get a probe on one of the leads, or do modifications.

Then was a matter of populating whatever part of the board I felt I needed for the experiments. 

For the videos you see and hear below, I used 2 experimenter boards in series, with a 1uF AC-coupled feedback paths between boards' input and output buffers. 

Anyone who wants to follow along, see the wiring setup, get PDFs and Eagle files for design so far, and so on, should please go to the github repository here. 

You can also get a gerber for the experimenter board from PCBWAY's project page here.

Benchomania: I used "tall trimmer pots" to manually dial in the offsets, which I then noted; I will make a 3rd revision of this board soon with the values for the voltage dividers approximated with resistors, hopefully making the board less expensive to fabricate.

It works....

So far, this month's experiments have yielded a satisfying timbre shifter when 0-5V CV was applied to the first 1J24B's PCB screen, and an buffered and bias offset 1-2V P/P is applied to the grid, but the offset voltages seemed too sensitive for this to be fully practical, so this design still has a way to go; perhaps multiturn trimmers will have to be designed into the next PCB iteration along with some other tweaks?  Probably.

The video is a single experiment board, built and calibrated, being modulated by a 0-5V control voltage. The bias offsets used throughout are roughly documented at the bottom of the post here, but each 1J24B seems a bit different.  Again trimmers might be the way to go here.

For the incomings signal I used a 1-2V ramp wave with about -3V DC offset, from a Siglent waveform generator, but I had to try different things; the bias offset and amplitude of a source audio signal greatly influences what you hear at output, and too much or too little offset and amplitude for the source can make the entire output go dead--but once it's dialed in correctly, it works, but the "timbre mod" is pretty subtle and is more visible on a scope than with my ears. 

Next I wired 2 boards in series, and got the feedback going between the two. CV modifying the timbre comes from the output of an "LFO PRIME".  In this configuration, you can hear a bit more drama in the CV sweep.....if you want to learn more about what I've gotten to work so far, a wiring diagram, notes, PDFs etc., of the setup you see in the video below, is available via github, here.

Since 2V P/P saw wave gets nicely transformed into other strange shapes reminds me maybe a bit of "West Coast" work by Serge--back in the early 2000's I built a clone of the Serge Waveshape modifier, this design sounds a bit like that. So far, so good.

OK! For the next revision I'd like the 2 tubes and the buffer board to be reasonably small, but the design can't be SMD (yet) because too many design points are still being worked out. Maybe by next month I will have rev 3 ready to go? Stay tuned.

 Update 3-3-22: boards are designed, I should have them back from PCBWAY next week.  Assuming I can make it work, I will post the updated design in an upcoming post.