Lightning on a Sunny Day.

Check the bolts!  This is Sun Storm prototype #5.  The onboard high voltage power supply makes enough heat that I felt it was necessary to add venting to the top lid.  Instead of some generic vent holes I came up with this idea to laser cut lightning bolts into the lid, back it with an EMI shielding vent screen, and backlight the scene with yellow LED's.  It almost begs you to place the Sun Storm someplace prominent.  Todd, the buyer of SS#5, said, "Beyond cool.  I hope that (lightning bolts) makes it to the production model."  It shipped off to him yesterday, but not before I had the chance to record some audio examples through it.  I need to spend some time going through what I recorded before I can post anything here.  It'll probably be a few weeks before I can scrounge up enough spare time to do that.  For now I'd like to continue with information about the massive updates in this prototype.   Pictured below is the same prototype before the vent mod to the hood.  It's here for reference of the current front panel layout.

Frequency Responose = +/- 1dB 7Hz-24.3kHz. -3dB at 5Hz and 29kHz.
​Attack Range = 0.5ms-56ms
Release Range = 51ms-292ms
Max Compression = 20dB
Noise Floor = -102dB A-weighted RMS
THD+N = 0.145% with a 50-Ohms 50mV 1kHz sine wave on the input and with the make-up knob at 100%.
Tube compliment = 12BH7 (V1), 12DW7 (V2), 12AT7 (V3).  All 3 tube types are currently in production and are widely available.  

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Small disclaimer:  The guts above represent the design of #5 before a number of final changes.  It's about 15% less developed above than the final outcome for #5, but it looks about the same. 

​There's definitely some key stuff to discuss here.  The most visible differences from the previous prototype is there's almost no space left inside the enclosure now and there's a 3rd vacuum tube, which is bigger than normal.  Lets start with the tube.  Actually, the only tube that carries over from the previous prototype is the 12DW7.  It's the perfect tube for splitting the incoming signal into the audio path and the control signal path (CSP), but it does a lot more than just that.  Mostly, for the CSP it supports several function, including the compression knob.  The 12DW7 is the one in the middle that we'll call V2.  To it's right is a 12AT7, V3.  The 12AT7 is the audio amplifier responsible for the make-up control.  It was a 12AX7 in all 4 previous prototypes. I came up with a much better amplifier design when I realized I don't need a lot of gain to make the make-up control work perfectly.  The 12AT7 expanded the frequency response of the Sun Storm, while also dramatically lowering the noise floor and the THD (total-harmonic-distortion).  Now about that big ass tube.  The 12BH7 is sort of like an overweight 12AU7.  It can be used to drive things up to a ridiculous 7 Watts peak if both triodes are used.  I'm only using 1 triode here, but its for that exact reason of wattage.  I had a hunch from the previous prototypes that I couldn't get faster attack times because there simply wasn't enough current flowing through the CSP.  Adding a bigass 12BH7 tube stage to the CSP just before entering the LBA Opto-Cell 1A (black box with white LBA logo in gut shot above) required a lot of supporting electronics, but it worked, easily speeding up the attack time from 5ms to 0.5ms.  It also happened to triple the amount of max compression.  This required a bigger, more complex triple high voltage boost converter.  It took me a couple tries to get the bugs worked out, but the power supply works great and is low noise.  It's a little power hungry and gets warm when powered on for medium-long periods of time.  The tubes themselves get much hotter than the power supply.  I'll have to print on the next one somewhere (keep away from flammable objects).  It's the heat that made me decide to add venting to the top lid.  I think that was a good move.  
I discussed the 12BH7, but now lets talk about the new controls.  On the next prototype(s) I plan to add an extra knob to the front panel, labeled "Time Const." for time-constant.  On prototype #5 it's 2 options for the T.C. - Fast and Slow.  It's actually the "Link" switch on #5.  Todd elected to omit the stereo link capability on this mono unit in exchange for ability to have a wider range of attack and release settings.  In the next prototype I plan to use a 3-position rotary switch to select fast, medium, and slow. These settings basically set a range for the minimum and maximum milliseconds for the attack and release.  On the back panel there's a ground-lift switch and a strange little limiter control.  The "limiter", if we may call it that, operates off the same CSP, but on it's own tube buffered branch.  It is therefore tube powered.  It affects the audio separately, and in parallel with the Opto-Cell 1A (pictured above in the gut shot.  Black box with white LBA logo).  The LBA Opto-Cell 1A is a photo-optical gain-reduction device.  The "limiter" is controlled by the same "compression" knob that controls the Opto-Cell 1A and in the same manner.  Turn up the compression knob and you will increase both the amount of compression and the ratio.  I think the limiter is very unusual at the very least.  It's a negative-feedback zener-diode soft-clipper.  It sounds hairy.  It definitely limits, but it sounds slightly distorted in a gentle fuzz sort of way.  I have mixed opinions about that.  It's certainly a tonal tool, as it has a "sound".  I found that sound to work really well on electric bass, and sometimes on vocals.  I'd prefer if it had less of that distorted quality, but that does make it very unique.  I'm thinking about filtering it for the next prototype so that the highs >6kHz are not affected.  I think that would smooth it right out like butter on hot toast.  I could do the same for anything below 50Hz so it sounds a little more open.  I noticed the limiter sounds better and more-open when the "side-chain" switch is engaged.  I'd have to double check the frequency, but I believe the side-chain is a 75Hz roll-off to the CSP.   
It's getting to be a long post, so I'll wrap it up.  If you made it this far, thanks for reading and following along on this long adventure.  It's been 10 months since this all began.  I need to take a little time off from the Sun Storm project to reset.  There's a lot of work building up that needs my attention.  The next Sun Storm will be a stereo unit.  Please check back.