3 Channel Switching - Needs verification/rejection

[panterafanatic]

Dropbox Amp Switching pdf

I've got it printed as a .pdf in my computer. The input/output MosFETs will turn on and off when whichever channel is high (the other two are low). I've gotta arrange the bias arrangement, but that can be done later.

The idea is that when one switch is closed, it turns off the other flip-flops, and turns on the channel I want. It could be done with positive or negative logic, either using 2 NAND or 2 NOR gates per flip-flop, or by utilizing the quad flip-flop arrangements of the 4043 and 4044. I'm not sure about bouncy switches or stepping on the same switch twice as to the output. The diodes provide isolation so that the resets can go high, but won't screw everything up. The 3 channels are independent, rather than dirty/clean-lead using 2 DPDTs. If this works it could ideally be used for any number of channels, however the gates are limited in output current so additional drivers would be needed for long chains; why someone would need more than 5 independent channels is beyond me.

Any comments would be appreciated.

[PRR]

No comment on the logic. I have a gut feeling it isn't that simple; also that the buffers and zeners are redundant, but I may be wrong on all counts.

How does the logic control the MOSFETs? Yank the gate up and down?

I do think the variable G2 bias (which you are not asking about) has way too much Silicon in it. There's no need to regulate; tube circuits work fine if all voltages scale together, and your voltages won't vary much over +/-10%. Small pentode current is small, G2 current even smaller, it seems excessive to use a MOSFET to buffer a pot. I have G2-fudged 6550 with just a 100K pot, albeit a 2W pot. Yes, a 0.5W 100K pot on full 300V will burn, but a 250K pot should survive. You may not need 300V on it, a 120K resistor on top will give a 200V range. The 47uFd is also excess, 5uFd is ample, screens were often bypassed with 0.5u in hi-fi, 0.1u for gitar.

[panterafanatic]

The zeners are redundant, the first buffers are to clean up any bounce, I'm not sure if momentary SPSTs suffer from bouncy switches or not. I'm not versed in logic at all except the simple gates and recent reading on flip-flops in my CMOS Cookbook. Those 3 buffers could be better suited to LEDs for the channels since bounce on the same latch should just keep it latched. I put a good bit of thought into it, but my inexperience in logic is what made me question it.

Yes, I was gonna change the gate voltage from about +30 to -20 or so. Give or take a few volts. I'm sure you know that the source will follow the positive one, so it can exceed the max rating of the gate-source voltage if needed, I might add protection with diodes, wouldn't hurt.

I wanted reliable control of the screen without having to worry about the compression effect of the varying screen. The "Edge" control allowed the cathode resistance to compensate for compression. One thing I don't like about having the variable screen voltage to select the input sensitivity is not being able to tailor the compression to frequency.

I habitually over design. 

[defaced]

I had a similar question a couple of years ago on a different forum, http://music-electronics-forum.com/t11908/.  I verified the 74LS374 circuit Enzo posted in the second post; this does exactly what you're looking for with one chip and a couple of other parts.  I ended up not doing anything past building a prototype because I wanted this foot switchable, and I was not interested in running 10-20 strand conductor wiring for all of the switches; seemed like a bad idea and prone to failure.  I ended up programming a PIC for logic and RS232 communication. 

[panterafanatic]

I'm actually recently a member there as well, a ton of tube info there. Thank you for the link, that's almost exactly what I need. I'm probably gonna go with a CD4042 or a more modern improvement of it since I've only got 3 I wanna select between. Plus the -Q output would allow me to use LEDs and not have it function off of the output that's being sent to the amp. I know nothing about PICs so that's above my head for the time being.

[panterafanatic]

Updated .pdf

I ended up going with the CD4042 and some inverters and buffers. The other latching switches are from R.G.'s article on using CMOS switches. Rather than using more inverters than needed or some other switching method it seemed the most simple to work with. Not a bad design IMO for 4 simpler ICs and a handful of other components. I'll make the foot switch necessary for simplicity. I'll most likely use MosFET gate switching for everything.