JCM800 2204 gain mods - opto-coupler switching JEL gain mods

[pjd3]

Thanks for stopping by.

A consideration for footswitchable JEL mods in current 2204 build.

When passing this consideration by one of the principal design engineers at Zoll Medical who also likes to play with Macintosh vintage tube amps, he recommended looking into resistive opto-couplers that could be simply switched on with the typically used relays. As someone who has built a couple of amps but has not played with or installed footswitchable gain mods, its hard to imagine that there wouldn't be any "pops" from hard switching relays. As noted, I could be completely wrong about that and in actuality have nothing to worry about. (or, plenty to worry about). What I liked about the "Advanced Photonix" line of resistive opto-couplers is that they have their own built in "time delay" of low-to-high resistance, especially from low to high resistance. The high-to-low resistance time transition is pretty fast (5 mS) but, nothing that couldn't be extended with the addition of a properly valued capacitor between the opto LED and current limiting resistor. As you've probably suspected, I attribute a "pop" to the very quick neutralization of voltage potential difference when a hard mechanical switch is activated, (if there is one). It just made some sense that a gradual rise or fall in the resistive element would allow a much slower discharge of potential and thus thwart a "pop" as the respective gain mod component is either gradually connected or disconnected to and from its destination, which is usually ground.

The JEL gain mod for the most part are resistors or capacitors that extend from the cathode of the 12AX7's to ground, usually acting as a cathode bypass cap, or a parallel resistor decreasing the cathode resistance in order to increase the gain of the stage. Another is just a large resistor voltage divider to decrease some overall gain. And another is a pico farad coupling cap to relieve some excess high freq as the result of so much gain and distortion.

What I'm mainly in the dark about is what to expect from the cathode of these various 12AX7 preamp stages. From what I have seen in data sheets, schematics, articles and threads etc is that a typical 12ax7 cathode is only a few volts, and these 12ax7 are very low current all around, inherently providing a voltage gain apart from a handful of milli-amps -except for one or two of the cathode follower type configs that appear to put 100 to 200 volts at the cathode. i expect that could require very elevated considerations than the typical 12ax7 gain stages found more closely to the input stages, V1 and maybe the first half of V2.

I just wanted to check in to see if there are any warnings or potential landmines that I'm presently not seeing and can take steps to consider. Of course, the opto-coupler data sheets have provided max currents, max power and voltages so I will be carrying out the math to establish a string of trial component values to hone in on the final implementation of the footswitchable gain mods.

Thanks for bearing with me as I force myself into new uncharted electrical territory!

Best,
Phil D.

[xtian]

Popping in any switching arrangement can be solved by using high-value resistors to allow DC to equalize. You'll see this in HADs designs, for example.

[pjd3]

Hey xtian,

I take it that a large resistor would not effect circuit performance but would be enough to constantly discharge potential differences. Should that be the case, this application may not need anything like the optocouplers I had in mind - which would be nice, they are quirky little components that require a few extra things.

Thanks, I'll look into your suggestion.

Best,
Phil D.