assessing power (watts) at typical 12ax7 cathode

[pjd3]

Hello,

I was hoping to get some feedback on the typical electrical action at the cathode of a typical 12ax7 in a typical guitar tube amp (if there is such a thing).

I'm about to delve into some switching mod experiments at the cathodes of 12ax7's for various reasons, mostly gain and some high pass eq selection. (different resistor values and bypass caps).

In particular, there is a component rated at 50mW and I'm looking to see if this will handle the electrical chores.

I understand (a little bit) that the cathode current in 12ax7's are very small on the order of a few to 5mA, maybe 10mA as the most you would expect to see. if the cathode resistor is say, 1500 ohms, then the voltage across the cathode resistor would be for example 1500 x .005 = 7.5 volts. For the power - 7.5 volts x .005 amps = .037 or 37mW.
Is this how you commonly assess the voltage and current at a 12ax7 cathode? Just simple ohms law? Being that this particular Opto-isolator is rated at 50mW it seems as it might handle that kind of power but, I believe that is only considering the No-signal bias state. I'm really not sure what happens when a typical signal is inputted into the grids of the 12ax7 and how the AC action will dictate the power.

Thanks. I'd like to make sure I'm not fooling myself about using these Opto-isolators for these kind of switchable mods at the cathodes. It would be nice if they did work!

Best,
Phil D.

[sluckey]

Opto-isolators come in many different flavors. It would be helpful to see a data sheet for the particular one you are considering. Also a schematic of what you have in mind.

[Stevem]

There’s something I think you need to know about how a tube is biased in regards to the value of the plate load and cathode resistors that are hooked up to it.

For a given power supply voltage being applied to the tube, you should not vary the resistance of either by more then 25%.
Like for example the common 1500 cathode and 100k plate load, especially if you still want the tube to reach cut off and saturation in regards to the wave form at about the same drive level.

[maxkracht]

If I'm understating your setup correctly, you would probably want a fixed lower limit resistor anyway. I'm guessing another resistor in series with the opto would give you adequate margin?

[pjd3]

Hi guys, thanks for your informed comments.

The mods I intend to experiment with are well known mods already used in various amps such as Fat cap mods, JEL mods, plus NFB and MID Raw.. The opto i'm referring to is an LED Driven resistive opto coupler. My work computer doesn't seem to allow posting certain things for cyber security reasons but, the link to one of them is:

https://mm.digikey.com/Volume0/opasdata ... 32SR2S.pdf

There are about 3 types in the series and they all are rated at about 50mW, 60 volt max across the cell AC or DC, and really the main difference between them is the rise and fall times of the resistor value and the LED current and voltage maximums.

The reason I like these things is that you have good control over the range and timing of their resistive range. With a simple RC branch you can set the time fluctuation of the resistive element. I like that. Plus, these things already have intrinsic rise and fall times, approximately 5ms rise and maybe 80ms fall. I see this as a good thing for already beginning the process of dissipating any differential charge non-abruptly, to avoid audible "Pops". This component was referred to me by one of the principal defib engineers here where I work. He left here to be an electronic consultant and likes to restore MacIntosh hi fi tube amps as a hobby. I figured he was pretty up on whats happening at the 12ax7 cathodes so I thought this was a worthy experiment to try. I have boards all set up and ready to go pretty soon as 2 of my amp projects are almost there. (A Fender type amp and a JCM800 2204 type amp) They would each have different things to switch for some tonal versatility.

So yeah, I'm willing to try this, even to watch it either fail or prove itself as a contending switching approach. I just need to get more versed on what to expect at the cathodes of these tubes primarily when they are amplifying a signal. And yes, I will be looking at creating range limits with series resistors that could optimize and keep thing out of trouble. And the opto resistive element would not be the prime cathode resistor. It would be either in series or parallel with the existing one. I always want to restore the "standard" value based upon the circuit/amp model that its included in.

I have alot to learn but, as far as I can tell, should these opto's meet the power/voltage/current max requirements, I really do think they are worth a try, even if it ends in realizing they are not prime candidate for the job. Hell. I do this for fun and learning too, not just for guaranteed success - except for my main gigging amps. I don't mess with them!

Thank you for your time!
Best,
Phil

[maxkracht]

https://www.taydaelectronics.com/photo- ... adial.html

I have been using these for a bit. They are cheap, clam better voltage and current ratings than what you are looking at, have very high dark resistance, and a fast response.

[pjd3]

Thanks for showing these max.

How did you interface these with an LED in terms of mounting and distance apart?

Thanks,

Phil

[maxkracht]

How did you interface these with an LED in terms of mounting and distance apart?

I've just been shoving everything in some black heat shrink, LED or neon right against the LDR, then pinch the ends when hot. The batch I have seems relatively consistent, each one I've checked was over 2M dark resistance, but I may have just been lucky. They are cheap enough to buy a big bag and sort anyway. Haven't tested thoroughly, but I think one of these and a 3mm red LED is pretty close to a vtl5c. If you wanted a more robust and higher wattage version, you could do a series/parallel thing and epoxy it all together before adding heat shrink.

[pjd3]

Thanks for explaining that Max.

I do like the added electrical robustness of these in power and voltage. Sounds like just a matter of getting a good number of them, testing them for ranges, and working up a little method of getting them securely fixtured for reliable/consistent performance. I'm a little surprised that there doesn't seem to be a larger selection of pre-made opto models with higher electrical spec but, maybe that's when folks decide that going to Mosfets is the thing to do. and maybe it is.

Thanks Max,

Phil Dl

[TUBEDUDE]

I used to epoxy CdS cells and LED's in pen barrel sections. Sturdy.