Cathode input gain stage?

[potatofarmer]

I've used a couple of those TY-250Ps and they've worked nicely. They are fairly sensitive to interference though, which I got around by just moving it away from, well, everything.

[Dave H]

I really don't know how to use the attached file.

Put the TubeLib.inc file in the same directory as your .ASC file. Run up LTSpice by clicking on the .ASC file and add the spice directive .INC TubeLib.inc to the schematic. To add the directive click on 'Edit' on the top bar then select 'Spice Directive' You can open the TubeLib.inc file with Wordpad to view the tube models it contains. The tube symbols on the schematic should have the same Value (or Spice Model) as the model name in TubeLib.inc. Right click on the tube symbol in the schematic to edit its Value.

I've done the above in the attached zip. Download and extract it and it should run.

cathode input.zip

[R.G.]

I just thought of another way to look at the input impedance at the cathode.

The cathode and plate have the same current coming through them. It has to be the same, as for all practical purposes, the grid doesn't conduct at all. So to put a signal in at the cathode, you have to have a signal source that can move the cathode up and down by enough voltage, while sourcing and sinking currents equal to the change in plate current.

[Chuck H]

Put the TubeLib.inc file in the same directory as your .ASC file. Run up LTSpice by clicking on the .ASC file and add the spice directive .INC TubeLib.inc to the schematic. To add the directive click on 'Edit' on the top bar then select 'Spice Directive' You can open the TubeLib.inc file with Wordpad to view the tube models it contains. The tube symbols on the schematic should have the same Value (or Spice Model) as the model name in TubeLib.inc. Right click on the tube symbol in the schematic to edit its Value.

I've done the above in the attached zip. Download and extract it and it should run.

[ATTACH]43725[/ATTACH]

I'll look into that. So far though I don't usually see the same directive options by right clicking. It was confusing because, at the very least, I expected right clicking to offer a place to plug in the model parameters I've gathered, but it doesn't seem to. Like I said, I'm not LTspice proficient. Maybe I even have an outdated version? I'll update the program and follow these directives and see how it goes. Thank you for addressing this aspect of the thread.

[Chuck H]

C'mon guys! This amp just happens to be a tube amp. I have nothing against SS devices (with the exception that I have to pigeon my way through the circuits). I would also like to keep the series stages minimized and I don't see the need to include additional and peripheral circuitry just yet. I mentioned this early on (though I knew the SS suggestions would come anyway ). Just to clarify something...

This isn't a project, per se'. My personal amp sounds great and uses the original design but I NEVER use the effects loop so it's detriment in use is moot. But I have a few amps in customers hands that use this circuit and I would like to solve for the effects loop problems and offer a sort of recall. Some of these owners wouldn't be happy with the addition of SS devices. That's it in a nutshell. And the addition of yet another triode for a single channel amp is something I just wouldn't prefer to do. Basically, the effects loop was only added to the design because I didn't like the hanging triode. I wanted to use it. So now I need to make it work with what's already on the table. I don't mind if anyone want to discuss the "appropriateness" of using SS devices in this application on this thread. It's all for the purposes of education. Just don't trouble with efforts to sell it to me for this particular circuit. Thanks.

I think playing the tubes own plate resistance against a load with no additional series resistor to achieve the low voltage level for the loop send should render a low enough impedance that HF losses aren't objectionable. I'll report on this when I get back to the project. Don't hold your breath

And please! Keep discussing this subject on any level. SS, tube or otherwise. I VERY MUCH enjoy the insight, interaction and education.

[nickb]

I'll look into that. So far though I don't usually see the same directive options by right clicking. It was confusing because, at the very least, I expected right clicking to offer a place to plug in the model parameters I've gathered, but it doesn't seem to. Like I said, I'm not LTspice proficient. Maybe I even have an outdated version? I'll update the program and follow these directives and see how it goes. Thank you for addressing this aspect of the thread.

You don't need to plug in any parameters. The .lib file contains the models along with the parameters. These were derived using a curve tracer and doing a best fit to the data.

[SoulFetish]

I just thought of another way to look at the input impedance at the cathode.

The cathode and plate have the same current coming through them. It has to be the same, as for all practical purposes, the grid doesn't conduct at all. So to put a signal in at the cathode, you have to have a signal source that can move the cathode up and down by enough voltage, while sourcing and sinking currents equal to the change in plate current.

Honestly, it seems that a transistor would really be ideal in this application This is kind of what they do. But, Chuck, I respect your design parameters. So, how about this?



I just threw a quick drawing of a concept together, You need to capacitively couple the cathodes unless you were going to design a set level. I'm sure its been done, but would this do what you want?

[SoulFetish]

Honestly, it seems that a transistor would really be ideal in this application This is kind of what they do. But, Chuck, I respect your design parameters. So, how about this?

I just threw a quick drawing of a concept together, You need to capacitively couple the cathodes unless you were going to design a set level. I'm sure its been done, but would this do what you want?

Wait.... I though about this. This is not the way to go. The potentiometer on the input side cathode basically just creates a variable high pass filter and loads down the driving signal. My B!
The voltage divider could go like this instead... hold on...

[SoulFetish]

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