It depends on where you are measuring that load resistance. If there is a coupling cap on your output after the point you are measuring, or a cap in the input of your SS amp, you won't get a true input impedance reading. But when a signal passes through those caps it does see the load. Your ohm meter won't read the DCR through the caps. .... if they are there.
True, I forgot about the SS amp might have a cap. I'll just put a resistor load on the output and see. Other than that there are no other coupling caps in the path.
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No, it's low gain setup with unbypassed cathode and 2.4k resistor. I will drop the resistor a bit so I can use volume control for extra gain if needed but at the moment it is about 1:1 with the tone control.
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I've tested it with just a resistor load and even just shortened the output. Only drops to 550k. Bellow is the Baxandal. Output is direct, no pot or cap.
An externally hosted image should be here but it was not working when we last tested it.
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This is what the curve shows with the way I've omitted the 38k and changed 1M bass pot to 500k pot on the tone control. The knob positions are slightly off but the curve looks flatter than the original. I have no way of measuring the curve in real time but by listening to it by A/B it sounds pretty much like it looks. I'm quiet happy with the way it works as a tone control. Only the load still bothers me a little but it is 570K, about the same as if I had 6V6 connected to it so I think it should be ok, I can't see what the difference is if the load is same.
An externally hosted image should be here but it was not working when we last tested it.
I can't add anything else to the Tone Stack to simulate it, I'll do it in real time. But I think it'll just drop level as if I turn down the volume control which is on the input. I'll try anyway and see what happens. I did a quick schema of my working circuit just to make things clear.
An externally hosted image should be here but it was not working when we last tested it.
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In the tone stack calculator you can set Zsrc. Set it to about 60k to represent your 12AX7 source resistance. Or download a proper circuit simulator.
The Duncan tone stack calculator does include a source impedance. It defaults to 38k which is typical for a fully-bypassed common cathode ECC83 with 100k anode load, etc. Not sure why but @6V6dude has set it to zero. (Also not sure why but it then shows up on the graphic as 0p rather than 0k or 0R.)
Edit: Simultaneous post with Merlin. For the circuit shown in post #29, I make the source impedance to be 56k.
Edit: Simultaneous post with Merlin. For the circuit shown in post #29, I make the source impedance to be 56k.
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I thought you were taking about the pot. I've replaced the 1M pot anyway because it was faulty and just used 50K pot. It works the same, no difference with any load so I'll leave it there. The preamp as it is works well, I'm happy with it. When connected to tube amps, I don't hear any compromise in sound, it is basically nothing more than tube amp gain stage in stand alone box. When it comes to connecting to SS amps and the higher load, there is some drop in volume about 10% and the bass needs to be turned up about 10% to get it somewhat flat. Not something that bother me since it's use will be for tube amps anyway but I did leave a space on the chassis for couple of extra tubes in case I want to add the buffers in the future for some reason. I'll send a pic when it's done.
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