# Valve (tube) amplifier schematic help

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505

The original answer to this has been moved (along with the discussion about how many Ohm's can dance on the head of a pin) by request to here:

https://www.electronicspoint.com/ohms-law-not-ohms-law-question-t252367.html

The non-controversial answer is that yes, there is an analogue in circuits using transistors. Where the load is a resistor, that load may be placed in the collector (or drain for FET's) circuit so that the change in current through the active device (valve, BJT, FET, etc) is converted to a voltage swing across the load (or load resistor).

#### Solidus

Jun 19, 2011
349
Now, Steve, I've had some rethinking on the things Chris and Kris have told me regarding impedance, and is the high impedance of the valve in my original circuit the reason all these resistor values are all 50K+?

Higher voltage, but very low current?

#### CDRIVE

##### Hauling 10' pipe on a Trek Shift3
May 8, 2012
4,960
Now, Steve, I've had some rethinking on the things Chris and Kris have told me regarding impedance, and is the high impedance of the valve in my original circuit the reason all these resistor values are all 50K+?

Higher voltage, but very low current?

If I find some time to spice an example for you, I will. For certain, Ohms Law will play a significant roll in what you'll see.

Chris

#### Solidus

Jun 19, 2011
349
If I find some time to spice an example for you, I will. For certain, Ohms Law will play a significant roll in what you'll see.

Chris

Chris, I was actually thinking of messaging you about this, but I drafted a schematic from one online and chained the back end output to a LM386 op amp to allow for headphone use.

Can you help me run a spice simulation on it to determine an adequate value for the output pot? The LM386 datasheet suggests 10K but I have my sincere doubts after looking at the rest of the valve amp schematic.

Eli

#### CDRIVE

##### Hauling 10' pipe on a Trek Shift3
May 8, 2012
4,960
Eli, I'm dead on my butt. I just finished 70% of building a stockade fence in the Florida sun. I can barely move right now but I'll get back to you when I feel better. If I don't respond fast enough maybe Kris will. He's very helpful and more than qualified.

66 year olds should be fishing not fencing!

Chris

#### CDRIVE

##### Hauling 10' pipe on a Trek Shift3
May 8, 2012
4,960
Your preamp would like to see a load of >=100K but I think your preamp will drive the 386 OK. I think the input impedance of the 386 is ~ 50K. If you do have problems you could build this.

Chris

#### Attachments

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#### Solidus

Jun 19, 2011
349
Chris, I think you will be happy to know that since I moved in last Saturday to college, I went about making the investment in "Fundamentals in Electrical Engineering". More transistors than one could dream of.

And that circuit is in from valve amp, out to 386?

#### CDRIVE

##### Hauling 10' pipe on a Trek Shift3
May 8, 2012
4,960
Yes, it feeds the input of a 386. I modeled that circuit to provide a wide audio bandpass into a 10K load. I think the 386 has an input impedance of about 50K but I'm not certain. If the input of the 386 is 50K then C2 can, but doesn't have to be, reduced to a disk cap of .2uF.

Chris

#### Rleo6965

Jan 22, 2012
585
After you finished building this tube amplifier and if you can hear annoying humming or 60 hz sound on the speaker. You need to use +12V dc on the heater of vacuum tube amplifier.
You can still use ac on heater but you must twist the wire and keep it away from wires or components of input circuit ( pin2 and pin 7). Keep this twisted wire closed to the metal casing or ground

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