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SET Headphone Amp DC Resistance Help


Paid Member
2019-12-14 6:20 pm
Cleveland, Ohio
Hi Everyone,

New member here, first post other than introductions. I am seeking some novice help. I am building a SET headphone amplifier, MH4 input, EL34 output. There will also be a switch to change the cathode bias resistor value to use KT66, KT77, and 6L6G tubes at various bias points. I'm not an engineer and I am learning things along the way, I am relying on previous math and physics education from my first degree (chemistry), so a lot of memory refreshing is happening.

Thankfully, I have a friend who is an engineer who is helping me along the way and checking my work. However, he is not always available to answer all of my dumb questions :)

Anway, with all of that being said, what I am trying to do is figure out the total DC resistance of my amplifier circuit for the sake of prototyping my power supply and using an appropriate dummy load resistor. I already have the transformer, a 250V 860mA high voltage secondary going into a hybrid diode-tube rectifer utilizing a FW4-500 tube with a 250mA DC max current output and a 35V drop.

With the transformer picked out and the total load calculated, I am hoping to put the correct dummy load into PSUD2 to estimate my B+ voltages for both stages and also use that load for my prototype and measurements.

The issue is, I am not sure how to calculate the DC resistance of my amplifier circuit. Do the tubes have a DC resistance that needs to be accounted for as well as the cathode and load resistors/output transformer primary? Grid leak and grid stopper resisitors I assume are not included as they are not seen as a load by the power supply.

What I was attempting to do was the following:

1) calculate the resistance of each stage individually
2) calculate resistsance of both stages in parallel
3) calculate the resistance of both channels in parallel
4) get total DC circuit load resistance

If someone could describe how I would go about doing this, I would apprecaite it. I know how to determine resistance in series and parallel, just unsure of the tubes' contribution if any. I've attached my draft of the amplifier circuit schematic. Please be gentle, it is my first design ever with no EE education! The B+ and B++ voltages listed are likely incorrect as the load I used in PSUD2 was very likely wrong. The power tube bias points are also assuming the B+ voltages listed.

Thanks for your help and I apologize if this is trivial or addressed elsewhere, I have not been able to find the information. I have the datasheets for all of these tubes handy if any more information is needed.

(Also, please let me know if I have posted this is the wrong area)


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Paid Member
2003-06-12 7:04 pm
Maine USA
> if this is trivial

Quite trivial. You have voltage and current numbers. I do not know where they came from. Or why they are different for each tube-type. (They could all be run with same conditions, since any of them is FAR more power than headphones need.)

Voltage is given as 280V. Currents run up to 100mA. MH4 plate resistor 56k drops 230V-145V= 85V so draws 85V/56k= 1.5mA. Total current 101.5mA.

280V/101.5mA = 280V/0.1015A = 2758.6 Ohms 28.42 Watts. Use 50W part for bench-checks, but >57W part for non-stop testing.

25 Watts in SE Triode should do 6 Watts out. Headphones need milliWatts, rarely over 100 of these, so it is about 60 times bigger than you need.

> 250V 860mA ...secondary

This is good for 500mA of DC, 2.5X what you need. It is enough iron for a four-6L6 guitar amp at 90 Watts output.