Simple Method To Measure Source Resistance?

[kelticwizard]

Wish to measure the resistance of a tube amp, (Rg).

The reason is that, when I model the loudspeaker to be used with this amp, I will add a series resistor equivalent to Rg in the louspeaker circuit. We also plan to include the DC reistance of any inductors in the crossover in the model.

Is there a simple way, preferably just using a meter, that I can measure the output resistance of a tube amplifier?

PS: The builder has just finished refurbishing a Dynaco ST-70, in addition ot other tube amps he already has. So the spec for Rg for the ST-70 would be helpful.

I include a pic of Small's Closed Box model to show which resistance I mean.

Thank you for any help you can give on this issue.

[poobah]

Hi Keltic,

Do you have a some BIG resistors lying around? Likw 5-10 Watt or so... around 10 - 50 Ohms?

Do you have a signal generator or something like a test tone CD or???

[kelticwizard]

Poobah:

thanks for responding.

The testing will be done by another member whom I am helping to design a loudspeaker. I don't even know which state he is in. Since he tinkers with tube amps, solid state amps and loudspeakers, I assume he has a multimeter or will be willing to obtain one.

The large resistors required are available at Radio Shack, or a combo can be made of Radio Shack resistors to suit.

Tone generator? I like to use this freeware online:

Freeware tone generator:
www.satsignal.net => Audio Tools => SweepGen

From there, it is a question of if his computer sound card has a Speaker Out, and what it's output resistance is. Less than 0.1 ohm would probably pass muster.

Or else, he can just hook up the computer to one of his solid state units for testing purposes.

I plan to ask him to please Right Mark his sound card to make sure it is functioning properly, (the first time I ran Right Mark I was shocked at how bad the response was-it was all right after I reinstalled).

Is that sufficient to test?

[poobah]

Hey Keltic,

Yeah that will work... anything that will produce a steady sine wave to feed into the amp. You want to have pretty low level to avoid needing a 100 Watt resistor on the output of the amp.

Now if this is SS amp it may have have really low output impedance so it might pretty hard to measure accurately
with a low-cost setup.

Lemme think about formula/procedure so I don't accidentally post a bunch of hooey.

Back at you in a while...

[jackinnj]

http://www.bcae1.com/dampfact.htm

[poobah]

Hey Keltic,

A good place to start wold be about a 10 ohm 5 watt (bigger wattage is better) resistor. You can use anything in the ballpark.

1. Measure the resistor and record the value as "RL".

2. Hook up the resistor to the output of the amp, apply 1 kHz to the input at a VERY low value.

3. Read the AC RMS voltage across the resistor, adjust the input signal until you get about 6 Volts RMS on the output. (This will give you about 3.6 Watts on a 10 Ohm resistor... the trick here is not to fry the resistor... do the test quick so you don't get the resistor real hot as that will change its value) now record that voltage as "VL".

4. Now, the input signal EXACTLY the same, remove the resistor and measure the AC RMS voltage at the output again. Record this voltage as "VO"

5. Plug all these numbers into the formula below; and there you go.

Rout = ((RL*VO)/VL) - RL

Now, don't panic if there is almost no difference between VO & VL, that just means your amp has a real low Rout. If that is the case; this method won't be very accurate... but if your Rout is that low... you won't care anyway.

Also, this only measures the resistive aspects of the output impedance... to measure all the reactive stuff takes more toys and $.

[Sheldon]

The way I read this, measurements would be taken with and without a resistor on the outputs. Wouldn't that be risky for the OPT's of a tube amp? - specifically the no load test?

Sheldon

[poobah]

Sheldon,

It would certainly be possible to do the test with two resistors; one that goes in and out of circuit, and one that remains connected at all times to prevent an open circuit.

I am somewhat of a newbie to tubes though... is there a problem with an unloaded tube circuit?

Are different topologies affected differently?

[Sheldon]

Originally posted by poobah
Sheldon,

It would certainly be possible to do the test with two resistors; one that goes in and out of circuit, and one that remains connected at all times to prevent an open circuit.

I am somewhat of a newbie to tubes though... is there a problem with an unloaded tube circuit?

Are different topologies affected differently?

Yes, to be safe you'd need a load at all times. The inductance of the secondaries can lead to very high voltages across the output with no load. This can cause the insulation to fail with internal shorting in the secondary windings. And the equation would be (my math skills are rusty so this would take a bit of doodling on my part)? Your measurement delta would be best with the common resistor of high value and you might be able to ignore its value in the calcs for rough estimates of impedence. How high? Maybe some of the experienced tube guys can tell us what a typical transformer would be happy with at your test levels - a couple hundred ohms?

Sheldon

[poobah]

I gotta think about this one... I keep forgetting they run these things open loop...

A delta v / delta I test would be the same... and would solve the problem.

Hey Keltic... hold up here until someone else responds... be nice if Eli Duttman would read this...