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Damping Factor

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Hi all,

Just curious if anyone has measured the DF of their Tubelab amp...

I measured my Tubelab SSE at a value of 4, into an 8-ohm resitive load (at 80 Hz). This amp has the big Edcors currently installed. I tried a range of mid-volume levels, and typically saw about 0.75 V drop between loaded and unloaded.

I used this formula: DF = E_no_load/( E_no_load - E_loaded), e.g.

3V / ( 3V - 2.25V ) = 4

Just curious if this seems right or common...

Thanks,

Dave
 
I try to avoid running my vacuum tube amplifiers with no load......I would recommend to anyone to keep the test short.

Time duration shouldn't matter. The "rule" comes from the fact that current through an inductor can NOT be instantly switched off. An unloaded OPT is essentially an inductor. At low signal levels the amp will behave normally without a load. As soon as you hit clipping the output tube will attempt to cutoff. This switches off the current flow through the OPT and the voltage will tend toward infinity. Stray capacitance and inductance will limit the rise time thereby limiting the absolute voltage rise, but often the voltage will rise to a point high enough to start an arc somewhere. The arc is then fed by the B+ and something will fry even if signal is removed.

I have seen it happen, and even provoked it to see what happens. The likely places for an arc to start is inside the OPT (fries it), inside the base of the output tube (may or may not be fatal), from pin 3 (plate) to pin 2 (heater) of the output tube socket (sometimes blows the power transformer since multiple KV is on the heater winding, but I have seen it blow a 12AX7 in a guitar amp).
 
Is there another appropriate method of measuring DF without having to unload the output transformer?

For the above test, do you input a sine wave and measure AC volts at the output both with and without a load?

Yes, input a sine wave... this article recommends 50 Hz:

Damping Factor with Calculator

But I noticed that my amp started to roll-off below 60-70, so I used 80 Hz.

Measure the voltage drop between loaded (I used a non-reactive 8 Ohm resistor) and unloaded.

Not sure I would do this again, unless I want new transformers...

Dave

p.s. Nice to meet you at BA 2001.
 
Is there another appropriate method of measuring DF without having to unload the output transformer?

Yes, you can measure the output voltage with any two different load resistors and use a formula. I am 1200 miles away from my lab so I don't have the formula handy now. I use two identical 8 ohm resistors so I can get 4, 8 and 16 ohms. Scope or measure the output to verify that the amp doesn't go into clipping ,or the numbers won't be accurate.
 
AX tech editor
Joined 2002
Paid Member
Yes, you can measure the output voltage with any two different load resistors and use a formula. I am 1200 miles away from my lab so I don't have the formula handy now. I use two identical 8 ohm resistors so I can get 4, 8 and 16 ohms. Scope or measure the output to verify that the amp doesn't go into clipping ,or the numbers won't be accurate.

I don't have the equation on the top of my head either, but you can do as follows.
You measure the Vout with two different values of load, and calculate the output current for the two situations. Then you take the difference between the two load currents as delta-I, and the difference between the two Vout's as delta-V. Rout = delta-V/delta-I.

jan didden
 
Thanks guys! I was assuming that two different loads would work; same as above except both R values are finite.......

OK next question......if using 2, 4, 8, 16 ohm loads, connect them all to the 8 ohm tap? (I have 4 and 8 ohm taps available).
 
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