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Old 12th September 2011, 03:28 AM   #1
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Default Damping Factor

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
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Old 12th September 2011, 06:20 AM   #2
rknize is offline rknize  United States
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Sounds reasonable. What tubes, what B+, and what primary impedance on the OPT?
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Old 12th September 2011, 03:52 PM   #3
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Originally Posted by rknize View Post
Sounds reasonable. What tubes, what B+, and what primary impedance on the OPT?
The B+ is around 430 V, with EH EL34s. Also using the recommended 5 kOhm impedance. Rk = 560 Ohm.

Dave
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Old 12th September 2011, 04:20 PM   #4
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Actually quite typical (and good) for designs where there is no global feedback in use. (There is the option for local cathode feedback in the output stage)
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Old 12th September 2011, 04:49 PM   #5
rknize is offline rknize  United States
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A little over 4 is about right for this amp with your setup.
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Old 12th September 2011, 10:25 PM   #6
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Quote:
Originally Posted by hartze11 View Post
I used this formula... E_no_load vs. E_loaded...
Hmm. As a general rule, I try to avoid running my vacuum tube amplifiers with no load. Mostly you are OK, but sometimes you fry an expensive output transformer. Just my two cents.
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Old 13th September 2011, 05:05 PM   #7
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Originally Posted by Ty_Bower View Post
Hmm. As a general rule, I try to avoid running my vacuum tube amplifiers with no load. Mostly you are OK, but sometimes you fry an expensive output transformer. Just my two cents.
I try to avoid it as well, but this was a quick test, on relatively low power.

Everything is still working fine, but I would recommend to anyone to keep the test short.
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Old 13th September 2011, 11:35 PM   #8
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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).
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Old 2nd October 2011, 04:40 PM   #9
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Default a couple of questions.......

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?
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Old 2nd October 2011, 08:14 PM   #10
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Originally Posted by boywonder View Post
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.
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