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Old 3rd June 2010, 12:24 AM   #21
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Originally Posted by chrish View Post
Along the lines of this discussion, I have tried to source some reasonably priced high wattage 8R resistors, but my usual sources are always out of stock. I do have a pair of 6R 50W that I have been using. Is this going to induce big errors in my testing?

G'Day Rob, I have moved again and I am literally just around the corner from you now if you feel like sharing ideas over a beer or need to use any of my equipment!

Chris
These were on sale not too long ago. I don't know if they ship overseas, however. Come to think of it, I don't know if they are entirely non-inductive either, being rather large wire-wound resistors...

8 OHM 120W POWER RESISTOR-MPJA, Inc.
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Old 3rd June 2010, 08:26 PM   #22
ChrisA is offline ChrisA  United States
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Originally Posted by torrence View Post
These were on sale not too long ago. I don't know if they ship overseas, however. Come to think of it, I don't know if they are entirely non-inductive either, being rather large wire-wound resistors...

8 OHM 120W POWER RESISTOR-MPJA, Inc.

I think in all cases you are best off to wire several higher value resistors in parallel. If they are inductive the inductance will be divided by the number of resistors used.
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Old 3rd June 2010, 09:32 PM   #23
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I use old Russian-made 25W and 50W wire-wound potentiometers (4.7 and 10 OHm) as dummy load. Their inductance is so small that it actually doesn't matter on audio frequencies. They serve as voltage dividers at the same time.
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Old 3rd June 2010, 11:20 PM   #24
chrish is offline chrish  Australia
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Another lesson learned the hard way...

Just repaired the blown power supply on my PP 6L6 MOSFET follower amp. Testing the outputs using 50W aluminum clad resistor. Hmmm, easier to clip the scope probes to the resistor leads than the speaker binding posts...

Several minutes of testing at just under clipping, hmm, what is that smoke?

It was the plastic on my (brand new) scope probes melting
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Old 3rd June 2010, 11:32 PM   #25
Loren42 is offline Loren42  United States
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Originally Posted by chrish View Post
Another lesson learned the hard way...

Just repaired the blown power supply on my PP 6L6 MOSFET follower amp. Testing the outputs using 50W aluminum clad resistor. Hmmm, easier to clip the scope probes to the resistor leads than the speaker binding posts...

Several minutes of testing at just under clipping, hmm, what is that smoke?

It was the plastic on my (brand new) scope probes melting
Unfortunately, they call that a lesson burned.
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Old 3rd June 2010, 11:56 PM   #26
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chrish, you are not alone in that lesson..
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Old 4th June 2010, 12:46 AM   #27
chrish is offline chrish  Australia
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Ha Ha, yes! Some lessons are best learned the hard way, or at least those learned the hard way are best remembered! Ask me why I ALWAYS use a bleeder resistor on my power supplies

Unfortunately, in some areas, the hard lessons are not survivable, eg high voltage. I note a Bonanza in your avatar, are you a pilot? - aviation is one of those areas too!

Regards,

Chris
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Old 4th June 2010, 01:18 AM   #28
Pano is offline Pano  United States
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Ouch, not good! I have a solder fume exhaust fan. Melted right thru it with a dummy resistor on top. Same "what's that smell?"
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Old 27th September 2010, 02:54 PM   #29
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Default Real Life Measurements on Power Amplifiers - Loadspeaker-Load Dummy instead Resistor

check out this article:
1) English
Stereophile: Real-Life Measurements
2) German (schematic page 56)
High-end Röhrenschaltungen - Google Bücher
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Old 27th September 2010, 04:21 PM   #30
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The article deals with SS amps with output stages not so frequency dependant when compared to Tube amps.
Every tube amp I've come across has the "rising thd with rising frequency" characteristic as the output tranny open loop response is deliberately tailored away by phase shift and the output transformer response when global feedback is added. This raises the effective output resistance of tube amps at both LF & HF extremes so the stiff impedance/load correction of a high octave crossover isn't liked by tube amps..
So, implying a tube amp performs the lowest HF distortion with only a simple passive 6dB/octave crossover network on the tweeter which gives the least loading effects. All agree to this ? This may suggest why tube amps are sonically better reproducing the top end.

The Kantor article gives only a partial picture of the lumped parasitics. The picture for tube amps is far more complicated than one imagines. The equivalent electrostatic speaker load is far more revealing and disturbing: my Dynaco goes in oscillation but the Williamson has no problems.


richy
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