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Old 27th September 2010, 04:44 PM   #31
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Quote:
Originally Posted by Wavebourn View Post
Both.

A dummy load as an attenuator for a speaker.





I have a lot of 20 Ohm resistors. If to connect 5 of them in parallel it is 4 Ohm in total, for higher power. If to add computer fans for forced cooling they will dissipate even more power without a problem.

http://wavebourn.com/forum/download.php?id=437&f=7

http://wavebourn.com/forum/download.php?id=438&f=7
Thanks for those pictures! I was just going to recycle an old small form factor PC. Now I have a use for it!
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Old 27th September 2010, 08:17 PM   #32
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If you use NFB over more than one stage this is an interesting experiment to do:

1) Create an A/B switch so you can switch between speaker and resistive load.
2) Connect a scope probe to a signal within the feedback loop
3) Setup a square wave (500Hz etc - to suit)

Then look at the wave shape change as you switch between the resistor and speaker!
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Old 28th September 2010, 03:17 AM   #33
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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?


Chris
Chris: At the risk of stating the obvious, if your output transformers have 4 ohm or 8 ohm taps, then a 6R load will not reflect the proper load to the output tubes, it'll either be a little high or a little low, depending on the tap. Pretty much like a loudspeaker...

So your theoretical load line/bias point and what you measure on the scope are going to be a little different.....and you've got to divide by 6 instead of 4 or 8 to determine output power.
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Old 28th September 2010, 12:46 PM   #34
chrish is offline chrish  Australia
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Since that post I managed to get some 8R 50 Watt resistors. I mounted each on a piece of heatsink I parallel them for 4R.
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