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Tubelab Discussion and support of Tubelab products, prototypes and experiments

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Old 5th June 2020, 07:00 PM   #1
brl0301 is offline brl0301
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So a few minutes ago I tried to let the magic smoke out of my output transformers and was fortunately unsuccessful. I have my speakers setup on a switch to play on either my tse ii or my avr. I didn't realize that it was switched to the avr and I started feeding a signal to the tse. I couldn't figure out why I was getting no sound from the speakers until I realized the switch was in the wrong position. The odd/scary/funny part was that I was still getting music from the amp but instead of coming from the speakers, it was playing straight from the output transformers.... Learned something new today and luckily didn't cost myself a few hundred bucks in the process.
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Old 5th June 2020, 07:44 PM   #2
mike567 is offline mike567  United States
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try permanently installing some 100 ohm resistors on the inside of the amp across the speaker terminals. CHEEP insurance that may save you $$$ during bone head moments.
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Old 6th June 2020, 12:00 AM   #3
Tubelab_com is offline Tubelab_com  United States
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In most cases a no feedback SE amp can be safely operated with no load as long as it is not driven to clipping. Note that I do NOT recommend this, but it is one of the methods that I use to quickly measure an amp's output impedance.

An amp's unloaded output voltage will increase above it's normally loaded output voltage. If this rise is enough to invoke clipping, there will be a period of time where no current is driven through the OPT, and this abrupt cessation of current is what causes the fireworks. It's also how the spark in your car's ignition system is generated and the principle that made Tesla famous.

I hook my scope across the speaker terminals, then crank up the signal generator to produce a few volts of output. I them start loading down the amp until the output voltage drops to exactly half the unloaded voltage. At this point the load impedance is equal to the amp's output impedance.

Yes, the OPT's will sing and talk to you. Usually one is louder than the other, and the sound quality sucks!
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Old 6th June 2020, 01:48 AM   #4
Jim the Oldbie is online now Jim the Oldbie  United States
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Not for nuthin, but didn't the old Fender guitar amps have a NC contact on the speaker output jack that would short the output to ground with no plug inserted, the idea being that a short is preferable to an open circuit in this case? Or did I just dream that?
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Old 6th June 2020, 11:34 AM   #5
Tubelab_com is offline Tubelab_com  United States
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Quote:
Or did I just dream that?
In most cases a tube amp will be safer with a short across it's output than an open. If a push pull amp was driven to full power into a short the tube current could be excessive, and into the red plate zone on some high power amps.....but the OPT in that same amp would be on fire if the load was open.
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