Speakers not connected on tube amp, good or bad? - diyAudio
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Old 4th August 2004, 10:02 AM   #1
s2kov is offline s2kov  Canada
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Default Speakers not connected on tube amp, good or bad?

When amp is switched on and speakers are not connected, will it damage the power tubes or OPT?
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Old 4th August 2004, 10:07 AM   #2
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It's possible to damage the output transformer if the audio level is cranked very high without a load on the transformer.
If the amp isn't passing any audio, it's perfectly safe.
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Old 4th August 2004, 05:30 PM   #3
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If it has heavy NFB (enough to destabilize... which may not be that much if it's a really bad design), it may oscillate without anything connected. If the speakers make a slight pop when you connect them, or you measure voltage on the output...

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Old 4th August 2004, 06:31 PM   #4
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I've always tried to keep a load on just to be safe.
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Old 4th August 2004, 11:14 PM   #5
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Hi,

Quote:
If the amp isn't passing any audio, it's perfectly safe.
You may have seen the writing on the wal by now:

While most tube amps will survive without any harm done, people tend to forget and accidents do happen...

Make it a habit to have a load connect to the secondary side of the OPT just as you would follow a turn on, turn off sequence to avoid clicks and plops in the speakers...Use a mute switch on the preamp if you have one.

Do all of the above and nine out of ten you won't ever experience burned out OPTs (they can be expensive to replace, especially as you'd have to replace the pair of them) or other silly defects you can hold only yourself responsible for.

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Old 4th August 2004, 11:58 PM   #6
arnoldc is offline arnoldc  Philippines
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i always use a 25W 8R resistor on my amps. it won't hurt to have this as a safety precaution. in addition, i have modified RCA plugs that are internally shorted which i also use (when getting voltage reading from the amp).
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Old 5th August 2004, 12:03 AM   #7
s2kov is offline s2kov  Canada
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Thanks guys for all your inputs.
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Old 5th August 2004, 05:28 AM   #8
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Default Spark safety gap to protect amplifier

A protection spark gap may be fabricated from two short pieces of solid #14 copper wire and wired across the primary winding of the output transformer using a standard eyelet terminal strip to launch the short #14 wires from that form the gap. Leave at least one unconnected terminal on the strip between the terminals used for the spark gap or the terminal strip will likely break down and lead to a conductive carbonized short before the spark gap fires. Depending on the power output of the amp and the plate voltages involved, and also the inductance value of the output transformer a gap spacing of from 1/16" to 3/32" should be sufficient to protect your amp in case of a voltage surge resulting from operating under no-load conditions.

If the gap fires while playing music loud there will be a very loud, very rude noise from the speaker. If this happens then the gap setting is too small and should be increased until it no longer fires during normal hi-powered operation. Best to set the gap by testing the amp into a suitable dummy load resistor on the bench after installing the safety gap.

Naturally all safety rues apply when working inside tube amplifiers like making sure the unit is unplugged from the mains and all electrolytic capacitors are discharged properly. NEVER use a screwdriver or shorting wire to discharge electrolytic power supply capacitors. A 10 ohm, 10-25 watt wirewound resistor on clip leads works well.
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Old 5th August 2004, 07:40 AM   #9
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Clever idea! I guess you could use a spark-plug, if you could find some convenient way of mounting it. It'd look pretty cool on top of the chassis.

I seem to remember that Mullard recommended connecting a 1k resistor across the speaker terminals in the 5-10 amplifier, to guard against damage in the event that the speaker was not connected. They omitted it in the 5-20 schematic, however.
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Old 5th August 2004, 09:30 AM   #10
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Hi,

Quote:
I seem to remember that Mullard recommended connecting a 1k resistor across the speaker terminals in the 5-10 amplifier, to guard against damage in the event that the speaker was not connected.
That's indeed very good practice.
Use a carbon comp. 1K/1W resistor.
By doing so your amp is still safe in case one of the speaker leads should accidentally come lose.

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