How to tell vacuum tube is gone bad?? - AVS Forum
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Old 10-23-2014, 06:05 PM - Thread Starter
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How to tell vacuum tube is gone bad??

I recently purchased primaluna prologue 3 and I connect with dynaudio bm6a.

First I saw is lights of three vacuum tubes are dim and

other lights of three vacuum tubes are bright.

Among 6, 1 of vacuum tube has little soot on wall.


When I connect output-2 with bm6a speaker (active speaker), the volume is very low even I tune max volume.

When I connect output-1 with the speaker, the volume is loud enough with 1/3 of preamp volume.


Are these symptom indicated bad vacuum tubes?

Any other way to check vacuum tube??
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Old 10-23-2014, 10:07 PM
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When I was a kid there was a vacuum tube tester in most stores larger than a mom & pop neighborhood store. Not very common these days. Anyway - soot on the inside probably indicates something burnt up. I'd get all the tubes tested and replace any bad ones. Look for a local electronics repair shop - many of them still have tube testers. Bring lots of money! Nice preamp!

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Old 10-24-2014, 07:01 AM
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Take it to a tech.

Dim may or may not mean anything; it depends upon the filament power and tube construction.

The "soot" may be the getter that is flashed to remove the final traces of gasses inside the tube. I couldn't tell without close inspection if what you are seeing is a problem or not.

If one channel is not working properly it may be a tube or something else. You could try swapping all the tubes between channels to see if the problem follows a tube or stays with the channel.

A tech can measure and assess the entire circuit.

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Old 10-24-2014, 10:41 AM
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Certain tubes do not get very bright, I have Tung sol gold pin 12AU7's in my Audio Research SP16. The tube by it's nature does not get very bright. My guess your larger Power tubes {EL-34's, KT-120's} are brighter than your smaller tubes {12 AU7's, 12AX7's}. Prima Luna amps have a feature that will indicate if you have a bad tube. Also tubes are not very expensive unless you get into exotic NOS versions.
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Old 10-24-2014, 02:53 PM
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There are two options.


1. use a tube tester.
2. swap for new tubes to see what happens.
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Old 10-24-2014, 05:21 PM
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PrimaLuna products have a BTI (Bad Tube Indicator) system. Each power tube has a red LED light next to it. If the tube is bad the red LED will come on and put the Integrated into protection. Are any lights on??
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Old 10-28-2014, 01:34 PM - Thread Starter
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i ordered new tubes matsushita? not sure if they are good. i like stuff made in japan

i din't think primaluna prologue 3 has BTI. can't see any led light except front power indicator.
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Old 10-28-2014, 09:37 PM
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Surprised Japan still made tubes. The only ones I knew that still did were former Soviet republics who still make them by hand. That is, brand new tubes, not NOS.
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Old 10-29-2014, 09:05 AM - Thread Starter
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my mistake. i used word "new" as replacement. i bought used matsushita tubes.
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