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Tube defect - What is the problem.
Tube defect - What is the problem.
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Old 28th October 2004, 02:36 PM   #1
gary f is offline gary f  Canada
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Default Tube defect - What is the problem.


Question: Last night, the left channel of my Dynaco St-70 serie II started to make tiny noises, after 2 or 3 minutes after started playing a song. The noise was like little morse code sounds: Tit! tit! tit tit etc... It is not the preamp because the amp makes these sounds even if there is no input.

I swapped the output tubes (JJ E34L) and the noise follow the tubes so I guess it is a bad tube. Duh!

My worry is that these tubes are quite new (less than 500 hours I guess). I bought a new match quad of J&J E34L a few months back. I think they are still under warranty anyway.

The Dynaco uses EL34/6CA7 output tubes, was it OK to use E34L and may this be the cause for the tube defect? Should I buy new output tubes? Do you have any recommendation?

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Old 28th October 2004, 11:26 PM   #2
mcs is offline mcs  Denmark
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The tubes would still be under warranty in Denmark (for another 1.5 years) - I don't know about Canada. If they are under warranty, just get them replaced. You are sure the bias current is set correctly etc?

The E34L tubes should cause no problems.

Best regards,

Mikkel C. Simonsen
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Old 7th November 2004, 07:17 PM   #3
kevinkr is offline kevinkr  United States
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Tube defect - What is the problem.
Definitely check with the original seller, depending on their policy they may or may not still be under warranty. Most sellers in the US warrant them for no more than 90 days. I would not consider 500 hours use as nearly new, particularly since I recommend replacement for current EL34 at the 1000/2000 hour mark in ST-70 applications. Sadly the sort of experience you mention is all too common, and I usually recommend a spare set in the event of failure. The EH EL34 is highly regarded in this application, but is no more reliable IMHO.. Replacements should be matched and burned in by the seller to assure reasonable reliability.
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