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Post subject: Degrading tube sound in TRR
Posted: Fri Jan 31, 2014 6:07 am
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Apart from catastrophic failure, how often do you replace your power tubes? My TRR with original Groove tubes bought new in 2001 has roughly 120 - 150 hours of play time at moderately high volume (~ 3.5). The sound doesn't seem as punchy or bright as a few years ago. I notice this now at gigs after playing for about 1 hr. Do most musicians just replace those tubes when there is a significant issue (like big drop in output power or "mushy" sound as some have described it)? Apart from any power cap issues, is there some typical "play time" in hours where one can expect degradation of power tube performance?


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Post subject: Re: Degrading tube sound in TRR
Posted: Fri Jan 31, 2014 6:34 am
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Do you have a method or tool to check the idle bias of those 6L6GC's? Prolly need re-biasing, after this length of time. If the tube won't bias into proper range --- time for new tubes.


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Post subject: Re: Degrading tube sound in TRR
Posted: Fri Jan 31, 2014 8:22 am
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yes I checked the bias a few months ago and they were all in correct range ~ 30mA. Will have to check and see if after an hour of operation if the bias drifts upward significantly (it does drift up slightly maybe 2 mA in the first few minutes of operation).


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Post subject: Re: Degrading tube sound in TRR
Posted: Fri Jan 31, 2014 1:23 pm
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MichelG wrote:
...how often do you replace your power tubes? My TRR with original Groove tubes bought new in 2001 has roughly 120 - 150 hours of play time at moderately high volume (~ 3.5)....


That is nothing as far as expected tube life. 5,000 hours is not unreasonable at all, and good tubes can last much longer than that. I might suggest that your speakers have just broken in well with that little amount of playing time, and you have plenty of life left in the tubes. It is possible that one or more tubes have a problem, possibly a weak preamp tube.

If you only notice the sound loss "after playing for about 1 hour", it might just be ear fatigue.

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Post subject: Re: Degrading tube sound in TRR
Posted: Fri Jan 31, 2014 5:37 pm
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MichelG wrote:
yes I checked the bias a few months ago and they were all in correct range ~ 30mA. Will have to check and see if after an hour of operation if the bias drifts upward significantly (it does drift up slightly maybe 2 mA in the first few minutes of operation).


30 ma per tube?

If so, there is your problem. At about 440 VDC plate voltage, that works out to only 12 watts per tube at idle or only 40% max plate dissipation. That is WAY too cold. How did you measure and set the bias anyway?

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Post subject: Re: Degrading tube sound in TRR
Posted: Sat Feb 01, 2014 5:21 am
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Good catch bluesky636. :idea:

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Post subject: Re: Degrading tube sound in TRR
Posted: Sat Feb 01, 2014 8:06 am
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Let's hope the OP is paying attention.

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Post subject: Re: Degrading tube sound in TRR
Posted: Sat Feb 01, 2014 12:58 pm
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+1

Thirty milliamps is downright frigid for a Twin Reverb. Milk that up to 38.0 mA or thereabouts and use that as a baseline datum point.

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Post subject: Re: Degrading tube sound in TRR
Posted: Sat Feb 01, 2014 4:41 pm
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Maybe, 30mA is all he can bias-up those tubes? Which means they are borderline bad or old. Checking the measured idle bias current passed in an amp is the best way to test output tube's strength and to match tubes.

I've seen old (I mean really old & used) 1950's Tungsol 5881 bias-up higher than brand new Sovtek 5881WXT. TS with the getter flash a reddish color, no longer silver. The TS still sound great and the Sovtek sound "cramped" and lifeless. FWIW. :D :mrgreen: :D


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Post subject: Re: Degrading tube sound in TRR
Posted: Sat Feb 01, 2014 7:48 pm
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The OP definitely has a problem whether he realizes it or not.

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