didjcripey
759 posts
Jun 20, 2014
9:43 PM
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Noticed my little 5 watt amp losing volume and sounding dull. Swapped out the preamp tube (the little one) and it is back to the great volume and tone I love.
If the preamp tube was worn out, would you expect the power tube would be too? ---------- Lucky Lester
Last Edited by didjcripey on Jun 20, 2014 9:44 PM
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Greg Heumann
2751 posts
Jun 20, 2014
10:06 PM
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Not necessarily. In some amps, pre-amp tubes are biased hit and run hard while power tubes aren't, in others it is the opposite. So the decline of one does not predict the decline of another. Tubes are truly reliable, amazing devices and can last years and years. Yours may have failed just because it was a poorly made tube in the first place - luck of the draw.
If you ever suspect that a tube is getting weak, it certainly doesn't hurt to swap in a known good one and see if it makes a difference. If it doesn't - put the old one back in! ---------- *************************************************** /Greg
BlowsMeAway Productions See my Customer Mics album on Facebook Bluestate on iTunes
Last Edited by Greg Heumann on Jun 20, 2014 10:07 PM
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didjcripey
760 posts
Jun 21, 2014
4:15 AM
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Thanks for your response Greg. Very good of you to share your expertise so freely.
I had about 5 years of pretty hard work from it, so I'm not complaining. It didn't go catastrophically in the middle of a gig either, so that's good too. It was NOS from a Chzech factory destroyed during the war, so not too many of them left. Good thing there are plenty of other good tubes available. ---------- Lucky Lester
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SuperBee
2081 posts
Jun 21, 2014
6:34 AM
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I have a small collection of tubes now, I have a couple of twin triodes with the designation missing. I'm not sure if they're 12ax7 or 12at7, though somehow in sure they're one or the other. Is there a visible difference, or do I just have to plug them in and measure the current draw? Also, I'm wondering about visible signs of wear in tubes. For instance, I have an EL84 with some brown staining inside the glass, particularly around where the mica contacts the glass. This tube seems to have run very hot; when I removed it from the socket, it seemed to have slightly welded itself to the socket material (not the pins, I mean the glass was stuck to the socket). Btw,this tube was in a valve junior type amp (a head with the Harley Benton type factory-fitted gain-sucking tone stack) which had seen duty in a studio for several years. The PT was set to run from 230vac, but here in Australia the wall voltage is nominally 240vac. The circuit board and component glue was noticeably scorched around the power tube. I'd not be surprised to find the tube had suffered.
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Greg Heumann
2753 posts
Jun 21, 2014
10:19 AM
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@Superbee - You can't tell much by looking - and although there are differences in construction from vendor to vendor I'm pretty sure you can't tell whether a tube is a 12AX7, 12AY7, 12AT7, 12AV7 or 12AU7 by looking. Sounds like your tube got GLUED to the socket by glue that softened when it got so hot. ---------- *************************************************** /Greg
BlowsMeAway Productions See my Customer Mics album on Facebook Bluestate on iTunes
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GEEZER1
213 posts
Jun 21, 2014
4:56 PM
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Here is an old Radio trick, on glass tubes we could not read the number, we would put the tube in a freezer or at that time the freezer compartment of a refrigerator. Let it get really frosty, then remove it and breath on it, sometimes the number would appear. It worked for me.
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SuperBee
2084 posts
Jun 21, 2014
5:16 PM
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Thanks Geezer, I'll give that a try.
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