Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: shattered on April 09, 2025, 07:08:29 am
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Hello.
While biasing a pair of NOS Orion EL34 tubes I noticed a not very unpleasant but clearly present smell that I never experienced from other NOS or new production tubes before. It's not a burning smell of plastic, no smoke, and I'd say it's even a bit likeable. Like an old library, antique electronics or an old vinyl recording.
With the amp turned on not more than 3 or 4 minutes at minimum volume and bias set to a safe range (50-70% dissipation), is it possible the smell comes from the OT insulation frying? I checked OT temperature once the amp some minutes after it was turned off and it was not hot at all. Maybe the smell comes from the cool art logo printed in the glass or some glue or product from the tube? I finally installed a different set of tubes and the amp sounds better than ever.
I read somewhere that tubes with bad gas emission could cook the output transformer, so I'm considering to get a uTracer3+ kit (not sure if it does this measurement and asked the creator) and I don't plan to use these tubes again until properly tested. I'm aware that checking emission is not good for tubes, and I'm not sure how safe is this tool in this regard. Currently I check for shorted pins with a Brymen meter in continuity mode, making sure just the heater pins have continuity.
Apart from this, and with the tubes out of the circuit, I noticed that _very_ gently tapping the glass with the fingertips near my ear I can hear a very light rattling sound in both tubes. There are probably no loose bits inside them since I can't hear anything moving when gently move the tubes upside-down. Not sure if this is neccesarily a problem since I checked this also happens with tubes I pulled from amps that work fine, and I read this does not indicate nothing since the metal expands with heat and sometimes they rattle when powered on and not when powered off and viceversa.
Your insight is much appreciated.
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I'm confused. You said the smell was coming from the tubes, then you said you suspect the OT. Which is it? Maybe you can get the tubes hot (ish) then quickly pull them so you can give them a sniff close up, so you know where the smell is actually coming from.
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I'm not sure where the smell comes from, I initially thought it was the tubes, but later I read they can affect the OT insulation. Since I waited for them to cool before removing them from the amp it was too late for this check.
I've just gave them a sniff in the base and maybe there's a very slight similar smell but I'm not sure. Will do that next time I test them while still hot, thank you for the useful advice.
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Clean the tube's glass
How do you know it is a NOS ?
1. NOS tubes are only brand new UN-used tubes.
If you buy a used tube which is used and supposedly "tests good" about 80% of the time the tube has some defect. Used tubes are not NOS tubes. Watch out for these "NOS" tube scams.
There is dozens of scams selling USED tubes and calling them "NOS." Don't fall for it.
2. A tube which tests 100% good can fail after it warms up. No BS.
3. Consumer Tube testers are not accurate. Tube testers will miss about 6 out of potential 10 problems.
Tubes which are pre-screened by professional tube labs are far more trustworthy.
Example: Eurotubes pre-screens every tube they sell.
4. According to me: the best way to test a tube is in the actual circuit. I don't trust tube testers unless it's done by a tube testing lab.
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While biasing a pair of NOS Orion EL34 tubes I noticed a not very unpleasant but clearly present smell ...
I read somewhere that tubes with bad gas emission could cook the output transformer ...
The "Gas Emission" you read about is not something you could ever smell.
It was talking about impurities in the metal electrodes, or on the surface of non-metal bits, of the tube. Or even a very small number of individual gas molecules (like oxygen, nitrogen, etc) from an imperfect vacuum.
These few atoms/molecules are physically free from the glass, metal electrodes, and support structure, and can move through the vacuum when pulled by electric fields.
The"gas emission" cannot be anything outside the glass, and so not anything you could smell.
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Thank you for the information.
I'm aware of NOS scams, but this time for some reason (and the cool Orion logo) I trusted the seller, the tubes came in their original box, were not announced as tested and the glass, coatings and getters look pristine.
Eurotubes in based in USA and I live in Europe, so for new production my actual choice is TAD, although I'm afraid of reliability issues of current production tubes (even if they've been screened).
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"more beans Mr Taggert?" sorry, thats all I got
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But are they NOS tho'? Something stinks.
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But are they NOS tho'? Something stinks.
To me, they are not NOS only used OS
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I've had some "used" tubes before which smelled weird, possibly cigarette smoke (or other smoke, I dunno! maybe they were stored in a wine cellar. . .), but after using them for a few hours they did not smell any more.
The main thing to watch for is the current passing through the tubes. If you can bias the tubes within specs, you're probably ok with your OT. I'm assuming there are no shorts -- but you would already know if there are.
Used or NOS, enjoy your tubes!
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Thank you for your insight. The tubes biased within range and didn't show any odd behaviour so I'll assume they are probably fine.
I wonder if there any risk in testing shorts between the tube pins (off circuit) checking continuity with a digital multimeter or this is safe for the tube. I check there is continuity between pins 2 and 7 (heater pins) and no continuity between any other pair of pins.
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Why do you want to check continuity between tube's pins ?
There are no continuity except for heater on EL34 as you see.
Your tests are safe for the tube
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I've had some "used" tubes before which smelled weird, possibly cigarette smoke (or other smoke, I dunno! maybe they were stored in a wine cellar. . .), but after using them for a few hours they did not smell any more.
Just wash/ clean the tubes.
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Why do you want to check continuity between tube's pins ?
There are no continuity except for heater on EL34 as you see.
Your tests are safe for the tube
I prefer to check if the tube has an obvious short off circuit rather than installed in the amp, and avoid unnecesary wear of the socket if there's no continuity between the heater pins of the tube.
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Why do you want to check continuity between tube's pins ?
There are no continuity except for heater on EL34 as you see.
Your tests are safe for the tube
I prefer to check if the tube has an obvious short off circuit rather than installed in the amp, and avoid unnecesary wear of the socket if there's no continuity between the heater pins of the tube.
Voltmeter can't test nothing in tube except heater
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Older amps have a smell about them. I chalk it up to years of depositions of participate, humidity, and oils. When you add the heat of operation then the smell comes out. It's not unlike old books--and I suspect for the same reason.
I don't think there's anything wrong with your tubes or amp.
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Research the terms "amp farting out." I suspect your answer lies there.
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I always kinda liked the smell of vintage electronics.
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as long as it isn't fumunda cheese
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I always kinda liked the smell of vintage electronics.
If that's the scent of cigarettes, I hate it.
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If that's the scent of cigarettes, I hate it.
Yeah? Maybe. I just figured it was the insulation used, or whatever they potted the transformers in, or like mentioned before, oils and time. I can't imagine every vintage piece of electronics was in a smoking environment.
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My Silvertone 1482 has the sweetest smell when those tubes get hot=============haven't tried to analyze it, just enjoyed it
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I know the smell well, smells like old books. I do a lot of conversions from old radios and turntable consoles. I think it’s the decades of dust on the tubes burning/diffusing/whatever.