How to Spot a Bad 12AX7 Preamp Tube by Eyesight

[@alcrook6662]

After seeing that vid Fran posted about the RCA ladies building tubes with their hands (and some complicated expensive machinery) in the sixties I get romantic about the best tube days. Good to see people teaching and learning the basics of owning and running a tube amp. Cheers !

[@vacuumtubesandguitars]

I absolutely agree

[@richb.4374]

A 12AX7A is a dual triode. It's literally two tubes in one envelope. Sometimes, one of the triodes can fail while the other one still works. You'll notice a drop in gain but the amp will still play. Bending the pins of the tube can cause loss of envelope vacuum when you straighten them back out. It causes hairline fractures in the metal to glass seal of the tube and the air slowly leaks in ruining the tube.

[@vacuumtubesandguitars]

You are 100% correct. A 12DW7 is proof of the the 2 in one, one triode is a 12AX7 and one triode is a 12AU7. I always use a pin straightener for just this reason, unless the pins are way too bent, then I just chuck it.

[@DMSProduktions]

GOOD tips! I've seen valves like this before!

[@vacuumtubesandguitars]

definitely no good and could harm your amp

[@DMSProduktions]

@@vacuumtubesandguitars Yep!

[@7venty2wos]

I see all these 5-15 minutes videos and you’re the first one to show me the same white preamp tube that I have on my Marshall dsl40c tube amp. I turns on and lights up the proper way, but I’ve switched out ALL my guitar cables and still no sound whatsoever. I was going to order $100 worth EL34’s , but if this one white bulb replacement will resolve my issue?? Then at least thank you for this ‘quick’ to the point video! Much appreciated! Now, do you know if I can just replace the one bulb? Or do they all have to be replaced like you would replace both sides of a vehicle’s headlights as an analogy? lol I’m hoping just the one to do the trick! But thanks again! - Nate Bragg

[@vacuumtubesandguitars]

Just the one is all that needs to be changed. I sell tubes on ebay at good prices - www.ebay.com/sch/fadedgypsy/m.html?item=194208877700&rt=nc&_trksid=p2047675.l2562

[@7venty2wos]

@@vacuumtubesandguitars right on! I appreciate that! I’ll check out the link for the tubes! Thank you

[@daleburrell6273]

...what amazes me, is that the tube even lights up!!! Once air gets inside of a vacuum tube, there's nothing to stop the filament from burning up!!!

[@vacuumtubesandguitars]

@@daleburrell6273 I have have tubes go bad also by chipping the glass while straightening a bent pin

[@daleburrell6273]

@@vacuumtubesandguitars ...that's easy to do, I'm sad to say.

[@maryannmoran-smyth3453]

I love vacuum tube amps and Kid-E-Cats… Thanks for the info… Keep on rockin brother

[@vacuumtubesandguitars]

Life is wonderful. Thank you very much

[@jeffreyyoung4104]

I sell nos and used tubes, each one gets tested before listing, and again before shipping. I have both US and China RCA 12AX7s, no difference in performance, but some people do care to only have the US made tubes. The China tube isn't listed at the moment, as I only have the one. If people get to the point that they no longer can or want to pay the high price for 12AX7s, you can rewire the sockets so you can run two 6AX7 with the heaters in series, as the specs are the same!

[@vacuumtubesandguitars]

I also sell tubes on ebay, and have been for 18 years. The tube deal is way over hyped these days. China and Sovtek are good tubes, JJ's too. But with new modeling technology being so good, tube amps and radios have no advantage

[@WilDBeestMF]

​@@vacuumtubesandguitarsAbsolutely true. I am mad at myself for having a really expensive all tube rig sitting there collecting dust because a moderately expensive plugin is doing a better job for most things. Yet, that's how the cookie crumbled the last decade or so!

[@vacuumtubesandguitars]

@@WilDBeestMF It is amazing at how good modelers and plug ins have become

[@mikemchugh3073]

Quick question - I have a 1994 Super Amp 4 X 10" that sounds fine on the clean channel but volume drops off on either of the 2 gain channels. Is a bad tube one possibility of the problem? (It has 2-6L6WGC/5881, 4-12AX7A, 2- 12AT7). Just wondering if a bad tube can be the >possible< culprit. Thanks for any advice you can share.

[@vacuumtubesandguitars]

It is possible a tube is going bad, but more likely is a capacitor dropping voltage

[@mikemchugh3073]

@@vacuumtubesandguitars Thanks for your reply and advice.

[@JamesWilliams-js4fo]

What is the best modern made EL34 tubes and what is the best modern made 12AX7 's to go in my Marshall DSL40 CR amplifier in your opinion?

[@vacuumtubesandguitars]

The TUNG-SOL 12AX7 Russian reissues are the best build and are my favorite. For EL34, Svetlana or the Sovtek Mullard reissues are the best in my opinion

[@JamesWilliams-js4fo]

@@vacuumtubesandguitars Thanks for that info it's much appreciated.

[@MrBurnzz.]

I just got a new amp head today, and when I installed the tubes it came with the pre amp tubes don’t light up at all, we’ll one of them does, there’s three pre amp tubes and only one of them lights up, do you think it’s just sum bad tubes?

[@vacuumtubesandguitars]

It is common for some tubes to not glow, especially the second, third or fourth tubes, since they are not the primary preamp tube. The first 12ax7 preamp tube in slot V1 most always will glow because it is doing the most work.

[@vacuumtubesandguitars]

If the amp is playing good and no crackling or weak tone then the tubes will be OK

[@MrBurnzz.]

@@vacuumtubesandguitars yea the pre amp tube on the far left has no glow, as the same with the middle one, the only one that glows is the one on the far right, I plugged in a pedal to effects loop to bypass the pre amp section and the amp finally started making slight connection noises

[@MrBurnzz.]

@@vacuumtubesandguitars yea but when plugging the guitar straight into the amp there’s no sound coming out of the amp, the red l.e.d turns on, but it doesn’t give any signal to through the cord, no buzz or hiss

[@vacuumtubesandguitars]

@@MrBurnzz. You could try reversing the order of the preamp tubes to see if the others light up in the first slot

[@kapioskapiopoylos7338]

funniest shit ever, i just seen an auction for a white top telefunken blackplate and i shit you not it went for 120$. it was hilarious

[@vacuumtubesandguitars]

absolutely insane. Some people are missing a few cells

[@lpa74]

i shit you not...youre my favorite turd

[@silviubadea84]

If a preamp tube (12au7a) is not glowing but i get sound out of my amp does than mean that the tube is faulty? Swaped it with the old tube and that one glow... The new tube, the one that is not glowing, is more powerfull than the old one that i see a glow on....

[@vacuumtubesandguitars]

Many tubes I have tested in amps do not glow and work just fine. A faulty tube will either not allow the amp to produce sound or will crackle badly or whine loudly.

[@silviubadea84]

@@vacuumtubesandguitars thank you for clearing it up for me, i had some troubles finding the specific tube, and when i installed it my jaw droped when i saw no glow, but still sound coming out of the speaker.

[@vacuumtubesandguitars]

@@silviubadea84 It's all good, keep on rockin

[@PeterDad60]

Loss of the vacuum is only one reason why any tube won't work. So the title should be amended to read differently. How about: Once a preamp tube has lost is vacuum and is white on top then it will not work. - Peter age 73

[@peskyparttimers8239]

If they are clear at the top rather than silver grey they are nearly always dead too.

[@vacuumtubesandguitars]

Not always, because RCA made side getter tubes where the silver flashing is on the side. If the tube has a top getter with no silver flashing then you are correct, it is 99% of the time bad.

[@alcrook6662]

@@vacuumtubesandguitars I agree, keep up the accuracy , cheers. (I worked for the Canadian Marshall, Orange, Ibanez , Eko and Hiwatt distributors on and off and had the honor of meeting Jim and his chief engineer briefly before he retired in the early 2000's if my memory for dates serves me right)

[@vacuumtubesandguitars]

@@alcrook6662 Wow that is awesome

[@alcrook6662]

@@vacuumtubesandguitars I'm still trying to get a youtube video made of facts and stories from the day that would blow the minds of moderns. I'm workin on it. Thanks.

[@vacuumtubesandguitars]

@@alcrook6662 That would be awesome

[@bobdimartino6738]

the reason i'm here is i bought a brand new black star head mkIII 100 watt when i turned it around i noticed one of the pre amp tubes was white so i figured it was blown i didn't even plug it in yet brand new broken

[@vacuumtubesandguitars]

It does happen, good thing you did not turn it on. My main amp is a Blackstar HT20 Studio

[@bobdimartino6738]

i returned it yesterday and ordered another new one they gave me 20% off so it was 999 i got the head and cab for 1600 @@vacuumtubesandguitars

[@vacuumtubesandguitars]

@@bobdimartino6738 way cool.

[@ambrusnemes4847]

These tubes are dead but punks don't!

[@vacuumtubesandguitars]

absolutely

[@justsumguy2u]

That's actually true of any vacuum tube; once they've gone to air they all turn white

[@vacuumtubesandguitars]

you are correct!!

[@7venty2wos]

Plus I subbed & liked you vid as well!

[@vacuumtubesandguitars]

Thank you, just contact me if you need other help, [e-mail]

[@luthiervandros]

The label will say sovtek. Jk

[@vacuumtubesandguitars]

amen

[@lpa74]

but be careful with old tubes because sometimes old dust and oxidation on the outside can look just like that so first wipe it down with some windex