12AX7 Tube Test Readings

[jrsdws]

I'm looking to by some old preamp tubes and the listing says they measure "70-75 on a Lafayette" tester.

I'm not familiar with test standards or equipment.

Is this a good reading making it a good usable tube?

[OldSchoolDave]

Is this a good reading making it a good usable tube?

Numbers only mean something when a range is provided. Lafayette made more than one model of tester. Their ranges could be different - I don't know. Ask what "Fail", "Minimum", "Good" and "New" readings are on that particular tester for a 12AX7/ECC83.

Now, for the even worse news: Those readings aren't the whole story. Some units have a "load test" which indicates, to some extent, the remaining life of the tube. You need to know if it passes emissions (i.e. no vacuum leaks) and shorts tests. Also, to my knowledge, no tester checks for microphonics. That can only be done in an amplifier. A visual check of the flashing can help substantiate if a tube is relatively new or well used.

My best advice to you is to ask a lot of questions. If the seller is unwilling to provide answers, it's a red flag. Ask if you can return a tube if it doesn't perform as advertised.

Once you find a seller you trust, give them preference for future dealings. If you find a tube lot that you like, stock up while you can!

Dave

EDIT: Here's a link to one model of Lafayette Tester: http://www.noobowsystems.org/backtolife ... -50-e.html
From the pic, it appears as though the readings are a percentage of perfect. In that case, 70-75 would seem to be usable, but past its half-life.

[jrsdws]

Hehe...well I had put in a max bid of $50 on a pair of Brimar CV4004 tubes and won them. I guess either it'll be a $50 lesson or a $50 bargain.

Here's the ended listing: http://www.ebay.com/itm/160667430660?ss ... 1439.l2649

Everything seemed in order as far as the pictures went.

[OldSchoolDave]

Everything seemed in order as far as the pictures went.

The pictures look great - and BRIMARs are excellent tubes. Considering the premium on CV4004 models, I think you did well.

Dave

[jrsdws]

Cool thanks!! I'm trying to gather up a small stockpile of tubes to run through the DR504 and a couple of other amps and I know the CV4004 was mentioned several times in other threads. I also got a NOS Mullard DV4024, a pair of tightly matched RCA 12AX7A's, and a pair of Baldwin organ labeled 12AX7's that I think are Sylvania brand.

Off to a good start while I shop for others. Once the Hiwatt tube arrangement is settled, the "other" amps get the left overs.

[azwhofan]

That's a pretty good score IMO. $50 won't even buy you dinner and a movie anymore ...

[OldSchoolDave]

I'm trying to gather up a small stockpile of tubes...

Good plan. Keep your eyes out for a Telefunken & Bugle Boy and you should have a nice set to work from.

Dave

[jrsdws]

Keep your eyes out for a Telefunken & Bugle Boy

Those two are already on the top of my shopping list!!

[mhuss]

One thing to watch out for with old tubes is noise. I have pile of preamp tubes that test fine but bang like hell when you tap on them, or are otherwise excessively microphonic. I'm not sure why I haven't chucked them (or sold them on flea bay )

--mark

[OldSchoolDave]

I'm not sure why I haven't chucked them (or sold them on flea bay )

I know you have too much integrity for the latter.

I suspect the former is because you secretly plan to open a flea market business selling nostalgic vacuum tube mobiles in your retirement years .

Dave

[Dr.HI-TONE]

most noisy tubes can still be used in V3 without any problems

[jrsdws]

Thanks for the info guys....always a wealth of knowledge available here.

I'm curious how the CV4024 would behave in our custom built handwired JTM45 clone. It's a nice little amp that nails that old Marshall and Fender Bassman tone pretty spot on. Either way I'm sure it will also benefit from some of these good older tubes.

[Zells]

The best way to test tubes in my experience is to install in an amp and use your ears. If the Brimar tubes work well in a Hiwatt or HITONE amp, then they should also be good for a JTM45 clone.

I have an old tube tester, but (I think) it only determines transconductance and possible leakage of air into the vacuum space. That helps to sort out bad tube, but it does nothing to judge signal to noise ratio or other tone related factors.

[jrsdws]

Ok...filled my current shopping list with a Tele and a Bugle Boy. I'm still eyeing a few auctions, but totally bargain shopping for OEM labeled stuff now.

I want to sample some stuff through the JTM45ish amp and take note before settling on my Hiwatt arrangement. If there is anything that we can't live without that the Hiwatt ends up with we'll know what to buy.

Through my father-in-law, who was a communications guy for the state police, I've located a few of his old buddies who repaired radios, etc., and did some builds many years ago. There are a few of them left with big stockpiles of old tubes I'm told. Time to find out what kind of beer they like and go visiting!!!!

[OldSchoolDave]

Time to find out what kind of beer they like and go visiting!!!!

Indeed! It'll help to know the equivalent model numbers for the popular tubes.

If they did TV repair, there could be some real oddballs in inventory.

Dave