When is a 12ax7 tube measurement too bad?

Freddie73

Freddie73

Active Member
Just received last week my Hickok 800 tube tester recalibrated by Hickok Meters. Severe weather is coming to MS, not good to go outside so I thought I’d recheck my 12ax7’s. I try to find vintage tubes with balanced triode , measuring above 1000, 12ax7’s for the phase inverters. 1250 is considered new with this TT . My question is how balanced do they need to be. I’ve been lucky as I have a few pairs of Sylvania long gray plates that are 1200/1200 to 1350/1400. Others 50-100 apart.
My other question is how weak is too weak? I have some pairs measuring in the 800-900’s still sounding great ( Telefunken’s).
As far as balance outsiders I have some measurements of 1400/1200 on the extreme spread but sound ok in tone controls?
How out of balance is too much? All tubes have no shorts or gas.
 
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Trust your ears. Weak testing small signal still can sound great. Am located in NE MS, we missed most of the bad weather, went south of us.
 
The way the inverters work in this, the sections do not need to match. Each half of the 12ax7 is doing something completely different.

honestly I rarely bother to test tubes unless I'm having a problem. If the voltages are right, its fine.
 
On service testers like the 800 the bottom of the "Good" band is typically 60% of new spec's.

It's considered to have a reasonable lifetime of use left.

I've seen tubes that measured 25% and still sounded fine.

Depends on how critical the circuit was designed.

Lots of tubes quit working in HF oscillator circuits at 60%.

Never heard a problem caused by a balance issue yet.

Hope that helps.
 
Thanks everyone. Most of my 12ax7’s running 700-900 still sound good. I’ve fortunately not thrown them away.
I’ve always read that the phase inverter position in a fisher amplifier needs to be as balanced as possible and vintage? I have run Genelex Gold Lion new production without issues in the phase inverters but they are well matched.
 
The inverters bias in a funny way and some of the modern tubes don't play well with it in as-built form. If you have done the "noose removal" modification it gets a lot less fussy.
 
Thanks for your replies
I’ve done the noose removal in my 400 and 800b with great improvement. I haven’t performed it in my 800c because, I think, it’s more complicated and I am running strong, balanced, USA tubes it sounds great.
As I move forward with this hobby I realize I need to purchase and become proficient with an oscilloscope.
So what I’m getting is the most important tubes to be strong and balanced are the Phase inverter tubes the rest not so much and go by performance in the circuit?
 
^ Absolutely!!!

Fwiw,

I acquired this explanation years ago, I wanna say from one of AK's finest tube guru's however I could be wrong

"The minimum Gm for a 12ax7 is 800, which is base for most Hickok-based circuits. A typical new 12AX7 will test between approx 1100 and 1400, with 1250 being considered “nominal” or average for a new 12AX7.

Of course, putting too much importance into a Gm score is misguided. Higher is NOT better in any practical way of course, we are assuming a tube that is within a reasonable ballpark of “nominal new”, a reasonable ballpark would be roughly +/- 15% of nominal new.

As an example, using the standard Hickok nominal new for a 12AX7 to be 1250 micromhos, it would be misguided to assume any superiority to a tube reading 1400 vs a tube reading 1100.

Either of those tubes could easily be NOS and any assumption that the 1400 tube is “better” (will sound better or will last longer) is simply ignorant of the realities and complexities of vacuum tubes."
 
Freddie73 said:
....Most of my 12ax7’s running 700-900 still sound good. I’ve fortunately not thrown them away...
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Good job!!

Years ago when I had access to a Hickok I had a couple of 12AX7's that tested low like yours (800-900). Around the same time I was involved with a lengthy back & forth with the late great @jaymanna who was schooling me on tube gear in general and tubes. When I asked about the low scoring ones I mentioned he said don't read too much into that, keep them, see how they respond in circuit and hold onto them.

I still have a few of those, I'm pretty sure one of them is still doing duty in a buddy of mines Fender tube amp and from what my ears tell me when I listened to it, it sounded fine
 
@jaymanna was a really nice guy. I just mentioned on a post that I was looking for a certain tube and he then sent it to me and wouldn't take a cent.
 
Freddie73 said:
And don’t throw away tubes based on measurements?
Click to expand...

depends on the particulars but if it works properly in circuit and the in-circuit voltage measurements are reasonable, for all practical purposes the tube is working fine and doesn't deserve to go on the cart.

I went through some the other day and pitched them though. One was a shame, a nice new-looking 5AU4 (its a 5U4GB for all practical purposes) that was absolutely dead on one plate. The other side tested 105%. Glad I tested it before using it. On the bright side the 5U4G that I got in a bag of stuff at the flea market for 5 bucks tested very strong on both sections, so that one is getting installed.
 
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