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Telefunken ECC83/12AX7/7025 tubes


captainbeefheart
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Still unboxing things from our move and I am on to my audio stuff. We had to prioritize getting rid of a lot of other things to downsize before the move but now I need the awful task of figuring out what should stay and what should go for audio stuff.

 

I only have one phono preamplifier that uses two 12AX7 tubes. Many moons ago lots of equipment used the 12AX7 and I always preferred European tubes, mainly Telefunken so I have the most of those. I also tinker and like to make amplifiers, I get the itch to make something every few years or so but I don't ever see myself making anything with a 12AX7 in it ever. I do like my phono preamp but I have far too many to be considered spares for an amp that might need new 12AX7 tubes every decade or two.

 

I really dig that there are a lot of tube users on this platform so I figured I would throw it out there if anyone on here might be interested.

 

I have about 25 that can go, planning to keep 15 or so for the future.

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13 hours ago, vulcan said:

I have a Jolida JD9 phono preamp that uses the 12AX7.

What would you recommend? How much for a pair of the Telefunken?

 

I have smooth plates in my phono preamp but both ribbed or smooth are great tubes for phono section.

 

I have tried every tube under the sun and always go back to telefunken for my phono preamp, they are the best.

 

What you want is ones screened for low noise and matched gain structures. I can match up a nice pair for you.

 

I am a little overwhelmed, I just signed on to look at the forums and have over 10 messages in my inbox for these tubes!! I am going to respond to everyone in order, private message me and we can go over details.

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Sorry guys I have been inundated with messages to respond to. Far more interest than I ever imagined for tubes. I guess they have gotten pretty rare looking at the market for pricing them recently.

 

I want anyone to fully understand that I am not your average fleabay tube vendor that knows nothing about tubes. I cringe everytime I see "tubes test at %80" or some rubbish like that. You have to be aware that tubes DO NOT have a linear lifespan you can relate to a % percentage, it's impossible. Tubes for the most part stay at their gm for their whole life and once emissions start to drop off at the very end of their life gm will start to decrease. A good tube will be around 20% +/- it's average transconductance. So for a 12AX7 that's roughly 1300uMhos to 1900uMhos transconductance values where 1600uMhos is average.

 

I test all tubes in circuit for transconductance AND gain AND distortion. I also test for noise the same way Brent Jesse does which is in a real circuit hooked up to an oscilloscope and speakers.

 

So it isn't just the tube you are paying for, it is the hours and hours of time I spend to weed through and only use or sell top tier tubes which is why they are worth every penny as you know it will be a great tube(s) that will perform excellent for many  years to come.

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  • 2 weeks later...

I have 6 smooth plates left.

 

A quick reminder how I test preamp tubes, normally one will either have a cheap emissions tester or better a transconductance tester, the latter being better but I do not think this gives anyone really any indication at life or performance in circuit. Why? First of all transconductance or "gm" is not constant, it changes and along with the plate impedance (rp) make up the mu. Now "mu" is the only constant and;

 

gm*rp=mu

 

So gm is only half the story when calculating gain in circuit, you need to know plate impedance which also is not a constant and changes like gm depending on specimen and operating point. You can have two sections, one with a gm of 1300 and the other 1500 but in the same circuit you will get the same gain. Having matched gain is most important in circuit.

 

I test for microphonics and noise, gas, leakage, and distortion. Distortion will give you an idea how linear the specimen is, this will be more prevalent the more output swing you need so having if you want your channels to be matched and perform similar you want matched gain and similar distortion profile.

 

Test circuit:

 

B+ = 250v

load = 100k

bias = 1.5v

 

Distortion measurements are taken at the same output voltage for all sections.

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There are now only 4 left.

 

Posts #14,16,17, and 18 are the ones left.

 

 

I am not sure if anyone noticed but I accidentally posted the same section distortion spectrum image twice for tube in post #16 twice. I went back and updated it with the correct one, it was triode #2 that was posted twice so triode #1 was added which is 2.19% THD and triode #2 is 2.49% THD.

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