Matched tubes verification

[the_nines]

Is there a way of verifying a matched pair of tubes besides taking the sellers word for it?

[sheltie dave]

Yes, there is. You could purchase the same equipment the seller used, at substantial cost, and duplicate their process. You could also have the seller mail the tubes to a third party tester, and again absorb another needless cost.

The path I would recommend is to use an honest tube seller of national stature who is known to be above board in their business practices, pay a slight premium, and trust them to do you right.

[the_nines]

Originally Posted by sheltie dave

The path I would recommend is to use an honest tube seller of national stature who is known to be above board in their business practices, pay a slight premium, and trust them to do you right.

Fair enough. I think you just described my story:

Just bought 4 new tubes from a local "mom & pop" store close to my house. I probably paid a bit more for them than I would have bought them on the net, but after factoring shipping, duties, taxes and a little bit of profit for the man I don't care. The store has been around since 1960 (same owner), this I know for a fact since I grew up in the same neighborhood. The man specializes in all things "tube & vintage". It's a "vintage Mecca" in there and the first thing that came to mind is "wow...the guys and gals in AK would have a field day in here". I only stumbled upon it yesterday because I knew he sold this kind of stuff but I never had the need to walk in as I had always bought solid state components (until now). I was looking for 7591's for my Fisher. He had them. He had a wall of tubes, all kinds (new & used).

He showed me two kinds he had in stock: a Scott and a JJ Tesla, new in their boxes, both were not cheap, the Scott being more expensive and also being new but sitting in inventory for a while.. He recommended the Tesla, I told him I was advised to stay away from Tesla. He asked me "why ..... this company has been making tubes for decades. Never had an issue with them, but I'll sell you whatever you want. Take the Tesla....if you're not happy, bring them back and pick up the Scotts". We talked a bit, told me he was in this kind business since 1950, first working with Grundig & Telefunken in Europe and then coming to Canada and opening his store in 1960.

I took the Tesla. He reaches up and picks out 4 Teslas from a stack on the wall.

I neglect to ask him if they were matched. I called him later and assured me they were. I don't think I got screwed because I can bring them back, but how did he know they were matched? Was there a lot number on the boxes, was it just his knowledge of tubes. I found it perplexing somewhat but I trust him. Am I naive?

[Bluelobster]

They could be matched by manufacture. When you ask for matched tubes do you know what your asking for? Do you know why you need matched pairs or quads? That would be more to the point. You need to check the bias of these tubes after they are in you system and warmed up a bit.
He is correct and your heresay about Tesla tubes is a wrong assumption. They are in fact pretty good, A few years ago when NOS European and US tubes could be had these were dismissed because of their third world communist origin. Now that stock and manufacturers are limited they have been reconized for what they are. Westinghouse black base they are not, but I think they are better than the replacements that JJ and Sovtec offer.
Take the guys word and try them, that kind of return policy is tough to beat. I'm sure after 50 some years he has an idea of what he's talking about.
I wasn't aware that European manufacturers made the 7591. I thought it was an audio developed tube made in the US. That is until Russian replacements were introduced a few years back.

[the_nines]

The tubes I bought the box reads:

JJ Electronic Vacuum Tube
Made in Slovak Republic

I google Tesla and come to TUBEDEPOT with this:

"TubeDepot.com is pleased to offer the current line of JJ Audio Tubes. Formerly known as Tesla, JJ Tubes are made on some of the original European equipment that was used in the heyday."

So is Tesla now known as JJ ?

[oldhifiguy]

nevermind

[Bluelobster]

Originally Posted by the_nines

The tubes I bought the box reads:

JJ Electronic Vacuum Tube
Made in Slovak Republic

I google Tesla and come to TUBEDEPOT with this:

"TubeDepot.com is pleased to offer the current line of JJ Audio Tubes. Formerly known as Tesla, JJ Tubes are made on some of the original European equipment that was used in the heyday."
So is Tesla now known as JJ ?

Yes they are. Now we're getting somewhere.
You will need to make some minor changes to your Fisher amp to use these safely. Jim McShane details what needs to be done on his site, about 1/3 down the page linked.
http://pages.prodigy.net/jimmcshane/tubes.htm

[kvflyer]

What type (number) tube/s are you trying to match? For the most part, matching is good. But a lot of circuits will work just as well if the tubes are "close" to each other. I personally don't worry about it. I have a Heathkit TT-1A and I know that matching is more than measuring emission/transconductance. But as I said, matching may not be as important as some would lead you to believe. Most circuits will tolerate a little difference in conduction curves.

JMHO

[Suzuki]

This might be wrong,,, but I recognize a pair as matched if the voltages, of the primary side of opt, between the center tap(+B) and P1, and P2 are equal. That means P1curent and P2cureent just offstet. (The measured voltage value)/(series resistance beween them)= plate bias current... Again, You only want to measure the each DC-volage and confirm they are the same or very close, then the two tubes are matched, in terms of the plate resistance. Still, to make sure they are a match as to gm, measure VACs of the same positions with a 0.1uF cap in series for DC-filtering.--Of course you need input 1khz sine-wave signal to the amp-- In just the same way, if the two measred values are the same or very close, they are a match. This is my idea, but I'm not completely sure. Please correct if this idea is wrong.

[Bluelobster]

"AKrs have told me 10% is considered a match."
10% of what? Too much supposition and not enough fact.
"listening test is the final word tho."
It will sound better if things are operating within design parameters to begin with.
" You could invest 2-300$ in a gm tube tester to satisfy your curiousity "
Not neccessary if you buy them from a reputable dealer that tests for gain and current draw
"Swap your tubes around in different sockets to see if anything audible shows up."
If you have matched pairs you run the risk of unbalancing the output circut. More current draw through one side of the push-pull pair.

[becketma]

You've discovered that "matched" doesn't have one definition, like the word quality.

Mu is defined, gm is defined.

My experience, with a balanced mono block, push pull output, is that when I matched the output tubes in the amp, clarity and depth increased a little. In this amp, a 6FQ7 is used as a cathode driver. If I changed the driver, the "match" changed. To match the tubes, I adjust the a variable resistor common to both tubes, so that the output voltage is the same.

I have about 30 output tubes, and will go thru many trying to get two that "match".

I have three different testers, one just for common small signal tubes. This one measures the db gain of each triode-something I can relate too.

I've measured my hearing, not matched.

I measured the room "gain" at my listening position from each speaker, also not matched at the test frequencies.

One time, I bought 250 Tungsram E88CC. I sold the tubes with gain matched triodes and gain matched pairs and use unmatched triodes and unmatched pairs.

One time, I bought around 30 mi surplus 6SN7GTA, identical tubes. Some were boxed 7 years later than the first boxed date--they all sounded the same.

Best
Bob

[the_nines]

So when you buy tubes off the net and the ad says "matched pair" or "matched quad" what procedure(s) are taken to be advertised as such?

Noob here .... so excuse the ignorance.

[ke4mcl]

the guy may have been in business for 50 years but not every tech pays attention to our weird nuances as audiophools. if he had more on the shelf how does he know which are matched to which? where the tubes or the boxes marked with any numbers (test readings).

if you go back and ask he may humor you and explain or he may brush you off. some old techs can get quite grumpy when questioned.

[archie2]

""AKrs have told me 10% is considered a match."
10% of what? Too much supposition and not enough fact." Quote from Bluelobster.
10% of the mutual conductance or Gm. as read off a mutual conductance tube checker, not an emissions checker.

[joshuahhhhhhh]

mutual conductance is 1 part of it but when we get into output tubes what you want are tubes that are matched for current draw this is not so much of a big deal when you have an amp with adjustable bias.