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Preamp Tubes matching in circuit

Hi guys,

I was wondering if matching preamp tubes in an actual circuit by measuring Ia (anode current) is a valid matching method.

I've ordered a bunch of Russian tubes that are supposedly matched. They come with a nice sticker and matching Ia and S numbers. When I place each tube in the same circuit and same position and measure Ia, I get various values that are sometimes 30% different than its matched tube. Does this mean that they're not matched?

Is placing the tubes one by one in the same position and matching them based on their Ia draw a valid way of matching them?

My circuit is the Aikido preamp using the Russian 6N2P and 6N1P.

Thanks
 
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Moderator
Joined 2011
That would be the most basic way, but it is more of an emission test than a signal gain test.
You might ask them how the tubes were actually tested.

Best matching would be for gm at a typical chosen quiescent operating point.
So in your circuit, you could check the mid-band signal gain at the operating point set by the circuit.
Some circuits will be more sensitive to gm than others. Cascode is more sensitive, cathode follower is less sensitive.
 
This concept of tube matching by sellers is a bit of a quagmire. Most match using an emission tester. which is not likely anywhere near the operating point of a pre-amp. There are a few Youtube videos that describe how to DIY a tube tester at operating points. I believe these are worth checking out to get a handle on the concept of what is required. Here are a couple of links worth reviewing:



I hope this helps
 
Hi guys,

I was wondering if matching preamp tubes in an actual circuit by measuring Ia (anode current) is a valid matching method.

I've ordered a bunch of Russian tubes that are supposedly matched. They come with a nice sticker and matching Ia and S numbers. When I place each tube in the same circuit and same position and measure Ia, I get various values that are sometimes 30% different than its matched tube. Does this mean that they're not matched?

Is placing the tubes one by one in the same position and matching them based on their Ia draw a valid way of matching them?

My circuit is the Aikido preamp using the Russian 6N2P and 6N1P.

Thanks

Matching tubes in circuit is pretty much a sure-fire way of determining whether they are doing their job or not. That said, the Aikido is not terribly fussy about matching. But if your Aikido is noisy, a weak tube may be hindering the noise-cancellation properties.
 
The reason I was trying to match them was to ensure they provide identical gain so both channels are balanced. Would matching guarantee same gain?

I realize that other components also affect gain but I've noticed, using an oscilloscope, that the output is not the same. Some tubes are better matched in output than others however the ones that are supposedly matched almost never have the same gain. The gain pattern follows the tube when I place it in the other channel.

I guess I'll try all tubes until I find the right combination that has close to identical gain. Matching plate plate or cathode current doesn't seem to guarantee matched gain. I'm still learning.
 
Moderator
Joined 2011
Make sure your probes are matched, and give identical output levels when
connected to the same signal. The probe compensation will affect this.

The L/R circuit gains will never be exactly equal. The circuit stage gives ~mu/2 gain,
so differing mu gives differing gain. And there's no loop feedback to adjust the gain.
The current being exactly the same in the upper and lower tubes does help the balance.

But that's what balance controls are for. Burn in the tubes for 100 hours or more and recheck.
See how much mu varies with plate current at lower currents (the currents that are normally used)?
 

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I agree with @jakruby matching in circuit is the way to go. If you have a small collection of tubes or another topology to check out matching at in circuit operating points is also a very good way to set up pairs or sets of tubes.
As I mentioned trusting tube sellers to match tubes is not always the best. Making your own tube matcher/tester is fun.
I hope this helps