[gort]

Is tube matching a BS? If it isn't why do most on-line dealers I found only sell unmatched tubes?

My X-Can V3 is on the way and I want to buy some spare tubes.

 

[ServinginEcuador]

No, it isn't pixie magic or anything else either. Most amps are not self-biasing, so if you stick in some tubes that are off gain wise they will likely provide unequal gain in each ear, causing it to sound weird, and destroy the soundstage. Matching isn't difficult, but to be done right it takes more than transconductance testing. Plate current and other factors, under operating conditions, are necessary to ensure equal amplification from each tube.

For an amp like the X-Cans you should either buy matched pairs, or at least buy ones with readings on them so you can pair them up yourself.

 

[Hirsch]

Quote:

Originally posted by gort Is tube matching a BS? If it isn't why do most on-line dealers I found only sell unmatched tubes?

In most cases, tube matching is going to be trivial. Matching tubes is expensive in terms of labor, and tubes. You need to have enough tubes to expect matched pairs to come out of the set. Then you've got to perform the testing. Then put the pairs together (you'll be lucky if a dozen tubes gets you three matched pairs). Dealers to sell them, particularly if they've got a lot of the tube. Those are the high-priced dealers, though. You do get what you pay for.

Transconductance matching is normally good enough, at least from a hobbyist standpoint. Yes, there are other factors, but it's probably not worth the expense and effort to measure them. In amps where it really matters, you'll often have a self-biasing setup or even a manual bias adjustment.

 

[gort]

Thanks for the answers. Not sure if X-Can are self-biasing or not. Hmmm...... I just visited their site (musicalfidelity.com) and they don't list email address, phone number, and mailing address. Weird. Wonder how a customer can contact MF about warranty repairs.

Edit: Does self-biasing amp need matched tubes?

 

[Mastergill]

Quote:

Originally posted by ServinginEcuador Most amps are not self-biasing, so if you stick in some tubes that are off gain wise they will likely provide unequal gain in each ear, causing it to sound weird, and destroy the soundstage.

We already had this discussion, tubes are not matched for gain and unmatched they will not provide imbalanced L and R channel

You match only POWER tubes that you need to use in a PUSH-PULL design when you cannot set the bias individually for each tube. Basically a push-pull run smoother with similar cathode current tubes.

It's very rare that you will need to match small preamp tubes that are used in headphone amp, this is why you don't see that offer from serious tubes dealer.

I don't know which tubes you are using in the X-Can, but if it's only preamp tube in a single ended design you don't need to match them.

 

[Hirsch]

Quote:

Originally posted by gort Thanks for the answers. Not sure if X-Can are self-biasing or not. Hmmm...... I just visited their site (musicalfidelity.com) and they don't list email address, phone number, and mailing address. Weird. Wonder how a customer can contact MF about warranty repairs. Edit: Does self-biasing amp need matched tubes?

Warranty repairs are usually done through the dealer you bought the unit from.

A self-biasing amp will compensate for unmatched tubes to an extent. However, biasing can only compensate for so much, and it's more accurate if the tubes are close to begin with.

Mastergill is correct. Tube matching becomes critical with power tubes in a push-pull design. There, if the amplification of one phase of the signal is too much greater than the other phase, you'll have an unstable situation that can potentially damage the tubes or the amp.

In something like the Fisher 400, which does not control bias voltage at all, matched power tubes are critical. Unmatched tubes are the cause of the "all over red glowing tube" phenomenon, which means that something very bad is about to happen.

 

[ServinginEcuador]

Well, I have to admit I was wrong. I did some searching around and found out some interesting facts on the net about tube matching:
Quote:

On the other hand, matching both sections of small-signal tubes such as the 12AX7, 12AT7 and 12 AU7 has proven to be a silly idea. I discovered the performance improvement, if any, was measurable, but not great enough to hear. We don't offer matched small-signal tubes for this reason. Save your money if you wish to buy from others!

Quote:

Tube matching is required when 2 or more tubes are operated in push-pull in an output circuit. This is especially important when the circuit is a fixed bias circuit and the tubes are operated at or even a little beyond their maximum ratings. This is typical in audio amplifiers as manufacturers would try to squeeze every watt of power out of the amplifier. If you don't use reasonably matched tubes in such a circuit, you run the risk of destroying one or both tubes as you could never properly bias the circuit for both tubes. They don't usually have to be EXACTLY matched; just within a few percentage points of one another. Also, if you have a stereo amplifier which run the tubes in 2 pairs, IT IS NOT NECESSARY TO BUY A MATCHED QUAD! 2 matched pairs will be fine. Tube tester matching is NOT normally effective matching because most tube testers operate the tube far below the normal operating parameters used in the amplifier. Tubes that match at typical tube tester voltages will NOT usually be matched in the amplifier. I use different types of test equipment to match tubes. I normally operate the tubes at typical amplifier voltages and measure plate current. Transconductance is NOT important when matching this way.

So, by taking transconductance and plate current readings of small signal tubes like a 12a?7 series tube, will the figures given just give you approximate lifespan left on the tube? If not, what does it give you?