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9am53 16th March 2005 09:15 PM

do pp output tubes have to be matched?
Bonjiorno, The guy who shipped me my amp didnt separately package the tubes and one of my 2 output tubes broke...he is sending me another one, but it wont be matched or anything...will I need to do something with the tubes...or should I just tell him to get me a matched pair?

hugonot 16th March 2005 09:22 PM

if you want to get lower disorion figures - yes, absoluely.
sound better, i think yes ( not have ocasion ).

Push-pull by its nature needs as much close matched devices as its possible .

percy 16th March 2005 10:13 PM

SHiFTY 16th March 2005 10:27 PM

If they are both new and of the same manufacture it should be ok.

Matched tubes are best but hard to properly do, and they often become different after a few months use anyway.

If your amp is adjustable bias then maybe bias them all the same, otherwise don't worry too much, it shouldnt make too much difference.

fdegrove 17th March 2005 12:22 AM



If they are both new and of the same manufacture it should be ok.
What's the manufacture to do with the tubes being matched?

If they're not from the same manufacturer they can still be matched but I don't recommend doing so as they're usually different enough on other parameters to not sound the same anyway.

Unless the guy at amp's manufacturer actually did his homework and matched a pile of tubes for that particular model of amp, the odds are against a new tube arriving that would match the rest of the set you have left....

For best performance tubes in a PP amp should be matched in the amp itself, no tubetester can match tubes for your amp unless it was specifically calibrated for that task.
To the best of my knowledge no manufacturer does that unless they send out an amp for a review or prepare for an exhibition but I haven't told you that....:angel:
Needless to say they only go through the trouble because they want the amp to sound its best..............


Matched tubes are best but hard to properly do, and they often become different after a few months use anyway.
If they're properly matched they usually stay that way.

Cheers, ;)

richwalters 17th March 2005 02:39 AM


Originally posted by fdegrove

If they're properly matched they usually stay that way.

Hi there....Thats the evidence I'm finding after so-called matched tubes been in high power amp circuits ....the quiescent currents do actually stay pretty stable..... but it's all down to who's turning the handle on the tester.
It's an illusion that matched pairs or quads are absolutely necessary as nfb can take good care of distortion variations. Put a screen in front of twp sytems one with matched tubes and the other unmatched and get people to listen and I guarantee they can't tell the difference.


9am53 17th March 2005 04:07 PM


Originally posted by percy

After looking through this old thread I am thinking maybe I can just match the current going into the cathode of each tube. Is this recommended? Would I just measure the current goign through the cathode resistors when the amp is on, and make up for any differences by changing resistor values...or using a pot?

richwalters 17th March 2005 05:15 PM

Hi there...others can round me up better on this one... adjusting the cathode current (s) is only one of several DC parameters....It doesn't do anything for the AC gain of the the gm (transconductance) will vary per tube. A common mistake is to simply adjust the I cath for quies draw..but if you looked at the full signal draw in fixed bias AB1/2, that figure could be wildly out. Cathode bias does automatically adjust this...a benefit ...however the AC dynamic control used on the anode stage drivers on some tube amps actually trimmed out the thd so each half p-p got exactly the right driving voltage.
A tube tester will check for dynamic gm and Icath for a matched pair otherwise there ain't much point doing it.

The old spec is to adjust for min thd by using the AC balancing potty on the driver stage when o/p stage is driven roughly 90% of full o/p when using a thd analyser. Okay for many without tools there's noway of measuring it....but another getaway' is to use closely matched resistors for the driver stage anode loads. A different example is the orig Williamson.. but better examples are the GEC 88-50 ; 30-100Watt amps.


Gabevee 17th March 2005 05:17 PM


While I advocate matched tubes... I do not think it is critical in many applications. Here's why I think so:

1) Many P-P amps are cathode bias and run at 60-70% of full potential power. Therefore the tubes do not get near their peak specifications where a mismatch would cause damage.

2) Many tube P-P amps are run class AB. Which means about 40-60% of the time they are both class A.

3) Most tubes specs should be within 10% of ideal design specs, or 10% of each other. I guess at most that could mean up to a 20% variance. But for the most part a variance of 5-10% is not bad.

4) A mismatch sounds better because there is less even order harmonic cancellation due to the mismatch. Actually, not better but more euphonic, which is better to those who like that kind of distortion.

Where matching is important:

1) In fixed bias amps running the tubes at close to full power potential. That is, a P-P amp running at a clas AB where it is mostly B. So the tube is biased at very close to the knee in the transfer curve (whatever that means). That way one can have up to a 95% efficiency of output power versus B+ supply (not counting the power drawn by the heaters). That is why fixed bias amps are adjustable.

2) Where one wants to make sure a close to perfect balance occurs for close to perfect signal symmetry (lowest distortion). This is why I get them for my amps. This symmetry also counts where the output transformer is concerned, where a slight difference will cause the OPT to see some positive or negative current flow (from the point of view of the secondary winding), causing unwanted residual magnetism in the core.

3) I guess there is a benefit for longevity also, where one tube is not causing the other to work harder, as it were.

Can't think of anything else not already covered here by others. I do know that when I matched pre and driver tubes also between channels the imaging and center voice or instrumentation was very precise. I also got a matched 12AX7 for an experiment, and again, it is definitely an audibly good experience.

My :2c:.


9am53 17th March 2005 05:40 PM

Hmm, I really dont understand any of the technical mumbojumbo, but I will take home the message that matched tubes sound less distorted, and last longer...iow they are better.

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