Mismatched power tubes

He's right, depending on what you mean by 'depth of tone and character'. Unmatched tubes will produce more harmonic distortion, and a more complex tone. Matched tubes give what I would call a deeper tone, though - clearer and cleaner, and with relatively stronger fundamentals.

Mismatched tubes also produce more hum, and you can only go so far before you get to the point where you can't bias both (or all) of them in the right range - without separate bias controls, anyway.

But there are two parameters commonly measured for tube matching - plate current (the current the tube idles at with a given bias voltage) and transconductance (the gain, crudely). If you get a set with matching plate current but different transconductance, that would enable you to get the increased harmonic distortion without the hum and with both tubes biased optimally.

Why are matched tubes so heavily marketed? Because for most amps - without separate bias controls - it's at least useful to be sure all the tubes will run correctly, and hum will be minimised. For cathode-biased amps with a single bias resistor it's important to ensure that one tube doesn't 'hog' current at the expense of the others, too (which will shorten its life disproportionately). But there is a lot of hype about it too - and certainly with matched preamp tubes, which are unnecessary in any amp.

 

The matched tube thing comes from the Hi Fi realm. Actually perfectly matching "Push Pull" power tubes effectively cancels out even order harmonics. So a little mismatch is desired tone wise. Tubes that are greatly mismatched will be less desirable since one tube is really cold and the other is too hot. Usually 5 and even 10ma apart is acceptable. So what Terry said was correct to a point

 

1) There's no consensus on what sounds best. If there were, there would be only 1 amp, one guitar, one pickup, one speaker, one overdrive box, one set of strings, etc.....

2) Does the article say "how mismatched" tubes should be to sound "better"? If not, he's basically not saying anything. It's very rare to be able to match tubes to less than 1/2 a mA. So does mismatched mean 1/2mA, 1mA, 2mA, 5mA, 10mA, or more?

3) Tube matching, at least closely, IS over rated. To be within 5 or 10mA is good because it keeps hum at a minimum and allows the tubes to wear out somewhat evenly.

4) The only reason I attempt to match tubes closely is that it's easier to do than to convince some people that it isn't necessary

5) Even with proper burn in before testing, tubes will drift in use and likely drift apart. So a "perfectly matched" pair of tubes won't stay perfectly matched for long. Starting off as closely matched helps to insure that they won't become "grossly mismatched" over time.

 

Mike if I have tubes that are Old Ge Black plates and they dont match in my amp,without a bias pot,is it best just to try different ones till I find a pair that are within 5ma,that should be ok is that correct?
But will those tubes also dirft apart after a few hours use,even though they where never new to begin with?
Thanks

 

5mA is good. Tubes drift...they can drift closer together or further apart or in the same direction staying the same distance apart. It's a roll of the dice. Well used tubes are less likely to drift than new ones.

 

so when you buy a quartet, are they just closely matched pairs? same with a sextet etc? Ive always seen the tubes i want in matched pairs but not quartets. Would it be safe to buy two matched pairs?

 

Pairs are pairs, quartets are quartets, sextets are sextets. 2 or more pairs may or may not match each other.

Where are you ordering tubes that doesn't offer quartets?

 

i actually havent changed my poweramp tubes yet, i have the TADs that came with the amp and have been happy, but ive been searching around online for what i might like instead of the TADs, and i saw the NOS JAN Phillips that the two rock guys use and will probably end up getting a quartet of those, but i meant tubes like the GEC KT 66 on your website shows only matched pairs.

Once i get a little more play money ill be retubing my preamp first, then my poweramp, so im sure if i need anything ill be coming to you first, since you have the widest selection and seemingly the most happy customers.

 

The GECs are sold out (look at the bottom on the listing). There are so few of these left in the world that finding a matched quad will be a major challenge.

 

ah, sorry. there has been a few of those kindsa instances where ive been looking around for stuff.

im sure youll be hearing from me sometime in the near future!

 

Interesting.
So, would be safe to assume and possibly fun if an amp I have )Twp Rock) has sperate bias controls for each tube, as long as I stay within spec, I could try some different combinations?
Tis is two fold:
Obviosuly, two different brands, obviously not matched of the same octal type? This is a no brainer if I read all of the above correctly.
Secondly, I know the answer is no, but HAVE to ask, what about say one 6l6gc and one kt66?

 

yes ive also read the diezels all have separate bias points so they suggest mixing and matching tubes. so you could just buy a bunch of high quality singles?

 

Ok, I'll throw a wrinkle in here. How about cathode biased (class A) amps. How important are closely matched pairs in them?

 

Do you mean cathode biased or Class A?



It's more complicated...

In a cathode-biased amp with shared cathode resistors, matching is very important, because if the tubes are mismatched the hotter one will pull more current, raise the bias voltage for both, and so make the cooler one run still cooler. It will eventually stabilise, but not before the difference is much larger than it would normally be. This is called 'current hogging'.

In a cathode-biased amp with separate resistors per tube, it doesn't matter much at all, because each tube will automatically bias to the same operating point, since its bias voltage will adjust independently of the other tubes.

This is the same no matter what the Class (which is almost always AB).

In theory, matched tubes are essential for Class A because otherwise you cannot ensure that all the tubes reach the maximum possible voltage swing without clipping one of them before the other at either forward or cutoff, even in a separate-cathode-resistor amp.

 

I haven't the technical knowledge to offer an informed opinion either way. All I can say his Terry has been providing me with outstanding tubes at excellent prices for a few years now. I don't question anything he says about valves.

Often, I'd order a quad and get five or six tubes, initially I freaked, which are the matched ones? Terry would say, just try them, swap them around and above all else, use your ears. He was right of course, which makes me think he's most likely 100% correct on this issue too.

 

Yes, as long as there's a bias pot for each tube and you're not trying to mix weak tubes with strong tubes.

 

There's nothing technically wrong with using mismatched brands of the same tube type/number. Some people have found, by accident, that they actually prefer this. Back when you could buy US made tubes in drug stores, this was common practice. If one 6L6 went, the customer would buy another not necessarily of the same brand. They also wouldn't bias or check for matching. That said, there were occasional mysteries like "my amp sounds better/worse than before. What happend?". Or tubes wore out prematurely, etc.

Mixing a KT66 with a 6L6GC is ok too as long as they bias similarly and the amp can handle the added current requirements for the heater of the KT66.

 

I have a set of mis-matched Tesla EL34 running in my Komet head...they are running at 37ma and 22ma (one hot and one rather cold)...

they sound GREAT! probably won't last as long as 2 tubes running at 30ma...but...