Munga
April 16th, 2006, 04:33 PM
I just find a very interesting little snippet about matched tubes and whether you really need to match them or not. Hope you guys enjoy!
Do I always have to buy matched pairs of output tubes? The issue of "matching" output tubes, either by buying carefully matched pairs or by tweaking the bias levels and drive signals per output tube is not a settled one. It used to be common wisdom to simply buy matched tubes. A few people noticed, however, that they had a favorite pair of output tubes, which made their amp sound much better than others. The common assumption was that these tubes were better matched somehow. When these tubes get measured, though, it usually turns out that they are NOT matched, at least not matched for AC gain characteristics.
The concept of matched output tubes comes to us musical amp types from the hifi community, where they are trying to get the lowest possible distortion. This was true from the start, when Fender was trying to build low distortion amps and copied hifi circuits. The concept has simply clung to us, largely through inertia. It is relatively well accepted even in the hifi circles now that even-order distortion is euphonic, sounds good to our ears. It is very likely that the even-order distortion produced when mismatched output tubes are used sounds better than perfectly matched tubes.
If you have modified your amp so you can independently set the DC bias and the AC drive signal, you can tune almost any pair of tubes into AC and DC matching. You can also tune in a selective amount of AC drive mismatch to experiment with the selective mismatching sound.
There are technical reasons for matching. Getting enough turns of wire on the primary of an output transformer to get the right primary inductance and still using as little iron and copper as possible to do the job properly is an engineering problem that almost always results in Class AB output transformers being smaller for proportional power outptu than a Class A output transformer would be. The (relatively) smaller transformer and wire size makes a class AB (most guitar amps) output transformer susceptible to burning out if one of the half-primaries carries too much current.
If one side of the transformer carries significantly more current (like double) than it would otherwise in "normal" operation, it is possible it will overheat or open, effectively killing the transformer. Tubes that are so mismatched that to get the right total current for a pair means that one is carrying more than 50% over the nominal DC current for a matched pair is getting into the region where you ought to worry about output transformer damage.
If you mismatch, try to get the DC current the same in both sides of the output transformer, and an imbalance in the AC gain of the tubes. The logical limit of this AC mismatching is to remove all the AC drive from one output tube, which is a technique used by at least one commercial amp maker. This effectively keeps the output transformer happy with respect to DC, and gives you a single ended output stage; this also costs you a large amount of your available output power, but, hey, we're after tone, right?
Note that the commercial tube suppliers have good reason for wanting to sell us matched sets at a premium. I would expect their opinion to be that matched sets are absolutely crucial. As in all musical matters, let your own personal ears be your guide.
If you have a set of tubes you know are not matched, or if you have modified your amp to be able to set the bias and drive levels on each output tube separately so you can either match or not match the tubes at will, you might want to try un-matching them and see how it sounds to you.
From: http://www.geofex.com/tubeampfaq/tube_amp.htm
Alec
Do I always have to buy matched pairs of output tubes? The issue of "matching" output tubes, either by buying carefully matched pairs or by tweaking the bias levels and drive signals per output tube is not a settled one. It used to be common wisdom to simply buy matched tubes. A few people noticed, however, that they had a favorite pair of output tubes, which made their amp sound much better than others. The common assumption was that these tubes were better matched somehow. When these tubes get measured, though, it usually turns out that they are NOT matched, at least not matched for AC gain characteristics.
The concept of matched output tubes comes to us musical amp types from the hifi community, where they are trying to get the lowest possible distortion. This was true from the start, when Fender was trying to build low distortion amps and copied hifi circuits. The concept has simply clung to us, largely through inertia. It is relatively well accepted even in the hifi circles now that even-order distortion is euphonic, sounds good to our ears. It is very likely that the even-order distortion produced when mismatched output tubes are used sounds better than perfectly matched tubes.
If you have modified your amp so you can independently set the DC bias and the AC drive signal, you can tune almost any pair of tubes into AC and DC matching. You can also tune in a selective amount of AC drive mismatch to experiment with the selective mismatching sound.
There are technical reasons for matching. Getting enough turns of wire on the primary of an output transformer to get the right primary inductance and still using as little iron and copper as possible to do the job properly is an engineering problem that almost always results in Class AB output transformers being smaller for proportional power outptu than a Class A output transformer would be. The (relatively) smaller transformer and wire size makes a class AB (most guitar amps) output transformer susceptible to burning out if one of the half-primaries carries too much current.
If one side of the transformer carries significantly more current (like double) than it would otherwise in "normal" operation, it is possible it will overheat or open, effectively killing the transformer. Tubes that are so mismatched that to get the right total current for a pair means that one is carrying more than 50% over the nominal DC current for a matched pair is getting into the region where you ought to worry about output transformer damage.
If you mismatch, try to get the DC current the same in both sides of the output transformer, and an imbalance in the AC gain of the tubes. The logical limit of this AC mismatching is to remove all the AC drive from one output tube, which is a technique used by at least one commercial amp maker. This effectively keeps the output transformer happy with respect to DC, and gives you a single ended output stage; this also costs you a large amount of your available output power, but, hey, we're after tone, right?
Note that the commercial tube suppliers have good reason for wanting to sell us matched sets at a premium. I would expect their opinion to be that matched sets are absolutely crucial. As in all musical matters, let your own personal ears be your guide.
If you have a set of tubes you know are not matched, or if you have modified your amp to be able to set the bias and drive levels on each output tube separately so you can either match or not match the tubes at will, you might want to try un-matching them and see how it sounds to you.
From: http://www.geofex.com/tubeampfaq/tube_amp.htm
Alec