Tube matching is one of the most misunderstood and abused aspects of tube testing. The fact is, a good number of tubes advertised as "matched" really are not matched at all. The problem? a lack of understanding, misinformation, and the use of inadequate test equipment.
Some truths and misconceptions on tube matching:
1/ Are tubes matched if they have the same construction? NO.
2/ Are tubes matched if they have the same codes? NO.
3/ Are tubes matched if they came from the same amp? NO.
4/ Are tubes matched if they are the same type? NO.
5/ Are tubes matched if they have close or the same Gm or emissions readings? NO.
6/ Are power tubes matched if the readings are within 5 - 10% when tested for mA at the correct voltages? YES!
7/ Are preamp tubes matched if the mu (gain) readings are close or the same when tested at the correct voltages? YES!
READ ON............................
About Testing:
Old service testers come in primarily two categories: Emissions and Transconductance (Gm) types.
Examples of the Emissions types are the Sencore Mighty Mite series, some Heathkit and Eico's, and some B&K's, to name a few. These testers are ok for hobby or home use to sort out bad tubes, but nothing more. Some were used in the field as service testers, but I will say no more about them.
Transconductance (Gm) testers such as the TV7's, Hickoks, Jacksons, Triplett, B&K, (there are too many to list here) are service testers designed to give a technician, serviceman or military personel the ability to diagnose problem tubes in circuits and used as an aid in trouble shooting. Transconductance (Gm) is easier to measure than the actual gain (how much a tube is actually amplifying). So transconductance became widely used in service testers, and thus has been generally accepted as a reasonable test for "quality". Only a few are able to measure mA (plate current draw) to match power tubes, but the voltages are far too low to be of any use. But contrary to popular belief, these testers cannot measure the actual output of a tube for the following reasons:
- The ACTUAL output of a tube is measured in "mu" which also called "gain" or "amplification factor".
- To calculate mu, you must multiply the transconductance (or Gm) by the plate resistance (Rp) so "mu = Rp x Gm".
- So, the transconductance (Gm) is related to the mu but it is NOT the actual mu.
- Because of the relationship between Gm and mu, and because it is easier to measure, it was widely accepted as a suitable test for quality.
In the "old" days, tester manufacturers were bound by patent laws and many different approaches were used to measure transconductance. Things got very confusing and manufacturers competed for a technicians business. The end result was that for decades we had little choice but work around the limitations of transconductance. Despite claims of the abilty to match tubes with these testers, the result was that the tubes were not really matched at all. Those old service testers did NOT apply voltages high enough to match those found in everyday circuits and manufacturer tube data sheets! But what they did well was give a very good estimation of tube quality and sorted out gassy and shorted tubes.
Then there are the modern units. New ones such as the Amplitrex, Maxi Preamp and the MaxiMatcher are much more reliable and use "real world" voltages and were designed for tube evaluation suitable for sellers for retail use. The old service testers simply cannot compete with the new technology. Modern testers such as the Amplitrex and MaxiMatcher have changed things. We can now properly match power tubes with the correct tests at the correct voltages.
Power (output) tubes are matched by mA (curent draw), NOT transconductance (Gm). Because this is what matters in a power tube. So why would you match tubes by Gm when it has nothing to do with whether or not a tube will bias properly in you amp?? It really doesn't matter what type of biasing circuit your amp has; The closer the mA, the easier they will match, the better they will sound, and the longer they will last!
For preamp tubes, matching by Mu (gain) is best, because transconductance is NOT the output! (as demonstrated above you must measure the Rp and the Gm to calculate the output). But because transconductance is PART of the picture, it was used to give an indication of tube quality; nothing more, nothing less. So now that we have modern testers such as the Amplitrex and Maxi Preamp to measure the actual Gain (how much the tube is actually amplifying) we now have a better understanding of what the preamp tube is actually doing in the circuit!
There is a school of thought that believes that tube transconductance (gm) "tracks" or follows the mA (plate current) and the gain (output). Sometimes it does, sometimes it doesn't. Matching by Gm, even at the right voltages, is simply NOT a reliable way to match tubes!
"So my tubes are properly tested; when are they matched"? Well, that varies with opinions. For power tubes, I think that most agree that mA (plate current) readings should be with 5 - 10% to be considered matched. Phase inverters have a very important role to play in a circuit, and it is probably better to have dual triodes that are strong and balanced for best performance. For other preamp tubes, it really is not that important because biasing is not an issue, but tube gurus with high end gear usually like matches that are very close; it really only matters how "perfect" you want to be. But unmatched preamp tubes are generally not a problem for circuits to handle.
In Summary:
- the old service testers are great for giving an indication of tube quality and finding shorted or gassy tubes. I believe they are fine if you are willing to accept their limitations. Only a very few can test for mA (plate current) but the voltages are far too low to be of any use. None test for tube gain (mu).
- when possible, preamp tubes are best matched by gain (mu) on modern testers such as the Amplitrex or Maxi Preamp.
- Power tubes should only be matched by mA (current draw) at the correct voltages such as on a Maximatcher or Amplitrex. Or buy Factory Matched tubes from a reputable dealer.
- When tested at "real world" voltages on testers such as the Amplitrex or Maxi series, readings are usually lower than those obtained on the old service testers.
- Tubes "matched" on the old service testers usually aren't matched at all when tested at the correct voltages on modern testers such as the Amplitrex.
It is my experience that finding vintage matched power tubes is very difficult and they have become quite rare. You need piles of tubes of the same construction to find actual matches. The odds of that are quite remote. But don't be fooled! I have seen many sellers advertising "matched" tubes because they look the same or have the same codes....Hmmmm....
Keine Kommentare:
Kommentar veröffentlichen