Tube Matching

You cannot convert a GT dynamic number to a static number. The GT number is based on where the tube distorts not just the idle draw. The only way you can do a close version of conversion is to have the latest or same batch of raw stock from which the tube was pulled (as each batch is different) and have access to the GT computerized GT stuff and run a side by side measurement. This will get you in the ballpark but there are many other factors considered on the GT scale.

The GT system also makes sure the tubes match under actual operation rather than at idle over the entire operating range of the tube. This is one reason that you might see a static match difference in a matched set at times of a milliamp or two but under operation, on a curve tracer, the two tubes will match perfectly across the entire amp’s spectrum.

When you have numbers such as “43” you need to know what this number is. Is it a plate current? Usually, that is the case. But if it is plate current you need to know what the plate voltage setting on their test equipment was and also know the bias voltage used. Otherwise, this number means nothing at all to the end-user.

As a side note… when I do tube tests I always use the original test spec settings as published in the RCA, GE, Sylvania, GEC, Mullard, etc., books. By the way… all these specs were the same and had the same target.

As an example… a simple tube matching unit such as the Maxi-Matcher will test a tube at 400 plate volts and -48 bias for 6L6 tubes. The numbers you see will be from 25-55mA or so. Using RCA book spec 250 plate volts is used with a -14.0 volt bias. A spec tube would produce 72mA. The tube is driven much harder as the bias is so much lower. This would be somewhat the same as testing at 500 plate volts and a -28 volt bias all things being equal. Most 6L6 amps have their bias at about -48v to -52v. -52 was the standard on many Fender amps and is the fixed bias of Mesa Boogie 6L6 amps.

Testing at true book spec pushes the tube much harder into its true operating range than the devices out there that seem to use higher plate voltages but with the higher bias keeps the tubes running at a much lower output. Less throw out when you do it that way. Some folks think GT stuff is too pricy. Well, the labor time in testing is much higher as is their reject rate which is above 50%. The cost and upkeep on the testing equipment is insane as well as the calibration frequency to assure that a #5 tube from 20 years ago is the same as a #5 tube today.

For anybody in my general area …. if you even want your tubes tested just bring them to me. I do not accept mailing by the way. I will test them for free for you. I can also do this on afternoons, evenings, and weekends as I have testing equipment at home. I can also tell you the true and actual gain of your preamp tubes which cannot be done with simple common testers including all of them that you see for sale on eBay. I can also tell you transconductance, current output, plate resistance, and run curves on the tubes for true matching rather than just matching a number like a transconductance which is what most folks do.

Conclusion

Once you know what the tubes in your amp are actually doing it is no longer a crapshoot at +/- 50% variability or more in the case of preamp tubes. You can then replicate your tone of you change tubes, move to a more gainy tube, less gainy one, tighter one, easier to compress faster one, etc. This is what blueprinting is all about in some of its areas …. knowing the specs and documenting what is actually going on in the amp.