a tube primer for guitar and bass players - Guitar Amplifier Blueprinting

The most important tube(s) in the preamp section are in the first stages, the tone and gain stages. Changing V1 in many amps will yield
the most results. Tubes used for current drivers as their primary function, such as reverb drivers or effects loop drivers, do little in most
amps to change the tone. This is generally not part of the tone or gain stage. The exception to this is the phase inverter or driver, of the
power tubes. There will be more on this subject later in this paper.
Moving on to the tubes
What we are looking for are specific factors in testing, and other factors in sonic aspects. Rather than list all the technical data for each
tube, where I have this data, I will provide additional information for gain, output, and consistency. The consistency factor I will call QA for
quality. For this “spec”, the lower number, the better. A perfect tube factory run would have 0% inconsistency. A factory run where the
tubes were +/- 20% off target factory expected specifications, would have a number of “20”. The standard specs that are the target
numbers for gain is 100, and output is 1.2 milliamps, in the case of the 12AX7/ECC83/7025 family. Output will be shown as a percentage
of factory expected target. If a tube has 1.2 milliamps of output, it’s Output would be 100%.
The most important factor in all of the above, is know and trust your tube vendor. Tubes need to be tested for microphonics, low output,
and noise. With today’s material inconsistency, a tube that is quiet today may become microphonic after a few hours of use, so a vendor
with a good warranty is one that one may wish to have in your corner.
NOTE: This test data is from bulk factory batches of large quantities of tubes, at times 1000 tubes of each type from each factory run. This illustrates
how random these tubes are, and shows what a chance you may be taking with untested tubes in general. These tubes, after testing using good
methods, will narrow this inconsistency considerably. In one Tube Company’s case, Groove Tubes, their reject rate on factory tubes can exceed 50%.
You can sometimes easily tell how well a tube has been tested easily … price. When there is a lot of test time and labor, and half of your stock hits the
trash bin, the end retail cost has to be higher. The data below was collected at one part of late 2002 and early 2003. For the most current data, see the
updated information elsewhere in this document.
20