Power Tube Testing and Matching: For twenty years, RAM Tube Works has stood as the singular reliable source for tested and truly matched power tubes. With our computerized testing procedures we are able to provide tubes that will allow your amplifiers to function their best. In virtually every instance, RAM Tubes will easily better the performance of the original tubes in your amplifiers. RAM did the original research to discover that if both bias and transconductance are matched at a particular operating point, then the tubes will remain matched over a variety of operating points. Please request or look on the web to see our paper: The Virtues of Power Tube Matching.
Power Tube Data
RAM Power Tubes are tested for the following parmaters at screen voltages of 300, 350, 400 and 450 volts to allow the use of the data for any power amplifier.
G1 Bias Negative voltage to achieve a 50 mA cathode current at a given screen voltage GM Transconductance at 50 mA cathode current in micromhos. GL Grid current in micro-amps. IG2 The G2 current in mA. IKP Multiply by 10 to get peak current in mA at G1=0 (pulse test).
Pairs of tubes are matched to 5% bias accuracy (G1 Bias) and 10% transconductance (GM). This assures low distortion and tracking of bias at different cathode currents and as the tubes age. Bias is selected at common operating screen voltages (V G2). For example, 6550s are matched with VG2=350V while EL34s and 6L6s use VG2=400V.
Our testing of large batches of tubes has produced the expected bell curve when plotting the range of their test results. It is interesting to note that tubes taken from opposite ends of the spectrum (as might happen) will result in a bias variation ratio of 2:1. In an amplifier with a single bias pot for the pair, one tube would carry 2/3 of the total current and the other tube 1/3. The 2/3 tube will obviously overheat and fail early. Distortion would be high due to the gain difference associated with unequal cathode currents. Even in amplifiers with individual bias pots, transconductance variations would cause a large increase in distortion.