How to Bias a Quad or Quartet of Output Tubes......Theory and Practice

In this technical video, we will discuss the theory and practice of how to bias a quad or quartet of output tubes. Basic biasing theory is covered, then applied to a double-ended pair of output tubes, for review, then finally applied to a matched quad set of output tubes. Calculations are discussed, as well as proper maximum plate dissipation values, and how they apply to cathode-biased and grid-biased circuits. In addition, safety practices are fully explained and encouraged. For a hands-on demonstration of this biasing technique being applied to a Fender AB-763 Twin Reverb, please click on the link below, in the video description.

NOTE: This video depicts a manual method to bias quad amps. If you are fortunate enough to own a Eurotubes Bias Probe, you can simply "march it down the line" of the four output tubes and read the biasing data for each individual tube......eliminating the shortcomings associated with "average" PD for each pair. Also, my suggestion to use the PV to detect disparity in a parallel pair won't work. Since the tubes are parallel, the PV will always be the same for each.

If you enjoy advertising-free videos like this one, please be sure to SUBSCRIBE to our channel and click on the bell to be alerted each time a new video is posted. Also, please consider becoming a Patreon patron of our channel by visiting this link: Patreon: uncledougsvintageamps  or making a PayPal donation to: [email protected]. Thanks so much for watching !!!

Link to Fender Twin Reverb video which demonstrates the biasing of quads:  1968 Fender AB763 Twin Reverb:   Eval...