Date: Thu, 27 Mar 1997 05:07:21 GMT From: Jeffrey Herman To: Multiple recipients of list Subject: Using a tetrode or pentode as a triode Thought this might be of interest to many of you; picked it off the ARRL email server. 7.3W, Jeff KH2PZ From: info-serv@arrl.org Subject: Re: Synthesized Triode from Tetrode or Pentode By Dave Newkirk/ July 28, 1993 Connecting together the screen and control grid of a tetrode (or the suppressor, screen and control grid of a true pentode) turns the tube into a high-mu triode. The resulting tube may be, but is not necessarily, a *zero-bias* triode. True zero-bias tubes are designed to be so, and must be operated according to their designer's specs to work well with zero bias. Too much plate voltage on a zero-bias triode can overcome its grid's control and make it draw too much plate current when idling. So a high-mu triode synthesized by connecting together the multiple grids of a screen-grid tube *may or may not* be operated as a zero-bias tube, depending on its resulting control characteristics and how you power the tube. Relatedly, a screen-grid tube connected as a high-mu triode may not necessarily exhibit the low-IMD characteristics of a good, designed-for-low-IMD zero-bias triode. In other words, you wouldn't necessarily synthesize a 3-400Z by high-mu- triode- connnecting a 4-400. (Even a 3-400Z is old hat. There are newer, better zero-bias tubes, where *better* equates to "lower intermodulation distortion.") I haven't mentioned so far what is probably obvious: Connecting a well-screened screen-grid tube as a triode means that it's no longer a screen-grid tube. An amplifier built with a tube connected in this way must be (A) neutralized if the tube is operated grounded-cathode or (B) operated in grounded-grid so its connected-together grids can act as a screen or (C) configured as a cathode follower (which is largely impractical if you want significant power output). This sets us up for the next question: What would happen to a beam-power tube, such as a 6DQ5, if its screen is connected to its control grid? It would act as a high- mu triode. *But*--and this is very important--it *couldn't* be operated grounded-grid (without neutralization, that is)! This is so because, like many beam power tubes, the 6DQ5's beam-forming elements are internally hardwired to its cathode. This bypasses the shielding afforded by grounding its control and screen grids. Beam power tubes that cannot be operated in grounded-grid without neutralization include the 807, 1625, 6146 and many TV sweep tubes. *Some* TV sweep tubes and RF beam power tubes (6KD6, 5763) bring their beam-forming elements out to a separate pin. *These* can be operated in grounded-grid without neutralization; you'd ground their control grids, screens and beam-forming plates for RF. You have the option of tying all the grids together for dc or feeding them separately. This gives on one last issue: There's one more important issue in using screen-grid tubes as high-mu triodes, grounded-grid or not. By construction, a tube's control grid exerts more control over its electron stream than its screen; it's closer to the cathode than the screen and made of finer wire. Further, screen-grid tubes are intended for applications in which the screen is *considerably more positive* than the control grid. *Because of this, severe grid overdrive will likely result* if the control grid and screen are merely tied together (for dc *and RF) during high-mu-triode operation because the screen operates at the same potential as the grid and therefore doesn't draw them past and away from the grid as it does when it's significantly more positive than the grid. *GE Ham Notes* (see enclosed) goes into detail on this, as does V. S. Campbell and W. S. Skeen, "Grounded Screen-Grid Operation for Tetrodes," *QST*, Nov 1959, pp 37-39. (The introductory box for this article goes like this: "A tetrode with control grid and screen tied together to form a high-mu triode for a grounded-grid circuit makes a very simple arrangement. However, this type of operation invariably results in excessive control-grid dissipation. This article shows a simple method of avoiding this difficulty.") It's therefore important to monitor element currents individually, at least until the circuit is finalized, when operating a screen-grid tube as a high-mu triode. Calculating the operating conditions for a tube operating in grounded-grid is relatively straightforward for true triodes, as explained in G. Grammer, "Input Impedance and Fed-Through Power in Grounded-Grid Amplifiers," Technical Topics, *QST*, Dec 1958, pp 32-35, 184. Calculating these parameters *isn't* straightforward if you want to run a screen-grid tube in grounded grid and apply normal screen voltage to the screen while grounding it for RF; experiment will likely be necessary to determine particulars in this case.