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previous: Mook A pair of 6V6s in fixed bias want t... -- 8/13/1999 12:58 PM View Thread

Re: Choosing an OT: Theory Q's

8/13/1999 4:31 PM
Re: Choosing an OT: Theory Q's
The 8k p-p load is normally seen in tube manuals as a suitable load for 6V6s run in cathode bias, but I don't know where you found the "4k p-p in fixed bias" recomendation for those tubes.  
Anyway, it's tough to be able to state some hard and fast rules in choosing the "optimum" load resistance for a certain tube type, just based on one parameter like how it's biased.  
Some of the parameters that are juggled to select a load resistance are: maximum plate supply voltage for the circuit, max plate current, tolerated distortion level, maximum signal power output, rated plate dissipation, class of operation, and of course tube type. In choosing an optimum load resistance for the performance characteristics desired, it's an iterative process, involving graphs & equations.  
In triode circuits, the maximum power transfer occurs when the load is twice the ac plate resistance. In pentode (or beam tube) circuits, higher loads will usually extract more power than lower value loads, based on the product of the change in plate current and voltage across the load. But there's a safe limit with respect to max plate dissipation. For instance, in the typical 50w 6L6GC class AB2 circuit, such as seen in fender amps, raising the load resistance above the optimum design value will increase distortion and may take the tube out of the safe operating area for plate dissipation. (This stuff doesn't matter at lower than maximum power levels, where the tube's max dissipation isn't approached.) Class AB2 amps are usually set up for the maximum power capability of a particular tube, so you'll see that a relatively low value of load resistance will be specified. Many circuits for lower power tubes must be run at lower anode supply voltages, and usually need a higher load resistance to extract safe maximum power at that operating point. Your cathode biased 6V6 circuit would be a good example.  
Anyone interested in seeing what's involved in choosing a power amp load should take a look at the front section of an RCA tube manual. One I have in front of me now is an RC-23, but most of the info there is similar or repeated in other versions (other printing dates). You can obtain a reprint of the RC-19 version from various sources, but AES ( has it for $12.95.  
It's ok to experiment, and pick the load that sounds right for the application. Erring on the low side seems to be safe. Erring on the high side, in maxed out circuits, could be troublesome. Fortunately, high power transformers with high impedance windings are very costly, and therefore not usually available. So unless you try to use, say, something like a 50w p-p 6L6 (6.6k primary) output transformer on a maxed out class AB 6V6 or 6BQ5 circuit with an 8ohm speaker connected to the 4ohm tap, and running it flat out, it should be fairly safe (IMHO) to experiment varying load resistance up or down within reasonable limits, to arrive at the tone you're after.  

Mark Knapp Doc,This is a very ... -- 8/14/1999 2:03 PM
Ken Doc said:q{In triode circui... -- 8/16/1999 8:09 PM