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FAQ - Upgrades and Tweaks Circuit and Component tweaks

 
 
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Old 6th May 2008, 11:58 AM
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Default Triode and Pentode Connection for KEL84

This is a basic connection guide to change the operation of the output stage from Ultra Linear to Triode and Pentode. It does not include details of any other adjustments that may be required to optimise the stage for best operation in these modes.

(If it is required to run without global feedback this can be achieved by removing the feedback wires from the Red speaker posts and reconnecting them to the Black speaker posts. The input of the amplifier will become very sensitive. It should be noted that the 750R feedback resistors R32/33 are also the cathode resistors for the first pentode valve. Therefore, any changes to feedback level should be made in a manner which keeps this 750R DC connection to ground intact. Reducing feedback is covered in a separate FAQ.)

Disconnect and isolate the pink and grey UL tap TX wires from tabs 1, 3, 16, 18

For Triode operation connect tabs,

1 to 2
3 to 4
15 to 16
17 to 18

For Pentode operation connect tabs,

1 & 3 to 11
16 & 18 to 12






Text from the original Kel84 introduction,

"The output transformer has Ultra Linear taps to allow experimentation with this mode of operation. These taps are at 12.5% of the total primary winding which is a little less than that used in vintage amplifiers, where 15% to 20% was commonly used for the EL84. Ultra Linear operation attempts to convert the pentode's characteristic to something similar to that of a triode. Hence Ultra Linear`s other commonly used name which is "partial triode operation". If one could imagine the screen grid being connected directly to the anode, then the valve would operate as a triode. At the other end of the scale, with the screen connected directly to the H.T. line then we have pure pentode operation. Ultra Linear mode takes the screen grid to a tapping point on the output transformer primary which is somewhere between the triode and pentode extremes, giving a similar power output to the pentode condition but with the triode`s mainly second harmonic distortion structure. It is entirely possible for the experimenter to change the operation of the output stage of KEL84 by moving the screen grid connection around. The kit is supplied wired for Ultra Linear, but by changing the screen grid connections of the output valves Triode, Pentode and Ultra Linear operation can be tried. With triode operation the power output will drop drastically however."

Last edited by Richard; 6th May 2008 at 12:25 PM. Reason: extra info
 

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