Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Scsoul on July 30, 2015, 10:13:09 am
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My new build which is basically a 18watt lite running kt66 @ about 400 425 on plates I'm wondering what range of screen voltage I'd need too make the tubes run safely happily, it has a pretty decent sized OT that I could run at 4 or 8k not sure which is best suited for kt66 run cathode biased?
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Can't find a "good" data sheet, here's a link for a new sheet;
http://www.drtube.com/datasheets/kt66-jj2007.pdf (http://www.drtube.com/datasheets/kt66-jj2007.pdf)
As long as you stay below the design max for G2, 3.5W from datasheet the rest is tweak n see.
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Right from the GEC datasheet, Vg2=300V, not going to be a "18W" though...
(https://dl.dropboxusercontent.com/u/1326040/KT66_PP_8k.png)
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KT66 is a European-design 6L6. While original-recipe KT66 had a lower plate dissipation rating, all new-made KT66 will be using the 6L6GC-like plate structure. Any 6L6 conditions will work.
In guitar amps we almost always run screens very-near plate voltage. Screen current is lower and screens amplify ripple, so we usually take one R-C or L-C filter between raw plate voltage and screen node. Choke filter will give negligible drop, R-C can be scaled for 2%-10% voltage drop. Basically, steal a screen-filter plan from a similar amplifier.
Because we often run screens higher than factory data-sheets, we must increase G1 bias voltage to keep the tube idle current reasonable. In the KT66 suggestion just above, G2 is run at 2/3 of supply (300V out of 450V). If we put G2 up at 90%-98% of plate supply, we expect to need roughly 3/2 or 1.5 times the G1 bias. Sheet says -27V from 2*500r, we pencil -40V with 2*750r (one 400r will usually work).
To use the 4K OT connection you must run fixed bias, *high* screen voltage, and you expect almost double the supply current, from 2*(62+9)= 142mA to around 280mA, and get around double the power. The Fender 50-Watt 2*6L6GC amps are a good model for KT66 at 4K load.
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I think to put it in simpler terms to wrap your head around, spec sheets show measured operating measurements/results from given stated parameters upon which the information was derived from but these "measurements" are not absolute maximum real world tube operating limitations that must be adhered to as PRR (& Leo Fender) demonstrated.
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To use the 4K OT connection you must run fixed bias, *high* screen voltage, and you expect almost double the supply current, from 2*(62+9)= 142mA to around 280mA, and get around double the power. The Fender 50-Watt 2*6L6GC amps are a good model for KT66 at 4K load.
Can a 18W Lite OPT handle the increase in output power? Better make sure before he cranks up the screen voltgage... And is there any advantage in running 4k over 8k for the primary impedance?