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Thread: Switchable b+ power?

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    Switchable b+ power?

    What about switch-selectable b+ in my PP 6L6x2 cathode biased paraphase inverter homebuild to make if more versatile for different volume needs? The power tranny has multiple hi V secondaries. Say 6L6 plate voltage of around 390 for louder and 330 for quieter situations? It is now running at about 390 V with a 200 ohm cathode resistor and 50 mfd cap. Do I need to switch in a different cathode resistor value with reduced b+ voltage?

    Thanks-
    Jim R

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    Its safe for the tube to use the same cathode resistor,as long you have a resistor tailored to the highest voltage.
    But you aint get the best sound out from it.
    You probably get a little earlier breakup, but thats what you want. I dont believe it's gonna be much lower volume.
    If you switch to lower voltage, and a 250-270 ohm, cathode resistor and use a pair of 6V6's i believe you get a better solution.
    It give a very big volume difference if you use a smaller speaker. I have my ampheads combinated with 2 - 3 different speaker cab combinations. It give me possibility to choose gear after situation.

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    Last edited by arnenym; 02-01-2009 at 07:46 PM.

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    Senior Member bnwitt's Avatar
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    You'll need to change the bias supply voltage as well along with the B+. The Fender Red Knob "The Twin" had a low/hi output switch that did just that. It actually dropped out half of the PT's secondary HT winding and changed B+ to the power tubes from 469 to 236 that way.
    Attached Thumbnails Attached Thumbnails The Twin schematic-parts.pdf  

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    Thanks for the reply. However, this concern does not pertain to a cathode biased amp (described in initial post).

    Jim R

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    Senior Hollow State Tech Bruce / Mission Amps's Avatar
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    Quote Originally Posted by chuggy View Post
    Thanks for the reply. However, this concern does not pertain to a cathode biased amp (described in initial post).

    Jim R
    Huh? What are you talking about? That is precisely what you described... a cathode biased amp with 6L6s...
    "...my PP 6L6x2 cathode biased paraphase inverter homebuild..."

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    Bruce

    Mission Amps
    Denver, CO. 80022
    www.missionamps.com
    303-955-2412

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    Sorry for the murky reply- it was directed at "You'll need to change the bias supply voltage as well along with the B+." This does not pertain to a cathode biased amp since it does not employ a negative bias supply voltage, unless switching in a different cathode resistance is also considered changing the bias supply voltage.
    Jim R

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    You shouldn't HAVE to go lower than 200ohms at anything over 300v (plate to ground)...but your plate current will drop some, so you may want to correct for tonal reasons? Old Dano/Silvertone/Wards used to run 6L6 at just over 300v with an effective cathode value of 180-ish ohms.

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