Cathode bias half power switching

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neun_elf
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Cathode bias half power switching

Post by neun_elf »

Hi,

When using 4 x cathode biased EL84's, is it possible to fit a half power switch to disconnect one pair of cathodes, in the same way as Mesa do with a quad of 6L6gc's to get their 60/100 watt switching, or will that not work with cathode biasing?

Thanks in advance.
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rock_mumbles
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Post by rock_mumbles »

It will work ...

To be all proper ... you should then change your speaker cabinet to the next lower impedance tap to have the proper reflected impedance ...

With four EL84's you want 4k reflected impedance with two you want 8k ... so plugging your 8 ohm speaker into the 4 ohm tap gives an 8k reflected impedance etc.
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Wayne_Alexander
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Post by Wayne_Alexander »

Look at the Ceriatone website for their layout for the "Dizzy 30" which is close to a Matchless DC30 in circuitry if not layout.

http://ceriatone.com/images/layoutPic/m ... iatone.jpg

That's a good switching system for half power in a 4 EL84 cathode biased amp.
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Plexi
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Post by Plexi »

But its really not "half power" so to speak. You may like the sound of one over ther other, probably the 4 tube over the 2,just guessing.
But if your thinking of it dropping to half the vloume of what you are hearing, It really doesn't do that.
Really a bit misleading. You will not hear much of a difference in volume between 4 tubes and 2.
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phsyconoodler
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Re: Cathode bias half power switching

Post by phsyconoodler »

No it won't work if the EL84's have one shared cathode resistor.You would need to have individual resistors/caps for each power tube to take advantage of a half power switch.
You can do it with fixed bias but not with cathode bias.
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whitey
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Re: Cathode bias half power switching

Post by whitey »

+1 for Wayne, thats the exact way I did it for my trainwreck rocket and it works great! Half power doesn't mean half as loud since decibel (human ear detectible) and power are not linear but logrithmic function. Check your el84 bias though if I recollect the bias is hot! and may not be suitable for modern tubes.
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