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Old 27th September 2009, 05:24 PM   #1
bst is offline bst  United States
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Default Question re paralleling common-cathode tube sections


Hope you'll forgive a not-quite-beginner's ignorance. I've recently been given scores of NOS 6J6 and 6J6A common-cathode dual triodes, including many Mullard 'Master Series' 10,000-hour premium tubes.

I'd like to explore the possibilities of using these as drivers or low-power output tubes by paralleling both triode sections. I realize that the plate resistance will be halved, and the current capability will be doubled. Unfortunately, the Miller capacitance will also be doubled.

There are many similar tubes which can be had for very little expense. Are there practical topologies or 'work-arounds' to optimize common-cathode tubes for audio use? Or are these best left to their original uses as oscillators and class-C radio components?

Thanks in advance for your comments.
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Old 27th September 2009, 09:07 PM   #2
m6tt is offline m6tt  United States
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Common cathode tubes work well in audio, there's nothing about this design that causes the "audio electrons" from making it to the plate . They make great LTPs, and can be used as good audio mixers (shared plate load, shared cathode load, each grid as input).

I am not sure how well the sections are matched within a given 6j6, or between different variants, so that must be considered during paralleling. I have noticed differences in gain with my two-6j6 sample in the same circuit. They sound fine to me, but they get damn hot!
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Old 27th September 2009, 11:01 PM   #3
bst is offline bst  United States
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Thanks for your reply! Mis-match between sections was one of my primary concerns. A little more digging this afternoon on the 'net yielded this interesting page on Alex Cavalli's website: In it, he describes using a current mirror on the plates and a CCS on the cathodes to reduce noise and distortion, and force the two sections into balance.

Now I have to do some more research on current mirrors!

Thanks again.
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