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Old 14th July 2012, 11:07 PM   #1
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Default Which pins to use first?

I'm no designer so I don't know why this is. I've noticed that when a twin triode such as a 12AX7 is used in the front-end of an amp, the higher number
pins are used first (6-7-8) and the others (1-2-3) next. This practice is not restricted to the 12AX7. I see this with many twin triode. It also does not matter how the tube is used. Is there a reason for using the second section of the tube first? Does it Matter?
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Old 15th July 2012, 12:13 AM   #2
ArtG is offline ArtG  United States
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Perhaps it has something to do with how the tube is viewed. A tube designer would probably visualize the tube looking up from the base into the tube. A designer of sockets, or circuits would probably see things from the opposite direction. On "twin" tubes the characteristics are supposed to be identical, although I notice that in certain tubes, the interelectrode capacitance sometimes varies from one section to another.
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Old 15th July 2012, 04:51 AM   #3
AJT is offline AJT  Philippines
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no two tubes are exactly alike, even if they are inside the same envelope....so that i use these sections as i see fit.....to my liking....
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Old 15th July 2012, 05:17 AM   #4
tomchr is online now tomchr  Canada
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I just use whatever works for the layout.
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Old 15th July 2012, 06:08 AM   #5
maxro is online now maxro  Canada
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Some otherwise "twin" triodes, like the 6DJ8, have different heater-cathode voltage limits on the two triodes. These need to be wired in a certain order when used as both halves of a DC coupled stage, i.e. common cathode to concertina phase splitter.
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Old 15th July 2012, 07:45 PM   #6
DF96 is offline DF96  England
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If an AC heater is used then hum may be better for one section. Read the datasheet - always a good idea!
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Old 16th July 2012, 07:10 AM   #7
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I find it just makes the layout a little easier (in guitar amps).
Experience is what teaches us to make a different mistake next time...
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