12AX7

[TIMBO]

Hi again, I am of the understanding that the two halves of a 12AX7 are the same, yet i have seen a couple of schems that show the pin connections to be a particular way but to run the connecting wires it would be better to do the opposite. Thanks

[rafe]

or you could do a 180 on the socket?

[TIMBO]

Thanks rafe, You are correct , not knowing the actual layout of these builds it may just as simple as that. So i am correct, their is no difference. Thanks

[eleventeen]

That's a good way to mis-wire something, and, in an upside-down chassis, make it easy to bend pins mis-inserting the tube somewhere down the line.

[LooseChange]

If I am understanding you correctly... The 12AX7 pin order is the same either way.
Pins 1, 2 and 3 are the same as pins 6, 7 and 8. When planning a layout I'll always consider the positioning when running my wires.  I would leave all tubes positioned the same way and just re-number the schematic.

[TIMBO]

Thanks LC, done that. Got quite a involved build with the little booga and got a spare 1/2 12AX7 and don't know what to do with it.

[LooseChange]

Maybe parallel the other half?

[FYL]

I am of the understanding that the two halves of a 12AX7 are the same, yet i have seen a couple of schems that show the pin connections to be a particular way but to run the connecting wires it would be better to do the opposite.

"With Vf applied to pins 9 and 4+5 and the centre tap of the heater transformer connected to earth, the triode section connected to pins 6,7 and 8 is the more favourable section of the triode with respect to hum", from the '54 ECC83 Philips datasheet.

[jeff]

 I don't know how much of a real difference it makes but I always use the 6 7 8 section as my first gain stage, especally in a high gain build. The 1 2 3 stage is said to be noisier so by using the 6 7 8  stage first your second stage amplifies less hum.

[TIMBO]

Boy, you guys have got the goods!! This does explain a lot , i did a small build that had the second stage Pins 6,7 & 8 at the input. I just thought it was that way just for ease of wiring.
Can anyone explain why a v1 (12AX7) would have DC power (6.3v ac converted to DC) to the heaters and all the other of the valves AC ??

[FYL]

Can anyone explain why a v1 (12AX7) would have DC power (6.3v ac converted to DC) to the heaters and all the other of the valves AC ??

Input signal is app. 100 mV, V1 gives 30 or 40 dB of gain in common amps, any heater-induced hum can be very noticeable. V2 and others work at much higher levels and are less susceptible to hum. Power tubes don't need DC heaters - common mode noise is nearly eliminated at the OT.

[TIMBO]

Thanks again, I have a project in the planing stage that is the dual rectifier peramp that has V1 dc heaters . I'll see how it goes with AC and convert it if need be. Thanks