Using B Side of Tube for Input

[Structo]

I noticed this a long time ago and if I heard the reason I've forgotten.

Why on the Dumble ODS is the second triode (B) of the first 12ax7 (V1) used for the input?

And on V2 (OD) it is done again with the B side coming first in the stage?

Is it supposed to be quieter doing it this way?

On my D'Lite the A side is used for the input as well as on V2.

I'd love to hear why this was done as I don't remember any other amps done like this.

[ampdork]

Not sure if he had a reason for that other than ease of wiring and a nice lining up of things.

However, if you are referring to the oddball ones where the second tube socket is purposely mounted 180 degrees from the first tube socket...that comes from some tube data sheets (mazda for one) that talks about doing this to lower hum (IIRC) in cascaded gain stages.

[Deric]

Towards the bottom of the first page...

http://tdsl.duncanamps.com/sheets/ecc83/ecc83.pdf

[talbany]

Sorry Miss Post


Tony

[Structo]

Thanks Deric, I knew I had read that some place before.

[Bob-I]

I tried this on a DUmble amp. First wired it in V1a on pins 1-2-3, then reversed it to use 6-7-8 on V1a see if the noise was reduced. I didn't actually measure anything but to my ears there was no difference. Maybe V2 would be more effective, and maybe it's a measurable difference but I didn't hear it.

[jaysg]

I believe the 'a' section is the 6, 7, & 8 and 'b' the 1, 2, & 3.

[Structo]

You know what? I'll be damned.
I was ready to say, yeah right, so I got to looking at some diagrams and I believe you are right!

I have never noticed before that the tube is laid out that way.

So actually Dumble is using the first triode for the input.

And Brown Note is using the second triode first.

[img:206:369]http://www.fmoaudio.com/images/amp-12ax7.jpg[/img]

[img:423:325]http://sub.allaboutcircuits.com/images/05309.png[/img]

[xk49w]

So what is it about the internals that makes one section quieter than the other (or whatever)?

[heisthl]

read the link again: http://tdsl.duncanamps.com/sheets/ecc83/ecc83.pdf it says pins 6.7.8 should be used first.

[Normster]

I read years ago that pin 6 was further from the heater filament and therefore less susceptable to hum. Like Bob, I tried it and didn't notice any difference. However, I wire all of my amps Dumble style with pin 6 first because it's easier to wire the heaters and puts the pin "gap" at the rear of the chassis (which makes it easier for me to line up the pins when installing tubes).

[toneman]

would that be for all 3 preamp tubes,,,Clean..OD...PI ?

[Structo]

Yes, it would appear so in looking at the #124 schematic.