Pre-amp design questions

[Hellhammer]

1. If one would design a 4 gain stage -> cathode follower pre-amp. Is it bad design to use the same double triode for the first stage and the CF?

2. Would a common-anode mixer be the best way to mix two separate channels?

3. If the channels were 3 and 4 stages they are phase inverted and couldnt be used in parallel? Is there another way to invert the phase than to add a 4th stage to the 3 stage channel?

Very thankful for any help I can get!

[guitarmike2107]

1. If one would design a 4 gain stage -> cathode follower pre-amp. Is it bad design to use the same double triode for the first stage and the CF?

2. Would a common-anode mixer be the best way to mix two separate channels?

3. If the channels were 3 and 4 stages they are phase inverted and couldnt be used in parallel? Is there another way to invert the phase than to add a 4th stage to the 3 stage channel?

Very thankful for any help I can get!

I once split the cathode follower between two 12ax7, it was noisy, now that might have just been the valves I was using, but since its so simple to wire a cathode follower on a single socket I have stuck with that way ever since.

Also I wouldn’t wire a 1st stage next to a 4th/5th stage in the same bottle, that’s sounds like asking for trouble.

You can mix two out of phase channels by using both inputs at the PI, assuming you don't have NFB of course.

[Hellhammer]

Thanx for the reply. This would be a stand alone pre-amp so no PI. I'm just tossing around ideas so far, but the way I picture it now is:

Shared first stage triode -> triode -> triode -> common anode mixer triode -> shared CF for ch1.
Shared first stage triode -> pentode -> common anode mixer triode -> shared CF for ch2.

I might want to use half a 12au7 for the first stage as well as the CF, that's why I'm asking if they can share the same tube.
Thanks

[Super_Reverb]

3. If the channels were 3 and 4 stages they are phase inverted and couldnt be used in parallel? Is there another way to invert the phase than to add a 4th stage to the 3 stage channel?

If you are going to mix two channels, that would be driven simultaneously, they need to be in phase. You could add a bipolar / MOSFET inverting gain stage with a gain of around 1 (hint unbypassed emitter/source resistor slightly smaller than collector/drain resistor. Could DC couple if you're creative.

cheers,

rob

[Hellhammer]

If you are going to mix two channels, that would be driven simultaneously, they need to be in phase. You could add a bipolar / MOSFET inverting gain stage with a gain of around 1 (hint unbypassed emitter/source resistor slightly smaller than collector/drain resistor. Could DC couple if you're creative.
cheers,

rob

Thank you! I'm not very creative as I know nothing about mosfets... I have some reading to do.

[rdjones]

Looping back around to the proximity of the first stage is not recomended, especially with lots of gain.
Consider a 12DW7 for the triodes and something like the 6AN8 for the pentode stage.

reddog