Long Tail Pair PI with signal to both grids

[rocket]

Is it possible to feed the signal from two preamps to the two grids of of a (dual) tube in LTP configuration (of course it is but does it work properly).

So one preamp would go to one triode and the other to the second triode.

The reason for doing this is that I want to build a dual channel amp with the preamps having a different number of stage and I want to avoid adding the up out-of-phase.

Do I have to expect any Problems, especially as on preamp will be a pentode with a high output impedance.

[Ray Ivers]

does it work properly

From a theoretical standpoint? No, not at all - instead of one gain stage cathode-driving a grounded-grid stage, you have what amounts to a 'common-cathode mixer', similar to a common-plate mixing stage sharing a single plate resistor between two gain stages. Of course, you may love the sound of it, who knows.

preamp will be a pentode with a high output impedance.

My advice would be to leave the PI as-is, and replace your pentode with a triode/pentode like an ECL82, ECL86, or similar (using the triode stage for input and the pentode as the 2nd stage, for better noise/microphonics/overdrive performance) - then mix both preamps thru resistors at the PI input, which has a super-high impedance in a LTP. That's just what I would do, YMMV, etc.

Ray

[AdmiralB]

The AC30 works this way - Brilliant/TB channel into one side of the PI, normal and vib-trem channel into the other side.

[rathmann]

The reason for doing this is that I want to build a dual channel amp with the preamps having a different number of stage and I want to avoid adding the up out-of-phase.

Are you planning on having remote channel switching? Or are you planning on just having different inputs per channel?

How many stages per channel? If they both have an even number of stages or both an odd number they'll have the same phase and can be connected to the same PI input. If there is a combination of even and odd number of stages you can do what you've suggested and connect then to opposite sides of the PI.

Regardless of the number of stages or where you connect each channel to the PI it would be highly recommend you provide a means to mute the high gain channel when not in use. Otherwise you'll get all the hiss and noise of the high gain channel even when it's not being used.

steve

[rocket]

I am planning 4 lowgain stages in one channel (med mu tubes like 6sn7) no CF and one single pentode. Master volume switchable and after the PI.

No channel switching, just 2 inputs, option of external switching with an A/B box

The noise from the unused channel might be a point to consider.

I don't want real high gain however.

I have the tubes and stuff and will just try it. If I don't like my design I can still build a deluxe amp (power amp will be PP 6v6).

Thanks for your sugggestions.