Combining outputs from 2 tone stacks

[W5FH]

Am looking at a build that will have two separate, identical preamp sections that will combine ahead of phase inverter. The preamps will be close copies of the Bassman 5F6A with a cathode follower driving a tone stack at very end of preamp, with output taken from treble pot wiper. I am struggling with a circuit to combine the outputs of the two tone stacks and not allow any interaction between them. Have been studying the 12AX7 used as a common anode mixer but I have read a fair amount of negative comments about them. Does anyone have a suggestion? I can provide a gain stage to make up for resistive mixing if that is the way to go. Thanks!

[sluckey]

Any resistive mixing will have some degree of interaction. Higher value mix resistors offer less interaction.

I think I would use a common anode mixer. The Gretsch Chet Akins amp did (see V3).

If you will use a LTP PI ***AND*** there is no NFB loop, you can simply connect one preamp to one LTP grid and the second preamp to the other LTP grid.

[W5FH]

Thanks Sluckey-
    I have a print-out of the Gretch mixer as used in that Chet Atkins amp. I forgot to mention that the preamps would rarely be used at the same time, the user simply wants to have controls set for two different instruments as he rotates between them at gigs. There will be a neg feedback/presence loop. In studying the common anode mixer in this application, with rarely ever a signal on the second grid, the stage is pretty much going to behave as an ordinary gain stage with its operating line shifted some from quiescent current of unused triode section, correct? Would it be best to separate the two cathodes if using this mixer? Going further with this, I saw a post (in the past) on this topic whereby someone said not to use the two triodes as a mixer (separate out their plates) and just combine their outputs with a resistive mixer. This is what has me wondering about the best circuit to use. I don't want a lot of gain and if using mixer or just resistive-combined triode stages I would need to reduce their gain.   

[sluckey]

Maybe all your guy needs is a SPDT switch. Zero channel interaction.   

[nandrewjackson]

+1 for either 2 mixing resistors or a mini toggle.


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[W5FH]

Thanks replies. Stumbled upon a version of the VOX AC-50 amp that is very similar to what I am wanting to do. The cathode follower driven tone stacks are combined with 470K resistors coming off of treble pot wipers. Anybody use this particular version of AC-50 or have played around with it? Just curious if much interaction between tone stacks. Thanks.

[sluckey]

Any resistive mixing will have some degree of interaction. Higher value mix resistors offer less interaction.

[acheld]

+2 on the mini-toggle.

No interaction, and no big deal . . .   presto magic!

[tubeswell]

A vote for common anode mixer if you have a spare dual triode.

[W5FH]

Thanks all-
   I appreciate folks taking the time to reply. The user wants the option to be able to use both preamps at the same time, but he anticipates it would be rarely done. That is the reason for considering mixing resistors or a common anode mixer. If I could pick your brain just a little bit tubeswell can you detail to me a recommended circuit of a common anode mixer? I found another example of one in a HiWatt preamp. Is it best to separate out the cathodes, each having its own resistor/bypass cap? Since I don't need much gain do you have ideas for a lower gain one that will do well in bass service? I just finished a build that uses a common anode mixer to mix the dry signal and wet reverb signal (Astroverb) and have studied adapting it to this amp. Thanks.

[tubeswell]

If I could pick your brain just a little bit tubeswell can you detail to me a recommended circuit of a common anode mixer?

Some suggestions courtesy of Merlin's 1st edition preamps book (Or you can play around with different/separate Rk||Ck variants on each triode. The sky's the limit really

[pdf64]

Just to note that anode mixing can be seen as another form of resistive passive mixer. The mixer resistors being the anode circuits.
Compared to a pair of mixer resistors, its benefits are there should be no channel interaction / loading, and the output impedance is much lower, but there’s still about a 6dB signal loss. And it uses an extra triode.

[tubeswell]

With passive mixing resistors, there is increased load compared to a common anode mixer. Sure, in a common anode mixer, the other triode minutely changes the way the load line behaves, but there is still net gain (compared to not having any gain stage there), whereas a passive mixing resistor circuit has net signal insertion loss.