Mixing Resistor Explanation (AB763)

[clbraddock]

Something I have never been able to wrap my brain around is what are the "mixing resistors" actually doing. I'm mostly thinking of Fender (AB763 versions) where there tends to be a 3.3M resistor and a capacitor connecting the dry signal and reverb return. I don't really understand what this is doing. Normally a resistor and cap together, I think LPF/HPF, but there is no ground connection. The cap could be a coupling cap to block DC, but then why the resistor? The resistor could be for a voltage drop, but then why the capacitor? And, how do these achieve "mixing"? I'm sure there is a simple explanation, but I am too dumb to figure it out. Can someone please explain to me what the mixing resistor section is doing electrically? Thanks!!

[tubeswell]

The wet/dry reverb mixing in a BF (e.g. AB763) amp is via a voltage divider, which is comprised of the 3M3 (as the upper resistor ‘R1’) and the reverb level pot in series with the 470k resistor (as the lower resistor ‘R2’). I.e the 470k and the reverb level pot together form a partly variable R2.

Formula for a voltage divider output is R2/(R1+R2) = Vout.

This voltage divider attenuates the dry signal while simultaneously providing a path for the wet signal to get from the reverb recovery stage to the grid of the mixing stage. The amount of dry signal attenuation is relatively big for the dry signal because it has been through 2 gain stages (albeit with the load of the Tone Stack in between these), and if not for this voltage divider, the dry signal at this point would drown out the relatively feeble wet signal coming from the reverb recovery gain stage (which is trying to amplify the tiny electrical impulses from the reverb pan output transducer- which is much weaker than your typical guitar pup), and you wouldn’t hear any reverb otherwise.

You can alter the default amount of dry signal attenuation (and wet signal gain) by changing the 3M3 (or the 470k*) (or the reverb level pot).

* the mixing stage needs a certain minimum grid load resistance for the input impedance function, and there is a maximum grid leak resistance on a 12AX7 datasheet, so you don’t want to eliminate this resistance altogether, nor make it too high.

[clbraddock]

Thank you!! Very detailed. I hadn't thought about the voltage divider. So then, in the schematic I attached the V-out is going to the 220K resistor to ground near the verb level pot? If so, then part of the dry signal is travelling "down" to that ground via the voltage divider (i.e. reducing the dry signal then going to the grid of next amplification triode) while at the same time the reverb return is travelling "up" the same wire to rejoin the portion of the dry signal going to the next amplification stage? When I say "down" and "up" I just mean relative to the schematic. That makes a ton more sense if I am understanding correctly.

[tubeswell]

In the schematic I attached the V-out is going to the 220K resistor to ground near the verb level pot? If so, then part of the dry signal is travelling "down" to that ground via the voltage divider (i.e. reducing the dry signal then going to the grid of next amplification triode)

Yes the 220k presents an extra 'signal load pathway' (which is in parallel with the variable 470k + pot). You don't 'need' the 220k in this circuit (but having it there provides more of a 'load shunt' to 'regularise' the input impedance of the mixing stage, and the output of the dry signal voltage divider - albeit that this loads the signal down more).

(The 10pF just provides a HF bypass for the 3M3 going to the grid of the mixing stage, and it's capacitative load is minimal in terms of the dry signal voltage divider)


These 2 attachments 'kind-of' illustrate signal pathways, but they are more 'intuitive' and don't reflect an accurate depiction of signal impedance. To factor signal impedance into this, we'd need to consider the AC load pathways to all DC voltage point connections (including plate resistors and tube plate resistance). But for the purpose of showing how the dry-signal voltage divider works, its good enough I suppose. (Please direct all criticisms to the site janitor)

[clbraddock]

Thanks! You guys are amp geniuses! Thank you for taking the time to explain for a newbie like me!

[HotBluePlates]

... what are the "mixing resistors" actually doing. ... The cap could be a coupling cap to block DC, but then why the resistor? ... how do these achieve "mixing"? ...

Several things are happening at once with mixing resistors.  The most important is "mix two or more circuit-outputs, keep each circuit from interfering with the other circuits."

If there were wires instead of resistors, the connection of a circuit-output would change what the other circuit delivers.  So the mixing resistor is there for "isolation."

A great example is when 2 channels have volume controls whose outputs (wiper) are connected to "mix channels."  If there is no mixer resistor on each volume control's wiper to isolate, turning one control all the way down mutes all channels.  This is what happens with the typical Les Paul-style guitar with both pickups on.

[PharmRock]

... what are the "mixing resistors" actually doing. ... The cap could be a coupling cap to block DC, but then why the resistor? ... how do these achieve "mixing"? ...

A great example is when 2 channels have volume controls whose outputs (wiper) are connected to "mix channels."  If there is no mixer resistor on each volume control's wiper to isolate, turning one control all the way down mutes all channels.  This is what happens with the typical Les Paul-style guitar with both pickups on.

Does this sort of explain the weird volume control interaction of the two channels in a 5E3?  I know its a little different in that the plates feed the volume controls' wipers, then the signal goes to the PI.  I guess the volume pots themselves end up being "variable mixing resistors" in a way?

https://el34world.com/charts/Schematics/files/Fender/Fender_deluxe_5e3_schem.pdf

[pdf64]

…
Does this sort of explain the weird volume control interaction of the two channels in a 5E3?  I know its a little different in that the plates feed the volume controls' wipers, then the signal goes to the PI.  I guess the volume pots themselves end up being "variable mixing resistors" in a way?

https://el34world.com/charts/Schematics/files/Fender/Fender_deluxe_5e3_schem.pdf

Yes, that’s pretty much it. The last  little quirk is that if one channel is turned up full, its anode impedance will be at the mixing point, and so will form another potential divider, reducing the effective signal level of the other channel.
Anode impedance will be about 15k for 12AY7, 40k for a 12AX7.
Hope that made sense!

[PharmRock]

Hope that made sense!

Yes it does...thanks!