Ch. Mixing resistors of 100K, 220K, 270K or 470K?

[Brownie]

Can anyone reveal the ways the various values of mixing resistors behave in the circuit? This early Traynor had 100K ones, but I substituted 270K ones, split the V1 cathodes and put a bunch of Marshall values into it for buddy client. That amp is finished and now I'm doing up my 5f2/5f3 Frankenbeastie with a stacked tone pot. What should I choose for those mixing resistors? Maybe I'll use 220K like on a Fender AB763

[stratomaster]

Everything is a filter (especially when adding caps in parallel with the mix resistors) and everything is a voltage divider.

While the divider ratio is the same between these, the overall resistance (impedance if the coupling cap is small enough) to AC ground is significantly different between the different values. You'll need to trace the circuit back through the coupling cap and plate resistor to HT, then parallel that with the functional triode residence to get a good calculation

As to how they'll behave, that depends a lot on the other values in the circuit.  In general, you'll have marginally more signal the higher you go, but there's also a voltage drop across the first mixing resistor, so there is a ceiling. There are also noise considerations as higher resistances are inherently noisier.

[Brownie]

Thanks for that. I guess I'll have to set up my board in order to make it easy to try various pairs of resistors. Then I can hear for myself

[dogburn]

So what is a 5f3? Do you mean 5e3? Either way, it would be interesting to see what you come up with.

[Brownie]

I was supposed to edit my post but I missed that error. It's a 5E3 shoehorned into a 5F2 chassis. A single input jack,2 volumes, master and a stacked tone pot. I was just supposed to make a Harvard out of it in the first place, but it'll work eventually...

[sluckey]

There are no mixing resistors in a stock 5E3. The two volume pots act as mixers.

[pdf64]

The mixing resistors are in the grid circuit, so as their value increases, the Miller capacitance LPF corner frequency decreases.

[sluckey]

The mixing resistors are in the grid circuit, do as their value increases, the Miller capacitance LPF corner frequency decreases.

Are you maybe thinking about the two 68Ks for the HIGH and LOW input jacks?

As for mixing the two channels together... There are no mixing resistors in a stock 5E3. The two volume pots act as mixers.

     https://el34world.com/charts/Schematics/files/Fender/Fender_deluxe_5e3.pdf

[shooter]

my 8th grade English-comp understanding of the OP;


"Can anyone reveal the ways the various values of mixing resistors behave in the circuit? This early Traynor had 100K ones


"That amp is finished and now I'm doing up my 5f2/5f3 Frankenbeastie with a stacked tone pot. What should I choose for those mixing resistors? Maybe I'll use 220K like on a Fender AB763




maybe OP's schematic might clear up the english??

[pdf64]

The mixing resistors are in the grid circuit, do as their value increases, the Miller capacitance LPF corner frequency decreases.

Are you maybe thinking about the two 68Ks for the HIGH and LOW input jacks?

As for mixing the two channels together... There are no mixing resistors in a stock 5E3. The two volume pots act as mixers.

     https://el34world.com/charts/Schematics/files/Fender/Fender_deluxe_5e3.pdf

I was thinking about the first section of the first post, the Traynor / Marshall scenario, as I couldn't make sense of it in the 5F2/5E3 context.

I tried to put that forward as a general consideration, so yes it also applies to the input mixers, though the 68k value there is pretty much ubiquitous.

The Miller capacitance will interact with the effective source resistance of the signal at the grid.
With the 5E3 type arrangement, that will vary depending on the settings of the volume controls, I suppose it's part of the control interactivity thing they're known for.

[HotBluePlates]

Can anyone reveal the ways the various values of mixing resistors behave in the circuit? This early Traynor had 100K ones, but I substituted 270K ones ... Maybe I'll use 220K like on a Fender AB763

The 5E3 Deluxe is a special case, and you are asking about "general use."  I will pretend we are talking about any amp other than a 5E3 Deluxe.

You look at a mixing resistor and think "100kΩ" or "220kΩ" or "470kΩ" but you should not think that.  Most of the time there are "2 resistors" because "2 signals are being mixed" and the signal-source sees those 2 resistors in-series.

   - If you have 220kΩ mixing resistors, the signal goes through one resistor, goes through the 2nd 220kΩ resistor, often sees a coupling cap, and a resistor-to-filter-cap (perhaps a "100kΩ" plate load resistor).

   - The signal therefore sees a total "resistance to ground" of 220kΩ + 220kΩ + 100kΩ (plate load) = 540kΩ.  This is a load impedance for the previous stage.

   - Higher mixing resistance ---> higher load impedance ---> somewhat higher gain for the preceding stage ---> BUT!! can create more noise.

   - Lower mixing resistance ---> less noise potential ---> BUT!! smaller load impedance ---> somewhat lower gain for the preceding stage.


The tradeoff is we want "as high as we can get away with" to "minimize gain-reduction due to loading" while also "not so big as to create excessive hiss."


Mixing resistors are typically equal-value so that 2 mixed signals are each reduced to about-half-original-strength, and the output signal is an equal-blend of the input-signals.

   - Sometimes you don't want "equal-blend" of the input signals, as in a Fender Reverb circuit.

   - Fender used a mixing network that greatly-reduced the Dry Signal, and barely-reduced the Reverb Signal because the Reverb Signal was much weaker.  The unequal-mixing was necessary to get the two signals to a proper relative balance (the mixing network is one of several places a tech could modify the relative strength of Dry and Reverb in those amps).

Can anyone reveal the ways the various values of mixing resistors behave in the circuit? This early Traynor had 100K ones, but I substituted 270K ones ... put a bunch of Marshall values into it ...

Traynor had 100kΩ at the output of Volume pots.

I am too young to have ever had 3 and 4 instruments plugged into a single tube amplifier.  There are members here who may have had the experience of several players using a single amp back in the early days when money was tight.

   - If your buddy turns down his Volume control, you don't want your guitar to get quieter, do you?

   - Mixing resistors coming out of Volume control wiper seek to prevent one Volume control setting from altering the loudness of another channel.  So they "isolate the effect of one Volume control" from another.

   - Traynor's Volume controls were 500kΩ, and they were linear-taper.
   - 100kΩ + 100kΩ = 200kΩ, which is 40% of the max-resistance of the "other-channel" Volume control.  That's probably high enough that anything above "all the way down" on the other-channel has little effect on your-channel's Loudness.


Notice that Fender used 220kΩ (or sometimes maybe 270kΩ) with 1MΩ pots ---> 220kΩ + 220kΩ is 44% of 1MΩ ---> similar ratio.  Once again the goal is "high enough resistance to do the job, but not any higher."

Others point out "filtering" and many locations will have high-frequency roll-off if the series-resistance gets too high due to capacitances-to-ground and/or Miller Capacitance at a tube-input.