Voltage divider b/t stages vs. resistor etc.

[EFK]

Can someone explain what the difference would be in the different methods of knocking back signal between triode stages? For example, let's say you want to reduce the signal a bit between V1 and V2 on any given amp - you could use a voltage divider and tailor the resistor values to a certain percentage of signal, right? Or, I've seen a single resistor dropped to ground. Or how about sticking a single resistor inline? Assuming you tailor the value of any/all of these resistive methods to yield a particular percentage of the original signal going on to the next stage, is there any difference between them insofar as effect upon the raw signal tone? Also, I know how to calculate the percentage of signal being dropped in a simple voltage divider, but how does one calculate what value to use if you are just hanging a resistor to ground off the 'line?'

[Enzo]

The signal is just a voltage, it just varies over time. Ohm's Law applies like anywhere else. All the laws apply like anywhere else. If you just stick a resistor in series, how much voltage will it drop? That is determined by the current. Ohm's Law. Now think about the signal coming from the coupling cap of the previous stage plate to the grid of the following tube. How much current flows through that grid? None. How much voltage drop would be across a resistor in series with it? None.

Now if I have that series resistor AND a resistor from that grid to ground, they make a voltage divider as sure as a volume control. And the ratio of resistances will tell you how much signal reduction you get.

If you see a grid with 470k to ground or something like that, the resistor is not there to reduce the signal, it is there to reference that grid to ground. It is your grid return or grid leak resistor.

Now there are subtle impedances and things involved, so one co9uld say I am ignoring certain elements of this, but in the sense of your question, we can ignore those.

[Wilder Amplification]

What you're speaking of are the different methods of "interstage coupling".

That being said...there always MUST be a resistor to ground at the grid of every preamp stage. This is the "grid leak resistor" and it is there to provide a ground reference (i.e. "zero" the grid) so that the voltage drop on the cathode resistor that is derived via the quiesent current flow through the cathode resistor elevates the cathode above the grid voltage for biasing. In grounded cathode arrangements obviously the grid leak resistor would reference to a negative voltage source to make the grid more negative than the cathode but the point is that the leak resistor MUST be there.

In a voltage divider arrangement, the bottom divider resistor is the grid leak resistor to the next stage. In most preamps you'll find that the gain knob provides this reference to the driven stage's grid.

With the loading resistor method (i.e. "hanging a resistor to ground off the line") the loading resistor is the grid leak resistor. What it does is load the driving stage's output, which reduces its gain and hence the amount of signal passed onto the driven stage's input.

[EFK]

Thank you both! That was very clearly explained! I'm interested to find out how either method - grid load or voltage divider - will effect treble frequencies....

[redelephant]

The divider will eat more treble.

[tubeswell]

Therefore you can bypass it (for A/C) with a cap

[Enzo]

A cap will make your divider freqiency selective in a number of ways if you want it to.

[Russ]

To avoid the treble loss from a divider, can you use a split load instead - that is two load resistors in series where you then connect the coupling cap at the junction of the two resistors instead of at the anode? Is there any treble (or bass) loss with this method?

russ

[redelephant]

Do u mean one resistor to gnd, the other to grid, and the input is at the middle? If so it's just like having a high value grid stopper which does exactly as described, cut treble.
The resistance driving the tube sets the frequency knee. Reason is the input capacitance of that tube, which is the tube's input capacitance times it's gain.

In my preamps I decided to use lower gain tubes instead of the standard 12AX7 and avoid having anything but a cap between stages. Not counting the gridstoppers that is, which are mandatory. But a divider can be a great way to tweak response since a small cap over the top R will give Hf boost.

[GainFreak]

The divider will eat more treble.

Please explain how a simple voltage divider between stages eats treble.

[octal]

I know that question was not directed at me, but a divider can eat treble due to the series element of the divider interacting with the "steve" miller capacitance of the following triode stage. It forms a low pass filter. If you keep the values of the divider resistors lower, the rolloff can be pushed above the guitar range - >5KHz.

Nathan

[Wilder Amplification]

I know that question was not directed at me, but a divider can eat treble due to the series element of the divider interacting with the "steve" miller capacitance of the following triode stage. It forms a low pass filter. If you keep the values of the divider resistors lower, the rolloff can be pushed above the guitar range - >5KHz.

Nathan

In some high gain circuits the treble killing action of a divider can actually work to your advantage by keeping the HF "fizzies" tamed.