Interstage voltage dividers in cascaded marshalls

[jack2911guitarblues]

Hi to all mad modders out there!
I'm just wondering if I could get some clarity on the subject of voltage dividers used in between gain stages?

Have done a fair bit of reading on the subject and understand that they are used (among other things) to keep gain down in between stages.  I have built a Plexi 50 Watter with internal Bridging between Bright & Normal Channels and actually run both Pre Phase and Post Phase Inverter Master Volumes on it, although now I'm leaning more toward having just the PRE Phase MV in place.  Besides having  true  Plexi sounds, I also have a switchable cascade circuit (based on a Bainzy circuit off the net).  Have  ended up with a Cascade circuit that's pretty well the same as a Top Hat Emplexador 50.

All running good, just fine tuning the Cascade side and wondering why it is that some Cascade circuits use JUST a Gain control outputting to grid of next stage, while other designs ALSO utilise a fixed Voltage Divider (2 x 470k res.) after the Gain control.

Am I correct in thinking that a 1 meg Gain control at 50% is electrically the same as 2 x 470 k resistors in series, with a centre tap?  I have 2 Gain controls, 1 after  V1A and 1 after V1B.
 
I understand  that the 2 Channel Plexi needs  470k resistors to mix both Bright & Nirmal channels, but Cascading Normal into Bright Channels, one wouldn't need to use these resistors? Or would they still be necessary?

So by having a 1 Meg Gain control at 50%, that would drop Gain down by half, and then a fixed Voltage Divider (2 x 470k res.) after that, would the Gain be then down to 1/4 of original ?
Just trying to increase my understanding about Voltage Dividers.

Hope My questions are clear enough!
Any theory is appreciated!
Jack

[jjasilli]

Am I correct in thinking that a 1 meg Gain control at 50% is electrically the same as 2 x 470 k resistors in series, with a centre tap?
That's the general idea.  But: (i)  In a voltage divider the 1st R is the Series resistor; the 2nd R is the Shunt resistor.  Note that the desired voltage put out by the divider circuit is not in series with the shunt resistor. (ii) The exact 50% mark depends on whether the pot is Linear Taper or Audio Taper.

I understand  that the 2 Channel Plexi needs  470k resistors to mix both Bright & Nirmal channels, but Cascading Normal into Bright Channels, one wouldn't need to use these resistors? Or would they still be necessary?
A schematic would be helpful.  But generally mixing resistors are mandatory to separate channels from one another.  In cascading circuits there are not 2 signals to mix.

So by having a 1 Meg Gain control at 50%, that would drop Gain down by half, and then a fixed Voltage Divider (2 x 470k res.) after that, would the Gain be then down to 1/4 of original ?
Yes, but note: a 12ax7 has a mu, or gain, of 60.  So if you put 1 volt in, you get about 60 volts out.  That's way too much for the input of the next gain stage.  So, something must be done to reduce this gain.

[SnickSound]

Am I correct in thinking that a 1 meg Gain control at 50% is electrically the same as 2 x 470 k resistors in series, with a centre tap?  I have 2 Gain controls, 1 after  V1A and 1 after V1B.

If you are using a log potentiometer (aka audio taper), then no. You have to crank that potentiometer to almost 9/10 to get 50% voltage reduction.

That said, a "JCM800" type circuit has a lot less gain than a cascaded Plexi, because of the unbypassed cold-bias 2nd stage. So effectively, a pot turned to 50% in between actually works pretty well to prevent a very fuzzy tone.

I know this because my latest build allows me all combinations (Cascade switch + Cold-Bias switch)

[jjasilli]

If you are using a log potentiometer (aka audio taper), then no. You have to crank that potentiometer to almost 9/10 to get 50% voltage reduction.
Yes, there is a clarity issue lurking here.  If the pot is at 50%, then that's where it is.  The thing to keep in mind is that the 12:00 o'clock position is not 50% with an Audio Taper pot.

[jack2911guitarblues]

Thank you guys, for some enlightening and helpful info.

So I measured the resistance on both 1 Meg pots, and indeed I got  about 460k on both outer lugs when the Pot was set at about 2 o'clock. So that was interesting.
 I'll make an effort to get a schematic up asap, but essentially I run a Plexi circuit and a Cascaded circuit together.
I use a 4 pole double throw switch to achieve the change from one mode to the other.  It's a monster switch but it works well!   So in CASCADE mode, it runs from the BRIGHT INTO THE NORMAL (series) and I use 2 x  3 way mini switches to adjust the Normal Channel  Cathode Caps & Resistors when in Cascade  or Plexi modes.
So I can choose between .68uF/None/2.2uF, and with the other mini switch, I can choose 10k (jcm800)/820ohms/4.7k
Cath Resistors.  IN PLEXI  I run: 820 ohms/2.2uF Norm Cathode,   And in CASCADE I run:  4.7k/0.68uF Norm Cathode
So Cascade Mode is not based on. a jcm800, but is nearly a dead ringer for Top Hat Emplexador Cascade circuit.
These mini switches are great, because sometimes if need be, I can switch from 0.68uF to NO Bypass cap if things get too bright or harsh, depending on the setting I'm playing in, or can cut down on Gain by going from 4.7k to 10k Resistors.
Just enjoying the learning curve, so ANY advice or experience is most appreciated!

Jack(down under)

[terminalgs]

With these types of voltage dividers between gain stages, you should also consider the input impedance (Zin) that the components have for the preceding gain stage's output.  I believe what you described using is drawn below in Fig.A. in which Zin would vary depending on the wiper position of P1.  If it's all the way "down", Zin = P1, but if it's all the way "up", Zin = P1||(R1+R2) (i.e. P1 in parallel with the combined impedance of R1+R2).. If P1=1Mohm, and R1 and R2 each  are 500ohm,  Zin would vary from 1Mohm down to 500ohm as you turned the volume up.  If you have a capacitor coupling the preceding gain stage to circuit, you'd want to make sure that cap is sized so you don't get unwanted low frequency roll-off when the impedance of the divider circuit drops.


Fig.B could do the same thing for you as Fig.B, while avoiding the change in Zin (that would say constant at P1+R1), not to mention save you 1 resistor (cheap) and some turret board space (not necessarily cheap...).

[jack2911guitarblues]

Thanks Terminalgs
I guess I still have lots to learn about ohms law and basic electrics!  But it's slowly all starting to make sense.  I guess basically I was trying to understand why it is that some designers utilise a fixed voltage divider after the Gain pot before the next stage. (ie like the Top Hat Emplexador).  As opposed to Hoffman's designed Hot Switch, where the output of that same Gain pot has NO extra voltage divider to further cut the signal. I would have thought there would have been rules as to how much the next stage in Cascade Gain circuit could handle.  But obviously there can be a large variation in this type of circuit. Jack

[terminalgs]

Thanks Terminalgs
I guess I still have lots to learn about ohms law and basic electrics!  But it's slowly all starting to make sense.  I guess basically I was trying to understand why it is that some designers utilise a fixed voltage divider after the Gain pot before the next stage. (ie like the Top Hat Emplexador).  As opposed to Hoffman's designed Hot Switch, where the output of that same Gain pot has NO extra voltage divider to further cut the signal. I would have thought there would have been rules as to how much the next stage in Cascade Gain circuit could handle.  But obviously there can be a large variation in this type of circuit. Jack

It depends whether you want a lot of clean headroom, a moderate amount of overdrive, or a tremendous amount of overdrive.

Each successive gain stage has an amplitude limit where a distortion free input signal will not result in a distortion free output signal.  You can purposely design the amp so that a gain stage is overdriven (input is too hot and the output is distorted),  or you can lower the signal's amplitude between gain stages so that the next gain stage isn't overdriven, so you have more headroom in the amp farther down the circuit.  If overdrive is what you are after, you might limit the  overdrive by using a 470K resistor on top of a 1M pot, or you might like the sound of the amp if the next stage is slammed with an even bigger signal, and decide to use a 1M pot and nothing else.

You might also decide you want clean signal to be supplied to the tone stack, and then overdrive a later stage, or maybe you want the overdrive before the tone stack.

[jjasilli]

Yes.  And you can also design a gain stage so that one half of the signal wave clips (or, saturates), while the other half does not.  This can be balanced-out, or only partially balanced-out for "synergy", in the next gain stage, making use of phase reversal from stage to stage.  This is one of the reasons for amps with multiple cascading stages.  Where to place the tonestack is another design feature.  Certain Marshalls, Randalls and SRO come to mind.