Master volume pot taper.

[FranciscoPerez]

Hi!!
A friend recently asked me to add a Post PI MV to his 1971 100Watt Marshall JMP ( 1959 circuit ).
I own several dual 1M Lineal pots and wondered if I could use them for this purpose changing its taper by adding a 220k resistor from wiper to earth. I've seen this done from time to time, but not in a MasterVolume.
Would that work without an issue???

I'll be using this type of MV:

Do you think this MV is a good idea for this particular amp? Pi coupling caps in actual circuit are 0.022uf and not 0.1uf

Thanks!!!!
Regards

Fran

[sluckey]

Would that work without an issue???

Yes. I'd try the linear pots without the resistors first. Only add the resistors if he's not happy with the linear taper.

Do you think this MV is a good idea for this particular amp? Pi coupling caps in actual circuit are 0.022uf and not 0.1uf

That's a very popular MV and will control the volume just fine. Some people like to make the additional caps about 10X the value of the existing caps in order to have minimal effect on tone.

[FranciscoPerez]

Glad to hear that Sluckey!
Thanks!!!

Cheers,
Fran

[kagliostro]

I didn't tried this

but, as Steve says

someone say that with large capacitors is better



K

[FranciscoPerez]

Great thanks!!
I'll try 100nF instead of 22nF, still not 10x but hope it's enough for I don't have larger values at hand.

Cheers,
Fran

[Willabe]

Hi Fran,

They use 10x for the 2nd pair of caps value to keep the bass ~90% of original stock. The 2nd pair of caps (10x) voltage rating only needs to be a little more than the -bias voltage so 100v cap would be fine. Keeps the cap size and cost down too.

The other way is to use 2x of the original stock cap uF value for both pairs (4 caps total) to get around the same bass (~90%) as the stock circuit. Again the 2nd pair of caps only need to be 100v.

So for a 22nF cap (4 caps total) you could use a 47nF (4 caps total) and you'd be even a little closer to ~100% of the stock amps bass sound.


           Brad          

[kagliostro]

I never investigated about that

thanks Brad

Franco

[Willabe]

 

[thelonious]

Just out of curiosity, when you've got multiple caps and resistors like this, how do you go about finding the corner frequency of the network?

I ask because it seems like having 1u caps right before the power tube grids might cause problems with blocking distortion, and if you could find the corner frequency, you could keep them to the minimum necessary size (Brad's idea about keeping both pairs of caps 2x the original value seems like a better solution to me, for that reason, than the 10x value 2nd pair). Also, since Fran is using 1M dual pot instead of 500k, could he use smaller caps and be ok? I just couldn't figure out how to calculate the corner frequency when you've got a more complicated RC system like this...

[Willabe]

Just out of curiosity, when you've got multiple caps and resistors like this, how do you go about finding the corner frequency of the network? I ask because it seems like having 1u caps right before the power tube grids might cause problems with blocking distortion, and if you could find the corner frequency, you could keep them to the minimum necessary size

Hmmm, good question.

(Brad's idea about keeping both pairs of caps 2x the original value seems like a better solution to me, for that reason, than the 10x value 2nd pair).

That's been posted here before and is in KOC's TUT4 (?) book in the master volume chapter.


            Brad   

[thelonious]

That's been posted here before and is in KOC's TUT4 (?) book in the master volume chapter.

One of these days I am going to scrounge up the $$ to buy a few of those books...!

[Willabe]

I would say buy TUT1 and if you like it then I'd say go for TUT3. Those 2 cover a lot of ground.




              Brad     

[thelonious]

I would say buy TUT1 and if you like it then I'd say go for TUT3. Those 2 cover a lot of ground.

Cool, will do. Thanks!

[Willabe]

If you're going to buy them give Kevin a call instead of AES then you can talk with him some. He's a pretty talkative guy.

London Power.com

             Brad     

[HotBluePlates]

Just out of curiosity, when you've got multiple caps and resistors like this, how do you go about finding the corner frequency of the network?

If the volume pot is full-up, the caps are in series; that's why if you added a cap of the same value as the existing one, the total effective capacitance is halved and the -3dB frequency moves up an octave.

  - Figure the value of the caps in series: 0.022uF existing cap, 0.22uF added cap = 0.020uF total.
  - If the volume is full up (and caps in series) then the 2nd cap is a smaller impedance than the pot, so ignore it. Use the total C and the R after the caps. f = 1 / (2*pi*220k*0.02uF) = ~36Hz.
  - When you turn the volume down, the caps are increasingly isolated from each other. Total capacitance goes up, with a limit at the value of the smaller cap. If the bigger cap is 10x bigger, the change in total C was already small, so the apparent change in C with volume change will be small. By this logic, the -3dB value moves downward slightly, so the full-volume condition is the limiting factor to consider.

[thelonious]

Excellent explanation, thanks!

[FranciscoPerez]

Thanks for the tips and explanation guys!

I installed the MV with a pair of 0.1uF caps and the 1M lineal pot with 220K resistors from wiper to ground (they were already soldered to the pot and I decided to try it that way first). The normal channel works fine, the bright channel also fine, but when I parallel the inputs of the amp I get a loud kind of whistle if the bright channel preamp volume is turned above 1 out of 10. What could be the cause for this??? I'm kinda worried about this, because my friend plays with paralleled inputs all the time.

I installed the MV pot in one of the normal channel input holes (removed the jack, the one that's not carrying the 1Meg resistor) and soldered the wires (that were unconnected after the jack removal) back together. Is this not the correct input wiring arrangement when installing the MV in place of one of the input jacks?

Thanks!!!
Kind regards,
Fran

[FranciscoPerez]

Just a couple of things I found out, without v1 there's no squeal , as I expected. Changed v1 just in case, and it squeals still.
With all tubes, no guitar plugged, and only the jack cable paralleling both channels, it squeals just the same as with the guitar plugged in.
It's driving me nuts, because the input wiring looks the same as it was before. The wires from the removed input jack connected the same way they'd be if the jack would be present.

[thelonious]

If you move the MV pot to a different location temporarily, keeping its wires away from the input jacks and preamp stages, does the oscillation/squeal still happen?

Did you use shielded wire to and from the MV pot?

Any wiring near the input jacks and V1 is going to be extremely sensitive. My guess is that even with shielded wire, you might not be able to place the MV pot in that location.

[FranciscoPerez]

Thanks Thelonius!
I'm not using any shielded wires :(
Anyway, why could be the reason that both channels work well on their own, but I get the squeal when paralleling?
I'll try removing the pot and placing it in a different location and see how it goes. But the above fact made me think it wouldn't make a difference.

[sluckey]

Anyway, why could be the reason that both channels work well on their own, but I get the squeal when paralleling?

Funny things happen when you run a high level signal back to the very sensitive input of the preamp. The solution is to move the MV away from sensitive circuits. You would benefit by using shielded wires to connect the MV also.

[FranciscoPerez]

Yes you were right, I moved the pot away and it stopped squealing. I decided to try shielded wires keeping the pot placed in the input jack and it worked!! There were some loose signal wires near the pot before, and I placed them away under the board.
The owner doesn't want to drill a hole in the amp, and though not optimal, I'm so glad I got away with this solution!

Thank you both!!!!
Cheers,
Fran

[thelonious]

So glad you got it working!

both channels work well on their own, but I get the squeal when paralleling?

Like sluckey said, weird things happen when you're dealing with a lot of gain and sensitive areas of a circuit. Someone more experienced than me can probably give you a more detailed explanation, but the basic idea is that you've got a weak signal coming in from the guitar (500mV? 2 volts?) and a bunch of preamp stages designed to make small voltages into big voltages. Along with the guitar signal, they also pick up whatever other interference is floating around in the amp and amplify that, too. By the time your signal gets to the master volume you installed, you're dealing with a much larger signal voltage than you started with. Run that big voltage through unshielded wires right next to your input jack, and you're asking for trouble. It's like sticking a microphone 2 feet away from a speaker; you'll get feedback. That's why you'll often see master volume pots on the back panel of an amp, with shielded wires run as short a distance as possible from the phase inverter coupling caps to the pot.

Why does it happen only when the inputs are paralleled? When only one channel has a jack plugged in, the other channel is shorted to ground. The noise and interference picked up by all the metal in the jack, wires, tube, and accompanying components is being dumped to ground instead of into your signal path. When you plug something in to both channels, now you have double the noise and interference getting shoved into your amp input. Now stick a the master volume pot and wires next to that... and bam, you've got squeal.

[FranciscoPerez]

Your explanation is great Thelonious, it makes a lot of sense! Thanks!

[HotBluePlates]

... I ask because it seems like having 1u caps right before the power tube grids might cause problems with blocking distortion ...

I forgot to address this earlier.

Pretend you put the bigger cap from the phase inverter to the master volume. Now put the smaller cap from master volume to tube grid. Problem solved.

You could look at doing other things to attack the discharge time of the coupling caps to reduce blocking (like making the bias feed resistors smaller), but then you're back to shaving bass.

[thelonious]

Now put the smaller cap from master volume to tube grid. Problem solved.

Good call. I didn't know if the order they came in was important or not---I guess not since they're in series!

[HotBluePlates]

I didn't know if the order they came in was important or not---I guess not since they're in series!

With the master volume full-up, it doesn't matter because the total capacitance is smaller than the smaller of the caps.

When the master is turned down, having the smaller cap between the master and tube grid still keeps the capacitance that the grid sees low. The resistance of a portion of the master volume pot isolates the other cap from the circuit.

[thelonious]

I think my thick skull has got it now... thank you HBP!