Changing value of potentiometer

[Mypalmurph]

What draw backs are there to changing the value of a potentiometer by placing a resistor between the track ends? for instance - 33k resistor end to end across a 250k pot to mark it roughly 30k

[tubeswell]

Solder tack the pot and resistor, connect multimeters between each set of luggs, turn the dial and measure the way R changes between input/wiper/output. Then you’ll figure out if the trick works for what you need. Hard to guess what the pros and cons are if you don’t tell anyone why you’re thinking of doing this.

[Mypalmurph]

Well I want to use a 25k pot for my mids on the Hoffman 6v6 plexi build I’ve got on my bench and I only have a 250K pot.  I would normally order what I need but I already did and the the post office has misplaced the package so any who.  I feel like maybe it’d work fine on a dimmer but maybe not the amp- I have time to work on my amp now my wife is busy and I can get into building uninterrupted- I don’t wanna swap the pot out if the resistor doesn’t work right- but…..

[pdf64]

I don’t think that will work out well as a mid control, sorry.

[dwinstonwood]

Mypalmurph, this Tone Stack Calculator might help to give you a visual idea of the frequency curves using different pots, etc.

https://www.guitarscience.net/tsc/marshall.htm#RIN=1300&R1=33k&RT=220k&RB=1M&RM=25k&RL=517k&C1=470p&C2=22n&C3=22n&RB_pot=LogB&RM_pot=Linear&RT_pot=Linear

The link goes to a Marshall tone stack (others along the top). First, click "Take Snapshot." Then, click "Edit," change RM to whatever value you want, and click "Apply." You will then see the two curves side by side.

[Dave]

The mid pot only uses two lugs of the pot as a variable resistor. It will work fine as a mid control. Personally, I would tie the resistor to the two lugs you will be using since it is being used as a variable resistor anyway. Another thing, you are not limited to 25k or thereabouts. You could play with the resistor value until you get the mid sweep that you like. You might even like 250k in that position, wouldn't hurt a thing, it would just give you tons of mid sweep all the way down to zero.


Dave

[bmccowan]

The mid pot only uses two lugs of the pot as a variable resistor. It will work fine as a mid control. Personally, I would tie the resistor to the two lugs you will be using since it is being used as a variable resistor anyway. Another thing, you are not limited to 25k or thereabouts. You could play with the resistor value until you get the mid sweep that you like. You might even like 250k in that position, wouldn't hurt a thing, it would just give you tons of mid sweep all the way down to zero.


Dave

I would leave out the pot for now and install a 10K fixed resistor in its place. Order the pot you need and quickly install it when your wife is not looking.

[sluckey]

The mid pot only uses two lugs of the pot as a variable resistor. It will work fine as a mid control.

Not so. This amp uses all three lugs just like most all old Marshalls.

[Dave]

So it does... I stand corrected.


Dave

[pdf64]

Mypalmurph, this Tone Stack Calculator might help to give you a visual idea of the frequency curves using different pots, etc.

https://www.guitarscience.net/tsc/marshall.htm#RIN=1300&R1=33k&RT=220k&RB=1M&RM=25k&RL=517k&C1=470p&C2=22n&C3=22n&RB_pot=LogB&RM_pot=Linear&RT_pot=Linear

The link goes to a Marshall tone stack (others along the top). First, click "Take Snapshot." Then, click "Edit," change RM to whatever value you want, and click "Apply." You will then see the two curves side by side.

But with the arrangement described in post 1, the actual response will be somewhat different to what the calculator shows.
eg consider that with the mid pot set electrically halfway, the resistance between the pot wiper and circuit common (and hence mid cap/RC filter frequency) will be over 60k, rather than about 12k.

[PRR]

All your medium to low values will be bunched-up in the last 1/10th of a turn.
Bright-Bright-Bright-Bright-Bright-DULL.

Why would pot makers make many-many values if it was just that easy?

[dude]

Could the resistor trick be used to make a dual pot’s both sides be equal as usually they are not. Say, for the master dual pot in a LaMar type MV?

[pdf64]

Could the resistor trick be used to make a dual pot’s both sides be equal as usually they are not. Say, for the master dual pot in a LaMar type MV?

I suppose so. But if achieving perfect symmetry is that important, a Dumble style balance trimmer is typically used.
Note that as soon as such a master volume is turned down from max, that symmetry / balance seems likely to be lost. As pot taper conformance to the nominal isn’t specified, could be anything.

[dude]

Note that as soon as such a master volume is turned down from max, that symmetry / balance seems likely to be lost. As pot taper conformance to the nominal isn’t specified, could be anything.

I used a R to balance the dual MV's pots in a 6V6 Plexi.  The balance (after using a R on one side) was perfect on both sides with pot on the table, seems to work well in the amp but I didn't check the balance in the amp when on and playing, I will do that when I open the amp soon and report.  Thanks for the tip on the Dumble balance trimmer.

[kagliostro]

....  a Dumble style balance trimmer is typically used. ....

May be here is late ..... but, what do you mean with Dumble style balance trimmer ???

Franco

[pdf64]

....  a Dumble style balance trimmer is typically used. ....

May be here is late ..... but, what do you mean with Dumble style balance trimmer ???

Franco

See https://el34world.com/charts/Schematics/files/Dumble/Output.pdf

[HotBluePlates]

... what do you mean with Dumble style balance trimmer ???

See https://el34world.com/charts/Schematics/files/Dumble/Output.pdf

Oh... you mean "Williamson-style balance trimmer" (see R12).   

[bmccowan]

Oh... you mean "Williamson-style balance trimmer" (see R12).   

It's remarkable how much goes back to Williamson. I've been contemplating building an amp similar to the Gibson GA-50 and wondering about the two chokes - and here they are in that schematic - almost identical spec.

[shaun]

Well I want to use a 25k pot for my mids on the Hoffman 6v6 plexi build I’ve got on my bench and I only have a 250K pot.

 

That gave me a good laugh precisely because I wondered that same thing once or twice years ago. But you'll probably get a shift in the way the pot works which probably won't feel natural to the end user. Plus, we're talking about signal path, so your signal would be effected by the dynamic between the pot and the added res. This may or may not be subtle, but in chasing tone, there is no single magic bullet; a great amp sounds great because of a hundred little tweaks and adjustments in the right direction. In my thinking, your short cut would probably be the opposite of that.

[kagliostro]

See https://el34world.com/charts/Schematics/files/Dumble/Output.pdf

Thanks Pdf64

That was remembering something to me ....

Oh... you mean "Williamson-style balance trimmer" (see R12).

Exactly

Thanks HBP

Franco

[passaloutre]

Don’t some people use a 250k in the Mid pot to act as a raw control?

[sluckey]

Don’t some people use a 250k in the Mid pot to act as a raw control?

Yes, in a Fender style TS. But can't do that in a Marshall TS.

[Colas LeGrippa]

Have you tried bypassing the tonestack ?
The frequency curve is way more rich than wirh a tone stack, even with all the controls set at 10.
From  the other side, you  loose the adjustment capabilities.
If this particular build is for you, give it a try:
hook up a switch to the gnd side of the tone stack to activate/desactivate it.
My last build is a Fender Showman (normal channel only, no tone stack,  PS  caps beefed up, 2X El34 ). The tone of this amp is incredible.
But, if you're not used to control your tone with your guitar, it is not the most versatile amplifier, but certainly one of the.most.best sounding.