1 meg vs 500k pots

[ALBATROS1234]

It seems mostly I see 1 meg values in schematics and I mainly use these. Upon looking at the supro 24 it's all 500k. First what is the difference totally is it like guitars where higher values provide more high end to pass ? I have a bunch of 1 meg and found that full size 500k linear are harder to find and double or more in price. Is this something that is of small consequence or A situation where 500k MUST be used?

[sluckey]

You can certainly use a 1M pot rather than a 500K pot for a volume or tone control. Doing so will likely change the tone of the circuit or the 'feel' of the knob. When I'm copying a particular circuit such as that Supro I always try to use the same value components as the original.

I don't know where you shop for pots but I find that 500K pots (linear or audio taper) are just as easy to find as 1M pots and the cost is the same. $1.60 for either pot...

https://www.tubesandmore.com/products/potentiometers_0?filters=Brand%3DAlpha

[nandrewjackson]

The difference between a 500K or a 1M may be small, depending on the context where it's used.
The rest of whatever 2 stages you are connecting surely plays a big role. If you're connecting two triodes with ; 0.01uF to a 500K resistor, to a 500k gain pot, then a 200 K grid stopper, it's gonna be way different than two triodes connected by a 0.04uF capacitor and a 500k gain pot.

[jjasilli]

See:  https://www.seymourduncan.com/forum/showthread.php?226642-How-do-the-values-of-a-250k-pot-vs-500k-pot-effect-performance


Though the above webpage is focused on pots is a guitar, the things said are true wherever a pot is used.  A higher value pot will preserve hi's, and signal voltage (gain); and vice-versa.  This will affect tone, and the "feel" of the amp in relation to playing a guitar through it.


Note that the pot may serve as the grid leak resistor for a gain stage, or be in parallel with a fixed value grid leak resistor.  Also, the stage may have a grid stopper, which is in series to ground with the net grid leak resistance.  The combined value of the grid stopper + the net grid leak resistance should not exceed the max total of grid leak resistance spec'd for the tube in the tube chart.


Note that if a 1M Vol. pot is in parallel with a 1M grid leak resistor, the net grid leak resistance is 500K (with the pot @ full rotation).  The lowest recommended grid leak resistance for a 12a_7 tube is 220K.


Tube type matters, as different tube types are spec'd for different grid leak resistances. 

[Ed_Chambley]

I would use a CTS, 500 Audio pot.  The taper type is slightly different than an Alpha and for me easier to adjust.


JJ's comments are correct, but thsi doesn't mean you cannot try.  If you used a 1 Meg Audio pot and like the results.  If not, change it or strap a resistor across 1 and 3.  For instance, when I put humbuckers in a 25.5 scale neck telecaster, I use 1 meg pots and tweak them.  The lower output humbuckers will get a airy top end and the single coil tone will show.  Seth Lover did not have a humbucker to match tone and the only thing of the day were single coils.  That makes me think he was trying to make a pickup that sounded like what the already had, but quite.


With 50's Les Paul wiring, 500k is brighter, but Les Paul wiring is difficult on a Tele with a PAF in the neck and a Single coil Tele bridge.  I use a super switch and when changing pups fron the bridge to neck, the neck receives 750K resistance.


In audio Hifi it is often a 100k volume pot.  Never hurts to try, but before I waste a lot of time, I would just jumper it with gator clips and see if I liked it.  If not, no solder removal.

[ALBATROS1234]

thanks guys certainly apreciate it, i know about the guitar 250k for single coils lessens brightness and 500k for humbuckers increaes brightness,or rather allows more highs to pass, i just normally see 1 meg pots in amps and noticed a few schematics had 500k which made me think this must be the same but i wasnt sure if there was more to it like the grid leak /bias thing mentioned above. sluckeys link also helped because i usually but 1 meg alpha pots  on ebay for like $2 but when i poked around i didnt see much in the way of 500k types

[jjasilli]

Maybe this is interesting:  http://www.geofex.com/Article_Folders/potsecrets/potscret.htm

[HotBluePlates]

thanks guys certainly apreciate it, i know about the guitar 250k for single coils lessens brightness and 500k for humbuckers increaes brightness ... i just normally see 1 meg pots in amps and noticed a few schematics had 500k ...

If you're copying a particular amp and want that amp's original voice, use the part values they originally used.

"Volume pot values vs guitar brightness" don't really apply as much when looking at a Volume control in an amp.  That's because the guitar has a pickup coil with inductance & capacitance, and the pot's resistance alters the resonance of that whole combination (which is how one value can be brighter or darker than another).  In an amp, we're generally dealing with extremely little inductance, mostly just capacitance, and care about how the pot loads a tube gain stage.

The larger-resistance pot will be a lighter load to the previous gain stage.  It can allow that prior stage to achieve a little more voltage gain and output.  If you hear much change in "brightness" there's a good chance it will just be due to a bit of extra distortion, and .

The larger-resistance pot will interact with coupling caps, shifting the frequency of bass roll-off lower.

Sometimes I think Valco got a deal on 500kΩ pots.  I keep thinking I'll modify my Supro Spectator to use 1MΩ volume & tone pots (and modify the tone control to match a tweed Princeton), just because the amp doesn't really break up much.  The 500kΩ volume pot makes the plate load of the 6SL7 effectively ~175kΩ where it would be ~212kΩ with a 1MΩ pot.

But I have not thoroughly investigated all the differences that will happen for such changes, and so far decided that this vintage amp oughta stay what it is.  It makes its own cool sounds and doesn't need to be converted into a Fender tweed amp...

[jojokeo]

In addition to HBP's loading comments and therefore slight resulting gain difference, there is also more resistance in the signal path by using a 1M pot over a 500K or 250K pot. For easy math consider all three pots adjusted halfway for 50% output and how much resistance the signal goes through which will affect high frequencies and possibly necessitating using a treble bleed cap when down below 50%:
1M = 500k
500K = 250k
250K - 125k

You may want/need a bright cap on a 1M vol or gain pot when you may not using a 250K but at the expense of a little less signal voltage. If you have plenty of signal voltage anyway then there's no worries dropping the value and another attribute may allow you to not need require a voltage divider going into the next stage thereby giving you double bang for the buck?

[jjasilli]

JoJo, I'm thinking it's the opposite.  A series resistor (IN the signal path) has little effect, because there's near -0- current in the signal path (Ohm's Law), EXCEPT for interaction with the capacitive Miller Effect inside the next tube stage.  Actually this part has me confused right now.


I'm thinking the shunt resistance has the most effect.  The pot is a variable voltage divider.  With the max series resistance, there's -0- shunt resistance.  It's the -0- shunt resistance that mutes the amp.  As the pot is turned clockwise the shunt resistance rises.  This lifts lo's, while hi's more easily bleed to ground and fail to make it to the next G1.  Until there's enough shunt resistance to lift hi's too.  Meanwhile the series resistance is there mostly to provide voltage division (except for that darn Miller capacitance.)


I think I need another glass of wine.  Somebody please chime in here.

[jojokeo]

JJ, don’t overthink it. What do you think a treble bleed / bright cap is for and where is it installed? With two resistors  instead of a pot - envision the ol’ plexi or Major (shut up Jimbo) circuit using two 470k and a 470pF where the cap goes across which resistor on the bright channel??? Trust me lad... 🙂 that cap makes a BIG difference.

[jjasilli]

JJ, don’t overthink it.    Nice try!

According to this site:  "The bottom line, in general, is that resistance of a length of wire goes up with frequency. In other words, a length of wire will attenuate high frequencies much more than it will low frequencies."  Reference https://www.physicsforums.com/threads/resistance-frequency-relationship.312315/


A series resistor is the equivalent of a long length of wire.  So it will attenuate hi's more than lows.  A bypass cap across the series resistors will pass hi's.  The shunt resistor is lifting hi's from ground.  You're right; I had it backwards. Isn't dyslexia wonderful!


I will write this 100X (somewhere else). A series resistor attenuates hi's, and passes lo's. A series cap attenuates lo's and passes hi's.  A shunt resistor bleeds lo's, and keeps hi's in the signal path. A shunt cap bleeds hi's and keeps lo's in the signal path.