pot value question

[pbman1953]

I have 2 amps with similar issues for the bass control on these amps:


Sunn 2000s- 1M audio pot


Fender Super Twin- 250k- audio pot


For both, as you turn up its all in you face right away. So for the ender I ordered a linear to try. For the Sunn I have a 1.5meg linear, will the extra 500k make a sound difference?


For the Fender the bass tone quantity. as you turn. is too much. Will the linear help that or is there a way to ease the effect to be smoother?




https://el34world.com/charts/Schematics/files/Fender/Fender_super_twin_180w_schem.pdf

[HotBluePlates]

... its all in you face right away. So ... I ordered a linear to try. ... Will the linear help that ...

Assuming the pot hasn't shown up yet, install it & you tell us how it works.  That will be more instructive than anything we could post here.

FWIW, I normally set the Bass knob around 1-2 (or almost-Off) on blackface & silverface amps.

[pbman1953]

... its all in you face right away. So ... I ordered a linear to try. ... Will the linear help that ...

Assuming the pot hasn't shown up yet, install it & you tell us how it works.  That will be more instructive than anything we could post here.

FWIW, I normally see the Bass knob around 1-2 (or almost-Off) on blackface & silverface amps.

You're right, for the Fender, when the bass control is that low there's almost no sound.


Any thoughts on the 1.5meg vs. the 1 Meg question?

[brewdude]

I would think that linear would behave faster/more abruptly at the beginning of rotation… but, I’m no expert.

[shooter]

here's some plots, no sound just pics 


Potentiometer Taper Charts | Antique Electronic Supply (tubesandmore.com)

[pbman1953]

Cool stuff, never knew they had that, thanks


Do you have an opinion if the extra 500k will make a difference from the 1, for the Sunn amp. Like mentioned the stock is an 1M audio. I have a 1.5 M linear. Like the Fender at 2-3 it's all done. From there , there's not much of a change.

[shooter]

install it & you tell us

[pdf64]

…

Sunn 2000s- 1M audio pot


Fender Super Twin- 250k- audio pot
…

The Fender notes a J taper, 35% I think. Linear being 50%, typical audio modern tapers being 10 or 15%.

So the J taper pot will come up much more quickly than if you stick an audio Alpha or Bournes in there.

[pbman1953]

That's it too quick, no real control. More like a switch to one spot and never changes after 3-4.


Just installed the 1.5M pot in the Sunn, not much changed. Bass starts at 2 and never increases.


Sounds great but is it supposed to be like that?



Any comments?

[PRR]

What does the ohm meter say? Smooth? Or jump-up?

[pbman1953]

Can I leave it connected while testing or disconnected?

[shooter]

take off the middle wire, then you should be good.


I've always wondered why guitarist need to mess with the amp once you're dialed in, don't the knobs on your guitar work? 

[uki]

I've always wondered why guitarist need to mess with the amp once you're dialed in, don't the knobs on your guitar work? 

In search for the holly tone !!!   

[sluckey]

In search for the holly tone !!!   

Quit looking. Here's all the holly-tone you'll ever need. Tell your friends...   

[shooter]

 
the music is always great when I herbal garden 

[sluckey]

Better blooms and foliage. Holly-tone will double the value of your pot.   

[PRR]

Better blooms and foliage. Holly-tone...

Spruce-food?? Who wants that?

Huh. Does claim to help blueberries "thrive", whereas growing anywhere near my spruce-weeds the blueberries can be scrawny.

[uki]

     doh  i've misspelled

[sluckey]

Blueberries love acidic soil, ph 4 to 5. Same with Hydrangeas and Azealia. Holly-tone is too expensive to waste on Spruce trees.

[pbman1953]

What does the ohm meter say? Smooth? Or jump-up?

1m Audio- stock pot
1- 17.5
2-36.9
3-130
6-409
8-801
10-976




1.5M Linear
1-139
2-282
3-362
4-646
6-1000
8-1.36
10-1.58

[shooter]

the original values indicate the signal feeding the pot is to hot for your liking, try an AU, AY prior to it.  The 1st tube after input jack is a good starting place.

[pbman1953]

the original values indicate the signal feeding the pot is to hot for your liking, try an AU, AY prior to it.  The 1st tube after input jack is a good starting place.

r




Besides using the tube, is there another type control to stretch the signal? That's been suggested before in the past. All it seems to do is cut back the signal and not effect the issue. I guess I need more understanding why a cut signal would help. Should I leave the 1.5m in for now?  Thanks

[pdf64]

I’ll try one more time.
To resolve your issue, try Alpha or Bournes audio taper pots.
Link to correct schematic for your Sunn?

[pbman1953]

I’ll try one more time.
To resolve your issue, try Alpha or Bournes audio taper pots.
Link to correct schematic for your Sunn?

Sounds like you know Alpha and Bournes, what's different about their taper?


I sent both the Fender and Sunn schematics

[shooter]

the J taper pot will come up much more quickly than if you stick an audio Alpha or Bournes in there.

[pbman1953]

the J taper pot will come up much more quickly than if you stick an audio Alpha or Bournes in there.

Thanks, I read back a bit and saw that.


Also I need you comments on what I wrote about using lower gain tubes, thanks

[pdf64]

…
Sounds like you know Alpha and Bournes, what's different about their taper?

As I explained in post #8, they’re a different nominal taper.
All ‘audio’ means in that regard is ‘not linear or reverse audio’, electrical output at 50% rotation should be less than 50%.

[pbman1953]

I think the stock pot was an Alpha

[sluckey]

I doubt that old Sunn originally had an Alpha pot in it.

[pbman1953]

I doubt that old Sunn originally had an Alpha pot in it.

Ok, what about a CTS, for the Fender, I have plenty. Is their audio taper in line with the Alpha or Bournes?


I need to buy the 1M Bournes

[sluckey]

The Bass pot in your Sunn would most likely have been the same as in my Sunn Sceptre. (see pic) Sorry, I don't have a pic that shows pot codes.

I suspect that the only thing wrong with your 1M pots is the taper. Something to try with your pots... Move the wire from one end of the pot to the other end. The pot will operate backwards but it may "spread out" the control better.

[pdf64]

…what about a CTS, for the Fender, I have plenty. Is their audio taper in line with the Alpha or Bournes?

I need to buy the 1M Bournes

I purposely left CTS out because I think their taper for the past couple of decades has been awful, no electrical change until 15-20% rotation.
But if you’ve got them anyway, you’ve nothing to lose, may as well use the darn things.

[pbman1953]

Probably won't, from what you wrote,  that 20-30% is exactly the issue

[pbman1953]

The Bass pot in your Sunn would most likely have been the same as in my Sunn Sceptre. (see pic) Sorry, I don't have a pic that shows pot codes.

I suspect that the only thing wrong with your 1M pots is the taper. Something to try with your pots... Move the wire from one end of the pot to the other end. The pot will operate backwards but it may "spread out" the control better.

I took a picture of the 1.5M pot I installed. It's still disconnected from the test before. The cap and resistor go to the middle leg and the right leg goes to the Contour pot's left leg. 


Based on that I'm not sure what you want done

[sluckey]

Disconnect the wire from the right lug. Reconnect to the left lug.

[pbman1953]

Run the Contour left to the Bass left?

[pdf64]

To cut more bass on the Sunn, I suggest to wire the contour control as a variable resistor, as per the mid control on the Fender.
From your description, 250k or 500k 10% audio would probably be fine for its bass control.

[pbman1953]

To cut more bass on the Sunn, I suggest to wire the contour control as a variable resistor, as per the mid control on the Fender.
From your description, 250k or 500k 10% audio would probably be fine for its bass control.

It's not a case of cutting bass. Concept this, you start the bass control from zero , 1,2,3, then after that there's no change.

[HotBluePlates]

... as you turn up its all in you face right away. So for the ender I ordered a linear to try. ...

... you start the bass control from zero , 1,2,3, then after that there's no change.

For circuits where a logarithmic-increase sounds like "smoothly rising" (as in Volume controls and your Bass control), a linear taper is the last thing you want.  All the action happens in the first 20-30% of rotation, then nothing.

When I said, "install it & you tell us," it was half because you'd already ordered the wrong part, and half so you'd hear the wrong-effect for yourself.  It helps internalize when the apparently backwards-wording will get you in trouble.

As for the Alpha or Bourns pots, most of those are a 10% audio taper (they make others, but not many stock every audio-taper variant).  A 10% taper is gonna be "opposite behavior" compared to your linear pots.

You didn't measure exactly halfway on the stock pot, but you did get a reading of ~400kΩ at 6.  I'm gonna go out on a limb and guess that was about 300kΩ at halfway, or a 30% taper audio pot.  True-logarithmic would be 10% at half rotation, though most pot-makers fake their curve to be "close enough."

So I'd say try a 10% audio taper Bass pot.  If that doesn't work, you're looking at component-value changes to trim the bass somewhere.

[pbman1953]

I re-tested the stock original Sunn 1M pot-


1-18
2-56
3-129
4-201
5-289
6-438
7-608
8-750
9-916
10-978

[dude]

Looks good to me, if you're not happy with the pot, look else where like HBP mentioned. My Fenders amp's Bass pot is usually never above 2 if that, especially with humbuckers, strat maybe 3.

[sluckey]

I re-tested the stock original Sunn 1M pot-

Like I said earlier, I'm really pretty sure that alpha pot is not original. I don't even think alpha was around when that amp was built. And Sunn was using all USA parts back in the day.

[pbman1953]

Sluckey

As far as the one I thought was an Alpha , you could be right. It looks like it has Alpha markings.


To be honest I'm confused-


1- I tried a lower gain tube , when I decided to tackle this. All I noticed is where the volume control ended up compared to a 12ax7. The bass control stayed the same. 1-2-3,boom right away then no change.


I decided to grab a CTS 250K-A, I keep for bass builds. If this one looks right, I'll try it the Fender because there's a 250k Audio -J in it now


Here's the results-
1-2.6
2-8
3-15.2
4-21.6
5-32
6-50
7-125
8-214
9-260
10-278




It's not that I'm un-happy with each pot. I wondering if this is the nature of the most bass circuits. You give a slight turn and that's it OR is there a way to make it more gradual? My Traynor YBA-3 Custom's bass control seems to work how I thought a tone control should work. From zero it's a gradual increase until about 1:00 , then it's done.

[PRR]

5-289
10-978

That's 29.5% at "5"; a 30% taper.

IMHO, not very tapered. It does have "in your face very quick", which may be impressive on the sales floor.

5-32
10-278

11.5% at "5"; a "10% taper", also lower total. If you don't want all the bass all at once, this may be your pony.

[pbman1953]

5-289
10-978

That's 29.5% at "5"; a 30% taper.

IMHO, not very tapered. It does have "in your face very quick", which may be impressive on the sales floor.

5-32
10-278

11.5% at "5"; a "10% taper", also lower total. If you don't want all the bass all at once, this may be your pony.

Interesting, for this example, it's more the taper than the pot value. So I could use this in both amps?

[pdf64]

… on the Sunn … From your description, 250k or 500k 10% audio would probably be fine for its bass control.

[pbman1953]

… on the Sunn … From your description, 250k or 500k 10% audio would probably be fine for its bass control.

Well gentlemen, it took me some time to understand what you were all try to get through but PRR showed me the light. I installed the 250K then low and behold a full, the bass control went from a switch like action to a pot. I figured to get closer to spec so I used a 500k instead.  I'd say a nice full span over most of the range, even until the end of the rotation. This totally changes the picture. Now , with the boost needed at times, it goes from boom to smooth bass. Just switch on the boost and add what you need.


Now on to the Fender.


Thanks to all, this is a game changer!

[pbman1953]

The Fender's a winner too!!


Thanks for the help and time!

[HotBluePlates]

Thanks to all, this is a game changer!

That's some good lernin' right there!

[pbman1953]

You bet and here's what's weird.


All the Fender pots have a "J" taper but the bass was the only one that exhibited this issue. All the others had a gradual increase, even the EQ pots.