What are the tripots of EHX Bassballs for?

[gigimarga]

Hello,

I want to build in a short time an EHX Bassballs and i saw on a schematic two trimpots at the emitters of the both 2N5088.

What are these trimpots for? How can i to set them?

Thx a lot!

[B Tremblay]

They set the sweep range.  You can easily tune them by ear.

[Mark Hammer]

The trimpots actually feed the base of the transistors and determine how much steady and changing envelope voltage is fed to them.  They can be mounted on the chassis as 10k pots, although keep in mind that only part of their range (the middle 60%) is actually useful.  It may actually even be more useful to make them 5k pots with a 1k5 on the ground end of the pot and a 3k6 on the other end.

I have also found that being able to retune both filters is not really necessary.  You can use a trimpot for the lower filter and set it, and use a chassis-mounted pot for the upper filter to stagger it apart from the lower one.

A nice combination of controls for the Bass Balls is like this:

1) Sensitivity (standard)

2) Decay time (replace the 330k resistor with 47k in series with a 500k variable resistor)

3) Stagger (chassis-mounted tuning pot for the upper filter)

4) Blend (replace the pair of 2k7 mixing resistors on the output of the filters with 1k resistors tied to each side of a 5k-10k linear pot; the wiper of that pot feeds the output)

Those four controls (plus fuzz/normal switch) will get you a broad range of sounds without having to use something bigger than a 1590BB-sized box.

[gigimarga]

WOW...thx a lot all!!!
I hope to have time to build and to modify it tonight!

[alex frias]

I use the two pots (for filter adjusting) outside the box. But Mr Hammet is, again, completely right as just some part of them are really usable.

One thing I particularly don't like that much was the fuzz, not bad, but not good either. So I follow another idea expressed here in this foum. It's about to use an antiparalleled pair of diodes to tune the fuzz. I think it works nicely for this circuit.

I remember to change some diode to Ge for sensibility improvment for guitars...

It's a very cool vocal sound you can get out of this pedal, very cool indeed!

[Mark Hammer]

The "fuzz" in the original is quite distinctive, but personally I do not care for it.  Inserting a diode pair to produce clipping instead of just the passive resistor network is a good idea.  Keep in mind that the "fuzz" signal is taken from the envelope follower section, so that turning up the sensitivity to get more sweep also gives you a louder output if you are in the "fuzz" setting.  Some people may find that they want to replace the output resistor to ground with a volume pot of the same value.  I think it may be possible instead to use a well-chosen pair of clipping diodes that will produce roughly the same output level as normal when you switch to fuzz.

[gigimarga]

I've built it tonight with all the mods suggested by Mark Hammer above and it sounds AMAZING!!!
Thx a lot Mark!
Also, after searching the forum, i found that you made another two mods:
1. attack pot
2. replaced 2 resistors which 2 diodes for clipping

Can you be more clear, because i didn't understand very well how to do them?


The only improvement is at the Blend pot because it's too much difference between the volume level when the Blend pot it's at minimum/middle/maximum (i will use a panning system from R.G.'s article from geofex which i tested some days ago and it worked fine).

If i change the Si diode with a Ge one what will be the differences? A better tracking or a faster tracking?

Thx again all!

[alex frias]

If you replace the diode with a Ge one you get a more "sensitive" auto-filter. I think it's an improvment when using single coil guitars.

[B Tremblay]

The only improvement is at the Blend pot because it's too much difference between the volume level when the Blend pot it's at minimum/middle/maximum (i will use a panning system from R.G.'s article from geofex which i tested some days ago and it worked fine).

In my Bassballs mods, I omitted the 2k7 resistors completely and used a 5k-linear pot, taking the output from the wiper.  Works very well, with minimal loss when centered.  However, I've only used it with guitar.

[Mark Hammer]

I've built it tonight with all the mods suggested by Mark Hammer above and it sounds AMAZING!!!
Thx a lot Mark!
Also, after searching the forum, i found that you made another two mods:
1. attack pot
2. replaced 2 resistors which 2 diodes for clipping

Can you be more clear, because i didn't understand very well how to do them?


The only improvement is at the Blend pot because it's too much difference between the volume level when the Blend pot it's at minimum/middle/maximum (i will use a panning system from R.G.'s article from geofex which i tested some days ago and it worked fine).

If i change the Si diode with a Ge one what will be the differences? A better tracking or a faster tracking?

Thx again all!

The single diode simply cuts off one "side" of the signal so that the envelope follower can do its job.  What kind of diode determines how big that half of the signal has to be to actually be conducted.  With a 1N914, the diode will not conduct until the signal reaches approximately a half-volt, and will stop any further portion of the signal from passing once the decay dips below that same point.  Using a diode with a lower forward voltage, like a Ge or a Schottky, will permit more of the entire note to be used to control the filters.

Is that important?  Hard to say.  With simple envelope followers, there can often be noticeable and unpleasant ripple at the end of the note when you let the string die out naturally.  So, having the diode in the envelope follower say "No.  I'm not interested in any more signal if you can't find a way to be larger than 500mv." can work to your advantage by cutting off the part of the decay that produces inferior sound.  At the start of the note, the 500mv minimum requirement may rob you of the first few milliseconds of attack, but it is not really that noticeable.

I used to be a strong advocate for installing an attack pot, but with experience, I found that you needed  big change in resistance to produce attack time differences that could be heard.  The highest resistances (for slower attack times) produced BIG changes in sensitivity and sweep.  So, either stick with 100R, or if you really want to have that option, use a toggle to select between 68R and 220R.

The R8/R9 pair you see here - http://topopiccione.atspace.com/pjimages/EHBassBalls.sch.gif - take the boosted signal from IC1b and bring the level down by about 2/3 so that it is closer to the non-fuzz level.  Since IC1b takes its signal from the wiper of P1, when you turn the sensitivity down the level of the Fuzz signal will drop and won't even be "fuzzy".  If you turn the sensitivity up to produce more sweep, it will be fuzzier, but it will also be much louder.  So, if you replace R8 with a small-value resistor, like 2k2-10k, and replace R9 with a pair of clipping diodes (e.g., a 1N914 and a 1N34, back to back), you will have more consistent clipping across most sensitivity settings, and will have more consistent volume too.  You may want to measure your diodes and select them so as to achieve a specific signal level that produces volume balance between fuzz/normal.  Yuo will be able to use the same PCB as you are already using.  Just piggyback two diodes together so they only occupy the space of one.

[gigimarga]

Thx a lot all, and especially Mark for his infinite patience!

I never had success to open the site http://topopiccione.atspace.com...it's a great mystery for me why!!!

So, Mark, R8 is the 100k resistor and R9 the 47k one to ground?

And where to switch from 68R and 220R? Instead of the 100R resistor?

Best regards!

[Mark Hammer]

Thx a lot all, and especially Mark for his infinite patience!

I have children.  It comes with the territory.

So, Mark, R8 is the 100k resistor and R9 the 47k one to ground?

And where to switch from 68R and 220R? Instead of the 100R resistor?

Correct on both.

[gigimarga]

I have children.  It comes with the territory.

I have a 4 years and 6 months old girl, but she eats constantly from my patience

Thx again!

[gigimarga]

I made the mods suggested by Mark above (2 diodes instead of R8/R9 and a 1k trimpot instead of the 100R resistor) and both sounds good.

Thx a lot!