Chorus with the vibrato scanner: please educate me.

[joostovermars]

As I understand, chorus is made by mixing a vibrato signal with a non-vibrato signal of the same source. That's how it's done in my L222 and by tweaking the circuit with the mods on Capt. Foldback's page (the L100 mods) I got a lush, deep chorus effect I like very much.

But now I have this E112 with scanner, which should have a WAY better vibrato & chorus. But -to be honest- the chorus is hardly a chorus, its more like a tonal variation of the vibrato setting. It doesn't sound like at all like the youtube B3 stuf I can find with chorus on. (I DO get vibrato, so the scanner is rotating).

Looking at the schematic I can see that the chorus switch is feeding the bottom of the delay line to ground through a 12K resistor instead of connecting it directly to ground. Exactly the same as in a A100. I don't understand how this can mix a straight signal into the vibrato signal. Please educate me on this!

Neither do I understand the Celeste I & II settings. I thought celeste is a slow moving chorus-like effect. Nothing slow with this "celeste", I just get another tonal variation.

Any reason to think of why I don't get the nice chorus I was expecting? Any help will be greatly appreciated!

[UncleClark]

I don't understand how this can mix a straight signal into the vibrato signal. Please educate me on this!

I"m familiar with the B2/C2/RT2 schematic so I'll use that as an example.

With the Vibrato selector turned to a "V" position the delay line components are directly grounded. So the scanner signal from every tap along the delay line is being time shifted via the inductors and capacitors.

With the Vibrato selector turn to a "C" position every tap along the delay line now includes the path to ground thru the chorus resistor. So on each tap there is now a path to ground both thru the inductor and capacitor, and also directly via the chorus resistor. The chorus resistor does not time delay the signal but the inductors and capacitors do delay the signal. This cause the output on each tap to become a mix of time-delayed and non-time-delayed signal.

The time delayed signal is equivalent to a frequency shift, either up or down, depending on which way the scanner is scanning. The resistor component is not frequency shifted.

Chorus is a mix of frequency shifted and non-frequency shifted signals. So that's how it's created, at least in the B2/C2/RT-2 consoles.

Sometimes the chorus resistor goes out of tolerance which can cause the chorus to become weak and ill-defined.

I hope this make sense.

[joostovermars]

Hi Uncleclark
This certainly makes sense! So, the value of the chorus resistor determines the amount of chorus. The Celeste resistors are doing almost the same, but in another way. It would be a nice experiment to replace the 12K chorus resistor with a 25k pot and see if that can act as a "chorus depth" control. Anyone tried this?

[UncleClark]

It would be a nice experiment to replace the 12K chorus resistor with a 25k pot and see if that can act as a "chorus depth" control. Anyone tried this?

Yes, I have tried it on my RT-2 console and yes it works as you describe.

[bnelson218]

In my X77-B rebuild, I added a scanner and line box in order to bring that essential B3-style vibrato to the X-77. I did away with the vibrato/chorus selector switch and instead wired the scanner/line box in the V3/C3 mode, since that's typically the setting many players use. I also added a 25K pot for an adjustable chorus and it works great.

[joostovermars]

Thanks Bnelson! Things are getting clearer! Still there is something that puzzles me: I assume that each LC network introduces an equal amount of delay. When I compare the scanner wiring scheme of a A100/B3 (V3/C3 setting) with the scheme of my E112 (full vibrato/chorus setting) I see a difference of the way the LC networks are distributed over the 9 scanner poles:


A100: 1-2-3-3-3-3-2-1
E100: 1-2-2-2-3-3-3-2

The A100 distribution resembles more or less a sine-wave kind of modulation, while the E100 distribution is more of a saw wave kind of modulation. So, am I right (assuming that the line boxes are similar) in that I can rewire the E112 setting to the A100/B3 setting to get a more B3 like vibrato/chorus?

[Bradley]

Thanks Bnelson! Things are getting clearer! Still there is something that puzzles me: I assume that each LC network introduces an equal amount of delay. When I compare the scanner wiring scheme of a A100/B3 (V3/C3 setting) with the scheme of my E112 (full vibrato/chorus setting) I see a difference of the way the LC networks are distributed over the 9 scanner poles:


A100: 1-2-3-3-3-3-2-1
E100: 1-2-2-2-3-3-3-2

The A100 distribution resembles more or less a sine-wave kind of modulation, while the E100 distribution is more of a saw wave kind of modulation. So, am I right (assuming that the line boxes are similar) in that I can rewire the E112 setting to the A100/B3 setting to get a more B3 like vibrato/chorus?

It does look like it could work but because you will be changing the tap positions of the first half of the scanner cycle it will also do something to (what my fuzzy schematic seems to call) the "small vibrato". Those switch contacts are shorted together for scanner positions 6 through 8. If you could get to them and break them out separately you could rewire that switch position correspond to the A100's V1 or V2. No one seems to use those but it would be nice if it functioned properly after the change.

[kkeys]

Neither do I understand the Celeste I & II settings. I thought celeste is a slow moving chorus-like effect. Nothing slow with this "celeste", I just get another tonal variation.
Any reason to think of why I don't get the nice chorus I was expecting?

The E- Series has no provision for that effect. There are two types of Celeste effects.

The H - Series has an additional scanner (Slow Scan Celeste) for a slow moving chorus-like effect as you describe.

The E and T - Series have just one scanner arrangement. My understanding is that a portion of the scanner output is recycled back to the input of the vibrato delay line. I'd say that the effect gives a more complex vibrato output. IIRC the manual for the T- 300 I once had described the effect as many vibratos at once or something to that nature.

[enor]

The E and T - Series have just one scanner arrangement. My understanding is that a portion of the scanner output is recycled back to the input of the vibrato delay line.

M-100 does this too.

[joostovermars]

Thanks guys! On my "get a console" errand ( there are not much of them in Holland and I didn't want to pay 10.000+ bucks for a B3+122) I once played a totally worn out H112. Half of it didn't work, But I liked the slow Celeste scanner VERY much! Especially because it delivers 2 out of phase signals to the stereo amp.
However. First thing to do is the Trevor Noon percussion mod. With the top lid off, I'll clean all tab contacts and see I whether I can get more chorus or vary the chorus depth with a 25k pot. To be continued.

[bnelson218]

I should mention that due to component value changes over time, you might have to experiment with the pot value to obtain the best chorus mix. If the 25K just doesn't get it, try a 50k, maybe even a 100k. You won't damage anything as you are simply elevating that line box above ground.

[joostovermars]

Thanks to this forum, I already gathered a lot of knowledge! It appears that maybe I'll have to flash the scanner to get rid of dendrites and get a better vibrato/chorus.

- - - Updated - - -

I should mention that due to component value changes over time, you might have to experiment with the pot value to obtain the best chorus mix. If the 25K just doesn't get it, try a 50k, maybe even a 100k. You won't damage anything as you are simply elevating that line box above ground.

Yeah, no problem! I do a lot of (guitar) tube amp building & restoring. I'll keep you guys posted as soon as I'm at it!

Joost