Why bother?

[Mark Hammer]

A Gibson G-115 amp was posted on a local 2nd hand site, and out of curiosity I thought I'd look at the schematic.  Couldn't find one for the 115, specifically, but the 105 seems awfully close and likely only differs by the number of output transistors and speaker complement.  It's an early-to-mid-'70s solid-state amp with a built-in 4-stage phaser.  One of the controls is labelled "Harmonics", so I took a closer look.  From what I can see here, in this preamp stage, it illuminates a bulb that shines on an LDR, which connects a diode pair to ground.  In other words, it adjust clipping "softness" (that's for you, Vivek).  It does not do so dynamically, or in response to anything else that I can see (e.g., the adjustment of some other parameter, like master volume).  In fact, the additional sub-circuit doesn't seem to do anything that could not be done with a simple pot between those diodes and ground.

So why'd they do it?  Is it because it achieves some sort of custom taper?  Does it result in a resistance value that potentiometers of the era could not achieve?  I'm baffled.  Seems like more than was actually required.

[Phend]

Don't doubt those who could design with pencil paper. Cool looking drawing by the way.
In engineering we did it all the time, add something, change something,  put a screw where it does nothing ( just kidding here).
Sales and upper management didn't even know what they were looking at.
Plus it kept us enginers employed. We made great looking ink drawings on mylar.
When cad came along I remember printing out a quarter scale drawing and the vp looked at it and said "those bearings look small".
Next day printed out a full size drawing and the vp said "that looks a lot better".
So maybe it did do something,  you would have to have been there.
PS all those I worked with there are great people.  We had fun.

[Fancy Lime]

The implementation of the Mid control in the James tone stack is also intriguing. I need to simulate that one. More of a bass-shift but may still be useful.

No Idea about the LDR. Looks pointless to me but that may well be my ignorance talking.

Andy

[stallik]

Drawing states channel 1 - possible that this would cause a bleed of the effect over to subsequent channels?

[anotherjim]

A lamp is bipolar and its filament has some remanence that will smooth the light output. I suspect that combined with the slow response of the LDR, it is envelope control of the clipping threshold.

[Vivek]

Mad unsupported foolish conjecture:

When you play this amp hard

It's rail voltage drops

so the Lamp brightness changes in a slow way

which changes the LDR with some light/resistance curve and some slow time constants

Leading to change in harmonics based on integrated behavior of how hard you played the Amp in the preceding moments.

Or changes in lamp driver transistor response due to heating up.

Like some kind of sag.

[Vivek]

A lamp is bipolar and its filament has some remanence that will smooth the light output. I suspect that combined with the slow response of the LDR, it is envelope control of the clipping threshold.

The only thing that can change the lamp brightness is fluctuations in the rail voltage

or heating of the transistor.

[Vivek]

It does not do so dynamically, or in response to anything else that I can see

Lamp brightness could change based on rail voltage changes

or temperature of the transistor




( oops, I thought I could delete earlier posts and merge into one, but now I find no way to delete my earlier posts)

[Vivek]

In engineering we did it all the time, add something, change something,  put a screw where it does nothing ( just kidding here).
So maybe it did do something,  you would have to have been there.

While studying the INTEGRATED PREAMP from TC ELECTRONICS

There was a bit of circuitry that originally helped reduce thump when the preamp was connected/disconnected

The guys who traced this might have got it wrong since they missed out on some essential connections, but they left some stuff behind, and that stuff accomplishes nothing at all.

But now there are more than 3 PCB manufacturers who have bits stuck at the input of their INTEGRATED PREAMP clone, that does absolutely nothing.

I did contact some of these kit manufacturers and they said, good point but we will let the unnecessary bits remain. It did not harm anyone in last few years that we are selling this kit.






So you have a point. This light / LDR thing could have been connected in some other way earlier, and did accomplish something more in a previous, more complete avatar. For example, it could have been connected to some type of envelope detector earlier.

[Fancy Lime]

Despite being subject to the human impulse to find rhyme and reason where none may exist as much as the next person, I agree with Vivek that, based on what information we have at this point, it seems most likely that the whole lamp-LDR arrangement is merely a bit of vestigial circuitry that lost its original function sometime during the evolutionary history of the design and simply wasn't expensive enough for the engineers to bother removing it and replacing it with a simple pot-as-variable-resistor.

[Gus]

One thing it might be

A photocell with out any light can be VERY high resistance so in the off range it is like a open switch

The range of resistance wanted for the harmonics is most likely MUCH lower than the no light on the cell setting

So if you did this with rheostat/potentiometer you would want a control with the switch that opens at the max resistance however most switches on controls work on the beginning of the rotation to turn something on.
So maybe this was cheaper than a custom control and/or they did not want to add another switch to the control panel.

I do like the old solid state preamps

[anotherjim]

It does not do so dynamically, or in response to anything else that I can see

Lamp brightness could change based on rail voltage changes

or temperature of the transistor

Ah yes, I didn't read the scheme right. And yet, is the scheme drawn right? As it is, the transistor avoids the need for a wire-wound pot I suppose.

[ElectricDruid]

Could the -12V be switched from somewhere else, like a footswitch? Then the LDR would provide a on/off action for the clipping.

[Elektrojänis]

One thing it might be

A photocell with out any light can be VERY high resistance so in the off range it is like a open switch

This is what I thought too... The highest resistance of the LDR:s was probably a lot higher than the pots available. If it needs that high resistance, it's one way to solve it.

[puretube]

It`s just a (noiseless/humfree) "remote-control" of the "knee" ...
(crackle not okay!)

[Mark Hammer]

One thing it might be

A photocell with out any light can be VERY high resistance so in the off range it is like a open switch

The range of resistance wanted for the harmonics is most likely MUCH lower than the no light on the cell setting

So if you did this with rheostat/potentiometer you would want a control with the switch that opens at the max resistance however most switches on controls work on the beginning of the rotation to turn something on.
So maybe this was cheaper than a custom control and/or they did not want to add another switch to the control panel.

I do like the old solid state preamps

I think Gus nailed it.  Although the question needs to be asked: just how MUCH resistance is required for there to be no audible clipping by those diodes?  I'm not disputing Gus's suggestion at all.  I'm asking for information.  In a number of things I've built, it can only take a couple of kilohms between the diode pair and ground to substantially reduce audible clipping.  But then, those are circuits powered by +9V, and this unit seems to be showing -40V going to the collectors of those input transistors, so I'm a little out of my league, here.

Ton is probably also correct in that the use of a bulb/LDR arrangement allows for the circuit to act a bit like a relay/switch, without actually being one.

[Phend]

You might want to google this; Peterson Vibrato and Vibrato Lamps for Rhodes Peterson Preamps.
I believe it uses a GE miniature lamp #19.

[Elektrojänis]

I think Gus nailed it.  Although the question needs to be asked: just how MUCH resistance is required for there to be no audible clipping by those diodes?  I'm not disputing Gus's suggestion at all.  I'm asking for information.  In a number of things I've built, it can only take a couple of kilohms between the diode pair and ground to substantially reduce audible clipping.  But then, those are circuits powered by +9V, and this unit seems to be showing -40V going to the collectors of those input transistors, so I'm a little out of my league, here.

The engineering department might have had the goal of having a minimum setting where it afffects the signal as little as possible and they did that. And no one just had the idea of testing if 10kohm or 100kohm pot would have reduced it enough for it to be inaudible. After all, distortion with guitars was probably not understood as well as it is now and they probably wanted a setting were there was as little of it as possible. Distortion figure of 0,02% would look a lot better on marketing sheet than 0,1%.

[puretube]

No, Mark: not as a switch, but as a variable resistor, remotely controlled (with the added possibility to switch the user-dialed "Harmonix" setting on & off, like ElectricDruid said).
Without the danger of picking up noise/hum by long wires from the panel to the circuit board, when the resistor is set to high (multi-meg) values.

[PRR]

..I believe it uses a GE miniature lamp #19.

I remember that wobbling preamp. If that is indeed the bulb, 100mA is a lot for a small pot to control. (LEDs were not even dreamed of yet.)