Heathkit TA-16 tremolo

[Helmholtz]

Full circuit is in post #1.

Problem is that the circuit is rather low impedance and even a minimum LDR resistance at full LED brightness of 2k will attenuate the signal by only a few dB.
It would be desirable to achieve a lower LDR minimum resistance for more tremolo depth.

It looks as if the LED and the LDR don't match well.
This could be due to low LED efficiency and/or the LDR having a narrow spectral sensitivity not overlapping with the red LED spectral line.

In any case it makes sense to try a higher efficiency LED.
Or get a Vactrol having an LDR resistance below 1k at 10mA LED current.

[J M Fahey]

I both have high efficiency crystal red clear Leds which will blind you if you look straight at them and lots of today hard to find CDS cells (forbidden in the EU for contamination reasons so probably also in California, simply because they ban everything ) so tonight will try a couple values and post results.

[Helmholtz]

You can use your scope to see how DC voltage or signal levels change with tremolo.

E.g. probe the Q13 collector. Use DC coupling (in calibrated mode) and watch the centerline move up and down. Read the voltage difference between highest and lowest position.

Also note that LEDs typically have a built in lens focussing the beam on axis. This needs to point at the LDR face.

[J M Fahey]

That test is valid but will need a working original spec lamp to be carried.
Q13 feeds current into it, not voltage.

None or random bulb, no or random voltage.

[Helmholtz]

That test is valid but will need a working original spec lamp to be carried.
Q13 feeds current into it, not voltage.

None or random bulb, no or random voltage.

Example with scope refers to LED in series with 1k.
LED current is proportional to the voltage drop across the 1k resistor.
Change of collector voltage is essentially proportional to change of LED current as long as the LED is on.

The voltage drop across the 1k resistor is calculated from collector voltage, supply voltage and LED voltage.
A red LED has an on-voltage around 1.8V.

Test will also show if the LED extinguishes at all.

[pontiacpete]

I put in a red LED with the clear case and pointed the LDR at the top of the LED and wrapped it twice with shrink wrap and it sounds great. Nice and strong. This is without the 330R. I’m going to leave it like this. Thanks for all the help.

[J M Fahey]

Just had 5 minutes free and did the design/math part, so I fully mimic the bulb behaviour: full brightness when dropping 6V and passing 25mA through it, no light output (for all practical means) with, say,5mA (at which point real bulb will be barely dark red, if at all).

See you made it work, happy with that.
Big part of success comes from using a high efficiency crystal clear Red Led, those have high Light output.

But in any case will complete the experiment in full (since I already started it anyway).

Here is the Math, 5AM here, will bench try it tomorrow and post *measured* CDS resistance values, might be useful for others, and in different circuits, say old Gibson amps or bulb driven phasers, wahs, etc.

EDIT: typo on full transistor current calculation.it says "6000mA", should say "6000mV"