Interesting tremolo circuit

[TenderTendon]

I just bought a 1965 Klemt S40 Showstar to mess around with. It's coming from Germany, so I have some time to prepare for its arrival. The seller states that the only issue (other than needing basic service) is that the tremolo occasionally doesn't work. Simple enough.. I located a good schematic and noticed it has a tremolo circuit unlike anything I've seen before. Interestingly, it interfaces via a cathode driven optocoupler that grounds the preamp signal, just like Slucky's TREMOR-LATOR. I have a pretty good understanding of vacuum tube amp operation, but I just can't figure out how they are achieving oscillation with the 2 triodes in the schematic. Could someone please explain the circuit in simple terms? Also, what is "GL1" in the tremolo circuit? Thanks!

[kagliostro]

GL1 seems to be a neon lamp


Franco

[sluckey]

The circuit is an astable (free running) multivibrator circuit. Very common in the '50s and '60s. A more modern ss version is known as an astable multivibrator or free running flip-flop. Use google for details. Or turn to page 107 in the attached pdf and read about multivibrators. The circuits discussed are transistors but the theory is the same for tubes...

GL1 is just an indicator lamp (probably neon) used for a visual indicator. It has no other function in the multivibrator.

[kagliostro]

The presence of the neon lamp remembered me this circuit where the Tremolo circuit was based only on the lamp

(I'm not sure but seems to me that the author furderly swapped the neon tremolo for something other)



Franco

[TenderTendon]

The circuit is an astable (free running) multivibrator circuit. Very common in the '50s and '60s. A more modern ss version is known as an astable multivibrator or free running flip-flop. Use google for details. Or turn to page 107 in the attached pdf and read about multivibrators.

Thanks. Interesting read. If I understand correctly, this circuit would be incapable of producing anything similar to a sine wave, so bias wiggle would not be a realistic application? Does the mutivibrator rely on the photo resistor to "smooth out" the output, making it more audibly pleasant? If not, what is in the circuit to make the "ON/OFF" output of multivibrator translate to a smooth, lush sounding tremolo (if that is even possible)? Lastly, is there any advantage using a multivibrator over the traditional Fender style oscillator? Sorry if any of these questions come across as stupid. I'm still learning...

[sluckey]

The presence of the neon lamp remembered me this circuit where the Tremolo circuit was based only on the lamp

That's a simple neon relaxation oscillator. You have a capacitor in series with a resistance (speed control) connected between B+ and ground. The neon is connected across the cap. The cap begins to charge through the speed control. When the cap reaches approx 85v, the neon bulb fires/flashes and discharges the cap. This results in a sawtooth waveform whose frequency is determined by the cap and resistor(s). This sawtooth is sent to an optocoupler and causes the LED to flash, which varies the resistance of the LDR. Meanwhile, the cap starts charging again and the cycle repeats.

Maganatone has a similar relaxation oscillator in a cheap amp...

     https://el34world.com/charts/Schematics/files/Magnatone/Magnatone_mp1.gif

[sluckey]

Does the mutivibrator rely on the photo resistor to "smooth out" the output, making it more audibly pleasant?

Yes. The LDR has a somewhat slow response to light changes. Also, the incandescent bulb in the optocoupler does not instantly turn on and off. It turns on pretty quickly but when the voltage is removed the filament slowly turns off.

[TenderTendon]

I think I am already understanding the limitations of this multivibrator and the clear advantage of the Fender style phase shift oscillator. I assume that getting a really good sounding low frequency tremolo (1-2 Hz) with this oscillator is going to be a challenge...

[kagliostro]

Didn't know that Magnatone used such a thing

Thanks for Sharing Steve

About the schematic I posted

the author decided to don't use that neon circuit because he wasn't satisfied by it

Franco

[sluckey]

That simple neon relaxation oscillator has special meaning to me. When I was 12 years old my older cousin built a little "magic twinkle light box". It consisted of a simple half wave line power supply and about 50 NE-2s with various caps and resistors. The lights seemed to randomly flash. Reminiscent of Spock's computer console on the original Star Trek tv series.     The magic part was that the lights continued to flash for a while even when the power cord was unplugged! I didn't know about filter caps at the time and all my cousin would tell me was "it's magic!" I was hooked! That simple neon relaxation oscillator box started me down the path to a long and rewarding career in electronics. I am still reminded of that moment every time I see a simple neon relaxation oscillator.

Thanks for the memory!   

PS... Here's more than you ever wanted to know about the neon relaxation oscillator...

     https://en.wikipedia.org/wiki/Pearson%E2%80%93Anson_effect

[kagliostro]

What you said is just amazing

I imagine the joy of seeing all those little lights go on and off

 

Thanks for the link

Franco