Univibe LFo at 9 volts

[Atodovax]

What kind of "work" do you tjink might be suitable for a bulb lamp to run at 9volts? Hehe

Hehe..
.
Ok i give up. Im going to stick with the LED.

[antonis]

Well isnt there an existing LAmp driver already there?

Mostly an LED driver rather than a lamp one..

[Rob Strand]

Hi Rob! I dont intend to improve it, its just that it was not working at 9volts with a lamp. I though maybe i could do an easy modification for that. I swapped the driver transistor gor a Darlington and while that made the lamp glow, the LFO now stalls at max speed or max depth

You should be able to get it to work.

The reason it stalls on maximum depth is because the input impedance to the lamp driver circuit is too low and is loading down the oscillator output.    When the depth control is backed off the depth pot resistance makes the impedance looking into the depth pot and lamp driver increase (essentially putting a series resistance between the two circuits).

Putting a Darlington there should help.  However you need to modify the circuit to take full advantage of that - otherwise you aren't removing all the loading effects of the lamp driver.  The resistors R29 (4k7), R33 (47k), R31 (100k) should be increased.   With a Darlington you could probably multiply all the resistor values by 10!  That's only going to be a ball-park as the circuit will need to be tweaked in terms of gain and sweep range (and offset).   That's what JC Maillet's link alludes to but someone needs to actual do the hard yards and tweak the values.

About the loading.  The output of the oscillator has 15k resistor.   That's driving a 25k pot, which is probably creating a loading problem from the start, maybe 50k will help.   It doesn't take much loading to kill these oscillators as the single transistor amplifier stages only just have enough gain.   Next is the impedance looking into the transistor.   When you only have a single transistor the value of R34 can cause loading at the base.  You might need a low value for R34 to get enough lamp drive.  In a circuit like this R33 also causes loading at the base.   With a single transistor you can't just raise R34, and R33 because there might not be enough base current to drive the lamp.   Changing to a Darlington allows you to raise all the impedance without the balancing act between lamp drive and input impedance.  FWIW R29 and R34 have some influence on the range of the sweep.    R34, R33 and R31 set the idle current through the lamp.

[antonis]

The resistor R19 (4k7) should be increased.

R29..!!
(typo fixing..)

[Rob Strand]

The resistor R19 (4k7) should be increased.

R29..!!
(typo fixing..)

Thanks.

[Atodovax]

Hi Rob! I dont intend to improve it, its just that it was not working at 9volts with a lamp. I though maybe i could do an easy modification for that. I swapped the driver transistor gor a Darlington and while that made the lamp glow, the LFO now stalls at max speed or max depth

You should be able to get it to work.

The reason it stalls on maximum depth is because the input impedance to the lamp driver circuit is too low and is loading down the oscillator output.    When the depth control is backed off the depth pot resistance makes the impedance looking into the depth pot and lamp driver increase (essentially putting a series resistance between the two circuits).

Putting a Darlington there should help.  However you need to modify the circuit to take full advantage of that - otherwise you aren't removing all the loading effects of the lamp driver.  The resistors R29 (4k7), R33 (47k), R31 (100k) should be increased.   With a Darlington you could probably multiply all the resistor values by 10!  That's only going to be a ball-park as the circuit will need to be tweaked in terms of gain and sweep range (and offset).   That's what JC Maillet's link alludes to but someone needs to actual do the hard yards and tweak the values.

About the loading.  The output of the oscillator has 15k resistor.   That's driving a 25k pot, which is probably creating a loading problem from the start, maybe 50k will help.   It doesn't take much loading to kill these oscillators as the single transistor amplifier stages only just have enough gain.   Next is the impedance looking into the transistor.   When you only have a single transistor the value of R34 can cause loading at the base.  You might need a low value for R34 to get enough lamp drive.  In a circuit like this R33 also causes loading at the base.   With a single transistor you can't just raise R34, and R33 because there might not be enough base current to drive the lamp.   Changing to a Darlington allows you to raise all the impedance without the balancing act between lamp drive and input impedance.  FWIW R29 and R34 have some influence on the range of the sweep.    R34, R33 and R31 set the idle current through the lamp.

Rob thanks so much! I will try that and report back!