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Suggestions for BJT Tremolo Oscillator
#1
Hey everyone I'm looking for a good BJT oscillator circuit to drive a LED LDR or Jftet. I'd prefer the LDR.... I've used the London Power Wide Range Tremolo in a  tube amp and like it but since all my stuff uses eyelet boards I'd like to find a BJT circuit to do something similar. I find discrete components easier to wire up.  There seems like a multitude of circuits out there. Does anyone know of a good design to start off of? I figure I can derive the voltage from the high voltage supply and use a Zener to get the right voltages.  Thanks for the help!
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#2
Hello Mr Makinrose

Vox has a single mosfet oscillator for the tremolo in their newer amps. They use it a lot so it must work okay?

Cheerio
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#3
(08-02-2021, 04:27 PM)Sherlok Ohms Wrote: Hello Mr Makinrose

Vox has a single mosfet oscillator for the tremolo in their newer amps. They use it a lot so it must work okay?

Cheerio
Thanks!  However I'm wanting to do a signal modulating circuit.  While the Vox work well but they are modulating the power tubes via the 220K grid-leaks.  I'm wanting to use these in the signal path to avoid stressing the power tubes and power supply plus it's for fixed bias amp.
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#4
Hello Mr Makinrose

May I kindly suggest that the oscillator itself can be interfaced with whatever you wish, as far as I understand. Yes, Vox likes to apply this signal to the EL-84 grids, which tells you the magnitude of the oscillation assuming the tubes can be driven to cutoff (10-15V peak).

Depending on the circuit point you wish to tie the tremolo to, you might consider using direct modulation, say to the the grid-leak of a triode gain stage. This would eliminate the need to have a 10mA or higher current available for the LED side of your preferred LDR interface. The B+ can certainly supply this but there will be a lot of heat in the dropping resistor.

I believe KOC suggested a nifty "current steering" interface where the LED in the LDR is always tied to the dropping resitor, and a modulation element is tied in parallel with it. The current is constant and goes through the LED or the element or is shared by both depending on the point of tremolo signal. The element could be a transistor, mosfet or jfet. The voltage is limited by the LED itself in the LDR. if you drove this from the Vox oscillator, then no specific voltages have to be derived from the plate supply - just have appropriate decoupling. The current steering is a method used to reduce "thumping" through the supply and into the amplifier. You can place a pot between the oscillator and the element.

I may be "talking out of my posterior", as they say, but I believe the above to be correct Smile

Cheerio
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#5
(08-02-2021, 06:49 PM)Sherlok Ohms Wrote: Hello Mr Makinrose

May I kindly suggest that the oscillator itself can be interfaced with whatever you wish, as far as I understand. Yes, Vox likes to apply this signal to the EL-84 grids, which tells you the magnitude of the oscillation assuming the tubes can be driven to cutoff (10-15V peak).

Depending on the circuit point you wish to tie the tremolo to, you might consider using direct modulation, say to the the grid-leak of a triode gain stage. This would eliminate the need to have a 10mA or higher current available for the LED side of your preferred LDR interface. The B+ can certainly supply this but there will be a lot of heat in the dropping resistor.

I believe KOC suggested a nifty "current steering" interface where the LED in the LDR is always tied to the dropping resitor, and a modulation element is tied in parallel with it. The current is constant and goes through the LED or the element or is shared by both depending on the point of tremolo signal. The element could be a transistor, mosfet or jfet. The voltage is limited by the LED itself in the LDR. if you drove this from the Vox oscillator, then no specific voltages have to be derived from the plate supply - just have appropriate decoupling. The current steering is a method used to reduce "thumping" through the supply and into the amplifier. You can place a pot between the oscillator and the element.

I may be "talking out of my posterior", as they say, but I believe the above to be correct Smile

Cheerio
You are right about the modulator being able to be used for other types of trem....I didn't have my thinking cap on!
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