Yesterday, 07:08 PM
Hi Guys
Many Vox amps have bias-modulated tremolo which adds a wrinkle to Power Scaling such amps. Fender has their Vibrosonic amp, as well. In both cases the modulation signal is applied via the Depth control to the output stage grid-leaks either directly, or through a bias pot. We need a way to make this signal shrink as we dial Power Scale down to low-power.
The easy solution is to use a p-jfet since it requires positive gate voltage to turn it 'off', wired as a shunt element across the Depth control. A series resistance is needed for the jfet to work against. This is the basis of our TREM kit. We used the last type of p-channel jfet that was available as a through-hole device, i.e. a component with leads. Unfortunately it is obsolete as are so many device suited to audio.
So what would be the basis for TREM of the future?
Ironically, the answer was another primitive device known as an LDR - light-dependent resistor. (The junction-field-effect transistor was one of the earliest semiconductors devised.)
To achieve a functioning identical to the jfet circuit the output of the LDR would have to be a high resistance that becomes a low resistance at low Power Scale settings. The LDR output is naturally 'high', so the control circuit has to provide more current to the LED input side of the LDR as power reduces. This requires that the LED be powered all the time but "held back" for full power and "let loose" at low power. This simply requires the use of current-steering as we saw in TUT chapter-9. Amps with bias-modulated tremolo have B+ of 350V to 450V, so I made the control range adaptable for 250V to 550V using two trimpots. The PCB for this circuit is 1.2" x 1.3" (29.4 x 31.9 mm) with space for a mounting bracket.
An alternative approach adds variable resistance between the Depth control and the modulation point. Here, the default resistance is 'low' and attenuated resistance is 'high'. Although this seems contrary to how the LDR is inherently, the control circuitry becomes greatly simplified in the Power Scale situation. The board size is reduced to 0.9" square (22.1 mm square). Again, trimpots allow the stock supply voltage to be 250V to 550V. This board is TREM-L (not on the site yet).
Many Vox amps have bias-modulated tremolo which adds a wrinkle to Power Scaling such amps. Fender has their Vibrosonic amp, as well. In both cases the modulation signal is applied via the Depth control to the output stage grid-leaks either directly, or through a bias pot. We need a way to make this signal shrink as we dial Power Scale down to low-power.
The easy solution is to use a p-jfet since it requires positive gate voltage to turn it 'off', wired as a shunt element across the Depth control. A series resistance is needed for the jfet to work against. This is the basis of our TREM kit. We used the last type of p-channel jfet that was available as a through-hole device, i.e. a component with leads. Unfortunately it is obsolete as are so many device suited to audio.
So what would be the basis for TREM of the future?
Ironically, the answer was another primitive device known as an LDR - light-dependent resistor. (The junction-field-effect transistor was one of the earliest semiconductors devised.)
To achieve a functioning identical to the jfet circuit the output of the LDR would have to be a high resistance that becomes a low resistance at low Power Scale settings. The LDR output is naturally 'high', so the control circuit has to provide more current to the LED input side of the LDR as power reduces. This requires that the LED be powered all the time but "held back" for full power and "let loose" at low power. This simply requires the use of current-steering as we saw in TUT chapter-9. Amps with bias-modulated tremolo have B+ of 350V to 450V, so I made the control range adaptable for 250V to 550V using two trimpots. The PCB for this circuit is 1.2" x 1.3" (29.4 x 31.9 mm) with space for a mounting bracket.
An alternative approach adds variable resistance between the Depth control and the modulation point. Here, the default resistance is 'low' and attenuated resistance is 'high'. Although this seems contrary to how the LDR is inherently, the control circuitry becomes greatly simplified in the Power Scale situation. The board size is reduced to 0.9" square (22.1 mm square). Again, trimpots allow the stock supply voltage to be 250V to 550V. This board is TREM-L (not on the site yet).