![[-]](https://theultimatetone.com/images/collapse.png "[-]")
05-05-2023, 06:24 PM
Hi Guys
An interesting thread showing a method for a fixed level of power reduction that would be aesthetically pleasing to many guitar players using tube amps. The mod is simply the cathode-bias version of PS-TT (Power Scaling Two-Thirds) where the plate voltage is not controlled. Output power is reduced and the high-power output stage effects are achieved at lower power and lower loudness.
The mod is not actually one that enhances sag per se; rather, it allows the output stage effects to be experienced at a lower power level / loudness.
Note that all guitar players realise that their instrument is only really "half an instrument" inasmuch as an amp or other electronics is required to produce The Ultimate Tone the player wishes for, just as has always been said in the TUT-series since 1994.
The OC3 is essentially a zener diode with an "ionizing heater" rather than the usual filament or heater+cathode. This allows it to be a 2-terminal device, but it has an octal base. The single component sub is a 100V zener. The OC3 is rated to conduct 4-40mA while maintaining 105V across itself. At 40mA there is a dissipation of just over 4W. As a zener we would want to use a 10W device if it is available, or go to series-connected lower-voltage 1W diodes.
The alternative is to use a low-wattage zener, such as 1N4764 and a pass element such as a BJT or mosfet bolted to the chassis. Yes, more than a single component BUT affordable, compact and available components. Certainly, for the aesthetic of "all tube" find an 0C3 or equivalent. Note the first digit is zero, not a capitol "o".
One advantage of using the series string of zeners is that a rotary switch could be added to select the reduction level. Of course, it can also be made fully variable as our kits allow.
We know that the tube is cathode-centric and in a tetrode / pentode there is an intrinsic triode comprised of k, g1 and g2, where g2 is the "plate". If you change the screen voltage (Vs applied to g2) then the idle condition of the tube changes in step. Assuming we are modifying and amp as the OP suggests, then we consider these issues:
In the case of a cathode-biased amp, the tube finds a new place to settle working against Rk in the new voltage environment. With conventional Rk biasing, the idle condition will no longer be as stock, as lower plate current flows and the supply voltages will be slightly unloaded, rising as a result. Maximum output power is reduced, which means the compression and clipping will be at a lower loudness level. Above the class-A transition, the single driven tube is working against Rk on its own which linearises what it does but ultimately restricts the output.
All cathode-biased guitar amps (except my own) have a bypass cap Ck in parallel with Rk. This allows a smaller number of tubes to produce a given output power and/or for more output than the full idle dissipation of the tubes otherwise allows, usually because there is a transition to class-B.
In fixed-bias, the drop in Vs reduces Ik as above. Without an adjustment of -Vg1, the tubes will likely be in cut off (zero current), and what is needed is a proportional reduction of -Vg1 to set the world back in order. As with the cathode-biased situation, these conditions for the output stage will produce lower output than stock.
In cathode-bias, we should note that adding the Vs reducer is only half the required mod. The supply will unload and the preamp and driver stages will all have higher voltage and have a brighter overall tone. To restore the stock conditions, a voltage regulator should be added for all those voltages. Really, the voltage needs to be clamped or restricted to not rise higher than the stock values, rather than adding hard regulation. Our VCK Voltage Clamp Kit does exactly this function for cathode-biased amps being Power Scaled.
There is a misstatement in the OP post regarding keeping Ea (plate signal) from falling below Vs. In every conventional output stage, the plate voltage swings close to ground at peak drive, never getting all the way because of the tube's internal resistance and Rk if the latter is present. Long before this point in the signal cycle, the plate voltage dips below Vs and everything is fine. Things are working as they should.
I believe it may be coincidental that when the OP's amp was modified to reduce Vs by 100V, that the signal swing on the plate "just happened" to never go below the reduced Vs value. This is normal in PS-TT since Va is left as stock.
Also, please attach the schemo as an attachment rather than inline if you want the text to be legible. Thanks
An interesting thread showing a method for a fixed level of power reduction that would be aesthetically pleasing to many guitar players using tube amps. The mod is simply the cathode-bias version of PS-TT (Power Scaling Two-Thirds) where the plate voltage is not controlled. Output power is reduced and the high-power output stage effects are achieved at lower power and lower loudness.
The mod is not actually one that enhances sag per se; rather, it allows the output stage effects to be experienced at a lower power level / loudness.
Note that all guitar players realise that their instrument is only really "half an instrument" inasmuch as an amp or other electronics is required to produce The Ultimate Tone the player wishes for, just as has always been said in the TUT-series since 1994.
The OC3 is essentially a zener diode with an "ionizing heater" rather than the usual filament or heater+cathode. This allows it to be a 2-terminal device, but it has an octal base. The single component sub is a 100V zener. The OC3 is rated to conduct 4-40mA while maintaining 105V across itself. At 40mA there is a dissipation of just over 4W. As a zener we would want to use a 10W device if it is available, or go to series-connected lower-voltage 1W diodes.
The alternative is to use a low-wattage zener, such as 1N4764 and a pass element such as a BJT or mosfet bolted to the chassis. Yes, more than a single component BUT affordable, compact and available components. Certainly, for the aesthetic of "all tube" find an 0C3 or equivalent. Note the first digit is zero, not a capitol "o".
One advantage of using the series string of zeners is that a rotary switch could be added to select the reduction level. Of course, it can also be made fully variable as our kits allow.
We know that the tube is cathode-centric and in a tetrode / pentode there is an intrinsic triode comprised of k, g1 and g2, where g2 is the "plate". If you change the screen voltage (Vs applied to g2) then the idle condition of the tube changes in step. Assuming we are modifying and amp as the OP suggests, then we consider these issues:
In the case of a cathode-biased amp, the tube finds a new place to settle working against Rk in the new voltage environment. With conventional Rk biasing, the idle condition will no longer be as stock, as lower plate current flows and the supply voltages will be slightly unloaded, rising as a result. Maximum output power is reduced, which means the compression and clipping will be at a lower loudness level. Above the class-A transition, the single driven tube is working against Rk on its own which linearises what it does but ultimately restricts the output.
All cathode-biased guitar amps (except my own) have a bypass cap Ck in parallel with Rk. This allows a smaller number of tubes to produce a given output power and/or for more output than the full idle dissipation of the tubes otherwise allows, usually because there is a transition to class-B.
In fixed-bias, the drop in Vs reduces Ik as above. Without an adjustment of -Vg1, the tubes will likely be in cut off (zero current), and what is needed is a proportional reduction of -Vg1 to set the world back in order. As with the cathode-biased situation, these conditions for the output stage will produce lower output than stock.
In cathode-bias, we should note that adding the Vs reducer is only half the required mod. The supply will unload and the preamp and driver stages will all have higher voltage and have a brighter overall tone. To restore the stock conditions, a voltage regulator should be added for all those voltages. Really, the voltage needs to be clamped or restricted to not rise higher than the stock values, rather than adding hard regulation. Our VCK Voltage Clamp Kit does exactly this function for cathode-biased amps being Power Scaled.
There is a misstatement in the OP post regarding keeping Ea (plate signal) from falling below Vs. In every conventional output stage, the plate voltage swings close to ground at peak drive, never getting all the way because of the tube's internal resistance and Rk if the latter is present. Long before this point in the signal cycle, the plate voltage dips below Vs and everything is fine. Things are working as they should.
I believe it may be coincidental that when the OP's amp was modified to reduce Vs by 100V, that the signal swing on the plate "just happened" to never go below the reduced Vs value. This is normal in PS-TT since Va is left as stock.
Also, please attach the schemo as an attachment rather than inline if you want the text to be legible. Thanks
