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02-08-2024, 10:09 PM
Hi Guys
Looselectron: I believe you went a little overboard with respect to checking that the amp works prior to installing Power Scaling.
In this amp, since you have an issue with the stock mosfet dying, the simple test wiring would be to remove the mosfet, then jumper across where D and S of the mosfet used to go. Done.
You can still twiddle the Watts pot and have the tremolo depth vary and this won't hurt anything.
As far as heat-sinking goes, usually VCK only carries the 12A_7 currents, so bolting to the chassis is sufficient. In this amp, there is a 6V6 as the reverb driver BUT it is powered from B+2, which is also the screen node for the output stage. We do not have to worry about this with respect to VCK.
It is the SV2 that needs big cooling, particularly Q3 which handles plate current for the output stage. Note that Q4 will pass the screen current and the reverb driver current, the latter which is only 5mA max.
Regarding the VHT-supplied mod: The diodes either side of the mosfet are there to protect the mosfet from capacitor discharge currents during turn-off. The input cap may drain faster than the output cap, as the B+1 and B+2 filters have no bleeder resistors where the input cap does. Add a bleeder on B+2. While you are at it, add a bleeder across B+5.
In the stock arrangement, it does not matter to the mosfet if the input cap discharges faster than the output caps do, as the mosfet has a reverse diode from D to S with the same voltage and current rating as the mosfet channel.
Looselectron: I believe you went a little overboard with respect to checking that the amp works prior to installing Power Scaling.
In this amp, since you have an issue with the stock mosfet dying, the simple test wiring would be to remove the mosfet, then jumper across where D and S of the mosfet used to go. Done.
You can still twiddle the Watts pot and have the tremolo depth vary and this won't hurt anything.
As far as heat-sinking goes, usually VCK only carries the 12A_7 currents, so bolting to the chassis is sufficient. In this amp, there is a 6V6 as the reverb driver BUT it is powered from B+2, which is also the screen node for the output stage. We do not have to worry about this with respect to VCK.
It is the SV2 that needs big cooling, particularly Q3 which handles plate current for the output stage. Note that Q4 will pass the screen current and the reverb driver current, the latter which is only 5mA max.
Regarding the VHT-supplied mod: The diodes either side of the mosfet are there to protect the mosfet from capacitor discharge currents during turn-off. The input cap may drain faster than the output cap, as the B+1 and B+2 filters have no bleeder resistors where the input cap does. Add a bleeder on B+2. While you are at it, add a bleeder across B+5.
In the stock arrangement, it does not matter to the mosfet if the input cap discharges faster than the output caps do, as the mosfet has a reverse diode from D to S with the same voltage and current rating as the mosfet channel.
