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02-01-2024, 01:01 PM
Hi Guys
You can see in the VHT circuit that the range for power reduction has been severely restricted inasmuch as the minimum power level equates to a still-too-high loudness. R65 needs to be reduced to 33k to allow useful sweep.
The SV2 we supplied has a tweaked resistor value to allow the use of the 1M dual pot in the VHT, as the other section is used to proportion the tremolo modulation signal applied to the output tube grids.
The amp lacks any means to control the rest of the circuit voltages to keep their tonal contribution consistent, which might be why they limited the amount of power reduction possible. This tends to be the way, as well, when combo-format amps are given poorly thought out power reduction schemes. Even Mavin Peal did that with their combos because the speaker loses fullness of tone at low-loudness, but also MP scaled too much circuitry. The VHT scheme seems influenced by Dana Hall and his vvt, which was a copy of a portion of the Classic-PS approach.
Physical construction of VHT amps we have seen is pretty tight, and sometimes opaque as to how to take them apart. Thermal management is therefore a bit compromised even for the tubes, let alone for fragile, heat-sensitive mosfets. This is the main weakness of power mosfets, is their need for very good cooling and heat extraction, and would explain why the original mosfet Q1 has failed twice. Mounting SV2 in the same place won't be more reliable.
In a combo amp, adding a fan is a good idea and fan noise will not be a problem. Use a 12Vdc fan and power it from 8Vdc derived from the 6Vac heater supply. A bridge and filter cap of 2,200uF minimum will suffice. The fan turns at reduced speed and you will not hear it at all from in front of the amp. The fan should be in the cold air path and push air past the tubes towards the rear. This is the same for stand-alone amps as heads and is shown in TUT and subsequent volumes.
Have fun
You can see in the VHT circuit that the range for power reduction has been severely restricted inasmuch as the minimum power level equates to a still-too-high loudness. R65 needs to be reduced to 33k to allow useful sweep.
The SV2 we supplied has a tweaked resistor value to allow the use of the 1M dual pot in the VHT, as the other section is used to proportion the tremolo modulation signal applied to the output tube grids.
The amp lacks any means to control the rest of the circuit voltages to keep their tonal contribution consistent, which might be why they limited the amount of power reduction possible. This tends to be the way, as well, when combo-format amps are given poorly thought out power reduction schemes. Even Mavin Peal did that with their combos because the speaker loses fullness of tone at low-loudness, but also MP scaled too much circuitry. The VHT scheme seems influenced by Dana Hall and his vvt, which was a copy of a portion of the Classic-PS approach.
Physical construction of VHT amps we have seen is pretty tight, and sometimes opaque as to how to take them apart. Thermal management is therefore a bit compromised even for the tubes, let alone for fragile, heat-sensitive mosfets. This is the main weakness of power mosfets, is their need for very good cooling and heat extraction, and would explain why the original mosfet Q1 has failed twice. Mounting SV2 in the same place won't be more reliable.
In a combo amp, adding a fan is a good idea and fan noise will not be a problem. Use a 12Vdc fan and power it from 8Vdc derived from the 6Vac heater supply. A bridge and filter cap of 2,200uF minimum will suffice. The fan turns at reduced speed and you will not hear it at all from in front of the amp. The fan should be in the cold air path and push air past the tubes towards the rear. This is the same for stand-alone amps as heads and is shown in TUT and subsequent volumes.
Have fun
