I intend to add a 6AX4 damper diode in series with the plate supply rail to my DHT SET amplifiers. The graceful turn-on of the 6AX4 will of course provide a reasonably slow ramp-up of the high voltage as applied to the plates of the tubes, and will allow the mercury vapor (MV) rectifiers to warm-up before full load is placed upon them. This is a requirement when using MV rectifiers.
I have two options as to where I can locate the damper diode within the power supply circuit; within the + plate supply DC rail immediately after the MV rectifier tubes and before the filter choke (choke input filter configuration); or at the negative return of the power supply, that being between ground and the center-tap of the plate transformer.
I am of the opinion that locating the damper diode tube at either of these two locations within the circuit will make no difference in terms of it's intended purpose, that being a graceful and gradual soft-start of the high-voltage.
However, to be safe, is there a preferred or recommended location for the damper diode rectifier tube for my application? If so, why? There may be some subtlety associated with either of these two approaches that I may be overlooking, that could reduce the effectiveness of the circuit, or degrade the performance of the power supply.
I have two options as to where I can locate the damper diode within the power supply circuit; within the + plate supply DC rail immediately after the MV rectifier tubes and before the filter choke (choke input filter configuration); or at the negative return of the power supply, that being between ground and the center-tap of the plate transformer.
I am of the opinion that locating the damper diode tube at either of these two locations within the circuit will make no difference in terms of it's intended purpose, that being a graceful and gradual soft-start of the high-voltage.
However, to be safe, is there a preferred or recommended location for the damper diode rectifier tube for my application? If so, why? There may be some subtlety associated with either of these two approaches that I may be overlooking, that could reduce the effectiveness of the circuit, or degrade the performance of the power supply.
Between the centertap and the ground return has the upside of one less heater winding as you can run it from the same winding as some of the amplification tubes.
Plus you dont need two high voltage insulated 6.3 Windings for two different tubes.
If you have a separate heater transformer for the 866's i would use a 50-100R 25W resistor in the Power line 117VAC side. and short it with a 555 based relay module after about a minute. Edit scratch that, its early and i was conflating TV damper rectification use.
However i would caution against gas filled tubes like the 866 because they are said to produce HF noise.
Plus you dont need two high voltage insulated 6.3 Windings for two different tubes.
If you have a separate heater transformer for the 866's i would use a 50-100R 25W resistor in the Power line 117VAC side. and short it with a 555 based relay module after about a minute. Edit scratch that, its early and i was conflating TV damper rectification use.
However i would caution against gas filled tubes like the 866 because they are said to produce HF noise.
Damper diodes don't have a long enough delay for mercury vapor tubes. Check out page 73 of the RCA manual as it really goes into detail on their operation. https://nvhrbiblio.nl/biblio/boek/RCA Transmitting Tube Manual TT-5.pdf
TV Damper Diode as a Soft-Start for Mercury Vapor Rectifiers
Thanks to those who have replied to my post!
I have one last question: When using a single half-wave damper diode (such as a 6AX4) as the soft-start element in series with the B+, does the indirectly heated filament (which is of course isolated from the cathode), need to have one side of the filament tied to ground, as a ground reference?
Thanks to those who have replied to my post!
I have one last question: When using a single half-wave damper diode (such as a 6AX4) as the soft-start element in series with the B+, does the indirectly heated filament (which is of course isolated from the cathode), need to have one side of the filament tied to ground, as a ground reference?
"need to have one side of the filament tied to ground, as a ground reference?"
NO!!!
Damper cathode-filament voltage maximum is only 100 Volts. In this case the damper cathode at B+.
NO!!!
Damper cathode-filament voltage maximum is only 100 Volts. In this case the damper cathode at B+.
I don't think the delay provided by the dampers is ideal. As they age the mercury rectifiers seem to need longer warm up times.
I have used MV rectifiers for years in my audio gear, and I have never experienced this issue you have described.
I am changing the soft-start circuitry in my power amps and preamp that utilize MV rectifiers, and my question pertains strictly to the use of a TV damper diode to provide this.
Thanks & Regards,
I am changing the soft-start circuitry in my power amps and preamp that utilize MV rectifiers, and my question pertains strictly to the use of a TV damper diode to provide this.
Thanks & Regards,
As such, should I connect the heater of the damper diode to the cathode?
The other alternative would be to put the damper diode in series with the center-tap of the HV winding of the power transformer to ground, yes? This way, the cathode is sitting at ground potential at all times, when the tube is fully conducting. In this situation, the plate of the damper diode would be connected to ground, and the cathode of the tube is connected to the center-tap of the transformer.
Last edited: