hmbscott
Member
I was motivated to devise some kind of heater soft start after reading that regularly switching tubes on and off stresses the heaters due to surge current and rapid heating. Heater life can be extended by leaving components on continuously (Tomer, 1960) although at the cost of electricity and potentially shortened useful cathode life as well as reduced life of other heat sensitive components such as electrolytic capacitors.
Thermistors are one approach, but introduce a voltage drop that may be excessive for an existing low-voltage heater transformer design.
I ran across a circuit outlined in TI’s LM317 Regulator datasheet that provides heater soft start without any voltage drop penalty. The datasheet describes the circuit thusly: “The capacitor C1, in combination with the PNP transistor, helps the circuit (see circuit diag.) to slowly start supplying voltage. In the beginning, the capacitor is not charged. Therefore, the output voltage starts at VC1+ VBE + 1.25V = 0V + 0.65V + 1.25V = 1.9V. When the capacitor voltage rises, VOUT rises at the same rate. When the output voltage reaches the value determined by R1 and R2, the PNP is turned off.
The circuit is applicable to the LM1085 regulator used in the Eros 2 and in the Moreplay Upgrade Kit and similar LM family regulated heater circuits. It doesn’t work for unregulated AC or DC heater circuits.
The version I implemented for my Eros 2 uses a slightly larger R3 (56K) and C1 (47uF) about doubling the heater warmup time. I measured heater voltage rise from 2 to 6.3VAC in about 9 seconds. I mounted the 4 new components on a 1” square proto board which I supported with a standoff from the Shunt Regulator board. Three leads tie the Soft Start Board to the appropriate locations of the Eros PSB (see pics).
One thing to keep in mind is that slow heater start does potentially subject the tube’s cold cathode to high voltage for a longer period, which could under some conditions contribute to cathode stripping or perhaps arcing. My Eros is equipped with a 30 second high voltage delay circuit, which gives the cathodes sufficient of time to reach temperature prior to application of high voltage. It probably is a good idea to implement a HV delay if you contemplate heater soft start
.
Thermistors are one approach, but introduce a voltage drop that may be excessive for an existing low-voltage heater transformer design.
I ran across a circuit outlined in TI’s LM317 Regulator datasheet that provides heater soft start without any voltage drop penalty. The datasheet describes the circuit thusly: “The capacitor C1, in combination with the PNP transistor, helps the circuit (see circuit diag.) to slowly start supplying voltage. In the beginning, the capacitor is not charged. Therefore, the output voltage starts at VC1+ VBE + 1.25V = 0V + 0.65V + 1.25V = 1.9V. When the capacitor voltage rises, VOUT rises at the same rate. When the output voltage reaches the value determined by R1 and R2, the PNP is turned off.
The circuit is applicable to the LM1085 regulator used in the Eros 2 and in the Moreplay Upgrade Kit and similar LM family regulated heater circuits. It doesn’t work for unregulated AC or DC heater circuits.
The version I implemented for my Eros 2 uses a slightly larger R3 (56K) and C1 (47uF) about doubling the heater warmup time. I measured heater voltage rise from 2 to 6.3VAC in about 9 seconds. I mounted the 4 new components on a 1” square proto board which I supported with a standoff from the Shunt Regulator board. Three leads tie the Soft Start Board to the appropriate locations of the Eros PSB (see pics).
One thing to keep in mind is that slow heater start does potentially subject the tube’s cold cathode to high voltage for a longer period, which could under some conditions contribute to cathode stripping or perhaps arcing. My Eros is equipped with a 30 second high voltage delay circuit, which gives the cathodes sufficient of time to reach temperature prior to application of high voltage. It probably is a good idea to implement a HV delay if you contemplate heater soft start
.
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