They have to be bypassed by resistors, say 1 MOhm that is enough to ignite one, then voltage on it drops, and another one ignites. No need for a sum of igniting voltages, just one glowing plus one igniting. Of course, the input voltage must be higher for higher ratio between "dropping" resistor and dynamic resistance of glowing tubes, for better regulation.
I never thought of that.... I have a line stage that uses two 150's per channel. It sounds quite good and is pretty in the dark. However, it has sat on the shelf for years now.
I have read before that VR tubes are sensitive to light. One person said he occasionally has to shine a flashlight on the VR tubes in his preamp in order for them to start conducting. Can anyone explain what is going on here?
Virtually all gaseous regulator glow tubes require a minimum voltage level to ionize the internal gas to begin conduction. Sometimes these tubes can drift off sensitivity specification with age. Visible light, which is a form of radiation, helps to trigger gas ionization. Some glow tubes even contain a tiny speck of a low level radio active isotope to aid in ionization if/when used in darkness. On the opposite end there are tubes, like the Raytheon 5886, that have extremely high input impedances that must be operated in the dark to avoid unwanted conduction.
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No need for a resitive voltage divider: If you put only one resistor in the 100k-ish range in parallel with one of both VR tubes, the total ignition voltage is the sum of the first valve's ignition voltage and the 2nd valve's regulation voltage. This recommendation is according to most datasheets for VR valves.
Best regards!
Best regards!
Take two VRs and a silicon diode, with sufficient reverse voltage rating to cope with the supply rail voltage (plus some margin for error). Wire them in series, so the diode will be forward biased under normal operating conditions. Put a resistor (1M?) from the top of the diode to the bottom of the lower VR (typically, ground). Put a resistor from the bottom of the diode to the top of the upper VR. This whole arrangement is fed from a resistor, as usual for VRs.
When you first apply voltage the VRs will be off, and the diode will be reverse biased by the two resistors. The two strings of 'VR+resistor' will be in parallel across the supply (except for the minor detail that one string is 'resistor+VR' - but that is OK). The VRs should fire. Once both are glowing the diode becomes forward biased, so then you get VR+VR+diode as the regulated voltage.
I saw this circuit online somewhere, but I can't remember where. A quick Google just now failed to find it. Maybe someone else can find it?
KP: this arrangement lets you go below the 'firing+glowing' limit of the one resistor method - you just need a bit more than 'glowing+glowing'.
When you first apply voltage the VRs will be off, and the diode will be reverse biased by the two resistors. The two strings of 'VR+resistor' will be in parallel across the supply (except for the minor detail that one string is 'resistor+VR' - but that is OK). The VRs should fire. Once both are glowing the diode becomes forward biased, so then you get VR+VR+diode as the regulated voltage.
I saw this circuit online somewhere, but I can't remember where. A quick Google just now failed to find it. Maybe someone else can find it?
KP: this arrangement lets you go below the 'firing+glowing' limit of the one resistor method - you just need a bit more than 'glowing+glowing'.
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