Delay relay
Several later tube-era Tektronix oscilloscopes such as the 549, 581 and 585 that already use semiconductor rectifiers for the anode voltages contain a thermal delay relay such as the 6N045T or 6N030T in a circuit that delays turn-on of the main supply voltages until the tubes are warmed up. This is normally not needed in scopes with tube rectifiers because of their inherent slow-start behavior.
Bruce Baur said:
The reason Tek resorted to time delay on the B+ is that the high current tubes, like the series regulators, did not like to produce current with a cold to warm cathode.
Until the cathode reached its normal hot running temperature there was a tendency for it to strip cathode material off of the cathode and shorten the life of the tube.
The 5U4 filament warms up fast, but the 6X4 warms up about the same as the other tubes and acts as a time delay for the reference that the supplies need to operate from the 5U4.
When the scope is turned on, a bi-metal strip in the delay relay is beginning to heat up. After the delay time (30 to 60 seconds depending on the instrument), the bi-metal strip closes a contact that energizes the main relay, thereby turning the anode voltages on. One of the main relay's contacts is used to make it self-holding, while at the same time cutting out the delay relay. At this time, the bi-metal strip is beginning to cool so it will be ready to provide delaying turn-on again in the event of a power outage.
The part number is decoded as: 6 − Heater Voltage, NO − Normally Open / C − Normally Closed, XX − delay time, T − B9A (Noval) miniature 9-pin base / no suffix − octal base). For example, a 6NO30T is a 6 Volt filament/heating element with a normally open contact and a delay time of 30 seconds, with a miniature 9-pin base. (The original literature used NO and N0 prefixes interchangeably.)
| Part # reference | ||
|---|---|---|
| Tek Part | MFR Part | Used in |
| 148-0002-00 | 6NO45 | 535,545,585,555 |
| 148-0006-00 | 26NO45T | ? |
| 148-0009-00 | 6NO15T | ? |
| 148-0021-00 | 6NO30T | 547,549 |
| 148-0023-00 | 18NO30 | 585A |
