Garey Barrell <[EMAIL PROTECTED]> made an utterance to the drakelist gang ---------------------------------------------------------------------- Amazing how much mis-information flows over the internet!!
Temperature stability is a function of temperature CHANGE, not absolute temperature. The reason a temperature controlled oscillator is NOT because it is heated, but because the heat is held constant by an internal, tightly temperature controlled heater.
Adding a fan to any electronic equipment, especially tube based equipment, will increase the life of the tubes and components, AND will also keep the temperature constant because of the constant flow of air. Again, the object is to stabilize the temperature, not raise or lower it, to maintain frequency stability.
A T-4X(any) will change cabinet temperature considerably from receive to transmit when there is no fan cooling. This is minimized by the addition of a fan, blowing air __OUT__. The object is to assist the normal convection cooling, (hot air rises,) by forcing air AWAY from the transmitter final compartment, AND the rest of the transmitter. Blowing air IN forces heated air throughout the chassis, including the PTO, causing the temperature to VARY, not stabilize. Blowing air into the transmitter reduces the heat in the final compartment, but INCREASES the heat everywhere else, varying the temperature of the PTO as well.
The TR-7 is DESIGNED to have air drawn out of the chassis, for the same reasons, and the heatsink is DESIGNED to have air drawn past it as well. The cooling SYSTEM is designed to transfer heat from the final transistors to the heatsink to air being blown AWAY from the area. You can hold a small source of smoke, (candle, incense stick, etc.,) near any of the cabinet slots and smoke will be drawn INTO them when the fan is installed as designed, blowing OUT.
Merry Christmas!! 73, Garey - K4OAH St Charles, IL Drake 2-B, 4-B & C-Line Service Supplement CDs <www.k4oah.com> [EMAIL PROTECTED] wrote:
[EMAIL PROTECTED] made an utterance to the drakelist gang ---------------------------------------------------------------------- Not sure I understand that comment about the drift, hi hi. The drift is a function of the thermal environment with respect to the PTO components, including any negative temperature compensating capacitors. While the transmitter will "work", it will have a warm up period, and then cycles up and down as a function of the transmit duty cycle. Compensating for the former is enough of an engineering challenge, but the latter, for tube gear of the 60s, well, it is harder to address although there were some good designs out there. From a theoretical perspective, I'm not sure why the rig would run better when it is hot. A temperature compensated crystal oven obviously works better at higher stabilized temperature (above room ambient) but conventional circuits, well I'm not sure. Drift aside, the tubes will last longer with improved cooling though.
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