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Re: Extending tube life?

Absolutely! You are doing the right thing to extend the life of not only the 7868 output tubes but also the other tubes and components as heat will shorten the life of caps, resistors, and even transformers.
All else being equal, you can conservatively expect to get more than double the usual life from your tubes if you use a fan to help ventilate the receiver.

If fan noise is a problem, the other poster was correct in suggesting using a 12 volt DC fan but operating this fan at a reduced voltage. I use a 3 inch fan from Radio Shack with my Fisher 400, attached to a small plastic kit box using double sided tape. The hobby kit box serves as a platform base that raises the fan up to the level of the back of the receiver chassis. Instead of operating this fan at the nominal 12 volts, I have it running at 6 volts. It is much quieter now, while admittedly running slower, but still moves plenty of air. This is easy to rig up as they sell these fans with bare wire leads in 2 inch, 3 inch, and 4 inch sizes. They also sell an accessory item called a universal hobby leads adapter. This part allows you to connect the bare wire leads of the fan to the bare wires on one end of this plug, which in turn fits their 1.5 - 12 volt universal DC power adapter. These come in both 300ma and 800ma rated adapters. I have had no problems running a single fan on the 300ma rated adapter. You can do better on price if you buy the fan, at least, online but Radio Shack is certainly more convenient for those of us who live within a few miles of one and maybe more than one of their stores.

The next item you may want to consider is a 5 amp, 7.5 amp, but not more than 10 amp rated variac (more than this would be overkill for running just one amplifier or receiver). These variable AC transformers will allow you to set the line voltage seen by the Fisher, lowering it from today's not uncommon 123 volts or higher down to a more relaxed 110 volts. When these vintage components were designed, 108-115 volts AC was the norm, and 118 or 120 volts was at the upper range of the expected line voltages for many of these older designs. Lowering the voltage to 110 volts results in a much cooler running receiver and in my experience the audio performance actually sounds better. With the fan running, and the line voltage adjusted down to 110 volts using a 5 amp rated variac unit, the top of the grill directly over the 7868 tubes feels just barely warm to my hand, and the rest of space across the case of the receiver actually feels cool.

If you decide to try the following, proceed at your own risk. I certainly don't recommend that you try this with your Fisher 400, but with my unit with the variac-reduced line voltage and the fan operating, I have been able to gently place a clean finger on the side of one of the 7868 tubes while it is running and it was not so hot as to feel uncomfortable, or burn my finger. Note that my Fisher 400 has been upgraded with all of the Fisher Doc modifications. If you decide to try this proceed at your own risk. I only use this as an example for how much cooler the Fisher 400 can run with a fan, a variac, and all of the Fisher Doc mods installed.




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  • Re: Extending tube life? - mkmelt 20:32:24 09/06/03 (0)


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