Garey Barrell <[EMAIL PROTECTED]> made an utterence to the drakelist gang ---------------------------------------------------------------------- Gene & Jason -
One critique of your otherwise excellent advice.
The 0.82 ohm resistor is actually a wirewound "flameproof" resistor. The wirewound is identified by the "double width" first color band. The value is also not particularly critical, since in this application it is really nothing more than a HV fuse that will explode on overload, clearing the circuit and protecting the power supply. NTE offers flameproof resistors in their replacement line, but anything under 1 ohm is difficult to find. A 1 ohm value is fine in the L-4B. The metal or ceramic oxide resistors are a second choice, at best. I don't know what they do under extreme overload.
Some power supplies include a resistor in this circuit to protect the power supply against transient shorts, such as a tube flashover, but these are a larger value and size (multi-ohm and multi-watt) just to limit the short circuit current.
Fuses DO have voltage ratings, which is the voltage they will interrupt when overloaded, which is why Drake used the resistor instead of a fuse. Small glass fuses are rated at 125V. The higher current automotive types are rated at 32 volts. I'm not sure, but I think that the reduced voltage rating is because the mass of the fusible link is enough to deposit on the inside of the small glass tube.
Collins used a proper HV fuse in the 30S1, that is about 6 inches long. They are expensive, and take a lot of room.
73, Garey - K4OAH Atlanta
Gene McCalmont wrote:
"Gene McCalmont" <[EMAIL PROTECTED]> made an utterence to the drakelist gang ---------------------------------------------------------------------- Hello Jason;
1. You will, of course, have to open the PS to replace R12. If at all possible, replace R12 with an exact value 2w carbon resistor. If unavailable, get as close as you can using only what is available in a 2 watt configuration metal oxide composition resistor. Remember, R12 is kind of like a thermal fuse so any condition that causes excessive current draw exceeding its 2w dissipation rating, will blow the fuse. Metal oxide resistors will be slower to "give it up" than carbon, so carbon is the choice. The value of R12 is important and fortunately, it is a commonly manufactured part.
Good luck and please keep me informed. Gene
W5DDW Gene McCalmont 270 Oak Ridge Ln. Argyle Texas 76226
-----Original Message-----
From: Jason Buchanan [mailto:[EMAIL PROTECTED] Sent: Monday, December 06, 2004 6:42 AM
To: [EMAIL PROTECTED]
Subject: Re: [drakelist] L4-PS R12 burned - .825 ohm 2W resistor on pin 6 of
cable harness
Thank you VERY much for your EXCELLENT advice - I will do some checking
inside the PS today. I did notice that one tube was dark while the other
was red, almost orange, when this happened. I noticed a small pop sound
coming from the power supply and when I turned my head to look at the supply
and start reaching for the power switch I saw that one tube appeared to be
handling the entire load while the other was not doing anything. Filaments
were lit on both tubes.
I will take a drive by the parts store today and dig up some of those resistors. And will take OUT that tube that went dark. No idea why it would go dark and the other pick up the load - would a grid-to-filament short cause this?
Thanks, Jason
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