"Gene McCalmont" <[EMAIL PROTECTED]> made an utterence to the drakelist gang ---------------------------------------------------------------------- Hello Jason; >From your email, it appears that one of your 3-500z tubes has "bit the dust". It would be difficult to say what caused the problem, but these tubes, while robust to a fault, are not indestructible and do have a definable life cycle. It is entirely normal for the tube plates to have a dull reddish glow while the amp is keyed, however, from your description, I would say you are correct that one tube was under full load while the other was not. While color descriptions are mostly subjective, an orangey glow means the remaining tube was pulling a lot of current when observed. Depending on how long this condition existed, it too is probably exhausted.
There is practically no way an average amateur can check a 3-500z unless the tube is put into service and observed. Fortunately, you had enough presence of mind to observe the tubes while switching off the power. Bravo! This is the way I would proceed: 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. 2. Observe the other PS components paying close attention to the filter capacitors, bleeder resistors and all three dropping resistors. These components may need attention as well. 3. Go to the bottom of the RF deck and observe the components and socket around the tube that was not conducting. Look for obvious problems such as swollen/cracked capacitors and resistors, burnt wire etc. At this point, I would not start wholesale replacing components. It is likely that any component failure at this point was catastrophic and should be easily discovered. 4. It is most likely that the one tube you observed had reached its end of life and will need to be replaced. When you remove the tube, mark it with a felt tip marker. What caused it to fail, having not discovered any obvious failed components on the chassis underside, will be difficult to determine. At this point, you have several options. Replace that one tube with another USED tube and get on with life, or replace BOTH 3-500z tubes with fresh examples. Although this would be a lot of work, the most optimal approach, since the amp is already down for the count, would be to freshen the PS with new capacitors and diodes, and a few other components, do a really good cleaning of the amp, service the fan motor, replace any obviously defective under chassis components at the RF deck, plunk in some new Taylor 3-500z tubes from RF Supply and Rock and Roll. You will have an awesome amp, capable of 1,500watts out, that will last you another 20+ years. 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 ---------------------------------------------------------------------- On Behalf of "Gene McCalmont" <[EMAIL PROTECTED]> Submissions: [EMAIL PROTECTED] Unsubscribe: [EMAIL PROTECTED] - unsubscribe drakelist in body Hopelessly Lost: [EMAIL PROTECTED] - help in body of message Zerobeat Web Page: http://www.zerobeat.net Brought to you courtesy of TLCHost.net http://www.tlchost.net/ ----------------------------------------------------------------------
