Carl, first of all it would be a sin to modify a nice piece of Drake equipment. With that said I do not like any Drake modifications, I like it the way it was, back when. The problem is finding the 6JB6 tubes, and if you know where to look new tubes are still available. We know lately there are several opportunities to buy those on eBay, both new and used. The problem is you really never know what your getting until they arrive, but then its probably to late if it was misrepresented. Those tubes just because they are in a nice looking box and/or labeled as a match pair, mean they are really new. In most cases the seller may not know for sure especially if it is part of an estate sale. All those are still "buyer beware" Also watch out, some of those deals maybe manufacturer 2nd's, those not quite meeting manufacturer specifications, and those probably will not work at transmitting tubes on the upper brands.
Also, do not go for the less expensive 6JB6 substitutes. I have tested several, none of which preformed well in the Drake radios. Some do well on the lower bands and even put out more power, but on 15 and 10 meters oscillate and will not neutralize. So stick with the 6JB6 tube. I have found staying with the Sylvania, GE or RCA tubes work well in the Drake. Other brands floating around like International (blue box) are nothing but garbage, even new out of the box. Match pairs if you can get them that way is nice, but having a match pair is not necessary if you follow a few basic rules. Always make sure all the tubes are the same manufacturer and are roughly the same age or have the same manufacture date code. If all that is unknown simply test each tube in the transmitter one at a time but removing the plate caps on all but one tube at a time. Note you need to keep all the tubes in the transmitter because the filaments are in series. Test each tube by measuring the idle current of each tube individually. Do not adjust the bias while testing the tubes, but one at a time record the idle current of each tube under test. Then just match the tubes that had the same idle current reading. Basically test all the tubes under the same test condition and then match the tubes with the same idle current for match pairs or triples. Testing the 6JB6 tube for performance is not too difficult. Sure signs of weak and bad tubes is the power output. The first sign tubes are getting weak is a fall off of power on 10 meters. A good set of tubes in a 4-line transmitter should put out 100 watts or more at 10 meters. As the tubes get weak you will no longer obtain that power level. As the tubes get weak lack of power output becomes noticeable starting at the highest frequency or bands and works towards the lower bands. Another test for tube performance is how well the handle the bias adjustment. A good set of tubes the bias adjustment should have plenty of adjustment room on both sides of the 60MA set point. Old weak tubes the bias setting is near or at the end of the pot rotation. A good set of tubes you should be able to turn the bias up to 120 to 150 MA of current. However, do that quickly and not more that a second, you don't want to ruin the tubes. Good tubes with good conductivity have the capacity to idle at much higher current levels. With that said, the normal bias setting for a pair of tubes is 60 to 80MA max and for three tubes 125MA in the TR4 series transceivers. I like setting the bias to 60MA for new tubes and around 80MA for the older tubes. Where to buy tubes, There are several tube vendors that still have the 6JB6 tube. Typically you can buy new NOS tubes in the 22 to 30 dollar range. You can check out my website under the "Parts & Service" for a few tube vendors I deal with. I know one of those vendor has a quantity of NOS GE tubes at $22 each. I just bought 50 from him and was well pleased with what I received. Ron / WB4HFN -----Original Message----- From: drakelist-boun...@zerobeat.net [mailto:drakelist-boun...@zerobeat.net] On Behalf Of Hulett, Russell Sent: Saturday, August 22, 2009 9:52 AM To: drakelist@zerobeat.net Subject: [Drakelist] Sweep Tube frustration Of four pair 6JB6 I own, only one will load on 15M and 10M. Only two will load above 40M. I'm using my T-4XB as the test rig ( the best pair (Sylvanias) are now in my T-4X ). All of the pair were "matched" by the vendors. I'm beginning to reluctantly think about doing the 6146 conversion. I hate to modify a nice rig, but expense of buying non-performing sweep tubes is becoming an issue for me. Have folks found the 6146 to be less problematic than the 6JB6 sweep tubes? In my younger days, I always found the 6146 to be pretty bullet-proof. All advice appreciated. 73, Curt KB5JO _______________________________________________ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist _______________________________________________ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
