Re: [Drakelist] TX4B cathode resistors
Hi Peter, IF you use ohms law to figure the current thru the resistors, you'll see that one tube is drawing about 28 ma., and the other is drawing 43 ma. They'll probably be in about the same ratio under higher load. Checking this is one way, when the resistances are equal, to determine if one tube is getting weak. You should replace both resistors with new ones rated at 1w or more. 73, Al, W8UT www.boatanchors.org www.hammarlund.info There is nothing -- absolutely nothing -- half so much worth doing as simply messing about in boats Ratty, to Mole On 9/18/2012 2:08 PM, Peter Ravn wrote: The two cathode resistors in the finals in the TX4B should be 15 ohms each, but R32 is 30 ohms and R33 16 ohms with the finals unplugged, so R32 is definitely bad. The difference in voltages on the cathodes of the two finals is, however, minimal. The voltage across R32 is 0.85 and 0.7 across the R33. What are the causes and consequences of the differences in the cathode resistors? Thanks. OZ8CTH Peter ___ 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
Re: [Drakelist] TX4B cathode resistors
- Original Message - From: Peter Ravn fas...@privat.dk To: Drakelist@zerobeat.net Sent: Tuesday, September 18, 2012 11:08 AM Subject: [Drakelist] TX4B cathode resistors The two cathode resistors in the finals in the TX4B should be 15 ohms each, but R32 is 30 ohms and R33 16 ohms with the finals unplugged, so R32 is definitely bad. The difference in voltages on the cathodes of the two finals is, however, minimal. The voltage across R32 is 0.85 and 0.7 across the R33. What are the causes and consequences of the differences in the cathode resistors? Thanks. OZ8CTH Peter I am not sure what the intended purpose of the resistors is. They have by-pass caps on both sides and are further connected to an RF choke which goes to the metering circuit. You can measure the total tube current of each tube by measuring the drop across the resistors. The voltage drop is very small compared to the fixed grid bias so I don't think they have much effect on it. I found the same thing in my T4XB, one resistor right on the nose, the other about twice the marked value. They were of different manufacture, the good one having a solid body, the bad one having a mold seam and different surface texture. I replaced both with metal oxide film resistors. I did the same in my TR-4. While all carbon composition resistors tend to increase in value with age and heat some are much worse than others. For the most part I've found that Ohmite and Allen-Bradley resistors tend to stay pretty stable. I don't know who made the seamed ones and don't see many IRC resistors in the stuff I've worked on. IRC have a sort of tree bark texture that was supposed to help dissipate heat and also were metalized, I have no idea of what that really was. The resistors with mold seams going the long way all seem to go bad. -- Richard Knoppow Los Angeles WB6KBL dickb...@ix.netcom.com ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
Re: [Drakelist] TX4B cathode resistors
They are in the meter cicuit...and if they are wrong..the plate current will not read correctly...and of course the current on both tubes will not be matched 73, Lee -Original Message- From: Peter Ravn fas...@privat.dk To: Drakelist Drakelist@zerobeat.net Sent: Tue, Sep 18, 2012 3:11 pm Subject: [Drakelist] TX4B cathode resistors The two cathode resistors in the finals in the TX4B should be 15 ohms each, but R32 is 30 ohms and R33 16 ohms with the finals unplugged, so R32 is definitely bad. The difference in voltages on the cathodes of the two finals is, however, minimal. The voltage across R32 is 0.85 and 0.7 across the R33. What are the causes and consequences of the differences in the cathode resistors? Thanks. OZ8CTH Peter ___ 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
