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In Reply to: My only recourse posted by jellobrain on January 21, 2001 at 17:12:30:
Hmm. Don't kill the good 45's, I agree with you. Maybe your others are still OK, though, tubes are harder to kill than you think.I think you are on the right track with the cathode R's - they may be overheating. mayeb they deveop a short afetr that happens - I don't know typical failure modes for resistors. Anyway you always want your resistors in a tube device to be rated for more (I say at least double) than you run them at. Cheap insurance against this kind of crap.
Leave it aside and solder in ONE new 1.5K/10W per channel when you get the parts delivery. Then we'll try again. You may have killed these 45's though. I'd advise getting the amp running reliably with a relatively (yeah, I know, that's still not cheap) inexpensive pair of 45's (or 2A3's) first before you put the Cunninghams in.
-j
Follow Ups:
Well after having come to my last "conclusion" about the cathode resistors I decided to do some reading about biasing in general. I have a RCA recieving tube manual from about 1959 and I downloaded some pages from an earlier version that had detailed info on the 45. WELL as I looked closer and then recalled the pinout diagram that I DID use when I put everything together, one that was on the internet, some kind of tube info page, anyway once I read "pin orientation shown from below" on the RCA tube manual page and then looked at my tubes and then back at the manual then back again then I DID smack myself in the forehead! Yup, you guessed it, I had the plate and cathode switched on the 45s! Well getting that right makes a BIG difference(DUH)!!!! They sound just fine. I did burn up the 1 45 and I think the capacitors on the cathode resistor (100V and I guess with things backwards the B+ was running through them, so now they are all swollen and I suspect bad, I replaced them) But NOW it works! I need to make some more efficient speakers. And I was thinking of a Class A SE with a 6550 in it.
Well after having come to my last "conclusion" about the cathode resistors I decided to do some reading about biasing in general. I have a RCA recieving tube manual from about 1959 and I downloaded some pages from an earlier version that had detailed info on the 45. WELL as I looked closer and then recalled the pinout diagram that I DID use when I put everything together, one that was on the internet, some kind of tube info page, anyway once I read "pin orientation shown from below" on the RCA tube manual page and then looked at my tubes and then back at the manual then back again then I DID smack myself in the forehead! Yup, you guessed it, I had the plate and cathode switched on the 45s! Well getting that right makes a BIG difference(DUH)!!!! They sound just fine. I did burn up the 1 45 and I think the capacitors on the cathode resistor (100V and I guess with things backwards the B+ was running through them, so now they are all swollen and I suspect bad, I replaced them) But NOW it works! I need to make some more efficient speakers. And I was thinking of a Class A SE with a 6550 in it.
You'll only ever make THAT mistake ONCE!(Of course there are plenty of others to make!)
I must say I feel a little less puzzled which is good (although it doesn't un-fry your 45.)
Happy listening - it sounds like it would be safe to throw the Cunninghams in now!
-j
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