 
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
|
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
In Reply to: FINALLY SOMETHING! posted by jellobrain on January 20, 2001 at 11:46:40:
The behavior of your 45 is not a really GOOD sign, no. Sometimes I have had a tube that will spark once and then settle down (it's bits of material getting caught beween two elements and getting a short-circuit through them) but not as a regular behavior. Is just one tube doing this? Maybe try and get your supplier to swap a good tube for the problem child.Measure these voltages, using the DC scale of your meter:
from pin 1 to ground - should be somewhere around 55V DC
from pin 2 to ground - should be somewhere around 325V DC
from pin 3 to ground - should be somewhere around 0 VDC
from pin 4 to ground - should be somewhere around 55V DCSo from pin 1 OR 4 to pin 2 should be about 270V DC
On the AC scale:
from pin 1 to pin 4 - should be 2.5V ACLet us know what you find. It certainly could be a bad tube. Also your source might be having trouble providing enough signal - it's not driving a headphone and it may not sound good driving a line level input. But this is a seperate problem to the tube's misbehavior. Try taking the output from a cassette deck or whatever you might have handy and see what you get.
-j
Follow Ups:
ok pin 2 to ground I get 316 VDC pin 3 to gnd I get about -.44 VDC
pins 1 and 4 to gnd are EACH about -.003 VDC. Between 1 or 4 and 2
are both 305 VDC. And 1 and 4 is about 2.3 VAC.
SO! There is NO 55 VDC to be found (that is the bias voltage right??)
and accordingly NO 270 VDC (which is the B+ minus the cathode bias
55, right???) So if I have learned anything so far! I guess it is
STILL not biased right? (AND WHY NOT? I am using only 1 1.5K ohm
resistor on the CT of the filament supply bypassed with a 100 uf 100V
electrolytic with the lead with the minus symbols on it going to
ground! MAYBE i have bad resistors??????
Is this measurement with the 45 in or out?If the tube is out, measure again with the tube in.
If you still get ~ 0VDC at pins 1 and 4, then something is wrong with the tube. If you do NOT get ~ 0VDC at pin 3 (the grid) then your 470K grid resistor is not soldered correctly. Everything else looks good.
("~" = "about" - by ~ 0VDC I mean the type of numbers you are getting, less than half a volt.)
-j
All those measurments were made with the tube in. AND yes the tube IS
bad, or at least NOW it is! Once I realized that(the tube was bad(now
the grid will not light up AT ALL)) I put in ANOTHER 45, ok so it
started up and worked fine. BUT about 3 or so minutes into operation
IT started to make poping sounds and sparking on the
inside!!!!!!!!!!!!!!!11 SO I turned it off and took it out and put it
BACK in its box (the first 45 was a used 1940's RCA ST-14 type, all I
have left are two much older Cunningham 345 balloon type that are FAR
to expensive to even dream about buring up!!) The only thing I can
think is the cathode resistors are not working properly, maybe
getting too hot, maybe they need heatsinks???? I don't know. But that
is about all I can figure out! The resistance on them measures fine
UNTIL I put them in the circuit and run them for a few minutes. SO I
ordered some ceramic type 10 watters, once they get here I will
solder those in and try again!
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
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
I am not sure how the whole cathode bias thing is supposed to work.
BUT in general what is the most common cause of little or no bias
voltage?????????
This post is made possible by the generous support of people like you and our sponsors:
Top of Page ]
[ Contact Us ]
[ Support/Wish List ]
[ Copyright © 2025, Audio Asylum, all rights reserved ]
