Best Regards,
Bill Grimm
Is your theory that somehow the catastrophic failure of one tube zapped the all the others? I don't visualize why an H-K short on one tube would take out a bunch more.
I like your hypothetical scenario, but I wonder if that unknown repairman, faced with a seemingly unfixable radio, went ahead and stuffed in as many dud tubes he had laying around - and returned it to the owner as beyond repair or put sold it to you in Michigan?
Actually, I have bought TRFs with all the '01As present - the seller said he had no way to test, so sold "as is." Then, I find that all the tubes are duds. Tell me how that can happen! The only "honest" expanation is that after one tube's filament blew, the owner kept the set energized (with a battery?) until another tube blew, etc., etc.
:Hi All,
: Rarely do I find tubes to be much of a problem in these radios. I used to be fond of saying that
:I have replaced more tube sockets than tubes. That was before my latest Zenith Y723, a 1955 vintage AM/FM.
: I picked it up in Michigan last year. The guy said he saw a white cap tube inside and closed it back up. When I eventually opened it, it presented vivid evidence of catostrophic failure.
: The radio has 7 tubes, 6BJ6, 12AT7, (2) 12BA6, 12AU6, 19T8, and 35C5. Rectification was done by a selenium rectifier, which I routinely replace. The 35C5 had shattered. A glob of hardened metal on its remains idicated that the shattering had been violent. The white capped, cracked tube was the 6BJ6. I took this as an indication that something really bad had happened in the filament circuit.
: Troubleshooting with an ohm meter yeilded the fact that the the filament circuit was grounded at the 12AT7. Looking at the schematic would then tell the tale of how the 35C5 and the 6BJ6 got in their wretched condition.
: When I opened the radio I noticed two other odd things. First, the speaker was disconnected. In
:this radio the only way to get the chassis out is to disconnect the speaker. Whoever put this radio back together knew that it was not going to work, so he didn't bother the connect the speaker. Second, eventhough the 35C5 had shattered there was no glass in the cabinet, nor any indication in the cabinet that an explosion had happened inside it.
: The filament ground at the 12AT7 turned out to be a hard H-K short between the cathode at pin 8 and the
:filament center lead, pin 9 in the Hytron 12AT7. I say hard short because the filament didn't have to be on for the short to be read by the ohm meter. The Hytron 12AT7, moreover, looked almost new. From this I suspect the following scenario. A repairmant takes out the chassis and puts a bench speaker on it. He finds that the 12AT7 was bad. He throws it out and get a the new Hytron 12AT7 out of stock an put it in, not knowing it has a short. Fireworks!! He puts it back in the case and says, "I am not messing with tube radios ever again!"
: There were two other bad tubes in the radio, too. The 12AU6 flashed on my tube tester. The tester had just tested the only good 12BA6, so it was set properly.
: Out of 7 tubes, 5 were bad, and not marginal, either, bad. I have never seen that before.
: Other than the devastation wrought upon the tubes, it was a routine restoration. The radio now looks great and plays wonderfully on an NOS 35C5.
: I know heat can reek havoc on tubes, but Michigan isn't known for hot weather.
:
:Best Regards,
:
:Bill Grimm
:
In my limited (18mos) experience with this old-radio-hobby I too feel that hardly ever is a tube the problem. I have almost never had to toss one away unles the filament is shot... and then if it's a 35Z5 I can usually find a use for it later in a radio that doesn't use a dial lamp.
However a few weeks ago I was working on an Air King 511-512
http://www.nostalgiaair.org/PagesByModel/458/M0000458.pdf
and it had very low volume even after recapping etc. Replacing the 12SQ7GT solved that problem.
.. I was really surprised and I put the questionable tube back in agin to make sure and sure enough the low volume was very apparent again... so I shrugged my shoulders, put the weak tube on the back of my bench and almost forgot about it.... That is until yesterday.
I have been restoring a nice little Bakelite six tube RCA 76ZX11.
http://www.nostalgiaair.org/PagesByModel/130/M0015130.pdf
All the metal tubes were pretty well rusted as well as areas of the chassis. After sanding the cleaning up the chassis and finishing the restore.... I tried sanding and repainting the metal tubes... well they look "OK" but not great so I decided to see what else I had laying around near by. Sure enough I found that "supposedly WEAK" 12SQ7GT and stuck it in to replace the rusty metal 12SQ7 ...expecting the volume to drop of just as in the Air King... but lo and behold it worked just great!!??
The only noiticable schematic difference is that the Air King ties both diodes together and The RCA does not.
I haven't dug out my tube tester yet to see what's really up.
.. but this just confirms my belief that tubes hardly ever go bad.. and even when you think they do.. they may still work well in another radio.
In fact I also had a 50L6 on the bench too that I had tagged as causing distortion ....( now I never tag a tube unless I've really proven the symptom to myself ) and when I put that "known-bad" 50L6 in this RCA in place of the 35L6 as an experiment.... No problem!!
... and no distortion at all either!
.... So unless the slightly lower filament voltage is responsible for better performance... again, I believe I should never toss a tube unless it blows up!!...lol
Hi Doug, Peter, and Marv,
I think the bad 12AT7 took out the 6BJ6 and 35C5 bacause the short in the 12AT7 shorted out the filaments in all tubes further down the filament string. The result was 120 VAC across the filaments of the 35C5 and 6BJ6, ka-boom.
I had a similar tube problem to Peter's in a Zenith 5S127. It played great with great sensitivity for 20 minutes. Then the sensitivity dropped. This wasn't noticable on local stations, but very noticable on weak signals. The problem turned out to be a bad 6K7G IF tube. Reluctant to toss an engraved base Zenith tube away, I kept it and tried it on another 5S127. Sure enough, the same problem happened.
I suspected a shrted filament, however, I tested it and it will draw 284 ma. all day long.
I am a little reluctant to try a tube that has become suspect. They can develop some subtle or sometimes intermittant problems. Usually tubes are cheap, too.
Best Regards,
Bill
A short in one filament can cause higher voltage across the others. This usually only takes out one other tube filament.
I've seen cases where a set of 01A's have open filaments. This will happen if high voltage, with enough current, is connected to the filament line.
Tubes testing weak can operate in a radio.
Most of the time when more than one dud is in a radio someone just filled sockets.
Norm
::
::::Hi All,
:::: Rarely do I find tubes to be much of a problem in these radios. I used to be fond of saying that.
::
::
::In my limited (18mos) experience with this old-radio-hobby I too feel that hardly ever is a tube the problem. I have almost never had to toss one away unles the filament is shot... and then if it's a 35Z5 I can usually find a use for it later in a radio that doesn't use a dial lamp.
::However a few weeks ago I was working on an Air King 511-512
::http://www.nostalgiaair.org/PagesByModel/458/M0000458.pdf
:: and it had very low volume even after recapping etc. Replacing the 12SQ7GT solved that problem.
::.. I was really surprised and I put the questionable tube back in agin to make sure and sure enough the low volume was very apparent again... so I shrugged my shoulders, put the weak tube on the back of my bench and almost forgot about it.... That is until yesterday.
::I have been restoring a nice little Bakelite six tube RCA 76ZX11.
::
::http://www.nostalgiaair.org/PagesByModel/130/M0015130.pdf
::
::All the metal tubes were pretty well rusted as well as areas of the chassis. After sanding the cleaning up the chassis and finishing the restore.... I tried sanding and repainting the metal tubes... well they look "OK" but not great so I decided to see what else I had laying around near by. Sure enough I found that "supposedly WEAK" 12SQ7GT and stuck it in to replace the rusty metal 12SQ7 ...expecting the volume to drop of just as in the Air King... but lo and behold it worked just great!!??
::The only noiticable schematic difference is that the Air King ties both diodes together and The RCA does not.
::
::I haven't dug out my tube tester yet to see what's really up.
::
::.. but this just confirms my belief that tubes hardly ever go bad.. and even when you think they do.. they may still work well in another radio.
::
::In fact I also had a 50L6 on the bench too that I had tagged as causing distortion ....( now I never tag a tube unless I've really proven the symptom to myself ) and when I put that "known-bad" 50L6 in this RCA in place of the 35L6 as an experiment.... No problem!!
::... and no distortion at all either!
::.... So unless the slightly lower filament voltage is responsible for better performance... again, I believe I should never toss a tube unless it blows up!!...lol
::
::
:
:Hi Doug, Peter, and Marv,
: I think the bad 12AT7 took out the 6BJ6 and 35C5 bacause the short in the 12AT7 shorted out the filaments in all tubes further down the filament string. The result was 120 VAC across the filaments of the 35C5 and 6BJ6, ka-boom.
: I had a similar tube problem to Peter's in a Zenith 5S127. It played great with great sensitivity for 20 minutes. Then the sensitivity dropped. This wasn't noticable on local stations, but very noticable on weak signals. The problem turned out to be a bad 6K7G IF tube. Reluctant to toss an engraved base Zenith tube away, I kept it and tried it on another 5S127. Sure enough, the same problem happened.
: I suspected a shrted filament, however, I tested it and it will draw 284 ma. all day long.
: I am a little reluctant to try a tube that has become suspect. They can develop some subtle or sometimes intermittant problems. Usually tubes are cheap, too.
:
:Best Regards,
:
:Bill
As far as the diodes are concerned, the detector circuit in a regular superheterodyne never draws enough current to be affected by a weak tube. The problem is almost always in the triode section.
Also, different radios use different voltages. Higher voltages may work better with weak tubes than low ones.
Finally, be sure that you replace all leaky condensers and drifted resistors. Condensers cannot have any leakage what-so-ever (not even in the millions of ohms). I have had radios that seem to get better when a better tube is put in, long before I discovered how critical it is to have good condensers (even more than having proper value resistors). Then I replaced all of the condensers in these radios with fresh leak-free ones, and now even the weak tubes, that caused trouble before, will work well now.
Thomas
I guess some things are sort-of unexplainable. Typically, though, if a circuit has lower impedances, fluctuations in tube conduction due to signal fluctuations on the grid will have less of an effect on the entire circuit (less of a voltage swing in the circuit). This is why lower impedances normally reduce gain. They'd further reduce gain with a weak tube.
T.
So that did not point to a defective socket.
The surprise came when I put the questionable tube into another radio and it worked fine.
Best Regards,
Bill Grimm
marv
:Hi All,
: Rarely do I find tubes to be much of a problem in these radios. I used to be fond of saying that
:I have replaced more tube sockets than tubes. That was before my latest Zenith Y723, a 1955 vintage AM/FM.
: I picked it up in Michigan last year. The guy said he saw a white cap tube inside and closed it back up. When I eventually opened it, it presented vivid evidence of catostrophic failure.
: The radio has 7 tubes, 6BJ6, 12AT7, (2) 12BA6, 12AU6, 19T8, and 35C5. Rectification was done by a selenium rectifier, which I routinely replace. The 35C5 had shattered. A glob of hardened metal on its remains idicated that the shattering had been violent. The white capped, cracked tube was the 6BJ6. I took this as an indication that something really bad had happened in the filament circuit.
: Troubleshooting with an ohm meter yeilded the fact that the the filament circuit was grounded at the 12AT7. Looking at the schematic would then tell the tale of how the 35C5 and the 6BJ6 got in their wretched condition.
: When I opened the radio I noticed two other odd things. First, the speaker was disconnected. In
:this radio the only way to get the chassis out is to disconnect the speaker. Whoever put this radio back together knew that it was not going to work, so he didn't bother the connect the speaker. Second, eventhough the 35C5 had shattered there was no glass in the cabinet, nor any indication in the cabinet that an explosion had happened inside it.
: The filament ground at the 12AT7 turned out to be a hard H-K short between the cathode at pin 8 and the
:filament center lead, pin 9 in the Hytron 12AT7. I say hard short because the filament didn't have to be on for the short to be read by the ohm meter. The Hytron 12AT7, moreover, looked almost new. From this I suspect the following scenario. A repairmant takes out the chassis and puts a bench speaker on it. He finds that the 12AT7 was bad. He throws it out and get a the new Hytron 12AT7 out of stock an put it in, not knowing it has a short. Fireworks!! He puts it back in the case and says, "I am not messing with tube radios ever again!"
: There were two other bad tubes in the radio, too. The 12AU6 flashed on my tube tester. The tester had just tested the only good 12BA6, so it was set properly.
: Out of 7 tubes, 5 were bad, and not marginal, either, bad. I have never seen that before.
: Other than the devastation wrought upon the tubes, it was a routine restoration. The radio now looks great and plays wonderfully on an NOS 35C5.
: I know heat can reek havoc on tubes, but Michigan isn't known for hot weather.
:
:Best Regards,
:
:Bill Grimm
: