It has really nice deep rich sound.
Every cap certainly needed changing ...but before I started on the paper/wax caps I first just replaced the filter caps and started listening to it.
Well after about 3-5 mins the audio would just start to fade away. ... Hummm nice easy trouble shooting practice I figured.
... I'll just locate the offending leaky paper cap and watch how nice it plays then... lol
I figured I should be able to find it easy.
Well the first thing I noticed was that cathode voltage drop on R20 the 68 ohm cathode bias resistor for the two 25L6 ouput tubes was drifting higher.. from about 5.5v to 7.5v then the audio faded away at that level.
So I figured that it must be drawing too much current and the offender was probably the .02uf tone caps on the plates...
so I changed them..... but same problem... humm.
Well anyway.. I kept replacing EVERY possible cap in the entire audio section that could possibly even remotely cause this... and nothing changed the results.
It was alright though because after all... I was replacing all the caps anyway... buy WHY was that cathode voltage always drifting up to 7.5 volts and killing the audio???
I changed every cap there was in the entire radio... all to no avail.
Even changed that 68ohm carbon cathode resistor to a film type.
( grasping for straws)
Well when there was nothing left.... I said...
all my logic tells me that there must be a short or a leak in one of the two 25L6 output tubes. Right???
Sure enough... changed both tubes and that cathode voltage stayed STABLE as a rock.
Ok... so why didn't I even suspect the tubes at any time?
I don't know... guess they just rarely ever are the cause.
Well I don't know if I learned much.. but I'm just ranting about it... lol
Well rant on. Timely piece. I spent hours yesterday doing about the same on a Philco 41-260. I replaced good micas, some very high reading resistors, and couldn't get the radio to stay on without cutting off, yet lots of static type noise. I left it for dinner. Went back out around 10, checked over my list of things done for readings on the tubes. The 7A4/XXL read just a bit over the average reading. I decided to try another one. Now loud and clear. End of my rant.
Bill VA
Forgot, two XXL tubes, it was the oscillator one.
Bill VA
I wonder what causes a tube to do that.
Doug
:Well when there was nothing left.... I said...
:all my logic tells me that there must be a short or a leak in one of the two 25L6 output tubes. Right???
:
:Sure enough... changed both tubes and that cathode voltage stayed STABLE as a rock.
:
Strange, too, that your cathode voltage increases as the sound goes bad. I wonder what would cause that to happen within the tube. Could it possibly be that the control grid is heating to emission, which would cause its negative bias to be given off? It takes a lot to cause emission with a metallic object that isn't oxide coated, but perhaps this is happening. If the tube was gassy, it'd be faulty all the time, I'd think. Maybe not.
I have had oscillator tubes that'd slowly fade out. This was due to a weak cathode, though, and wouldn't likely cause an increase in cathode current. These were typically 1LA6 tubes, which I'd bombard with the high B current setting of my tube tester, which would then rejuvenate them for a while. Incidently, I did this to rejuvenate the '27 and '45 tubes in my Majestic, which play quite well. They were all in the replace region when I got them. I play the radio almost every day for many hours at a time. I start with too high a filament voltage and turn the tester selector to #4 (200 volts, high current, whatever it is....the setting isn't used often except for H-V rectifieres and eye tube targets). Then I turn the heater voltage back to normal and run until I get the highest reading possible. Sometimes I turn down below normal heater voltage (possibly off, and allow cathode to cool while holding merit button) to cause the cathode to spark. Then I return heater voltage to normal, and when I achieve the highest reading possible at #4 with normal heater voltage, I switch the selector to #3 and proceed for the highest reading. Then I test the tube normally. This doesn't always work.
T.
OK.. update:
I replaced the two tubes one at a time and found the culprit. I also noticed a note I had written on the tube down by the base... "leaky" .... lol
Why didn't I see that lat night at 4am..??
.. Also why did I choose to have the tube in my spares drawer?
I think because some tubes fail only in some radios but seem to perform perfectly in others.
I'll put this one back in my tester later and see why I thought it was "leaky"
So because there was one missing output when I started this re-cap, the questionable or "leaky" 25L6 was the one I put in there.
Tonight...I put that questionable tube in my Precision model 10-20 tube tester... to see what's going on.
With all the settings set properly, the tube seemed to simply show performance on low side end (20% into) the green scale.. (compared with other used but good working ones that showed closer to the low side of middle (40%) of the green.)
There didn't seem to be any "hot cathode" leaks initially.
So I cranked up the filament to 35v for a while then 50 volts for a min or so.
During that time there did appear to show a leak at pin 5 the grid. As I reduced the filament back to 25-volts eventually that leak didn't show again either.
Anyway afterwards I put that tube back into the circuit in tube position 5 ( that's the output NOT fed by the inverter)
And sure enough again the audio slowly faded as the cathode voltage crept up toward 7 v.
So I tried that "bad" tube in tube position 6 and while in that position, even though the cathode voltage elevated slightly, it never failed and the audio performed properly for about 1/2 hour.
Curious about this I returnred it to position 5 again and began taking grid voltage readings ( relative to the joint cathode resistor junction)
On the GOOD tube in position 6 the grid voltage was a steady -5.6 volts.
The BAD tube by comparison started out with -5.6 volts on the grid but slowly started drifting upwards toward a less negative voltage and eventualy ended up at only about -0.5 volts as the audio volume slowly lowered too.
I don't know why or what contributes to that grid tending to drift more positively.
If it is, in-fact, due to a cathode-to-grid leakage.. then I guess it does make sense that the grid becomes closer to the same potential as the cathode as the tube heats up... but that would be driving the tube closer to saturation? Right?
... not cut-off
So then why don't I hear distortion from over driving instead of slowly fading audio as I would expect??
Wouldn't slowly fading audio point more toward a cut-off situation?
Huh huh huh?
Also, if the grid wires physically bend from the heat, they could be touching the cathode, which would make the grid more positive. If the grid is being measured with respect to the cathode, it'll be less negative. With respect to the chassis, it'll be more positive than the chassis. I have had tubes with grids that touch the cathode once hot. I have quite a few 6SN7s with this problem (the short light will light somewhat once the tube gets nice and hot). What I've done to them with much success is set my tube tester's selector switch to number 4, which is the highest current setting. I then quickly press the merit button a few times. Sparks fly and the offending part of the grid/cathode is burned away. The tube often functions normally afterward.
I do know that when I have a radio with leaky bypass condensers (such as the one feeding the output tube grid), the audio will slowly get quieter and quieter, and will sound distorted. Perhaps the same thing is happening due to the above, or perhaps the signal is actually being shorted out. You'd think that with the grid going positive, if an actual short wasn't occurring, that the signal would just get distorted as the grid went more positive, but I guess not.
thanks Thomas... that's my thoughts too.. but doesn't make sense.. right?
The grid in those tubes is really close to the cathode.
T.
Problem could be leakage due to a lot of usage. It may also be a gassy tube? Can you see a blue glow inside?
When a tube is gassy it ionizes. This causes the blue glow and acts like a leakage path between cathode and plate. Since the grid is between the two it's positive in relation to cathode.
Norm
:Well, maybe the grid is actually shorting out. Put the tube in your tester with the levers set appropriately. Run the tube for the length of time that it takes to get quiet. Run with the line adjustment turned up all the way, so that the tube gets nice and hot. Watch the shorts light.
:
:The grid in those tubes is really close to the cathode.
:
:T.
T.