I'm working on this Sonora-KM450 (Firestone S-7403-4)
It's a relatively "standard" AA5 with a 35L6 output tube.
http://www.nostalgiaair.org/PagesByModel/328/M0006328.pdf
I've recapped it entirely and replaced all but 3 resistors.
It plays quite nicely... however I noticed that the 35L6 output tube seems to start drawing more and more current at it warms up.
It is definitely that particular tube because others I've swapped in are rock steady.
With no signal volume...and 115vDC on the plate The cathode sits at 6.5 volts at turn on but it slowly rises to 10.5v over the next few minuets.
That means ( with the 250 ohm cathode resistor) that the initial current is 26ma drifting up to about 42ma.
I can't detect any discernible distortion or anything.. and all seems generally fine but I know that the cathode voltage is spec-ed on the print at 6.5v... and it drew my attention only when I measured it high.
The tube specs claim that 40ma is normal with no signal ...to max at 41ma with signal.
Initially the grid measure -6.5v relative to the cathode but that drops to -3 or -4 after the current on the cathode rises.
Any other good 35L6 in that socket stays perfectly at 6.5v on the cathode.
So what is really going on here? ..is the tube starting to fail?
And ..btw... Why is the cathode resistor spec-ed at 250 ohms in this radio which can only normally draw 26ma.. compared to most other AA5s that use a 150 ohm cathode resistor?
Is there a malfunction of the tube?
I'm puzzled.. which is easy for my feeble lil brain.
:I've posted this question on ARF as well.. but for those who may not frequent that forum.. I'll repeat it here too.
:
:I'm working on this Sonora-KM450 (Firestone S-7403-4)
:It's a relatively "standard" AA5 with a 35L6 output tube.
:
:http://www.nostalgiaair.org/PagesByModel/328/M0006328.pdf
:
:I've recapped it entirely and replaced all but 3 resistors.
:
:It plays quite nicely... however I noticed that the 35L6 output tube seems to start drawing more and more current at it warms up.
:
:It is definitely that particular tube because others I've swapped in are rock steady.
:
:With no signal volume...and 115vDC on the plate The cathode sits at 6.5 volts at turn on but it slowly rises to 10.5v over the next few minuets.
:
:That means ( with the 250 ohm cathode resistor) that the initial current is 26ma drifting up to about 42ma.
:
:I can't detect any discernible distortion or anything.. and all seems generally fine but I know that the cathode voltage is spec-ed on the print at 6.5v... and it drew my attention only when I measured it high.
:
:The tube specs claim that 40ma is normal with no signal ...to max at 41ma with signal.
:
:Initially the grid measure -6.5v relative to the cathode but that drops to -3 or -4 after the current on the cathode rises.
:
:Any other good 35L6 in that socket stays perfectly at 6.5v on the cathode.
:
:So what is really going on here? ..is the tube starting to fail?
:
:And ..btw... Why is the cathode resistor spec-ed at 250 ohms in this radio which can only normally draw 26ma.. compared to most other AA5s that use a 150 ohm cathode resistor?
:
:Is there a malfunction of the tube?
:
:I'm puzzled.. which is easy for my feeble lil brain.
:
:I'm working on this Sonora-KM450 (Firestone S-7403-4)
:It's a relatively "standard" AA5 with a 35L6 output tube.
:
:http://www.nostalgiaair.org/PagesByModel/328/M0006328.pdf
:
:I've recapped it entirely and replaced all but 3 resistors.
:
:It plays quite nicely... however I noticed that the 35L6 output tube seems to start drawing more and more current at it warms up.
:
:It is definitely that particular tube because others I've swapped in are rock steady.
:
:With no signal volume...and 115vDC on the plate The cathode sits at 6.5 volts at turn on but it slowly rises to 10.5v over the next few minuets.
:
:That means ( with the 250 ohm cathode resistor) that the initial current is 26ma drifting up to about 42ma.
:
:I can't detect any discernible distortion or anything.. and all seems generally fine but I know that the cathode voltage is spec-ed on the print at 6.5v... and it drew my attention only when I measured it high.
:
:The tube specs claim that 40ma is normal with no signal ...to max at 41ma with signal.
:
:Initially the grid measure -6.5v relative to the cathode but that drops to -3 or -4 after the current on the cathode rises.
:
:Any other good 35L6 in that socket stays perfectly at 6.5v on the cathode.
:
:So what is really going on here? ..is the tube starting to fail?
:
:And ..btw... Why is the cathode resistor spec-ed at 250 ohms in this radio which can only normally draw 26ma.. compared to most other AA5s that use a 150 ohm cathode resistor?
:
:Is there a malfunction of the tube?
:
:I'm puzzled.. which is easy for my feeble lil brain.
:
:I'm working on this Sonora-KM450 (Firestone S-7403-4)
:It's a relatively "standard" AA5 with a 35L6 output tube.
:
:http://www.nostalgiaair.org/PagesByModel/328/M0006328.pdf
:
:I've recapped it entirely and replaced all but 3 resistors.
:
:It plays quite nicely... however I noticed that the 35L6 output tube seems to start drawing more and more current at it warms up.
:
:It is definitely that particular tube because others I've swapped in are rock steady.
:
:With no signal volume...and 115vDC on the plate The cathode sits at 6.5 volts at turn on but it slowly rises to 10.5v over the next few minuets.
:
:That means ( with the 250 ohm cathode resistor) that the initial current is 26ma drifting up to about 42ma.
:
:I can't detect any discernible distortion or anything.. and all seems generally fine but I know that the cathode voltage is spec-ed on the print at 6.5v... and it drew my attention only when I measured it high.
:
:The tube specs claim that 40ma is normal with no signal ...to max at 41ma with signal.
:
:Initially the grid measure -6.5v relative to the cathode but that drops to -3 or -4 after the current on the cathode rises.
:
:Any other good 35L6 in that socket stays perfectly at 6.5v on the cathode.
:
:So what is really going on here? ..is the tube starting to fail?
:
:And ..btw... Why is the cathode resistor spec-ed at 250 ohms in this radio which can only normally draw 26ma.. compared to most other AA5s that use a 150 ohm cathode resistor?
:
:Is there a malfunction of the tube?
:
:I'm puzzled.. which is easy for my feeble lil brain.
I would suspect the coupling cap. to 35L6 grid. Harold
It is certainly not the coupling to the 35L6GT. Your swap out of the 35L6GT of course proves that.
It sounds to me that your 35L6GT has a filament that progressively shorts while it heats. A while ago I had a radio in here where the 35C5 had shattered. I pulled out the filament and found several bad, brown, spots on the filament. I would have expected shorts in those places to be possible.
A while ago I obtained a group of poor quality 35C5 (I use 35C5 a lot). The sound quality was poor. I checked their filament currents and found them all to be close to spec, indicating that the filament wasn't the culprit on poor sound quality.
Your 35L6GT is most interesting in that it retains excellent performance while overloading the filament string. Its filament may not be the first to go while the radio is in use, too.
I used to check this kind of thing before releasing a radio into the wild, but got away from it since I wasn't finding problems. your story will have me going back to it.
Best Regards,
Bill Grimm
AND.. . . . with the supplied info on the aggressiveness of the situation being exacerbated by heat onset, the most likely cause will
be either a gassy tube or more likely grid emission, upsetting the micro 1st grid biasing level being presented to the tube and then
a proportional shifting of the class of operation of the tube and an increased power consumption.
Ref:
>
73's de Edd
The article about the 5881 is interesting. There is a pair of them up on eBay. I wonder if anyone knows they are delux 6L6's.
Your hypothesis of secondary emission doesn't fit the filament taking more current. Peter mentions that the biasing goes strange, but sound quality reamins good, too. How odd.
Could his 35L6GT have a shorted filament and secondary emission?
Best regards,
Bill Grimm
Right up to your usual proficient assistance.. thanks for helping to clear it all up and un-feebile-ize my brain a little bit.
Give me a dope slap. I misread Peter's post. By taking more current he meant cathode to filament flow, not filament.
I take it this is classic secondary emission.
Best Regards,
Bill Grimm
: Give me a dope slap. I misread Peter's post. By taking more current he meant cathode to filament flow, not filament.
:
: I take it this is classic secondary emission.
:
:Best Regards,
:
:Bill Grimm
Hi Bill:
I wasn't referring to "filament" at all.
The current flow from cathode to plate starts at 26ma and increases to 42 ma.
The cathode resistor is 250 ohms. The voltage drop across it starts out at +6.5 volts (26ma)... but the problem is that the voltage drop slowly increases to +10.5 volts ( 42ma)
:: Give me a dope slap. I misread Peter's post. By taking more current he meant cathode to filament flow, not filament.
::
:: I take it this is classic secondary emission.
::
::Best Regards,
::
::Bill Grimm
:
:
:Hi Bill:
:I wasn't referring to "filament" at all.
:
:The current flow from cathode to plate starts at 26ma and increases to 42 ma.
:
:The cathode resistor is 250 ohms. The voltage drop across it starts out at +6.5 volts (26ma)... but the problem is that the voltage drop slowly increases to +10.5 volts ( 42ma)
Give me a second dope slap. I meant to write cathode to plate flow, not cathode to filament.
You must have thought I was really lost.
Best Regards,
Bill Grimm
Thank you,
Vinny
:I've posted this question on ARF as well.. but for those who may not frequent that forum.. I'll repeat it here too.
:
:I'm working on this Sonora-KM450 (Firestone S-7403-4)
:It's a relatively "standard" AA5 with a 35L6 output tube.
:
:http://www.nostalgiaair.org/PagesByModel/328/M0006328.pdf
:
:I've recapped it entirely and replaced all but 3 resistors.
:
:It plays quite nicely... however I noticed that the 35L6 output tube seems to start drawing more and more current at it warms up.
:
:It is definitely that particular tube because others I've swapped in are rock steady.
:
:With no signal volume...and 115vDC on the plate The cathode sits at 6.5 volts at turn on but it slowly rises to 10.5v over the next few minuets.
:
:That means ( with the 250 ohm cathode resistor) that the initial current is 26ma drifting up to about 42ma.
:
:I can't detect any discernible distortion or anything.. and all seems generally fine but I know that the cathode voltage is spec-ed on the print at 6.5v... and it drew my attention only when I measured it high.
:
:The tube specs claim that 40ma is normal with no signal ...to max at 41ma with signal.
:
:Initially the grid measure -6.5v relative to the cathode but that drops to -3 or -4 after the current on the cathode rises.
:
:Any other good 35L6 in that socket stays perfectly at 6.5v on the cathode.
:
:So what is really going on here? ..is the tube starting to fail?
:
:And ..btw... Why is the cathode resistor spec-ed at 250 ohms in this radio which can only normally draw 26ma.. compared to most other AA5s that use a 150 ohm cathode resistor?
:
:Is there a malfunction of the tube?
:
:I'm puzzled.. which is easy for my feeble lil brain.
:Thank you,
:Vinny
:
http://antiqueradios.com/forums/viewtopic.php?t=99947
Vinny
::Mr. Balazsy's, what is the address of the arf forum.
::Thank you,
::Vinny
::
:http://antiqueradios.com/forums/viewtopic.php?t=99947
The tube has gas, this is common for these output tubes. Just replace it.
Dave
I came to that conclusion last year when I first posted this .. so I'm curious.. how did you happen upon this thread from last year?
Did it somehow get re posted by error?
:
:I came to that conclusion last year when I first posted this .. so I'm curious.. how did you happen upon this thread from last year?
:
:Did it somehow get re posted by error?
Now that is a good question, I just opened the forum and there it was, I never noticed the dates. Maybe some time of a warp in the Internet continuum.
Dave