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 Post subject: What’s the scoop with tubes 35C5, 50C5, 50EH5?
PostPosted: Nov Mon 21, 2022 2:25 pm 
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I recently picked up a 60’s Panasonic “Matsushita” RE-784 tube radio that uses a 50C5 output tube. The radio worked, but the sound was distorted when it was working with the 50C5 and then it finally stopped working not long after acquiring the radio and I didn’t have another one, so I popped in a 35C5 and the old radio has never sounded better and no distortion is present like before.
I also tried a 50EH5 substitute and the distortion and muffled sound returned. The RE-784 calls for a 50C5 so why does it sound better with a 35C5?


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 Post subject: Re: What’s the scoop with tubes 35C5, 50C5, 50EH5?
PostPosted: Nov Mon 21, 2022 2:59 pm 
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Because the tube now has much higher filament voltage, and won't last long.

The first number is the filament voltage. They can't be interchanged. The tube filaments are in series, and need to add up to 110 or 120V. The output tube at 50V takes almost half of that.

The other tubes will see higher filament too.


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 Post subject: Re: What’s the scoop with tubes 35C5, 50C5, 50EH5?
PostPosted: Nov Mon 21, 2022 3:07 pm 
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The 50C5 uses a 150 ohm cathode resistor. This has to be changed to 62 ohms for a 50EH5 as it is a higher gain tube.


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 Post subject: Re: What’s the scoop with tubes 35C5, 50C5, 50EH5?
PostPosted: Nov Mon 21, 2022 4:36 pm 
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Thanks for the quick replies gents....I have a couple NOS 50C5 tubes on there way as we speak and I’m going to change out the old Matsushita.047uf 600v cap and the two smaller 5uf 25v caps at the same time. I have a couple 6.8uf 25v here and 4.7uf 35v here....which would you recommend for replacement?
Even as is....this 60’s radio still sounds stellar for never being touched or updated.


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 Post subject: Re: What’s the scoop with tubes 35C5, 50C5, 50EH5?
PostPosted: Nov Mon 21, 2022 11:21 pm 
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The scoop is that all of those tubes had a high failure rate, due to heater to cathode shorts. The 50's series tubes were worse than the 35's.

At any rate, that's why the 50C5 sounded bad---it's shorted

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 Post subject: Re: What’s the scoop with tubes 35C5, 50C5, 50EH5?
PostPosted: Nov Tue 22, 2022 9:50 am 
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tubes4life wrote:
The scoop is that all of those tubes had a high failure rate, due to heater to cathode shorts. The 50's series tubes were worse than the 35's.

At any rate, that's why the 50C5 sounded bad---it's shorted


Thanks William! I have some 50C5’s on the way...hopefully have the old Panasonic sounding good real soon!


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 Post subject: Re: What’s the scoop with tubes 35C5, 50C5, 50EH5?
PostPosted: Nov Tue 22, 2022 3:23 pm 
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tubes4life wrote:
The scoop is that all of those tubes had a high failure rate, due to heater to cathode shorts. The 50's series tubes were worse than the 35's.

At any rate, that's why the 50C5 sounded bad---it's shorted

And on that note, often those 50 volt filament tubes would not test shorted initially. But when in place in the radio, the hum might creep up after the set was on for a while. Used 50C5/50L6 tubes are always suspect.

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 Post subject: Re: What’s the scoop with tubes 35C5, 50C5, 50EH5?
PostPosted: Nov Tue 22, 2022 8:27 pm 
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Tubes of that type, as well as their octal and Loktal base counterparts were designed specifically for AC/DC radios where the 120-volt line was rectified and used for B+. Since the grid bias voltage is subtracted from the B+ voltage in such circuits, they tried to make AC/DC radio tubes work linearly with only a few volts of grid bias derived through a cathode resistor so most of the B+ could be applied to the plates. But when this is done, the tubes become extremely sensitive to any leakage in the coupling capacitors, leakage in the tubes themselves, and to the values of the grid return resistors. Very often the tubes themselves were not at fault. The coupling capacitors tended to go leaky which would bias the grids more positive. This causes the tubes to run very hot, slowly boiling occluded gas molecules out of the metal. The gas spoils the normal operation of the tube, resulting in distortion. But many owners just lived with it as long as they could and changed the tubes from time to time. Back in the day those tubes only cost a buck or so, while taking the radio to a shop and having the capacitor (and tube) properly replaced might cost several dollars.

I think a lot of AC/DC radios turn up today with gassy or weak 50L6s and 50C5s because after the tubes had been replaced two or three times, many owners decided to park the sets and buy new ones the next time they needed a new 50C5 or other output tube.

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 Post subject: Re: What’s the scoop with tubes 35C5, 50C5, 50EH5?
PostPosted: Nov Wed 23, 2022 10:46 am 
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Chris108 wrote:
Tubes of that type, as well as their octal and Loktal base counterparts were designed specifically for AC/DC radios where the 120-volt line was rectified and used for B+. Since the grid bias voltage is subtracted from the B+ voltage in such circuits, they tried to make AC/DC radio tubes work linearly with only a few volts of grid bias derived through a cathode resistor so most of the B+ could be applied to the plates. But when this is done, the tubes become extremely sensitive to any leakage in the coupling capacitors, leakage in the tubes themselves, and to the values of the grid return resistors. Very often the tubes themselves were not at fault. The coupling capacitors tended to go leaky which would bias the grids more positive. This causes the tubes to run very hot, slowly boiling occluded gas molecules out of the metal. The gas spoils the normal operation of the tube, resulting in distortion. But many owners just lived with it as long as they could and changed the tubes from time to time. Back in the day those tubes only cost a buck or so, while taking the radio to a shop and having the capacitor (and tube) properly replaced might cost several dollars.

I think a lot of AC/DC radios turn up today with gassy or weak 50L6s and 50C5s because after the tubes had been replaced two or three times, many owners decided to park the sets and buy new ones the next time they needed a new 50C5 or other output tube.

.
Thanks for the extra info Chris!
All the tubes inside the RE-784 looked original and were all branded Matsushita Japan. I’ve got the whole string of tubes replaced now except for the 50C5 tubes that I’m waiting on. I changed out the leaking .047 600v coupling cap yesterday and installed a couple new 6.8uf 25v caps as well. I popped the old 50C5 tube back in and it did work for a bit...the radio sounded much better with the new caps installed. The cathode resistor off the 50c5 was reading 135 ohms....would that be a little out of spec?
Here’s the leaky cap...


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 Post subject: Re: What’s the scoop with tubes 35C5, 50C5, 50EH5?
PostPosted: Nov Wed 23, 2022 1:14 pm 
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That coupling cap says ‘Oil’ but it is really paper in oil. Great caps when they were new, but after about 10 years they started going bad just like any other paper caps. That one is decidedly past its best used by date. The cathode resistor may have changed value due to constant overheating once the cap became leaky and drove up the grid bias. Unfortunately, there’s nothing you can do with the 50C5 except replace it. Once they get gassy there’s no going back, they will always produce distortion when they get hot.

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"Hell, there are no rules here--we're trying to accomplish something!"

Thomas A. Edison


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 Post subject: Re: What’s the scoop with tubes 35C5, 50C5, 50EH5?
PostPosted: Nov Wed 23, 2022 2:56 pm 
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BikenSwim wrote:
The 50C5 uses a 150 ohm cathode resistor. This has to be changed to 62 ohms for a 50EH5 as it is a higher gain tube.


Would it hurt anything to run the radio for now with the 50EH5 or should I just put it on the shelf til the 50C5 tubes show up?


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 Post subject: Re: What’s the scoop with tubes 35C5, 50C5, 50EH5?
PostPosted: Nov Wed 23, 2022 3:40 pm 
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Chris108 wrote:
That coupling cap says ‘Oil’ but it is really paper in oil. Great caps when they were new, but after about 10 years they started going bad just like any other paper caps. That one is decidedly past its best used by date. The cathode resistor may have changed value due to constant overheating once the cap became leaky and drove up the grid bias. Unfortunately, there’s nothing you can do with the 50C5 except replace it. Once they get gassy there’s no going back, they will always produce distortion when they get hot.


Before I recapped it would take forever for FM signal to begin....now it’s about 10-20 seconds after start up, so I’m thinking the new caps helped quite a bit. I will give it a good run once the 50C5 tubes show up, maybe I’ll have to go back in and change the cathode resistor at some point and get it back up to 150 ohm. I’m still getting a bit of hum even with the volume on low.
Glad I got the new caps in there...the smaller ones were actually cracked open at the base leads!


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 Post subject: Re: What’s the scoop with tubes 35C5, 50C5, 50EH5?
PostPosted: Nov Wed 23, 2022 4:07 pm 
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Johnnycanso wrote:
BikenSwim wrote:
The 50C5 uses a 150 ohm cathode resistor. This has to be changed to 62 ohms for a 50EH5 as it is a higher gain tube.


Would it hurt anything to run the radio for now with the 50EH5 or should I just put it on the shelf til the 50C5 tubes show up?


The 50EH5 normally uses a 62 ohm cathode resistor. With 135 ohms, it would draw less plate current and run cooler. It might distort at high volume but it won't be harmed, and it won't harm the radio. Enjoy! Change to 150 ohms when you get the 50C5 tube.

Replacing the PIO capacitors will help, I have a Kenwood tuner that had them, and the tuning meter was sluggish. The PIO caps have "dielectric absorption" and hold a charge. You can short it out for a moment then measure again, the voltage charge is still there! After replacing the caps, the meter moves quickly as I tune, and the tuner sounds great.


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 Post subject: Re: What’s the scoop with tubes 35C5, 50C5, 50EH5?
PostPosted: Nov Wed 23, 2022 5:36 pm 
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Well....It’s outta commission now, tuner string just broke. Tried a drop of glue and no luck, even tried to restring with floss and no luck. On the shelf it goes! :(


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 Post subject: Re: What’s the scoop with tubes 35C5, 50C5, 50EH5?
PostPosted: Nov Wed 23, 2022 6:14 pm 
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Do you have any fishing line? That is the right size. Your photo will help, as even the direction you wrap the line around the shaft is important.


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 Post subject: Re: What’s the scoop with tubes 35C5, 50C5, 50EH5?
PostPosted: Nov Wed 23, 2022 9:36 pm 
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BikenSwim wrote:
Do you have any fishing line? That is the right size. Your photo will help, as even the direction you wrap the line around the shaft is important.


If I don’t i will get some this week...thanks for the tip!


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 Post subject: Re: What’s the scoop with tubes 35C5, 50C5, 50EH5?
PostPosted: Nov Thu 24, 2022 5:46 pm 
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BikenSwim wrote:
Do you have any fishing line? That is the right size. Your photo will help, as even the direction you wrap the line around the shaft is important.


Thanks so much...grabbed some 50Ib braided line this morning at Walmart.
The 50Ib line is a little thicker and easier to work with when knotting and restringing. It Worked perfect the first time! Happy to say the Panasonic radio is back up and running.


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 Post subject: Re: What’s the scoop with tubes 35C5, 50C5, 50EH5?
PostPosted: Nov Thu 24, 2022 6:05 pm 
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Great news, and yet another reason for being Thankful on Thanksgiving.

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 Post subject: Re: What’s the scoop with tubes 35C5, 50C5, 50EH5?
PostPosted: Nov Thu 24, 2022 6:14 pm 
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Don Cavey wrote:
Great news, and yet another reason for being Thankful on Thanksgiving.


It is a great day all around Don for sure!
Enjoy


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 Post subject: Re: What’s the scoop with tubes 35C5, 50C5, 50EH5?
PostPosted: Nov Fri 25, 2022 2:56 am 
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Johnnycanso wrote:
BikenSwim wrote:
Do you have any fishing line? That is the right size. Your photo will help, as even the direction you wrap the line around the shaft is important.


Thanks so much...grabbed some 50Ib braided line this morning at Walmart.
The 50Ib line is a little thicker and easier to work with when knotting and restringing. It Worked perfect the first time! Happy to say the Panasonic radio is back up and running.


Congratulations! Nice job! All Panasonic products are the best, this will last a very long time.


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