Vacuum Tubes and 110 / 115 / 117 / 120 / 125 volts

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drcampbell

drcampbell

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The longevity of vacuum tubes, like that of incandescent lightbulbs, is strongly correlated with operating voltage & current. A little extra voltage can mean a large reduction in longevity.

Line voltages have been creeping up over the years.

Somehow, (D'Oh!) I never put two and two together until this week, when I stumbled upon this voltage-reducing box on eBay.

If you're a vacuum-tube aficionado and would like your antique electronics to last longer, powering them with a buck transformer that reduces line voltage down to the 110 volts they were designed for seems like a good idea. Maybe even 105 volts. (a'la the hundred-year lightbulb in the firehouse)

This is not an endorsement of Dynaco products.

Screenshot_20231201_21:43.jpeg
 
As a ham radio operator, and "boatanchor radio" collector I found that item interesting. I do have 1 "issue" with it..... how much current will it handle. I looked up "buck-boost " transformers to handle 5 amps, and found i needed 150 vA to buck or boost 5Amps. I dont see how the unit he has could handle more than a couple amps.

Great idea, but I think it might be a little small for some equipment.

Howard
 
This is the first I'm hearing of his so I'm wondering if it's a real problem. If I were worried, I would set the line voltage using a Variac to the name tag voltage rating then measure the filament voltage- voltages at each tube if a series string- to see if it's what the filament is rated to operate at. I'm willing to bet that what the name tag says is pretty much like now- the operating voltage is only a nominal voltage.

As an example, take automobile radios. They use 6 or12v filaments supplied right off the battery. In the case of a 12v electrical system, that voltage can go up to 14 or more volts depending on the generator charging the battery.

Also, keep in mind that, although these are tubes it doesn't mean that they are unobtanium. Common if not most tubes are still available at reasonable prices.

-Hal
 
It would be cheaper to take total of the filament voltages and current, then calculate a small resistor to drop the remaining voltage (most likely about a 100 ohm 5 watt resistor). Think even with inflation you should be able to find one for less than $90.
 
RCC1 said:
It would be cheaper to take total of the filament voltages and current, then calculate a small resistor to drop the remaining voltage (most likely about a 100 ohm 5 watt resistor). Think even with inflation you should be able to find one for less than $90.
Click to expand...
In large power tubes, such as the ones still used in older FM broadcast transmitters, the most common filament voltage control is a big honkin' rheostat. In these tubes, adjusting the voltage properly to the thoriated tungsten filament grid can increase tube life by many hundreds of hours. If a tube's filament is rated at 7 volts, the voltage is set at 7 volts and run that way for 100 to 200 hours. Then, the voltage is reduced to the point just above where a noticeable decrease in power output is seen, which can be 6.4 to 6.6 volts. It then runs that way until the transmitter can't make full power output, then the filament voltage is slightly raised until full power is made again.
 
Still have a 1950 era 5 tube AM FM radio in my garage. Still works.Some of the tubes are 35W4 ( rectifier tube ) , 50C5 and something like 12AV6. Smart engineering how the five heater tubes are in series and connected to the 120 VAC line thus saving a transformer. Back in the 1980's while working in a large candy plant they still had level controls & metal detectors that had tubes.Both devices had a tendency to drift after calibration. Now retired and thinking about purchasing a tube radio kit to assemble.
 
Largest tube I ever dealt with was the 4CX15000A used in a Harris FM-20 ( 20 kW) transmitter


Filament was 6.3 volts at a whopping 160 amps, leads from filament transformer to tube socket were # 2 welding cable (to get the flexibility needed to hook it up.

Plate volts were 7800 at 2.9 amps for 17.5 kW output.........separate 3' W x 6' L x 5' H cabinet for the HV supply
alone.
 
garbo said:
Still have a 1950 era 5 tube AM FM radio in my garage. Still works.Some of the tubes are 35W4 ( rectifier tube ) , 50C5 and something like 12AV6. Smart engineering how the five heater tubes are in series and connected to the 120 VAC line thus saving a transformer. Back in the 1980's while working in a large candy plant they still had level controls & metal detectors that had tubes.Both devices had a tendency to drift after calibration. Now retired and thinking about purchasing a tube radio kit to assemble.
Click to expand...
50C5, 35W4, 12BA6, 12BE6, 12AV6 was the usual line-up
 
I can recall electronic tubes. Just. These were in a paper mill with thyratrons up to 300V dc. I moved with the times since.
 
hbiss said:
This is the first I'm hearing of his so I'm wondering if it's a real problem. If I were worried, I would set the line voltage using a Variac to the name tag voltage rating then measure the filament voltage- voltages at each tube if a series string- to see if it's what the filament is rated to operate at. I'm willing to bet that what the name tag says is pretty much like now- the operating voltage is only a nominal voltage.

As an example, take automobile radios. They use 6 or12v filaments supplied right off the battery. In the case of a 12v electrical system, that voltage can go up to 14 or more volts depending on the generator charging the battery.

Also, keep in mind that, although these are tubes it doesn't mean that they are unobtanium. Common if not most tubes are still available at reasonable prices.
Click to expand...
The seller uses the excessive B+ voltage as his concern.
 
RCC1 said:
It would be cheaper to take total of the filament voltages and current, then calculate a small resistor to drop the remaining voltage (most likely about a 100 ohm 5 watt resistor). Think even with inflation you should be able to find one for less than $90.
Click to expand...
Many collectors prefer to preserve their antiques in original condition. Which doesn't preclude using a voltage-dropping power resistor external to the device instead of a buck transformer.

garbo said:
Still have a 1950 era 5 tube AM FM radio in my garage. Still works.Some of the tubes are 35W4 ( rectifier tube ) , 50C5 and something like 12AV6. Smart engineering how the five heater tubes are in series and connected to the 120 VAC line thus saving a transformer. ...
Click to expand...
The original motivation for not using a transformer wasn't savings; it was so the radio could operate on either 110v AC or 110v DC.
 
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