Tube Markings 4cx300a, how old?

[Naysayer]

I got the volts & currents squared away. About to install two “new” 4cx300a I got from 2 different eBay Sellers. I used an old pair of 300y's to do rough tuning.

Filament V tweaking (6v) had me learn the Buck-Boost wiring of the AL-572 transformer, it provides more V options than I expected. Previously, I followed advice from someone already familiar (Thank you). I’m ready to install the “new” 300a’s paused to read the tube markings. One has no scratches and appears unused. The other tube shows light pin-tab marks as if it was once in a socket but otherwise appears new (no anode marks or no heat tarnish, etc). Pictures below.

Stamped in the metal: CHSA2285 and THJA0089. Printed in ink: “87 38" and “0234". Best guess is 1985 & 1989? There’s also a graphic printed on one tube that I don't recognize. I thought maybe "CPI" but CPI was not founded until 1995. IDK, relics perhaps. Anyway, my sockets are getting cold. Time to start the burn-in period.

neil, nyc

 

[nomadradio]

"8738" on one tube is the 38th week of 1987. Don't know about the other tube. But that's what you're looking for on Eimac tubes, four digits without an alphabetic prefix.

73

 

[Naysayer]

Thank you,
I've been running the Fils for several hours. Had to rewire blower and chassis fans so I could adjust speed, noisy amp for sure. A few hours to go then I'm back to RF testing

 

[SuperLid]

Eimac 1987, CPI 2002.

 

[nomadradio]

Wow! Only 15 years apart. With any luck they'll balance the load well enough.

73

 

[linearone]

Thank you,
I've been running the Fils for several hours. Had to rewire blower and chassis fans so I could adjust speed, noisy amp for sure. A few hours to go then I'm back to RF testing

Un popular opinion, all these hamsters who say you need to burn them in by running the filament for a magically divined length of time are wrong if the tube isn't conducting at some level it won't "getter" or do anything really.

 

[nomadradio]

This depends on the quality of the vacuum when the tube was made. I have seen 30 year-old Eimac-made 3-500Z that still had a perfect vacuum, better than a new chinese type. Surplus USSR tetrodes would show measurable gas current from the control grid when first hooked up, and show major reduction in gas current after cooking them with only the heater energized. The tube's heater is energized and a negative 100 Volts DC more or less is applied to the control grid. A 1-megohm resistor goes between the bias-voltage source and the tube's control grid. A voltmeter across this resistor will show you 1 Volt per microamp of gas leakage. We let the tube run like this until the leakage reading stops falling. At that point you have improved it all you can.

I have seen the suggestion that you apply screen-grid voltage next, to see if the grid leakage reading increases in response. I have never taken the process to that extreme, so I can't comment on what it will tell you.

As the old saying goes, "One accurate measurement is worth a thousand expert opinions."

73

 

[linearone]

This depends on the quality of the vacuum when the tube was made. I have seen 30 year-old Eimac-made 3-500Z that still had a perfect vacuum, better than a new chinese type. Surplus USSR tetrodes would show measurable gas current from the control grid when first hooked up, and show major reduction in gas current after cooking them with only the heater energized. The tube's heater is energized and a negative 100 Volts DC more or less is applied to the control grid. A 1-megohm resistor goes between the bias-voltage source and the tube's control grid. A voltmeter across this resistor will show you 1 Volt per microamp of gas leakage. We let the tube run like this until the leakage reading stops falling. At that point you have improved it all you can.

I have seen the suggestion that you apply screen-grid voltage next, to see if the grid leakage reading increases in response. I have never taken the process to that extreme, so I can't comment on what it will tell you.

As the old saying goes, "One accurate measurement is worth a thousand expert opinions."

73

It's interesting that tetrodes showed you an issue . Countless Russian triodes have been perfect for amp builders though. I literally popped open some 1980s gi6bs last week, hooked them up and keyed the box. I have an enormous amount of faith in communist tubes.