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NOS vs well-used 4cx tubes

Naysayer

Well-Known Member
Mar 6, 2020
177
135
53
New York
I’m a newbie when it comes to ceramic tubes. The 4cx300 variety is what I’m trying to use. I was fortunate to get several used 4cx300y tubes from a generous Ham out West for little more than shipping (callsign added later when I dig it out) and I purchased some NOS tubes from eBay: Some sealed in cans and some in unsealed boxes.

For this Post, I tried two eBay tubes (different Sellers) sold as NOS 4cx300a. They came in the boxes with the 1-piece, yellow plastic insert. One 4cx300a had no marks whatsoever and looked pristine. The other had light marking on the tube pin/tabs but otherwise showed no sign of heating or anode clips, etc. Almost pristine.

The two used 300y tubes were well-worn with obvious signs of heating, pin/tab and anode clamp scratches, heavily tarnished, hard to read the text, etc. I used the worst pair of 300y tubes for first Power-up attempts. After I had about 130W from an old CB and addressed several issues that arose, I thought it was time to try the NOS 300a tubes. I switched the Bias, Screen and Plate voltages and began the conditioning process I read about online.

I found articles advising to run only the filaments w/ blower on low for at least 12 hours when installing new tubes that were stored for long periods. It’s a new build and requires babysitting as this would be the first long stretch of Power On. I managed about 6 hours uninterrupted and a few more hour-long periods with just the filaments On, tube blower running low. Admittedly, a lot less than 12 hours/overnight. This process required more than a week as cooling issues required revising the 40mm fans moving air underneath the chassis.

Long story short, the NOS tubes made zero output. Like dead, wtf? I put the old 300y’s back in and they still produce. Stop, backup... re-verify the different settings for the 2 tube types and do it all over again. Same damn result. Another filament conditioning only stretch? Granted, 5-6 hours is far from overnight but completely dead? Time to take out my can opener? IDK Have one of each and need 1 more can but buying NOS is beginning to feel like buying a lottery ticket.
neil, n2eye
NYC
 

like buying a lottery ticket.
Roger that. I was lucky enough to buy a purpose-built tester for the 150/250B type tubes when its builder exited the amp-building business. Buying the cheapest "can't test" tubes on fleabay has produced good tubes and total duds both. Never had one in-the-can do this, however. I remember a tester for the 300-family tubes showing up either at Fair Radio or fleabay a while back, but just what it was called escapes me. Would allow a search, at least.

And if your amplifier is protected from overloads well enough, it will serve as a tester. Really sounds as if it just did.

Random thought. These aren't airborne-equipment versions of that tube that want 26.5 Volts on the heater, are they? If so, a 6.3-Volt heater supply would not wake them up one tiny bit.

Ruined a perfectly-good pair of 4CX250B plugging them into a box that was built to take advantage of then-cheap airborne versions. 26.5 Volts popped the 6-Volt heaters pronto. The airborne "F"-suffix tubes had gone from cheap to expensive in the decades since it was built. Ended up putting a 6-Volt heater transformer on the back of it so he could use the conventional tube.

Just looked up the 4CX300Y/8561. Says it will dissipate 400 Watts. Heater is 6.0 Volts it says.

Dern. Can't find an airborne-voltage type like that listed at all.

73
 
Roger that. I was lucky enough to buy a purpose-built tester for the 150/250B type tubes when its builder exited the amp-building business. Buying the cheapest "can't test" tubes on fleabay has produced good tubes and total duds both. Never had one in-the-can do this, however. I remember a tester for the 300-family tubes showing up either at Fair Radio or fleabay a while back, but just what it was called escapes me. Would allow a search, at least.

And if your amplifier is protected from overloads well enough, it will serve as a tester. Really sounds as if it just did.

Random thought. These aren't airborne-equipment versions of that tube that want 26.5 Volts on the heater, are they? If so, a 6.3-Volt heater supply would not wake them up one tiny bit.

Ruined a perfectly-good pair of 4CX250B plugging them into a box that was built to take advantage of then-cheap airborne versions. 26.5 Volts popped the 6-Volt heaters pronto. The airborne "F"-suffix tubes had gone from cheap to expensive in the decades since it was built. Ended up putting a 6-Volt heater transformer on the back of it so he could use the conventional tube.

Just looked up the 4CX300Y/8561. Says it will dissipate 400 Watts. Heater is 6.0 Volts it says.

Dern. Can't find an airborne-voltage type like that listed at all.

73
Wonderful feeling when meter swings right 1st time - Good News!
Got it working much better. Still using old 300y tubes and mourning my NOS 300a’s but today made up for it.

AB1 grid driven, 300Ω resistive input w/ L-network (inductor & V cap)
Eldico 2x 4cx250b RF deck & blower, box
W8JI HVPS board
G3SEK control boards

Drive/Load: Unmodulated AM carrier from Radio Shack TRC-451 I'm seeing 250w carrier on 28.370 into DL.

All meters are Hua model 851c panel mount
Grid current: None that I can see on 20-0-20mA meter
Screen current: -4mA (under load)
Plate Volts:
No load: 2.3kv
Loaded : 2kv
Plate Current: almost 100mA
Tank settings for posted result
Tune: 34pF
Load: 500pF

Notes
L-Network Input adjustment were rough at best;
Load cap was fully meshed so I need to strap in another section to complete final --tune process.
Tank coil & band switch (80-10) ad hoc wired for 10M using 2" of 20ga hook-up wire, more tweaking to go.

I’m still bummed about the 300a tubes but today’s result made up for it.
It's working despite the old tubes.
neil
 
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