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Author Topic: Difference between 250T and 250TH tubes?  (Read 3689 times)
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WB6NVH
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« on: July 11, 2022, 03:50:23 PM »

This past weekend I rescued a large mid-1930's homebrew AM transmitter, two full size racks that must total out at 800 pounds (!)  Needless to say this is going to be a major project to preserve and rebuild.  The way it seems to be configured now is a pair of 250T's modulating a set of 250TH's in parallel.  Someone has stuck a 250TL in with a 250TH in the PA, which I will correct, but anyway, my question is, what does the 250T compare to, the TH or the TL?  I wasn't able to find a datasheet for the plain, early "T" model.  I know the old 150T was re-rated and improved and became the 250T in that configuration, but should I treat them as a TH or a TL?

I am presuming the modulator had the 250T's and the PA the 250TH's.
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Geoff Fors
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« Reply #1 on: July 11, 2022, 04:43:56 PM »

iirc, the TH is a tube with mo' gain. The TL is lower gain.
Aka takes less swing to drive.

What the "T" is? Best to go back and find some pre-WWII tube catalogs (they're online) and
look at the specs for the early 250T, assuming there was such a thing, and the second letter is not
just rubbed off or missing?!?!

 Who is the manufacturer?

                                  _-_-bear                       
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_-_- bear WB2GCR                   http://www.bearlabs.com
K9MB
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« Reply #2 on: July 11, 2022, 05:34:58 PM »

http://n6jv.com/museum/250t.html

Look at the filament stem for a L or a H to determine whether it is low or high mu triode.

Later, they just put the T or L on the envelope.
73, MB
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WB6NVH
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« Reply #3 on: July 11, 2022, 06:33:53 PM »

They are Eimac.  They pre-date the TH and TL.  I will check for early tube catalogs, forgot about that. 
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Geoff Fors
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« Reply #4 on: July 11, 2022, 07:35:42 PM »

They are Eimac.  They pre-date the TH and TL.  I will check for early tube catalogs, forgot about that.  

Here is an image of the text from the N6JV museum link I just posted. It explains it all in detail.
The tubes in the image indeed are Eimac brand, by the way

http://n6jv.com/museum/250t.html

250T

“The 250T replaced the 150T triode by 1935. It was then rated at 250 watts. The 250T came in two types. One with a high mu (32) and a second with a low mu (13). A "HI" or "LO" was printed on the filament stem. It is usable up to 40 MHz at full rating. The single filament lead press is typical, but early tubes like the first example, had the 3-way grid support as found in the 150T. Both examples are "HI" versions. Late production of this tube was called a 250TH or 250TL in data sheets.

Length = 10.13" Diameter = 3.81" Max voltage = 3000 Max current = 350 ma Fil voltage = 5 Fil current = 10.5 amps Data sheet:”


* A2FDFB1B-CF2B-41E7-B8B3-EE4169B03FBE.jpeg (1199.68 KB, 2401x1810 - viewed 170 times.)
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K9MB
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« Reply #5 on: July 11, 2022, 08:40:44 PM »

I was curious when Eimac started promoting the 250T,
So I went back to 1936 and looked at their ADs in QST every month and the 150T was there until december of 1936 and then the next month, January of 1937, they had their first AD for the 250T.
They talked about the 250TL and the 250TH being variations on the 250T.
The only difference between a 250T with a HI in the filament stem and a 250T with a LO on the filament stem was the mu of the tubes and later, apparently they just printed it in the envelope according the the Museum article.

i am attaching the January 1937 QST AD below:
Very interesting…


* 3109F58D-9C59-43C3-8D95-4FBA441AB5A9.png (1088.83 KB, 2732x2048 - viewed 204 times.)
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WB6NVH
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« Reply #6 on: July 11, 2022, 09:57:27 PM »

Ah yes, indeed, the filament stems have what is probably a batch number on them and those numbers end with "-Hi" so that matter is solved. Thank you.

I am quite surprised that after 80 years of knocking around, these tubes are still physically intact without any filament bits rolling around inside and so on.  It is just too bad that the audiophool vultures have ruined the pricing and availability of the TH series tubes so that finding spares is difficult.

A future project will be putting together a jig to see if I can de-gas these specimens.
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Geoff Fors
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K9MB
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« Reply #7 on: July 12, 2022, 12:09:38 AM »

Ah yes, indeed, the filament stems have what is probably a batch number on them and those numbers end with "-Hi" so that matter is solved. Thank you.

I am quite surprised that after 80 years of knocking around, these tubes are still physically intact without any filament bits rolling around inside and so on.  It is just too bad that the audiophool vultures have ruined the pricing and availability of the TH series tubes so that finding spares is difficult.

A future project will be putting together a jig to see if I can de-gas these specimens.

Good luck on the gettering project.
I was thinking today that the late 30s was a golden age for vacuum tube development.
RCA developed the 807, 810, 811 and 813 in these years and all those tubes, or variations are still used and some still manufactured.
I like to read the old RCA Ham News for this period. Quite impressive that these devices have lasted over 80 years and counting.
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W3SLK
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« Reply #8 on: July 12, 2022, 06:13:55 AM »

Weren't the anodes in the 250Tx and their subsequent successors made of platinum?
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Invisible airwaves crackle with life, bright antenna bristle with the energy. Emotional feedback, on timeless wavelength, bearing a gift beyond lights, almost free.... Spirit of Radio/Rush
K9MB
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« Reply #9 on: July 12, 2022, 11:00:47 AM »

Weren't the anodes in the 250Tx and their subsequent successors made of platinum?

Seems like it was tantalum.
See AD below from October 1936 QST
Melting point is 3000C (5500F)


* 9E873908-0855-4467-8650-F170E982E111.jpeg (682.85 KB, 1344x2035 - viewed 176 times.)
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W2PFY
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« Reply #10 on: July 15, 2022, 07:02:51 AM »

I have used the 250T and the 100T tubes in my BC-610. The problem with the tubes I had was that the plate cap became loose and the tubes turned into display models. They all worked fine up to that point and it was fun running them. I didn't know that the 250T came out in 1937? Good stuff!
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K9MB
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« Reply #11 on: July 15, 2022, 03:04:27 PM »

I have used the 250T and the 100T tubes in my BC-610. The problem with the tubes I had was that the plate cap became loose and the tubes turned into display models. They all worked fine up to that point and it was fun running them. I didn't know that the 250T came out in 1937? Good stuff!

I got a couple of 810s that the caps had come loose on, though the seals looks ok through the glass envelope.
I have wondered if a high temperature cement made for furnaces might be used to reattach those loose caps to the glass?
Seems like the cap is soldered through a hole too-not certain, but if it could be bonded with high temperature cement and soldered tight- maybe an old and unreplaceable tube might be saved? That is a lot of if! 😉 MB
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K8DI
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« Reply #12 on: July 15, 2022, 06:05:55 PM »

I’ve desoldered, cleaned, glued, and resoldered quite a few smaller tubes with complete success. You don’t even necessarily need to glue and resolder, depending on the application you can solder the lead directly to the wire sticking out the top. No reason to toss an otherwise good and rare tube!

Ed
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Ed, K8DI, warming the air with RF, and working on lighting the shack with thoriated tungsten and mercury vapor...
W2PFY
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« Reply #13 on: July 15, 2022, 08:14:37 PM »

Well the problem with the T series Eimac tubes is the way the anode is connected to the plate cap. IIRC the 250T has three wires coming up through the glass and they become deteriorated probably by time itself and perhaps environmental air quality issues such as a damp cellar etc. They have a tendency to brake off right at the glass.I have not had a problem with the grids on either 100T's or the 250T. Another problem in these types of old transmitting tubes is at the plate connection, there is a well and the wires are recessed in that glass well aiding corrosion. If you have a 250T or 100T that has a solid connection to the anode cap, treat it extremely carefully. Maybe there is a way to treat the wires on top of the tube before use? Now to be silly, there may be a way to operate a tube that has a well with a detached cap like the 250T by using a metal that has a low melting temperature deposited in that well but the drawback would be that you would have to wait till the metal melted before you could use the tube:) Yes these tubes are your great great grandfathers tubes and you need to be creative to use them! As far as more modern types like 813 and 810, there are a lot of different approaches to reattach the plate cap but that ain't so with these old jugs!
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« Reply #14 on: July 16, 2022, 10:33:34 AM »

Interesting info on the anode connections of the 250T.
I just looked at a pic and it is difficult to see the details except the 3 wires coming through the glass….

Maybe wait a bit before using the bad ones for target practice, though. Someone might figure it out.

By the way, the 810 and 813 are hardly “modern” tubes, since they were both introduced by RCA in the late 30s, just a bit after the 250T.
Seems like 38 or 39.
RCA hit a grand slam with the 813. Extremely rugged and still loved by a lot of guys including myself. Thanks to WWII war production, millions were made, I guess.
The 810 is also an amazing high perveance triode that is perfect for class B modulators and audio amps and will produce a lot of power at low to medium plate voltages. RCA was amazing in the 30s. They also introduced the 807 and 811- two more Hall of Famers…
The 1930s was the golden age of tube design, IMO
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