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 Post subject: Never seen a 24 like this. Early type??
PostPosted: Jul Sat 15, 2017 10:45 pm 
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Picked up this tube today. Thought it was interesting. I do not recall seeing another 24 constructed like this. Is this an early version??


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 Post subject: Re: Never seen a 24 like this. Early type??
PostPosted: Jul Sat 15, 2017 10:48 pm 
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Deforest always did things differently

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 Post subject: Re: Never seen a 24 like this. Early type??
PostPosted: Jul Sun 16, 2017 1:10 am 
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I have seen those perforated plates in a number of DeForest tubes. I have also found them to be defective more often than other brands, for whatever that's worth.


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 Post subject: Re: Never seen a 24 like this. Early type??
PostPosted: Jul Mon 17, 2017 10:46 pm 
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The perforated part is probably not the plate. It appears to be a shield.

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 Post subject: Re: Never seen a 24 like this. Early type??
PostPosted: Jul Tue 18, 2017 1:30 am 
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It would be early with the engraved base and the 3 digit designation. I thought there were other early ones with 3 digits depending on maker. Before long they dropped the first digit apparently to avoid confusion.

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 Post subject: Re: Never seen a 24 like this. Early type??
PostPosted: Jul Tue 18, 2017 3:32 pm 
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That tube was made ca. 1929 by the De Forest Radio Company of Jersey City, NJ, which was the last one he was associated with. https://repository.duke.edu/dc/adaccess/R0645 David Sarnoff bought the company out that year and added it to RCA's tube business more or less as a favor to Dr. De Forest; it certainly didn't have anything at that point that RCA really needed or wanted. They kept the De Forest name a couple more years then quietly retired it.

The first digit, '4,' was originally reserved by American consumer radio tube makers in the 1920s to indicate an independent manufacturer ('1' was Western Electric, '2' was RCA, and '3' was Cunningham). The original purpose of this was to identify who sold the tube to prevent people from returning them to the wrong dealers with claims of defects. By the late 1920s there was increasing pressure on RCA to license other companies under its patents, resulting in more independent tube manufacturers, so the old three-digit system became unworkable and was dropped.

RCA released the type 24 in 1929 but ran into trouble with it. High secondary emission from the plates gave the tubes a large negative resistance region where they could easily break into oscillation. After some hastily accomplished research, RCA got the secondary emission down to manageable levels. The plates had to be darkened so they would radiate heat more effectively. The type 24 was withdrawn and replaced with the 24A. It would be interesting to plot some characteristic curves and see if the perforated shield in the De Forest 424 makes it more like the original RCA 24 or the later 24A.

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 Post subject: Re: Never seen a 24 like this. Early type??
PostPosted: Jul Thu 20, 2017 9:06 pm 
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Isn't there a Crosley radio which uses a 24 that oscillates by design, so use of a 24A will render the oscillator dead on this set? I believe that the oscillator circuit in the Crosley was known as a "dynatron" and was used in model 122.


Last edited by Chuck Braun on Jul Fri 21, 2017 1:49 pm, edited 2 times in total.

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 Post subject: Re: Never seen a 24 like this. Early type??
PostPosted: Jul Thu 20, 2017 11:04 pm 
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I have a hand full of globe 24s with the perforated plate. I don't think they are DeForest branded though.
Need to pull one out and check it.

Edit: The tubes I have are indeed marked Deforest 424 exactly like the one Dave pictured.

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Last edited by processhead on Jul Fri 21, 2017 12:16 pm, edited 2 times in total.

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 Post subject: Re: Never seen a 24 like this. Early type??
PostPosted: Jul Fri 21, 2017 12:59 am 
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Hmm Interesting. I thought the A version was just a version with faster heater warmup. And an ST bulb. I will keep that oscillation tidbit in the card file.

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 Post subject: Re: Never seen a 24 like this. Early type??
PostPosted: Jul Mon 24, 2017 4:06 pm 
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Quote:
Isn't there a Crosley radio which uses a 24 that oscillates by design, so use of a 24A will render the oscillator dead on this set? I believe that the oscillator circuit in the Crosley was known as a "dynatron" and was used in model 122.


The dynatron circuit was not unique to Crosley or to the 24 tube; any tetrode can be made to exhibit negative resistance (which leads to oscillation) if the screen grid voltage is higher than the plate voltage, and both are high enough that secondary emission from the plate can occur. The problem with the original 24 tubes was that it was all too easy to meet those conditions because the plates had a lot of secondary emission!

RCA certainly investigated and tested the dynatron oscillator circuit before the 24 tube was placed on the market; their senior tube engineers decided that the effect had too much variation from one tube to the next for them to support it as a recommended application. Nevertheless, Crosley used it anyway. When RCA fixed the problem in the 24A and discontinued the 24, a lot of Crosley owners were left high and dry. Crosley sued, but RCA successfully argued that since they never supported dynatron operation of the 24 in the first place, they were not obligated to support it in the 24A. Fortunately, in most cases it was just a matter of changing a resistor or two to get a 24A to oscillate in those radios.

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 Post subject: Re: Never seen a 24 like this. Early type??
PostPosted: May Mon 27, 2019 12:01 am 
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The 'Mighty Mite' superheterodyne receiver found in the March 1931 issue of Radio News used a dynatron oscillator:

https://www.americanradiohistory.com/Archive-Radio-News/30s/Radio-News-1931-03-R.pdf

Circuit:

Image

Chuck Braun wrote:
Isn't there a Crosley radio which uses a 24 that oscillates by design, so use of a 24A will render the oscillator dead on this set? I believe that the oscillator circuit in the Crosley was known as a "dynatron" and was used in model 122.


The circuit of the Crosley 122 does indeed use a dynatron oscillator. The circuit is here:

http://www.nostalgiaair.org/pagesbymodel/410/M0043410.pdf

Chris108 wrote:

RCA certainly investigated and tested the dynatron oscillator circuit before the 24 tube was placed on the market; their senior tube engineers decided that the effect had too much variation from one tube to the next for them to support it as a recommended application. Nevertheless, Crosley used it anyway. When RCA fixed the problem in the 24A and discontinued the 24, a lot of Crosley owners were left high and dry. Crosley sued, but RCA successfully argued that since they never supported dynatron operation of the 24 in the first place, they were not obligated to support it in the 24A. Fortunately, in most cases it was just a matter of changing a resistor or two to get a 24A to oscillate in those radios.


On the other hand:

radiotechnician wrote:

A 32 makes a dandy dynatron.



The Mighty Mite used a 32 tube for the oscillator.

Good points to know!

More info here:

http://www.theradioboard.com/rb/viewtopic.php?f=3&t=8753

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Last edited by Norm_N_Tam on May Tue 28, 2019 4:49 am, edited 4 times in total.

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 Post subject: Re: Never seen a 24 like this. Early type??
PostPosted: May Mon 27, 2019 12:56 am 
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RCA did implement Dynatron oscillators using 24's for the scan oscillators in their 1931 television even though the 24A came out in 1930 so they were still experimenting with them.

Darryl


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 Post subject: Re: Never seen a 24 like this. Early type??
PostPosted: May Mon 27, 2019 1:36 am 
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Chris108 wrote:
The perforated part is probably not the plate. It appears to be a shield.

The outer cylinders on most pre-war tetrodes and pentodes are actually connected to the outermost grid -- in this case, the screen grid. If you use a light to look upskirt at most of these tubes, you might be able to see the actual plate, which is about 3/8" to 5/8" in diameter. I think these outer cylinders were used to catch stray electrons.

BTW, this is why many manufacturers' schematics show two connected grids on either side of the plate, like on V1, V2 and V3 of the schematic Norm_N_Tam posted above.


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 Post subject: Re: Never seen a 24 like this. Early type??
PostPosted: May Mon 27, 2019 1:58 am 
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Chris108 wrote:
The perforated part is probably not the plate. It appears to be a shield.
Correct. I have a de-based early version that the shield is coiled like a grid structure. The grid cap is raw brass rather than nickle plated. I do have the base but as yet not decided what method to re-attach. There are no lumps of adhesive left to glue down the bulb.

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 Post subject: Re: Never seen a 24 like this. Early type??
PostPosted: Jun Tue 11, 2019 12:00 pm 
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Quote:
The outer cylinders on most pre-war tetrodes and pentodes are actually connected to the outermost grid -- in this case, the screen grid. If you use a light to look upskirt at most of these tubes, you might be able to see the actual plate, which is about 3/8" to 5/8" in diameter. I think these outer cylinders were used to catch stray electrons.

BTW, this is why many manufacturers' schematics show two connected grids on either side of the plate, like on V1, V2 and V3 of the schematic Norm_N_Tam posted above.


That's right. With five pins and the top cap already claimed for other elements, there is no external connection for a shield. But in most RF and IF circuits, screen grids are well bypassed to ground to keep the tubes stable. So by connecting the outer shield around the plate to the screen grid, the shield is also effectively grounded for RF purposes.

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