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 Post subject: 201A Solid State
PostPosted: Sep Sun 21, 2014 10:35 pm 
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I have made a number of solid state replacements for vacuum tubes over the last few years for the Zenith Transoceanic and was asked recently if I could make 201A replacements. I have no equipment that uses these so I am trying to gather information on the tubes and the equipment that uses them.

The goal is to make replacement tubes that work as well as or better than NOS 201A vacuum tubes. At this point I'm thinking of taking duds apart, inserting a circuit card into their bases, and then gluing the tube back together again.

I have a couple of questions for the group. First, does anyone know a good source for dud 201A tubes? Second, can anyone recommend a radio that would be a good test bed?

Thanks!
Edward


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 Post subject: Re: 201A Solid State
PostPosted: Sep Sun 21, 2014 11:46 pm 
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Hi Edward,

The 201A (aka 301A, '01A, et al) was the most common tube in the early years of radio, before about 1930.
In fact it was the only widely-used tube before the pentodes were introduced.

The A suffix indicates a 5-volt 1/4-amp filament. The non-suffix '01 tubes had 5-volt 1-amp filaments.

This is an extremely simple basic triode, with the filament also serving as the cathode.

Plate characteristics: Eb max = 135v
Eb = 135v, Ib = 3.0ma with a bias of -9v; Rb = 10,000; gm = 800.
Eb = 90v, Ib = 2.5ma with a bias of -4.5v; Rb = 11,000; gm = 725.

Amplification factor for both was 8.

Pretty much any radio before 1930 would be a suitable test bed.

I would suggest the Atwater Kent Model 20C (20 Compact) as a good choice.
All circuitry is mounted to the metal front panel, which can be removed from the cabinet.

Get one that has been properly restored, and it should be a very sensitive and reliable set.
You will need an AC power supply to substitute for the batteries that originally powered it.

======

In addition to its common use as an amplifier, from audio to RF, the 201A also saw service as a detector.
While retaining a triode connection, this function is quite different. Eb usually = 45v, or even 22½v.
If you intend for your replacement to fully replace the '01A, you should also test it in this circuit.

- Leigh

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Last edited by Leigh on Sep Sun 21, 2014 11:59 pm, edited 3 times in total.

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 Post subject: Re: 201A Solid State
PostPosted: Sep Sun 21, 2014 11:55 pm 
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I wonder if there would be much demand for a solid-state '01A replacement. Used ones are plentiful and can often be found for about $10. Weak ones can be easily rejuvenated on a tube tester by running the filaments for several hours at a couple of volts over rated.

But, anyway, you wouldn't be limited to modifying dud '01As - you could use any 4-pin tubes of your favorite shape. In the case of '01As, duds have an open filament, and are normally thrown away, I think. Weak ones aren't duds because they can be rejuvenated.

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Last edited by dcriner on Sep Mon 22, 2014 12:05 am, edited 1 time in total.

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 Post subject: Re: 201A Solid State
PostPosted: Sep Sun 21, 2014 11:59 pm 
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for a starter set I would use the RCA RC set, of course you may want other types such as a TRF, or Superhet to test.

Image


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 Post subject: Re: 201A Solid State
PostPosted: Sep Mon 22, 2014 12:00 am 
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Here's one that was going around.

Attachment:
fet_tube.jpg
fet_tube.jpg [ 131.9 KiB | Viewed 4936 times ]


As you see it does nothing with the filament circuit which is essential with rheostats for volume, etc.

This may give some ideas.
http://www.greenhillsgf.com/Project_SS-Tubes.htm

I've seen a schematic online using this approach.

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 Post subject: Re: 201A Solid State
PostPosted: Sep Mon 22, 2014 12:02 am 
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Try this--

viewtopic.php?t=81553

Also--

http://www.greenhillsgf.com/Project_SS-Tubes.htm

This second one has photos and pcb pattern if you click on the pictures at the lower right of the page.

Ed.


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 Post subject: Re: 201A Solid State
PostPosted: Sep Mon 22, 2014 12:15 am 
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dcriner wrote:
I wonder if there would be much demand for a solid-state '01A replacement. Used ones are plentiful and can often be found for about $10.


I am not sure I agree with that. If all of the 201A's were put back into the radios they were taken out of to maximize profits when the radios were sold there may be a shortage.

I collect 1920's radios and I cannot afford to buy enough 201A's to fill all the empty sockets in my collection even at $10.

Ed.


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 Post subject: Re: 201A Solid State
PostPosted: Sep Mon 22, 2014 12:41 am 
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Ed Kraushar wrote:
I collect 1920's radios and I cannot afford to buy enough 201A's to fill all the empty sockets in my collection even at $10.

Ed, I get that, but how much do you think a solid-state '01A replacement would cost, considering labor, parts, etc?

I have the luxury of having a supply of rejuvenated '01As that I bought at reasonable prices, so all of my TRFs are filled with good tubes. But with a solid-state replacement, as Bill says, something needs to be done to replace the filament resistance to accommodate volume control.

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 Post subject: Re: 201A Solid State
PostPosted: Sep Mon 22, 2014 1:07 am 
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Well, the one guy says he can make them for less than a buck in parts. Hmmm, labor not included.

But the supply remains good at present. And yes, rejuvenation works for some but not all. Prices have maintained themselves accordingly and indeed costs about $10/tube for good ones. That's a helluva lot better than what I have seen happen to some other tubes in the past 15 years of my puttering around on the net.

I appreciate the 'labour of love' that goes into the hobby even if its not realistic. If I had a collection of empty 1920s radios I'd want the real thing or nothing at all. So far, fortunately I've been able to keep up.

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 Post subject: Re: 201A Solid State
PostPosted: Sep Mon 22, 2014 1:13 am 
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The good thing about the solid state tube is that you don't need a special power supply. The downside is that each tube has to be customized for each stage, or at least that was my experience when Bud Bedker was making them back in the 1990's and asked me to test them in a 3 tube radio.


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 Post subject: Re: 201A Solid State
PostPosted: Sep Mon 22, 2014 1:28 am 
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This discussion reminds me of something. I try to avoid buying antique radios without tubes. Even if the set doesn't work, often all the tubes are good or maybe just one tube is burned out. For a set without tubes - often a set of tubes costs more than the value of the set with good tubes. I suspect that some sellers pull the tubes and sell them individually, and then sell the set separately.

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 Post subject: Re: 201A Solid State
PostPosted: Sep Mon 22, 2014 1:45 am 
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mrx wrote:
The good thing about the solid state tube is that you don't need a special power supply.
Consider a common 5-tube, battery-powered, TRF using '01As (which are the main uses of that tube type). With solid-state "tubes," how do you control the volume with the set's filament rheostat? Without a "special power supply," where do you get the B+ plate voltage, not to mention the 5-V dc filament supply and the C bias voltage.

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Last edited by dcriner on Sep Mon 22, 2014 1:58 am, edited 1 time in total.

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 Post subject: Re: 201A Solid State
PostPosted: Sep Mon 22, 2014 1:53 am 
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The volume is typically controlled at the filament. That's where these funky opto-isolator circuits come in. Not that hard to accomplish but the question remains - why bother.

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 Post subject: Re: 201A Solid State
PostPosted: Sep Mon 22, 2014 1:56 am 
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Bill M wrote:
The volume is typically controlled at the filament.
That control requires proper current flow through the circuit, either 1 amp or 1/4 amp per tube.

- Leigh

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 Post subject: Re: 201A Solid State
PostPosted: Sep Mon 22, 2014 2:09 am 
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Leigh wrote:
That control requires proper current flow through the circuit, either 1 amp or 1/4 amp per tube.
I guess that a power resistor could be incorporated into the glass envelope of a dud, replicating the original tube's filament. That would assume that maybe just one tube of the five were solid state. But, as Bill said, why bother?

Bill Turner offers tube reproductions, but they are for very rare tubes worth big $$ - not '01As.

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 Post subject: Re: 201A Solid State
PostPosted: Sep Mon 22, 2014 2:33 am 
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dcriner wrote:
I guess that a power resistor could be incorporated into the glass envelope of a dud, replicating the original tube's filament. [/quote]

That's not the same. Gain of 01As was very often accomplished by controlling filament voltage. I don't know what the 01A draws for current at 3.6 volts but nor do I know if that's material for practical operational purposes either.

Although it was designed for 5v nobody had any compunction to make it always that way in a variable gain scenario since varying the voltage was a simple gain control solution.

Leutz did a lot of work playing around with this and found improvements on SNR, etc in the 3.5-4.0 volt range. Graphs are in his books Modern Radio Reception I have experienced this as well when tweaking some of my HB stuff.

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 Post subject: Re: 201A Solid State
PostPosted: Sep Mon 22, 2014 2:36 am 
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Gain control via DC filament voltage in a filamentary triode is rather involved.

For one thing, the filament voltage sets the tube's operating bias.

As you decrease the voltage you decrease the bias, which increases the gain.

But as you decrease the voltage, you decrease filament emission, which reduces the gain.

- Leigh

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 Post subject: Re: 201A Solid State
PostPosted: Sep Mon 22, 2014 2:42 am 
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Yes, you are correct, Leigh.

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 Post subject: Re: 201A Solid State
PostPosted: Sep Mon 22, 2014 3:36 am 
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Thanks for all the great info. From the photos I looked at it seems the 201A tubes had pins on the side of their bases and were held in their sockets like bayonet light bulbs, rather than plugging in like more modern 4-pin tubes. Is this true? hat is the reason I thought I needed dud 201A tubes

Edward


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 Post subject: Re: 201A Solid State
PostPosted: Sep Mon 22, 2014 3:47 am 
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eea101 wrote:
From the photos I looked at it seems the 201A tubes had pins on the side of their bases and were held in their sockets like bayonet light bulbs, rather than plugging in like more modern 4-pin tubes. Is this true?
That is true, but the pins are not needed or used for later applications.

The pins were used for the non-suffix tubes in early sets.
The socket contacts were flat springs, laid horizontally below the tubes.
The tube was inserted through an open cylindric sleeve, pushed down, then rotated to hold it in position.

This type of socket was abandoned in the mid-20's in favor of modern sockets with side-wipe contacts.
The pins were retained on the tube body so they could be used in older sets.

If you did replacements for the 1-amp (non-suffix) '01 tubes, I think you would need the side pins.
The distance from the side pin to the bottom of the tube pins must be a specified value (I don't know it).

- Leigh

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