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 Post subject: Reading Older Schematics
PostPosted: Sep Fri 25, 2020 1:50 pm 
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Joined: Aug Thu 06, 2020 4:41 pm
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I am somewhat confused reading the attached schematic. The tube symbols are not what I am used too. What is the grid, plate, screen etc? Is there a site that shows old vs. new symbols. Also the resistors are designated with a 'M', does that mean meg-ohm? Finally the chart gives voltages but to undefined A, B, C. The screen is called out. I presume that A is filament since the voltage is low? Any help would be appreciated.

Thanks, Jerry


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Model 26 Schematic.jpg
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 Post subject: Re: Reading Older Schematics
PostPosted: Sep Fri 25, 2020 2:10 pm 
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N3RBW wrote:
I am somewhat confused reading the attached schematic. The tube symbols are not what I am used too. What is the grid, plate, screen etc? Is there a site that shows old vs. new symbols. Also the resistors are designated with a 'M', does that mean meg-ohm? Finally the chart gives voltages but to undefined A, B, C. The screen is called out. I presume that A is filament since the voltage is low? Any help would be appreciated.

Thanks, Jerry

The grid is the wiggly line on the left, the plate is the box, and the screen is the U around the plate. The filament should be obvious and the remaining connection is the cathode.

M is 1,000 in old schematics. It is the same as we use K today. They usually use MEG for megohms.

A is the filament as you guessed, B is the plate, and C is the grid in the voltage charts.
It isn't always obvious how C is measured. Sometimes it is measured across a resistor in the power supply. In this schematic, the 50 volts for the 45 grid is measured across the 1000 ohm resistor on the power transformer center tap.

Jay


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 Post subject: Re: Reading Older Schematics
PostPosted: Sep Fri 25, 2020 2:24 pm 
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At least they didn't draw the tubes upside-down.

John


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 Post subject: Re: Reading Older Schematics
PostPosted: Sep Fri 25, 2020 2:33 pm 
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There's actually some logic behind that depiction. In a tube amplifier, we can think of the grid as the input, and the plate as the output. But--at least with a triode--the more recent method still conveys the same concept.

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 Post subject: Re: Reading Older Schematics
PostPosted: Sep Fri 25, 2020 5:22 pm 
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IMHO, I find interpreting schematic fun, like trying to read Hànzì..

FWIW the RF tubes are "Screen Grid" 24a's. The depiction of the grid "outside" of the plate is the perforated shield over the active elements of the '24'...

The most difficult schematics to read IMHO are the Radiola VI/VII, Grebe Syncrophase. The later RCA with the tubes within a double enclosed circle, upside down. Schematics using a mixture of conventional symbols and drawn as a diagrammatic where signal flow and power distribution run amok. As well as those draftsman that developed their own, unconventional symbols...

On early diagrammatics it is far easier to diagnose an issue by re-drawing the schematic in a conventional manner.

Most of the time, "YMMV" Chas

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 Post subject: Re: Reading Older Schematics
PostPosted: Sep Fri 25, 2020 5:31 pm 
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Quote:
As well as those draftsman that developed their own, unconventional symbols...

Yes, sometimes I have to study the symbols and deduce what the draftsman was trying to convey by tracing out the connections. Usually grid become obvious as an input signal from previous stage, which in turn defines the plate. Screen grid by the way it connects to a supply voltage.
Then you have the tube drawn without any circle, how the heck do they keep the vacuum ?
And then there is the tube shown with the grid on one side of the filament and the plate on the other. No idea how the electrons move to the left thru the grid, and then back to to plate on the right.
Just having some fun with early drawings styles, remember these were all drawn by hand and the draftsman may have had considerable leeway on how to depict parts.


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Last edited by pauls.ironhorse on Sep Fri 25, 2020 6:01 pm, edited 1 time in total.
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 Post subject: Re: Reading Older Schematics
PostPosted: Sep Fri 25, 2020 5:53 pm 
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In Paul's example and other similar RF stage schematics the orientation of the RF coil plate/B+ and Grid/Gnd are, at times, indicative to how the coil is terminated. A more precise indication is the more modern transformer where there is a large dot to indicate the start of the winding. Mix in wireless era and European schematics/diagrammatic with resistors and transformers as empty rectangles... Chas

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 Post subject: Re: Reading Older Schematics
PostPosted: Sep Fri 25, 2020 6:29 pm 
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Everyone THANK YOU !!
This forum is the nicest I've ever belonged to.

Jerry
"If I understood what I was doing I could be dangerous"


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 Post subject: Re: Reading Older Schematics
PostPosted: Sep Fri 25, 2020 6:46 pm 
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I believe A, B and C are references to the A, B and C batteries uses in earlier battery powered radios, where the A battery powered the filament, the B battery powered the plate circuit, and the C battery is the grid bias voltage. There was never a screen battery so it is called out separately. The A reading is just across the filament, B may be either to ground or to cathode, and C is the plate to cathode voltage. This looks like a convention which never became a convention.

M is the roman numeral for 1000, as opposed to the metric multiplier k, which also means 1000 and is used today. So on these old schematics, 10M means 10k, not 10MEG


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