Part 18 (1/2)

When a signal is heard adjust to the position of _s_{1}_ and _s_{2}_ which gives the loudest signal and then closing _S_ start to tune the secondary circuit To do this, vary the capacity of the condenser in the secondary circuit Don't change the priet the signal with good volume, that is loud You ant to vary the position of the pri, for you will get sharper tuning as they are drawn farther apart Sharper tuningon wave lengths near that which you wish to receive Reduce the coupling, therefore, and then readjust the tuning It will usually be necessary to e in both circuits, in one case with switch _s_{1}_ and in the other with the variable condenser

As soon as you can identify any station which you hear sending make a note of the position of the switches _s_{1}_ and _s_{2}_, and of the setting of the condenser in the secondary circuit In that way you will acquire information as to the proper adjust your set by the knoave-lengths of distant stations

After learning to receive with this siood audion tube Ask the seller to supply you with a blue print of the characteristic[11] of the tube taken under the conditions of filae which he recoe battery and a small slide-wire rheostat, that is variable resistance, to use in the filament circuit Buy also a bank of dry batteries of the proper voltage for the plate circuit of the tube At the sao between the plate circuit of your tube and the pair of receivers which you have It will usually be advisable to ask the dealer to show you a characteristic curve for the transformer, which will indicate hoell the transfore It should operate very nearly the same for all frequencies between 200 and 2500 cycles

The next step is to learn to use the tube as a detector Connect it into your secondary circuit instead of the crystal detector Use the proper value of C-battery as determined from your study of the characteristic of the tube One or two s-post terminals are convenient C-batteries If you think you will need a voltage much different from that obtained with a whole nurid as we did in Fig 86 of Letter 18 In that case you can use a few feet of 30 German-silver wire and make connections to it with a suspender clip

Learn to receive with the tube and be particularly careful not to let the filament have too much current and burn out

Now buy sorid condenser of about 00002 o with it you canpaper and two little metal eyelets

The eyelets can be punched into the paper Between them coat the paper with carbon ink, or with lead pencil ht to serve nicely You will probably wish to et satisfactory operation in receiving by the grid-condenser ohohms

For this method you will not want a C-battery, but you ish to operate the detector with about as high a voltage as the manufacturers will reconal, which decreases the plate current, can produce the largest decrease It is also possible to start with the grid slightly positive instead of being as negative as it is when connected to the negative terreater change in grid voltage To do so connect the grid as in Fig 115 to the positive ter 113]

About this time I would shop around for two or three small double-pole double-throitches Those of the 5-ae toSuppose, for exa 113 so that by throwing theht the crystal as a detector You can listen for a minute in one position and then switch and listen for a minute in the other position, and so on back and forth That way you can tell whether or not you really are getting better results

If you want a rough measure of how ht see, while you are listening to the audion, how much you can rob the telephone receiver of its current and still hear as well as you do when you switch back to the crystal The easiest way to do this is to put a variable resistance across the receiver as shown in Fig 113 Adjust this resistance until the intensity of the signal when detected by the audion is the same as for the crystal You adjust this variable resistance until it by-passes so h the receiver, that the ”audibility” of the signal is reduced until it is the same as for the crystal detector Carefully made resistances for such a purpose are sold under the name of ”audibility meters” You can assemble a resistance which will do fairly well if you will buy a s by steps of ten ohms from zero to one hundred ohms At the same time you can buy four resistance spools of one hundred ohms each and perhaps one of 500 ohms The spools need not be very expensive for you do not need carefully adjusted resistances assee of 0-1000 ohms by steps of 10 ohms The cheapest way to114 After a while, however, you will probably wish to mount them in a box with a rotary switch on top

[Illustration: Fig 114]

To study the effect of the grid condenser you can arrange switches so as to insert this condenser and its leak and at the sa 115 sho You can ain in audibility at the same time

[Illustration: Pl X--Audio-frequency Transformer and Banked-wound Coil

(Courtesy of Pacent Electric Co)]

[Illustration: Fig 115]

After learning to use the audion as a detector, both by virtue of its curved characteristic and by the grid-condenserthe same tube as an amplifier First I would learn to use it as an audio-frequency amplifier Set up the crystal detector circuit Use your audio-frequency transforrid Put the telephone in the plate circuit Choose your C-battery for amplification and _not detection_ and try to receive

You will get better results if you can afford another iron-core transformer If you can, buy one which ork between the plate circuit of one vacuurid circuit of another siht equipe audio-frequency amplifier If you buy such a transformer use the other transformer between plate and telephones as you did before and insert the new one as shown in Fig 116 This circuit also sho you can connect the switches so as to see how116]

The next step is to use the audion as an anal before its detection Use the proper C-battery for an amplifier, as determined from the blue print of the tube characteristic Connect the tube as shown in Fig 117 You will see that in this circuit we are using a choke coil to keep the radio-frequency current out of the battery part of the plate circuit and a small condenser, another one of 0002 mf, to keep the battery current froure how you can arrange the switches so as to find whether or not you are getting any gain from the amplifier

Now you are ready to receive those C-W senders at 275 meters You will need to wind another coil like the secondary coil you already have Here is where you buy another condenser You will need it later If before you bought the 00005 size, this time buy the 0001 size or vice versa

Wind also a small coil for a tickler About 20 turns of 26 wire on a core of 3-1/2 in diameter will do Connect the tickler in the plate circuit of the audion Connect to the grid your new coil and condenser and set the audion circuit so that it will induce a current in the secondary circuit which supplies the crystal Fig 118 shows the hook-up

[Illustration: Fig 117]

You will see that you are now supplying the crystal with current fronals and the local oscillator which you have formed The crystal will detect the ”beat note” between these two currents

To receive the 275 nals you will need to make several adjustments at the sa of the antenna circuit and of the crystal circuit about where you think right because of your knowledge of the settings for other wave lengths

Then I would get the local oscillator going You can tell whether or not it is going if you suddenly increase or decrease the coupling between the tickler coil and the input circuit of the audion If this motion is acco

[Illustration: Fig 118]

Now youby slowly changing the capacity in the tuned input circuit of the tube Unless the antenna circuit is properly tuned to the 275 et no results If it is, you will hear an intermittent musical note for some tune of your local oscillator This note will have the duration of dots and dashes