The resistance given (200 ohm) is the DC resistance of the primary coil of the transformer. Most data given in tube data books and in modern literature has to do with the coil's AC impedance. When an alternating current signal (audio signal) is fed to the transformer, the impedance of the coil increases due to choking action. Chokes impede alternating currents, and the impedance rises with frequency. Things that vary the amount of frequency impedance are the amount of turns of wire and also what kind of core (if any) is used on the choke or transformer. The core would be the iron or powdered iron item which the transformer is wound around. To increase the ratio between AC and DC impedance, you could increase the size of the wire while still keeping the same turn count. The heavier wire would have less DC resistance, but the same turn count would have the same AC resistance, and so the ratio between the two (and also between low and high frequencies) would increase. A coil naturally impedes AC currents because of various factors including inertia and time and some other things I will explain here. When electricity starts passing through a coil, it creates a changing magnetic field. The change ceases when the electricity ceases to change rate or direction of flow, which is why DC does not get impeded by coils. As we all know, when you move a magnet past a coil of wire (or otherwise change its position in some way), you generate electricity. The changing currents of alternating current create changing magnetic fields. Since a choke is made up of many turns of wire, the magnetic fields generated by each turn of wire affect the other turns of wire, and create electricity in the other turns. The magnetic fields generated do not collapse necessarily as fast as the current changes, and so cancelling currents are generated which cancel the flow of the electricity. Placing a metal core on the coil allows the magnetism to travel further more efficiently, so that it reaches all turns of wire. The metal core also retains magnetic fields more easily, causing them to collapse more slowly. The slowly collapsing magnetic fields also absorb new magnetic fields which are produced. The whole process creates a mushiness which alternating (or changing) current cannot flow through easily.
Coils which are "tuned" to certain frequencies are tuned to have a natural magnetic build-up and break down that resonates at a certain frequency, that is, "works with" the frequency. For this reason, most high frequency coils lack metal cores, or have very small metal cores. The small movable metal (or powdered iron) cores of RF transformers can be moved to just the right position so that the affect the coil only so much.
Regarding your transformer, then, what you want to do is purchase a single ended universal output transformer from www.tubesandmore.com, that is capable of about 5 to 10 watts. Most modern companies and literature give the AC impedance for transformers, where-as most old circuit diagrams give the DC impedance. The numbers you will find, then, at www.tubesandmore.com, will be much higher than 200 ohms, since they will be the AC impedance values. Incidently, the AC impedance for audio components (output transformers and speakers) is usually taken with an a frequency of 400 cycles. At any rate, purchase your transformer. Pick a secondary impedance that will correspond to the speaker you choose. If it is a new speaker, and the speaker has a value given (4 or 8 ohms or some other figure), select the corresponding number on the transformer. If you are using the original speaker and have no clue as to what its AC impedance is, select the 4 ohm tap for now. Usually older speakers are rated at 3.2 ohms, but that is a terrible generalization, because actually they are all over the map. Select the primary tap that has a DC impedance around 200 ohms (use your ohm meter to find this), or start with the tap that has the HIGHEST impedance, and work down until you get agreeable power and audio quality as well as appropriate voltages on the tube plate, etc. Lower impedances will yield more power but usually slightly less bass. Higher impedances will yield less power but usually more bass. There's another explanation for why bass notes like high impedances which I will save for later. Anyway, pick a tap that works well. Then pick the secondary tap that gives the loudest and most agreeable tone from the speaker. Changing taps should have no normally noticeable affect on the output tube's voltages, so do not worry about selecting the wrong secondary tap. Go for good tone.
You are finished now. Enjoy the radio. Before you replace the output transformer, however, make sure that you at least replace all leaky condensers, if not all condensers. Leaky condensers are those which have anything less than infinity resistance. Electrolytics are good if their leakage is above about 5 or 10 megohms. Anything less is unacceptable. Drifted resistors (anything over about 10 percent) should be replaced as well.
Thomas