I've used the SPECO 7010s in parallel feed applications (as does Bottlehead in their original entry level 2A3 based "Paramour" and 6DN7 based "SEX" amps, these work amazingly well for what was then a $4 transformer. It just happens that this particular brand and model of line matcher trannie is a lot better than most low cost line matchers.
FWIW, Radio Shark used to sell a very similar line matcher, I am not sure if they still carry that model or not. Our 70 volt 10 watt line transformer has wattage taps. It has 10, 5, 2.5, 1.25, and 0.625 watt taps. It also offers color coded leads.
To figure out the equivalent primary impedance, just divide 5000 by the wattage "tap", for example, 0.625 W is 8K; 1.25W = 4K; 2.5W = 2K; etc.
These have both 4 and 8 ohm secondary taps. Where did I get 5000? That would be
70.7V squared, and
Z = V^2 / P.
These trannies are
not designed to be used as
series feed OPTs, as the primary current ratings, and more importantly, the insulation between the windings is not rated for high voltage or direct current. They may be OK for a low voltage (150V or less) B+ applications, but it would be best to try out a "breadboard" prototype first, before committing the design to hard metal..
In parallel feed, the plate current is fed through a choke, and then the audio "AC" signal is capacitor coupled to the OPT's primary winding. This assures that no DC is present on the primary winding, and that no core saturation can occur. For 8K, 1uF is a good value, 4K, 2.2 uF is good, and for 2K, 4.7 uF.
These SPECOs work best in an "auto transformer" mode, in which the "cold end" (black wire) of the primary winding is connected to the secondary's 8 ohm lead, and the speaker output can be taken off either the 4 or 8 ohm output as required.
This "autoformer" connection improves the bass response, and reducees the internal losses to some degree. Bottlehead discovered this trick, I can't claim credit. As they say, "
imitation is the most sincere form of flattery"
I've used these SPECOs as 4K (1.25W) and 8K (0.625 watt) OPTs, with a 10H to 30H plate choke, with sufficient current capability for the power tube's requirements.
HTH
Update 29 Oct 2011:
SPECO has significantly changed (let's just call ir "cheapened") these line transformers, and unfortunately, they are no longer suitable for use as parallel-feed OPTs. THis has forced Bottlehead to re-design their 6DN7 based "SeX Amp" and to design new OPTs for them. /ed B