[MrFixr55]

7K should be fine. While not a perfect match (This transformer is meant for 6V6 tubes), Antique Electronic Supply (AES) has a nice (meaning CHEAP!) transformer, meant for small single ended Fender amps using a 6V6 output tube. AES P/N P-TGO-001, price $20.00. Input taps for 5K Ohm and 8K Ohm. (For 6V6, the primary should be 5K Ohm with 250V B Supply but 8.5K Ohm with 315V on the plate.) Output for 4 and 8 Ohm speakers. Power handling is 5W. This transformer is used with amps that may have a plate voltage as high as 350 VDC, so it should definitely handle the voltage and current of a single 42 or 6F6. Use the 8 KOhm Primary. Use whichever secondary (4 or 8 ohm) that gives you the best sound. Mismatch is relatively minimal. Another transformer that would work is AES P/N P-T291, rated at 8W with 4K and 9K Primaries and 4, 8, 16 Ohm secondaries. This one is $30 but has specs that justify its use for 41, 6K6, 42, 6F6 and (interesting that they group this with the others) 6V6, using the 9K primary. $10 more but more flexible.

Hope this all helps.