The 7DJ8 is basically the same as the 6 but use 7 volts. The 6922 is very close also as is the 7308. They are all interchangeable. The 6922 is the premium version of the 6dj8. The 7308 is the premium version of the 6922. You can also add the 6h23 in the mix.
Did I mention I bought 4 more of the...