Can anyone here tell me if there are two versions of the 6F6 ie a early version and a later version identical to the 6V6?.
I have been told there is!

The 6F6 is a pentode while the 6V6 is a beam power tetrode, so as far as I know they are not the same valve. The 6V6 can however be substituted for the 6F6, but not the other way round. The 6V6 is supposed to give a little less power but also less distortion.

The 6F6 is also seen in a black metal can type.
And electrically the same as glass version.
JJ

In practical terms a 6F6 (actually a rebased 42) has less gain and needs more bias than the 6V6. Ideally it needs a 7k load vs 5k for the 6V6.

To use a 6F6 in a 6V6 socket you can just increase the bias resistor (cathode or back-bias) by about 50%. Otherwise it'll run a bit hot.

I don't know the answer to the original question, however if it's labelled 6F6 one would assume that the specifications for a 6F6 would apply.

The lower the load impedence, the hotter the valve gets. So both valves can use 7k, but a 6F6 would not be so happy with 5k.

Also, the lower the grid bias, the hotter the valve gets. Each type should use the bias that is in the specs.

So, any of a dozen kind of output valves can fit into (and work) in a 6F6 socket, so long as the operating conditions are adjusted as required.

I have a couple of black metal 6F6 valves, although they are getting a bit rusty.

This question has got me thinking again about trying a 6L6 instead of the usual 6F6 in my STC830 radio. Haven't tried it because of the 200mA higher filament current, however removing a couple of dial lights could be done for the experiment. Have tried a 6V6 with no problems.

The black 6F6 is a metal type, that's why it has no "G"; G is glass. These are a trap for the uninitiated. I did have a 6L6 metal in a KT-66 hole.

The factory had wired as for the KT-66 glass and used pin one as a B+ terminal of convenience. On Philips Octal and the RCA Metal's & others with shields, the metal body & metallisation are its shield, in the latter ones, but not always, the shield is pin one.

Therefore subbing a glass tube with metal can be rather nasty, as with the 6L6 where its body became alive. Its amazing what ends up on pin one. Never sub before looking. Pin one in these circumstances should be wired to ground, and a clue that if its wired to ground, it should be a metal tube.

As noted 6V6 is a Beam Tetrode (Renode); 6F6 aka #42 is just a power Pentode.

I did find in a Midwest with four 6F6 in PP the metal ones did actually work better than Glass.

As noted in my "valve testing" thread, you can try various output valves as an experiment. If you do decide to keep an alternate, then you'd need to make sure the operating conditions are correct (and make sure pin1 is earthed).

For example, the radio I was testing with uses a EL33, but I tried 6F6, 6F6G, 6V6G, 6V6GT, EL33A, EL37, 6L6, 6L6GA, EL3NG, 6Y6 and KT66 with complete success.

The 6L6 I have is metal so will take care with making sure that the shell isn't live.

As I fix, I have fixed hundreds of radios in over 5 decades. I keep saying "Ad Nauseum" that you assess anything even a battery set before you even think about power.

I got that one in pre Covid. It also had seven dry joint a short in the heater line that 6L6 a damaged mains cable & other dodgy wiring dial cable issues, and was supposed to have been restored & worked: Crap!

Caveat Actor applies always.