7199/6GH8A

I know it's common to substitute a 6GH8A for the 7199 after the appropriate pinout revision/adapter.  But in reading the tube data for the two tubes, the specs seems substantially different to me.  Is it that the difference has no effect on the specific performance needed from the tube?  There are some substantial differences - for example, one of the capacitance specs is 1.7 Pf for one tube and 5Pf for the other; another is triode plate voltage - 125 vs 215.  There are many other differences if I am reading the tube data correctly. http://tubedata.milbert.com/sheets/049/6/6GH8A.pdf http://frank.pocnet.net/sheets/049/7/7199.pdf

I don't know diddly about circuitry so I don't understand how tubes that SEEM to my uneducated eyes to be quite different are actually interchangeable in the same circuit.   ????  

You bring up a good point but not for the reasons you give. The most important capacitance is probably the grid to plate and they are almost equal. As far as the voltage the data is only examples. Both tubes will handle 330 volts if necessary but neither is run to the limit. I did however notice that there is a big difference in required bios at certain voltages for the pentode section. I also noticed that the triode section has a considerably higher amplification factor. However this is not very important because the triode section is used as a splitter and in either tube the triode amplification factor in a Dyanco circuit will be around .9 or less than what went in. However the higher amplification factor would cause the plate resistance to the next stage to be higher while the cathode would be almost the same so not quite as good. But probably not noticeable. I think the 6GH8 was chosen as a substitute because of the pentode amplification factor which if properly set is almost the same as the 7199. I assume that the people who suggested this substitute did the math and found no change necessary at the voltage the pentode was set to operate at for and the bios setting that is set by the cathode resistor.
I could do the math but I am going to assume someone did it before suggesting the change in a Dynaco. There is also the Sovtec 7199 however is is not what it seems. The Sovtec was probably another pentode/triode tube that was repined to plug into a 7199 socket without an adapter. It will operate in the Dynaco 70 without a problem however it will squeal when turned on and is close to unstable in the SCA35 where the amplification factor of the pentode section is pushed to it's limit.