It is an extreme safety hazard actually - if there should be an insulation failure between the primary and secondary B+ could appear on the secondary constituting an electrocution hazard to anyone coming in contact with the speaker cables, XO or driver.. NOT recommended! 
Advocating hazardous practices like this is actually a violation of forum policy.You need a better transformer or reconfigure as a parafeed amp with the primary grounded - then you can float the secondary safely. In which case you can probably get a parafeed type made with really good HF performance even with the tube's high rp since it does not have to handle plate current and can be made more compact with high perm magnetics.
As I mentioned a parafeed configuration with the primary grounded would allow you safely operate with the transformer secondary floating. For the reasons I stated above whether or not it works satisfactorily you cannot safely leave the secondary floating at these plate voltages.
If parafeed is anathema then you need to look for an OPT specifically designed to work with the high rp of the 75TL.
If parafeed is anathema then you need to look for an OPT specifically designed to work with the high rp of the 75TL.
well, let's see, how many dB is the drop from 1.0v to 0.4v?
try the freq response with a 16 ohm resistor, that makes the reflected primary impedance look higher...
also, are you at all sure that you have the xfmr strapped right yet??
To me it is very odd that grounding one side of the secondary would make any difference at all...
... while you are at it ground the other side (run it "out of phase") see if that makes any difference.
Let's try an experiment - "ground it" for testing purposes with a largish capacitor.
Are you showing no DC with the secondary floating on the secondary??
If you have two windings on the secondary, try grounding the center tap, no matter if that is the preferred strapping or not, set it up that way and test the freq response... hopefully that IS the way it is strapped.
_-_-bear
You are saying that it has the same freq response droop with all the other transformers??
try the freq response with a 16 ohm resistor, that makes the reflected primary impedance look higher...
also, are you at all sure that you have the xfmr strapped right yet??
To me it is very odd that grounding one side of the secondary would make any difference at all...
... while you are at it ground the other side (run it "out of phase") see if that makes any difference.
Let's try an experiment - "ground it" for testing purposes with a largish capacitor.
Are you showing no DC with the secondary floating on the secondary??
If you have two windings on the secondary, try grounding the center tap, no matter if that is the preferred strapping or not, set it up that way and test the freq response... hopefully that IS the way it is strapped.
_-_-bear
You are saying that it has the same freq response droop with all the other transformers??
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Almost -8dB @ 20kHz relative to 1kHz. Pretty severe roll-off..
These transformers are designed for tubes with lower rp than the ~5K ohms (?) of the 75TL. I suspect interwinding capacitance between the primary and secondary, it would only need to be 3.5nF or so for this to be the case. (With a 300B this transformer would be down about -1dB @ 35kHz or less)
You would replace the output transformer with a large choke plate to B+, preferably several hundred henries of inductance, and in this case you would use a high voltage film capacitor of several uF between the output transformer and the 75TL plate, ground the other end of the primary.
I would talk to Jack Elliano at Electra-Print about a suitable parafeed choke, and possibly a suitable OPT as well. Alternately he may be able to build a conventional transformer that works.
I'm using a custom pair of his OPTs in my GM70 amp running at 1kVdc. (Transformer family: 40W/150mA/7K) They work rather well but the GM70 plate resistance is a lot lower (1.0K - 1.5K) than the 75TL.
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