A very cursory search suggests the plate imedance of a 75T is around 5k. At the ~60% dissipation of your circuit it's significantly higher still. The amp's damping factor must be pretty low.
An outside possibility for the top end problem is a defective cathode bypass cap. Quick and dirty sanity checks are soldering another across it or better yet scoping the cathode voltage with varing frequency.
Another thing to try is to lift the transformer from chassis or wired ground. WITH THE POWER OFF! 
Then with it blocked up on some wood, plastic or other insulator, try it again...
Might tell you something...
Also , use a dummy load resistor to test, not a speaker.
Changing the resistance of the dummy load will alter the reflected impedance on the primary, fwiw.
_-_-bear
PS. as I said, try the amp without an output tranny, just an appropriate resistor matching the plate Z... skip the output iron, see if you get flat freq response - this will tell you 100% if the problem is in the iron or in the circuit.
Then with it blocked up on some wood, plastic or other insulator, try it again...
Might tell you something...
Also , use a dummy load resistor to test, not a speaker.
Changing the resistance of the dummy load will alter the reflected impedance on the primary, fwiw.
_-_-bear
PS. as I said, try the amp without an output tranny, just an appropriate resistor matching the plate Z... skip the output iron, see if you get flat freq response - this will tell you 100% if the problem is in the iron or in the circuit.
I tried with 1 OPT removed from the chassis and just sits on the side, doesn't affect anything.
Ok, I will get an 8 ohm resistor.
The strange thing is the different level of voltage drop with the OPT secondary floated and grounded. This is the same with both Lundahl and Tamura OPT.
Does anyone have any idea why this is ?
Ok, I will get an 8 ohm resistor.
The strange thing is the different level of voltage drop with the OPT secondary floated and grounded. This is the same with both Lundahl and Tamura OPT.
Does anyone have any idea why this is ?
My guess is that there is some undesired coupling from the primary to ground through the secondary when the secondary is grounded. If this is the case, you will see a relatively large signal voltage between the secondary and ground when the secondary is floating.
Do you have a scope or AC voltmeter you can use to measure at the AC voltage from the secondary winding to ground when the winding is floating? It may be hundreds of volts peak to peak at full signal so I would use a high range first and low signal.
that could be ripple on the B+ getting coupled.
Measure either side of the secondary to ground with a normal load connected across the secondary (floating).
How much as you increase the output signal towards 1V?
What seems to be happening is this 15V part of the primary signal is getting coupled common mode to the secondary. This is capacitive coupling which becomes significant above 2KHz. When you ground the secondary you're shorting out part of the high frequency signal on the primary through this coupling. You're losing some high frequency through this coupling even when the secondary is floating.
Whether the coupling is normal for this transformer or not is an open question. I would expect this transformer to be well sectioned and have low enough capacitance to work with the 75TL internal resistance. Maybe connection B would work better, assuming you don't have a defective pair of transformers. It acts like the windings are connected in the wrong order, but we checked that...
I'm curious, does the sound you hear from the OPT also increase when you ground the secondary?
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I would try connection B next then. Maybe you already know to be careful with the pins on the Lundahl. The brown board material gets soft when heated and the pins tend to come loose if overheated during desoldering.
It's a basic problem with high resistance triodes and no feedback. The F-5003 is meant to reflect 5K ohms but if you're using the 4 ohm output as shown on your schematic, then you're actually reflecting 10K to the 75TL. Still, it was meant to be used with a lower resistance tube than the 75TL so the roll off isn't surprising.
The Lundahl was designed for high resistance tubes, therefore I would expect better performance. That's why I think connection B might work better.
to be clear - my suggestion is to replace the transformer entirely with a power resistor equal to the suggested load resistance per the tube manual's specs. In this case you could probably be happy with any power resistor in the range of 5k-10kohms. 25+ watts is enough for testing I would think... unless ur quiescent bias current produces more watts... you need to equal that wattage.
If you get a flat frequency response (cap couple it to keep ur scope safe, and keep in mind that the cap will get a nice DC charge on it - or you can hang a high impedance voltage divider off the junction of the plate and the resistor and drop the measured voltage that way... low watts there... THEN you know the issues are not the rest of the amp.
As far as testing the transformer and the proper taps - if you have a solid state amp, then drive the secondary with it, put that same 5-10kohm power resistor (but you can use less wattage, if you don't crank the driving amp) and see what the response is across the primary + resistor... you may find that you have to pick the right taps AND also get the phase right. So, don't start with anything strapped. Pick just ONE pair of secondary terminals (one winding), and one pair of primary terminals (one winding) and see how that works. Then go to two on the secondary, finally, figure out how to do the primary taps...
Be sure to not be running signal when you touch the primary taps/wires - High Voltage.
_-_-bear
If you get a flat frequency response (cap couple it to keep ur scope safe, and keep in mind that the cap will get a nice DC charge on it - or you can hang a high impedance voltage divider off the junction of the plate and the resistor and drop the measured voltage that way... low watts there... THEN you know the issues are not the rest of the amp.
As far as testing the transformer and the proper taps - if you have a solid state amp, then drive the secondary with it, put that same 5-10kohm power resistor (but you can use less wattage, if you don't crank the driving amp) and see what the response is across the primary + resistor... you may find that you have to pick the right taps AND also get the phase right. So, don't start with anything strapped. Pick just ONE pair of secondary terminals (one winding), and one pair of primary terminals (one winding) and see how that works. Then go to two on the secondary, finally, figure out how to do the primary taps...
Be sure to not be running signal when you touch the primary taps/wires - High Voltage.
_-_-bear
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