I just measured an EDCOR GXSE15-8-3.5K at 56 ohms. I doubt the 1629SEA will run much more than 150 ohms DC resistance considering the ratings of both OPTs. I expect it will be less.
The Edcor transformer is only a 15 W 100mA bias transformer. The Hammond is rated at 30W with 100mA bias. Taking the ratio of the impedance divided by the SqRt of 2 ( [6500/3500]/2sqrt) gives a change of turns ration of 1.3:1.
1.3*56=73 Ohms. even twice that would be less than 150 ohms.
The Edcor transformer is only a 15 W 100mA bias transformer. The Hammond is rated at 30W with 100mA bias. Taking the ratio of the impedance divided by the SqRt of 2 ( [6500/3500]/2sqrt) gives a change of turns ration of 1.3:1.
1.3*56=73 Ohms. even twice that would be less than 150 ohms.
These kinds of questions are why you want to build the thing on plywood. The DCR of the output transformer and the plate voltage of the driver stage set your output stage current. You may need to add a series resistor (bypassed with a cap) between ground and the primarily of the OT.
I do also agree that you're going to get a mouse fart's worth of power out of this circuit.
Thank you for the suggestions. Do you think that with this amp thermal problems can easily occur not using a solid state rectifier?
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1W or so is enough 
****The one caution here and others should jump in - for a DC coupled circuit it may be mandatory to use a tube rectifier to get sensible slow turn-on behaviour as the output tube can experience some pretty rough start-up currents otherwise.
The tube rectifier draws a lot of heater current and most tube power transformers run fairly hot if you use them anywhere near their ratings. The tube rectifier it'self will also run fairly hot. A SS rectifier will not have any thermal requirements, it'll run without issue even hidden inside the box.
****The one caution here and others should jump in - for a DC coupled circuit it may be mandatory to use a tube rectifier to get sensible slow turn-on behaviour as the output tube can experience some pretty rough start-up currents otherwise.
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The declared wattage of this amp is around 4.5W. I'm not going to use this as a guitar amp and I was looking to a small but high fidelity amplifier that I can build as second attempt (I have already a 15W modified heathkit A7-5 tube amp that I usually use at half of the capable volume).
Do you have other schematics of vintage 6v6 or 6l6 hi-fi amp that you think it might be better build?
Thank you for the suggestion. Unfortunately I do have some experience in building but very little in modifying circuits. Would you be so kind to upload the modified schematic for ultra-linear?
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Here attached the original article. I underlined the major aspects of this amp that seem not realistic considering your opinions.
Seems that the only problem is obtain a 200ohm of primary resistance of the output transformer but as some of you stated is easily obtainable adding some resistors in series with bypassed with a capacitor. (advisable values of wattage and capacitances?).
Do you want add any other observation? Is this amp worthwhile to construct (not in term of costs - i.e. I know that the psu can be replaced by SS and the choke can be removed etc. - but in terms of performance)?
Seems that the only problem is obtain a 200ohm of primary resistance of the output transformer but as some of you stated is easily obtainable adding some resistors in series with bypassed with a capacitor. (advisable values of wattage and capacitances?).
Do you want add any other observation? Is this amp worthwhile to construct (not in term of costs - i.e. I know that the psu can be replaced by SS and the choke can be removed etc. - but in terms of performance)?
The numbers seem a little off to me.
The bias current of the output stage is mostly set by R3 and R5. Ignoring the roughly half milliampere screen current of the 6SJ7, and the OPT primary resistance you will get 62.7mA of current. At 100R primary resistance you will get 58.4mA, and at 250R pri you will get 52.95mA.
This is assuming that the cathode of the 6V6 is at 85V.
R3 and R5 provide negative feedback to the screen of V1 which stabilizes the bias of V2, but also reduces the gain of V1.
The bias current of the output stage is mostly set by R3 and R5. Ignoring the roughly half milliampere screen current of the 6SJ7, and the OPT primary resistance you will get 62.7mA of current. At 100R primary resistance you will get 58.4mA, and at 250R pri you will get 52.95mA.
This is assuming that the cathode of the 6V6 is at 85V.
R3 and R5 provide negative feedback to the screen of V1 which stabilizes the bias of V2, but also reduces the gain of V1.
It's too bad that Bates wasn't able to provide some basic performance measurements, but this circuit is certainly promising enough to construct on a breadboard at least. Direct coupling is a major advantage of this design. It also presents a low impedance to the output transformer primary, and it includes a short feedback path to the driver tube screen, bypassing the OPT, which should keep it stable regardless of OPT substitution.
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I have just found the Edcor GXSE10-5K as output transformer (5000ohm primary inductance with 250ohm of d.c. primary resistance) which is the closest in comparison to the other output transformer mentioned right? (EDCOR - GXSE10-5K)
As for the choke the Hammond 157L (http://www.mouser.it/Search/ProductDetail.aspx?R=157Lvirtualkey54600000virtualkey546-157L)
And as power supply the Hammond 373BX (http://www.mouser.it/Search/ProductDetail.aspx?R=373BXvirtualkey54600000virtualkey546-373BX).
Can you please confirm these components so I can buy them?
As for the choke the Hammond 157L (http://www.mouser.it/Search/ProductDetail.aspx?R=157Lvirtualkey54600000virtualkey546-157L)
And as power supply the Hammond 373BX (http://www.mouser.it/Search/ProductDetail.aspx?R=373BXvirtualkey54600000virtualkey546-373BX).
Can you please confirm these components so I can buy them?
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So sorry but may I ask you what are these files? I use a Mac as pc and the extensions are unknown.
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