Thanks for the replies regarding my question about integrating the TL-404 into a Mainline amplifier! I have indeed ordered a pair of these transformers, so I will definitely give this project a try and report about my experiences (both technical and soundwise) on the forum. I am still a beginner (with a background in physics though, but with no experience in electrical circuit design), so this will well take some time. Additionally, I am just building the Mainline, so I want to spend some time with the stock design first before changing it. (I am also feeling guilty to think about a relative layman doing modification of the OT-3 output, which is built on purpose, sophisticated and thought through, even before having actually heard it. It was just that the TL-404 seem so rarely on offer, are so beautifully built and get such rave reviews. And experimentation is half the fun of DIY...)
As for Paul Joppas comments (great, thank you!), I have done some thinking and reading and would like to post my thoughts (hoping some of this may be helpful to others, as well as perhaps getting some new ideas back):
1. magnetic hum-pickup(1): From pictures of other Bottlehead kits, I deduced that different transformers inside one device always seem to be oriented with the axes of the coil's windings perpendicular to each other. I interpret this as a simple consequence of basic physical laws: The magnetic field of a coil (with current flowing through the windings) should be strongest in the direction of the axis (that runs central through the wound core). The voltage this field induces in another coil is bigger if the magnetic flux lines intersect this second coil's wound core perpendicularly, rather than lying in the plane of the second coil's windings. So the induced voltage should be largest if the coils are close to each other and have the same orientation (like birds on a wire) and smallest if they are farthest apart and their axes are perpendicular. Since the orientation of the TL-404 coil axis relative to the mounting holes seems to be different than in the OT-3 (as far as I can see from images of the TL-404), I presume I will have to at least mount it rotated by 90°.
2. magnetic hum pickup(2): To experimentally find (or verify, see above) the best position for the tl404, I got the idea that I may just take one of the TL-404 transformers, hook it up with it's full (autoformer) winding to a voltmeter ac input and use it as an antenna to pick up the stray field of the power transformer. I could move it around close to the live amplifier (without big hazard, since it is not connected to the circuit) to get ideas at which position/in which orientation it is least influenced by the power-transformer's field.
3. new value of parafeed capacitor for use of TL-404 in Mainline: As I understand it, the coupling capacitor, output transformer, plate load and tube form a "network" that will let only certain frequencies pass to the ouput (i.e. it is a filter). I have read in a post by Paul Joppa (thread is here:
http://bottlehead.com/smf/index.php/topic,35.msg79.html#msg79 and there are serveral more for this topic) that there is a formula to at least estimate a good candidate for the parafeed capacitor value in case a plate choke is used. However, there is a constant current source in the Mainline, and (as I understand it) Paul states that there is no simple way of calculating an appropriate capacitance in this kind of circuit. So I assume the suggested 5-8uF for the parafeed capacitor in combination with the TL-404 is either from experience or from a more sophisticated model (which I think I have read somewhere Paul has). From my simple understanding of an LCR-circuit (oscillator circuit), the resonance frequency is proportional to the product of L and C. Since the TL-404 has a 5000 winding primary, and the OT-3 seems to have a 4000 winding primary, I expect the inductance L of the TL-404 to be higher. So this may be an approximative simple explanation for using a smaller capacitance C - to have the product of L and C not changed too much? In several posts by Paul Joppa, I read that also capacitance values between double and half the value that is calculated by the simple (yet in this case not applicable) formula should yield good performance. I also read that in the end, quantitative measurements and critical listening are necessary to find the optimum. So perhaps I should experiment with 2 or 3 reasonably priced capacitors of different capacity before probably getting a more "boutique part" with just the right capacity.
I do not want to "clog" the post with too much theoretical/hypothetical considerations and do not want to go off-topic. I am happy about feedback and promise to let you know about my practical findings as soon as I have verything ready and have had the chance to experiment.
