Midway through the build of my TU-8600S with Lundahl transformers. Some thoughts and questions. Keep in mind I am a rank amateur so take what I say with a grain of salt. Open to correction and suggestions.
Well thought out kit. This is my first build of any kind. Complete beginner. Tedious and meticulous work but if you pay attention to detail and can solder it can be done. My soldering skills are minimal, so I bought a few practice kits.
Here we go:
Through hole solder pads are single-sided with no copper lining the hole. Also, the pads do not go to the the edge of the hole. (There is a fraction of millimeter of bare plastic PCB board around the hole). Unless you bend the leads perfectly flat to the board, which is near impossible, there is no metal to metal contact between the leads and the solder pad
The entire speaker level output of both channels goes through a rather flimsy looking spring metal contacts inside a tiny cheap headphone jack before it goes to the speaker binding post. When you plug the headphone in, it breaks the contact. Seems rather a waste to have high end cables and binding post when the entire signal is passing through a tiny point of contact in a cheap jack first. An argument can be made for listening to a tube amp from the transformer secondaries (speaker out) to get the magic of the output tubes and transformers, but this seems a poor way to implement it. I would’ve preferred a high-quality switch.
Question: I can bend the output leg from the speaker “ohm selector” switch up (not attatch it to pcb board) and wire from there directly to the speaker terminals. Importantly, this mod requires NON conductive binding posts!!! (The posts screw into a pcb board and electrical contact is made when tightened. This will negate the bypass and do who knows what damage if headset plugged in! Again brass terminals, including those in the kit, will not work.) There is then the issue of the negative terminal. I have a plan but won’t expound. This will bypass the headphone jack, cheesy contacts and all. Of course, then I would have no headphone output. I don’t own headphones…but like the option. I’m still on the fence. Should I? Thoughts? I’ve made this mod seem complicated. It’s not.
Resistors. Kit comes with half watt miniature resistors. Unknown quality or type. Often recommended to replace, but the hole spacing it’s too close together for most upgraded 1/2 watt resistors. Meticulous looped bending of the resistor leads is required to upgrade. Ridiculous. Please, space the holes wider!
My prior understanding was that the TU-8600S had limited local feedback. There is an output from the primary side of transformer (apparently unique to this kit) that runs back to the output tube labeled “cathode NFB” on the schematics. However, there is also a line going from the output transformer secondary windings (speaker side ) all the way back to the cathode of the first input tube. I have very limited knowledge on such, but this seems to me like it is a global NFB feedback loop.Q: Hoping somebody smarter than me can enlighten me. Is this global NFB?
Against advice, I have gone with metal film takman resistors throughout with two exceptions. (The carbon film takman throughout is the “favored” upgrade).
My thoughts on this. Carbon equals noise. Hiss. (The only thing I hate worse than hiss is hum) and no benefit throughout much of the circuit.
The “magic” from carbon is that the resistance varies slightly with voltage. Yes, this is a form of distortion. But so what? Tube audio is about the subtle distortion that our brain likes. Auditory heroin. If we were after a perfect signal, we would not be listening to SET tube amplifiers. These voltage induced resistance changes are most pronounced with carbon composite resistors. Carbon films still exhibit this effect, but less. Here is the catch: At lower voltage, this variable effect is negligible to nonexistent. At low voltage all you’re getting are looser tolerances and noise from carbon. Not what you want the input stages, feedback loops or power supply. The place where you have the high-voltage and voltage swing to induce this effect are on the plate load resistors. (R105, R205 and R112, 212 in this kit) Oh, importantly, you do not want to up the wattage of these resistors as this will negate the desired effect. I used takman 1/2 watt carbon film in the first and AMRG 2 watt carbon film in latter. Reality? This is likely a load of crap! Probably no audible difference. But it willsound better! Because I did it! Besides, If it sucks, I only have to swap four resistor.
I left all of the electrolytic capacitors stock—Nippon Chem-Con 105 degree. Good reliable capacitor.
For coupling caps I’m using Audio Note Copper Foil first and V caps to output tube. These capacitors will functional electrically in either direction, BUT you want to hook the outer foil to the incoming signal in the inner foil to the outgoing signal. The Signal coming out of the inner foil is shielded by the outer foil, thus noise is reduced.
Thoughts, questions and ridicule all welcome! I’m about to wire in the headphone jack! Input on this needed soon!