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!
I built the same amp... did not have the the problem with the volume pot , but mine had the "larger" blue alps than the cheaper one that 1st came out in the kit, that said your TDK-=50K and the alps=100K. reading your other comments about un needed gain -it's not really that it's a gain issue as all the tubes are needed 1/2 of a 12AX7 and a 12AU7 in parallel to get 200v p-p swing (without clipping) that's needed to drive a 300B. Amp was Designed by a well know designer in Japan and he worked with Mr. Lundahl to develop the output transformers specifically for this amp. I been thru many tube amps and this is by far the best sounding I had though it's my 1st 300B.. I have come to love the low power SET's over the higher PP amps.. just me, but you'll need efficient speakers. Was alittle dumbfounded after I had the kit noticed the specs say 10%thd@ 9 or 10W? I'll have to run it thru my audio analyzer but no matter what it is on paper the sound is fabulous! When I 1st built it looked at it on the scope and saw symmetrical clipping at 23-24V p-p .. so it's ok up until 8.2Watts or so - square wave shows some ringing. It's like the the best of both worlds - new/old that really works- Really a great kit and very easy- like painting by numbers a little easier than when I built my 1st tube amp in the 70's a Dynaco ST-70 (I sill have 2 ST-70's) this amp with a constant current "no sag" the SS rectified DC is just as good as with tube rectification and the B+ delay relay works out great for the tubes. it also utilizes an auto bias ckct. $2K without tubes is not cheap but it can compete with some costing much much more. I really really want to build Skunkie's 300B amp!!!! with the point to point it's better to me to modify and work on than a pcb and want to stay a little more true without all the SS. And will be soon - hope she has a parts kit out soon - it would make it so much easier. One can never have too many amps
Update! Absolutely floored by this amplifier! … After two changes.
After completion I thought this was a nice amplifier but not an endgame. It did not live up to the hype for me. The sound was nice, but somehow filtered and constrained.
As you can see above, I put AN Copper coupling caps in the first stage and V-Caps CuTF in the second. Having read the AN caps were smooth and rounded, and the V-Caps were detailed and airy I thought this would be a good blend.
On a very expensive gamble, I pulled out the audio note coppers and replace them with another set of V-Caps. Hoping to open up the sound. Wow! Dumbfounded at the difference.
Even with no burn in the on the new caps the difference was immediate. They did get better with time. Keep in mind, I already had one set of V-caps installed and burned in for four weeks. The manufacturer claims a very extended burning time so if you install all four V-Caps at once it could take over a month to reach the manufactures recommended burn In time. Not certain I buy into extended burn in, but can’t hurt.
This was no subtle change. I have spent years experimenting with different cables and power conditioners to very little if any effect. Even tube rolling can be hard to differentiate. This was not that. This was profound.
I don’t know if the difference was because the V-Caps were that good or the AN were that bad (in this amp). You may not have to spend as much as I did for V-Cap CuTF but I would not recommend the AN copper coupling caps for this amp.
The different couple in caps and the volume control modification (as described in my DIY post) and this will be with me for a very very long time. The hype is real.
Yeah that is the problem with some older designs and modern sources.
Final thoughts. Great experience building and I learned a few things along the way. Enjoying the amp. However if I had do overs, I would have spent the extra on the TU-3900VK that ditches the unnecessary 12AX7 tube.
The 3900 has just the two 12AU7s which gives it an input of 1Vrms which is Much more compatible with today’s 2V sources. The 3800, with the additional 12AX7 has 0.270Vrms input making it way to sensitive. As stated in earlier posts, the volume control is almost an on/off switch. Too soft to too loud within a couple of degrees.
I came up with a work around that I’m happy with posted in “questions“ section of forum. The real solution is less needless gain. I suspect the manufacturer was well aware of this issue and thus the 3900.
Next build is a Skunkie 300B!
A lot of old tube amp schematics show "input 300mv" which clearly would be way too sensitive for modern sources. It's why I think I can get away with possibly a single stage driver on my 47 amp. BTW is there a schematic for this amp?
Amp has waaaaay too much gain for my DAC which has a 2 volt output. Volume control almost on/off switch. Better match with my phono preamp.
Seems to be an issue with all tube amps I’ve tried. This one is worse than most. Many tube amp designs predate the high output sources of today. Maybe that’s the problem—designed for old school lower output phono section??? Dunno.
Going to try a voltage divider in front of volume pot per Goldpoint websit. Hard to recommend this amp if your source is 2 volt out. Going to post some “questions” regarding gain and such.
Bottom of board. Two resistors carbon film (plate load—AMG and pink), handful of z foil, rest takman metal film.
Top of board. TKD pot upgrade.
My one moment of inatention. Brushed red wire with soldering iron :-/ Also of note, OT blue and white wires. 300B ”cathode NFB circuit”
Anxious to listen—Cover off yet to install! Speakers are BLH fostex kit from Madisound with Lii Audio 8” drivers. Denafrips Aries II from Allo USB signature bridge. Computer in hall closet (next to router) running roon. REL T/7X filling in below 80 Hz
Project “complete”. Excellent sound. Staging, clarity and detail far exceeds any of the budget Chinese amps I’ve tried. I also had a very expensive Polish manufactured 300B for a very short time. I returned it do to hum issue (clearly heard across the room) so I can’t make a direct comparison but from memory I prefer the Elkit amp. This amp has near zero hum. Can barely hear it with ear against driver with very sensitive full range Lii audio drivers.
i’m very pleased with my selection of components (See above)) One thing I did not mention, I did use vishay Z foil resistors at the input (to the first stage grid and ground from the pot) and on what appeared to be a global negative feed back circuit (output transformer and cathode of first tube)
The one issue. I can barely crack the volume open before It becomes too loud. 7 o’clock is louder than I like. Beyond 9 o’clock drives me out of the room. Very difficult to fine tune volume with such a limited range. This issue is not unique to this amplifier for me (I own/have owned far too many amplifiers) but is more pronounced than my other amps. This may be a combination of my speaker sensitivity and the Denafrips DAC higher output. I’m going to see if I can install a 2 resistor attenuator circuit, per Goldpoint Level Controls website, on the input using Vishay Z foil resistors.
As for the single-sided solder pads on the board I mentioned earlier: they do have purpose. First, they deter impatient or inexperienced builders (me) from accidentaly putting a component on the wrong side of the board. Secondly, if you want to later upgrade components it’s much easier to remove the old ones. Double sided through hole pads can be a bear to remove a component, especially if it has multiple leads. I have tried to add pictures, but unable.
Thanks for the review!
After further examination of the headphone jack with magnification, the contacts inside look fairly substantial and I decided to leave the headphone jack in circuit. It’s a fairly simple fix if I ever want to take it out.
Their is much to like about this kit. From the integration of heat sinks and ventilation holes into the rather substantial PCB board, to actually integrating A twisted pair into the board from the input jacks to the volume control. Power supply lines and signal lines are also very well separated. The power supply is beefy and well regulated and has separate substantial ripple filters for the left and right channels.