Nice!

Looking forward to watching the progress!

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.

More “final thoughts“ I absolutely love this amplifier now. When I first finished it I thought it was pretty good, but felt it did not live up to the hype...Not a keeper. While very enjoyable, the sound could best be described as slightly filtered, maybe constrained. I thought I would enjoy it for a while then move it on. No longer. After one more simple change, it is a keeper! I originally installed Audio Note Copper Foil coupling caps in the fist stage and V-Caps CuTF in the second. Haven’t read much about both, I thought this would be a perfect blend. The Audio Notes were described as smooth and rounded while V-Caps were called precise and airy. On an expensive whim I decided to switch to all V-Caps. I removed the AN Caps in the first stage and replaced with V-Caps. Absolute shock! The difference was profound! In all my years of experimenting with different cables, power conditioners and such I really never heard much difference. Maybe if I tried hard enough I could convince myself there was a subtle change. Even tube rolling changes can be subtle. This was not subtle in any way shape or form. I don’t know that it was because the V-Caps were so much better, or that the AN caps were incompatible with my system. You may not need to spend as much as I did, but I would definitely not use AN caps in this amplifier. Keep in mind, the V caps sounded much better immediately, however there was an improvement with time. Also, unless you’re running a very low output source, I highly, highly recommend the volume pot mod. I have the upgraded 50k ohm pot. I used 62K Ohm resistor inline with the signal and 24K Ohm to ground. See the Goldpoint audio website DIY for details. As well as a more functional volume knob, I honestly do think there was a subtle improvement in sound quality…maybe (but not even close to swapping out the capacitors). This amplifier now lives up to the hype. It’s a keeper.

Okay. Guess I don’t have it figured out. Any recommendations for chassis punch sizes?

Think I figured it out. Ordered 3/4” and 1 1/16”.

Might give it a shot. 100V 100uF is not available. Could do 100 V 94uF. Is this close enough?

I’m looking at something like this: Mouser #:598-476AVG100MGBJ

Stunning! Would it be possible to share your parts list? Details and hints would be great! I am about to embark on this build. Thank you, Rob.

Excellent work Henryk! Absolutely stunning! I’m about to embark on this project. Would you mind sharing your parts list? Any tips would be greatly appreciated.

Final thoughts. This amplifier simply has too much gain. 0.270Vrms input is too sensitive for most sources. I suspect the manufacturer knows this. They have ditched the 12AX7 tube in the newer 3900 model giving it a much more reasonable 1 Vrms input which is compatible with typical 2 volt 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!

Modification complete! I’m calling it a “partial volume pot bypass“. Okay, not my idea. It’s a simple old school voltage divider mod I found on Goldpoint’s website. But the name is appropriate. You are literally bypassing a chunk of low quality potentiometer resistance with uber high quality z-foils. An improved signal path. Audible? Eh. Increase range of volume control. Definitely. It’s easy to think in this in terms of smashing the signal down twice only to amplify it more or inserting more components into the path. As an absolute novice, that’s what I thought, so I shunned the idea. Truth is you can “smash” the source signal down entirely with a “plastic carbon composite“ potentiometer, or you can ” gently lower” it with with ultra high end vishay z foils and fine tune with the pot. The voltage allowed into the amplifier and amount of amplification is identical either way for a given listening level. As for adding components, yes you are adding a resistor, but this allows you to use less of the pots resistor. A wash at minimum. A win if higher quality resistor. This mod is not necessary and of no benefit to most. Keep in mind my purpose for the mod. This amp has way too much gain for my source. My DAC has 2 Volts output. This is common of many modern sources—Some are higher. The input sensitivity of this Amplifier is 0.270 volts. The volume control was “zero to blast you out of the room” with with 2 degrees of rotation. Here we go: I first experimented with roon “headroom management” with various music to determine the appropriate dB reduction. Then I did some calculations using online voltage divider and parallel resistor calculators (based on Goldpoints modification dB tables) to figure out the voltage reduction vs dB reduction. Did some “breadboard” measurements using cheap resistors and alligator clips to volume pot. Looked the at results, read some tea leaves, spit in the wind, and decided on resistance values. Bought the vishay Z foils. A look at the mod: Top of volume pot module. On the right, Vishay Z foils added to left and right channel input pins (pins cut). The pot on left is for comparison and left stock as a spare. Bottom of volume pot board. Vishay Z foils added from input to ground after the input resistors. Reassembled. Time for a listen. Fingers crossed. Nothing! No sound. Fu@k!!! Whoops. Forgot to install jumper between pot board and main board. Easy fix. Installed. Try again. Yes! Sound!!! But wait. Pretty soft. Turn it up. 12 o’clock. Better. But let’s try more. Oh no. Full volume for satisfactory levels. Damn. The the numbers were all good. I think. Did I eff up the math??? Too much reduction? Crap! Light bulb moment. I still had 14 dB digital reduction set in roon! Digital reduction removed…Perfection! The volume control is now fully functional from 0 to 2 o’’clock with “normal” levels around 10-11 o’clock. Sounds sweet!!! Zero degradation and I swear there is a little extra sparkle to the high hats! Success

To clarify my thought process: Top Drawing WITHOUT voltage divider the signal travels further through pot “lower quality“ resistor vs lower drawing. The 2 volt input signal sees the exact same resistance in both circuits. Just more “high quality“ resistance and less “low quality“ resistance for the exact same 1 Volt to the amplifier output in bottom circuit. Not saying this will sound better, but, contrary to intuition, that no harm comes to the signal doing this. In fact, even if inaudible, an argument can be made, at a there is a benefit. IF your amplifier gain is too much for your source this will certainly give you increased “range” on the volume control. (There are effects on the overall impedance seen by the source, but this can be negated, or even used to advantage by careful selection of resistor values vs dB reduction and source output impedance. That’s for another post.) Thoughts?

Thanks for input. No bypass cap. Tried subbing 12AU7 which helped a little with gain but sounded like crap. For now, I’m using roon software to reduce gain which works but I still feel quality suffers. There is an entire debate about this on the Roon website. In theory it should not affect the quality, but I swear it does. At first, I was amongst the “it can’t possibly affect the quality” crowd, but the more I listen, I swear it does. Maybe it’s in my head. But I don’t like it. I’m going to implement a voltage divider mod ”in front“ of the pot per Goldpoint website. Gain better matched to source would be ideal, however, there’s an argument to be made for high quality resistor voltage divider in front. Here is my newbie understanding. Please let me know if I’m wrong. For a given listening level, say 70dB from seat, you need a given input voltage to the amp. Let say it’s 1 Volt. If your source is providing 2 Volts that has to be reduced (divided), in this case, by half. That’s what the volume pot does. It’s a variable voltage divider. At lower volume setting, the signal passes through the more of the pot’s “low quality” resistor. Inversely, at higher the volume settings the signal goes through less of the pot’s resistor. Reducing the signal with a voltage divider made with high quality resistors (vishay Z foils?) allows you to use higher volume settings. In effect, you are bypassing part of volume pots “low quality” resistor with higher quality resistors. To be clear, I doubt putting the divider in front of the pot will cause an audible increase in quality, however it should NOT degrade the signal. The signal, in theory, should be slightly better. So there is no downside to this mod and a possible benefit. Keep in mind the original premise. Too much gain for the output of my DAC. This would give me the increased range on the volume control I seek with no degradation and a slight, if inaudible, benefit.

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.

Nice! So many projects I want to do and so little time! You’ve just added to the list!

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.

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.