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roblund7
DIY Champ
DIY Champ
Mar 31, 2022
In Show and Tell
Long overdue Review of my A12 personally modded by Skunkie In a side-by-side listening comparison to unmodded A12, well, there is no comparison—Skunkie modded blows it away! Wow! End of review. Thanks Skunkie!
Lucky Owner of A12 From Video Series content media
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roblund7
DIY Champ
DIY Champ
Feb 05, 2022
In Questions & Answers
What are the required punch sizes for 4 pin, 8 pin and 9 pin sockets? Also, any recommended sources. All I seem to find are for electrical conduit work.
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roblund7
DIY Champ
DIY Champ
Feb 01, 2022
In Questions & Answers
Gathering info for my upcoming Skunkie 300B build. I have a question on the 300B cathode bypass cap. I just recently completed a TU-8600S kit build. This amp is really starting to grow on me. (Will be updating my review shortly). Seems part of the “secret sauce” for that amplifier is the use of low ESR capacitors in key locations. This is emphasized in the instructions. Low ESR (equivalent series resistance) caps are also called Aluminum Polymer Electrolytic. Apparently they “respond” faster than the standard Aluminum Electrolytic cap for crisper audio and less smearing. Wondering if a low ESR cap could be used in place of the 100uF 100v 300B cathode bypass cap? Here’s the rub: The highest value 100 V polymer cap is 47uF. I was wondering if it would be acceptable to run a pair of these in parallel for 94uF? I imagine this would further decrease the ESR also. I have read, but don’t understand, in some circuits too low of an ESR can be a bad thing. Is there any risk from doing this? Another thought would be to run two of the typical electrolytic capacitors in parallel. This would reduce the ESR also, but nowhere near as much. I would love to see this set up scoped but I don’t have the equipment or technical know-how. Skunky? Are you up for it?😊 I know you are very busy! However any thoughts would be helpful.
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roblund7
DIY Champ
DIY Champ
Jan 22, 2022
In Questions & Answers
Hi I posted some thoughts and photos on this amp on show and tell page. Overall pleased but, as I mentioned in that post, the gain is way to high for my source. Looking for suggestions to tame it. The amp is a 300B design. The first section is a single 12AX7 split between channels feeding a pair of 12AU7s paralleled to each channel. My source is a Denafrips DAC with RCA output at 2.0Vrms, 625 Ω Options: 1. Lower gain tube in V1. I tried a 5751 (which has 30% less gain than the 12AX7) in the front. Their was an almost indiscernible decrease in level. I also tried an 12AT7 with similar results but a slight loss of quality. No help. 2. Any thoughts on changing out resistors or caps to lower voltage/gain in pre section while keeping tubes in designed operating parameter? Don’t know if it helps. Here’s the spec idle voltages from schematics: 12AX7: Cathode: 0.8V. Anode: 110V. Grid: ?? 12AU7: Cathode 8.7V. Anode 175V, Grid 0V. 300B: cathode???, anode 360V, grid -70V 3. Voltage divider in front of volume pot. Looking at something similar to Goldpoint website. They have tables with VERY specific values based on pot impedance. Is there a reason for this? If I step outside of these values will there be impedance matching issue? These are not common values and I do not want a gaggle of resistors. Any recommendations for about a 12 dB decrease? Current pot is 50 Kohm. Source output is RCA at 2.0Vrms, 625 Ω Thanks
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roblund7
DIY Champ
DIY Champ
Jan 13, 2022
In Show and Tell
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? My modifications. 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! Thanks all Rob
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roblund7

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