I appreciate the offer Craig but its too nice to part out. Its pretty much all chrome and all there, chassis and all , even have the goofy pin out connectors for it. Thats alot of "ALL'S"
I got some wiring done after work tonight. I wish the pictures would show more depth... that is the cool part about building tube projects point-to-point like this. You can bend wires and use 3-D space to keep wires far apart:
A little more progress... this resistor ties the tube screen to the tube plate and makes a pentode tube act like a triode tube (the tan resistor sticking up between terminals 3 and 4). I could type a bunch of BS about how you need to keep the resistor as close to the screen terminal as possible but this picture is good enough mofos!
Hi guys, I finished up wiring this preamp today. I powered it up and tested all voltages, everything is A-OK. So I hooked it up in my always revolving system... currently my Yamaha CD player, Rotel power amp, and The Industrials MTM's. A couple of very obvious things I heard right off the bat. This preamp with about 23 dB gain sounds much more effortless and natural driving my Rotel power amp compared to my cathode follower (zero gain) tube line stage preamp. This preamp has awesome dynamics, the imaging and sound stage are fantastic, and it has zero hum or hiss. The downside, these 6V6 tubes are super mocrophonic. If I tap on the wooden case I can hear an audible thud coming through the speakers. This probably won't be a big deal. Right now the case doesn't have any feet, so some rubber feet will help. They also make some high temp silcone tube rings that are supposed to absorb tube microphonics. And I haven't tried any other 6V6 tubes...
Some final pictures to come. And after some more listen time I will add coments on its sonics. So far I like what I hear. I want to build the ClassDaudio amp and see how this preamp and it sounds together.
Good question. Since some of my parts were scavanged from an old radio I would need to find readily available replacements. That shouldn't be too hard. Just give me a little time... I've already started the next project(s) which in fact have a time line, my son and daughter's pinewood derby cars (small boy scout pack so they allow sisters to compete as well).
A while back in this build thread I mentioned that there are some very important safety steps that these high voltage amps need. The first one was the solid ground wire with star washer from the earth ground to the aluminum top plate. The next one is a low impedance path from the high voltage power supply common to earth ground. There are a few different suggested methods. The one I feel is very safe is to use a X2 rated capacitor in parallel with a 10 ohm 5 watt resistor from the power supply common directly to the earth ground. Here is a picture of an X2 rated capacitor (see the X2 marking to the right of the 0.22 uF rating and all the CSA and UL markings):
Here is a picture of that cap in parallel with the resistor and soldered between the PS common and the earth ground. These components will protect you if something from the secondary side of the power transformer shorts to the top plate. Since the B+ of this amp is 340 VDC (read lethal) this is not something you should skip just to save about $1.50 in parts.
Ok here is the next to last safety step. It is a power supply bleeder resistor. This ensures that all of the high voltage potential stored in the power supply caps is bled off to zero volts in a reasonable amount of time. I used a 220K Ohm, 2 Watt resistor. During power on it only dissipates 0.5 Watts. When the power switch is turned off it drops the voltage down to a safe level in under 60 seconds. Here is a picture (the small green power resistor):
The last safety step is of course a bottom cover to keep anyone's fingers away from lethal voltages. I don't have a picture of my bottom cover yet (sorry) but this project has one for sure.
On a side note one could argue that a top cage to keep kids' fingers away from the blistering hot tubes is absolutely necessary. I won't say that it isn't, but while a burnt finger hurts really bad, it will heal. A lethal electrical shock is, well, lethal. If you build something like this and have little visitors over... you decide
A couple last shots. I like to use the 3D space available to keep good distance between signal carrying conductors and everything else (white solid conductors), and shielded cable is super effective.
And finally here is the money shot. I wish I had Bill S. photo skills... this is the best my cell phone would do. A little blurry up front but I think you can see the glowing magic
PS - this old school volume knob is just temporary until I anodize the big fat aluminum knob.
PPS - In summary, this project has been a blast to build and share with you guys. Good morning, and in case I don't see ya, good afternoon, good evening, and good night!
Thanks Craig, very nice. And thanks for explaining the safety tips and photos of it. This will make it easy and SAFE to follow.
A lot of online circuits, etc, gives the safety warning, but doesn't tell what to do, a most common statement is "Lethal Voltage, if you don't know what you are doing, you are DEAD". Great!
Could you also high light the safety features in the circuit diagram, so it's doubly clear... !
Oh ... that looks saWeeT! There's more tube goodness in the background I see.
No big hurry on the build sheet, there are a ton of unfinished projects to keep me busy in the meantime. As soon as everything thaws, I've got sawdust to make.
Fabulous job on this, Craig. You sure have a knack for the art of point-to-point wiring! I wish all my old tube projects looked that nice under the hood.
Thanks Tom! It's by far my favorite part. The 12BH7 preamp you brought to Iowa and showed me looked very good too. It wasn't the typical "rat's nest" of point-to-point wiring I often see and cringe at. I've pulled apart a bunch of vintage tube gear to scavange parts and thought "how in the wide world of sports" can a company produce something put together like this? My heart goes out to the repair guys of yester-year that had to trouble shoot those messes.
This is my last tube project for 2018. I have two really exciting speaker projects for this year. Hopefully one of them is done in time for InDIYana, DDIY, or a Meniscus GTG... for sure Iowa!
I have greatly reduced the 6V6 tubes' microphonics by adding these high temperature silicone rubber o-rings. There are places on line that sell the exact same "damping" rings for $5 each. I got these from McMaster-Carr... package of 10 for $6.69
The bottom cover is also grounded, or it simply doesn't touch anything? Can it be wood or do we need ventilation inside the box - doesn't look like there is any hear producing components inside...
Nothing inside gets too hot. The plate load resistors drop a little over 2 watts each, so they're pretty warm.
The silicone rings mechanically dampen the tubes. Those are actually power tubes which generally are very quiet as output power tubes because they aren't used to provide much voltage gain in that final amplifier stage. But in this circuit they do have a lot of gain, so any vibrations "rattle" their fragile parts inside (the screens and grids) which can be heard through the speakers.
Comments
Some final pictures to come. And after some more listen time I will add coments on its sonics. So far I like what I hear. I want to build the ClassDaudio amp and see how this preamp and it sounds together.
Here is a picture of that cap in parallel with the resistor and soldered between the PS common and the earth ground. These components will protect you if something from the secondary side of the power transformer shorts to the top plate. Since the B+ of this amp is 340 VDC (read lethal) this is not something you should skip just to save about $1.50 in parts.
On a side note one could argue that a top cage to keep kids' fingers away from the blistering hot tubes is absolutely necessary. I won't say that it isn't, but while a burnt finger hurts really bad, it will heal. A lethal electrical shock is, well, lethal. If you build something like this and have little visitors over... you decide
PS - this old school volume knob is just temporary until I anodize the big fat aluminum knob.
PPS - In summary, this project has been a blast to build and share with you guys. Good morning, and in case I don't see ya, good afternoon, good evening, and good night!
A lot of online circuits, etc, gives the safety warning, but doesn't tell what to do, a most common statement is "Lethal Voltage, if you don't know what you are doing, you are DEAD". Great!
Could you also high light the safety features in the circuit diagram, so it's doubly clear... !
No big hurry on the build sheet, there are a ton of unfinished projects to keep me busy in the meantime. As soon as everything thaws, I've got sawdust to make.
The bottom cover is also grounded, or it simply doesn't touch anything? Can it be wood or do we need ventilation inside the box - doesn't look like there is any hear producing components inside...
The silicone rings mechanically dampen the tubes. Those are actually power tubes which generally are very quiet as output power tubes because they aren't used to provide much voltage gain in that final amplifier stage. But in this circuit they do have a lot of gain, so any vibrations "rattle" their fragile parts inside (the screens and grids) which can be heard through the speakers.
http://diyparadise.com/w/