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Little Sweetie Forum amplifier project


henry4841
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11 hours ago, rigma said:

I can vouch for this, as well. I've reamed out many a vintage cymbal mounting hole from the old 7/16" to the new 1/2". This demurring tool works well to break the sharps.

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I had hoped one of you smarter than me guys would jump in and explain how one of these tube circuits worked. I am 73 and self taught about tube circuits in my 60's with a lot of help from Maynard. I admit my memory is not what it once was and I am too lazy to do all the searching for info again. But I am going to try and explain the 6Y6 tube circuit operation from memory best I can. If I make mistakes or am wrong someone jump in and help me. I am going to do this as if talking to my teenage son. 

 

Print out the 6Y6 tube pinout from the 6Y6 tube data sheet, google, to follow along. On the schematic of the 6Y6 tube at the bottom you will see an arrow looking thing with 2 pins. This is the heater, pins 2 & 7 of both tubes. This is where we are going to attach the 6.3V AC from the transformer. This is what makes the tube glow but we are not doing this for light but to heat the next thing from the bottom up called the cathode pin 8. When the cathode gets hot enough it starts shooting electrons everywhere inside the tube. We want to catch some of those electrons with the plate which is pin 3. The cathode is negative charged and the plate positive charged and with electron flow from negative to positive it catches some electrons. Too many. If we do not stop so many getting to the plate it is going to burn up. This is where the control grid, pin 5 comes in. It is negatively charged but not as much as the cathode. Negative meeting negative electrons repel each other much like in a magnet. This limits the amount of electrons reaching the plate keeping it from burning up. We are not going to cut off the flow of electrons but limit the amount going to the plate. This is called biasing. This tube is good for 12.5W and we are going to make it use 9W of dissipation to have good sound and long tube life. Do not take my numbers as fact. Just an example. Using 9 watts of the tube will get us 1 clean watt of audio power. This is class A operation, wasteful but keeping the audio signal as intact as possible with no manipulation as other forms of amplification do. The bias circuit is make up of R8 and R7 along with the C5 cap that is used to give the circuit more gain. Attach these parts to the pins in the schematic. I am not positive what function the other grid on pin 4 with R9 resistor does. It has a positive charge less than the plate because of resistor 9. Possibly to speed up the electrons after going past the grid. Help guys. But whatever it does the main thing to know is about triode amplification. A true triode tube does not have the other grids, only the control grid pin 5. The C4 cap is prevent DC from the other stage getting on the grid pin 5. You cannot have DC from the source or another stage on a control grid for it to operate properly. The plate pin 3 is going to be connected to the OPT by the wire. On the 6SJ7 tube we are going to tie the pin 3 grid to the cathode making it much like the 6Y6 tube. Other than that the circuits does pretty much the same thing with different numbers. 

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A tip for you first time builders. Put those RCA inputs close to the potentiometer. Less chance of ground loops, noise, this way. You will not have to use shielded cable to connect them to the pot this way. Like this picture of an early build of mine for my personal use. Notice the output jacks are mounted on the sides as well for short wire hookups. This is a SET 45 tube build that does not get much use, notice dust. Throwed a lot of money, for me, into this build. To me the sweetie sounds better. Just me and my ears. Do not get me wrong. This SET 45 sounds really good it is just to me the tone is better in the Sweetie. I think Maynard really hit the bullseye with the Sweetie design. 

 

 

P1030990.JPG

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I've always understood the logic behind placing the inputs thusly, but never could live with the execution. I don't want RCA cables running to the front of the amplifier. Just doesn't look tidy enough for me. I'll just have to live with whatever I get with rear placement of inputs.

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If done correctly will work just as good and definitely look better in the rear. The one I am building has the all connections in the rear. In the front just makes it easier for a first build. One can do the next build, after they learn more, for appearance. Just thought I would throw that tip in. In the front is not going to hurt the appearance in my room. Only close friends and relatives can enter.  🙂

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Another reason for the tip is because I want the beginners first build to not have the dreaded hum many first builds tend to have if one has not learned all the fundamentals of amplifier building. I want as many first time builds to be successful as possible sonically. KISS again. 

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24 minutes ago, Shakeydeal said:

I've always understood the logic behind placing the inputs thusly, but never could live with the execution. I don't want RCA cables running to the front of the amplifier. Just doesn't look tidy enough for me. I'll just have to live with whatever I get with rear placement of inputs.

If you ground properly and use shielded cable inside the chassis, you will not get noise or have any issues. A shielded microphone cable (Belden) works very good for input wiring. 

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I wasn't alluding that there would be problems with rear mounted inputs. But I know some builders like to keep the signal path short and tout the benefit of having them there. Like all the early Dynaco amps. I just don't like the aesthetics of them mounted on the front panel. Some don't mind.

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4 minutes ago, Shakeydeal said:

I wasn't alluding that there would be problems with rear mounted inputs. But I know some builders like to keep the signal path short and tout the benefit of having them there. Like all the early Dynaco amps. I just don't like the aesthetics of them mounted on the front panel. Some don't mind.

I agree. I don't like front mounted either. I especially dislike the chassis's where they put the speaker & RCA's on the top, then the cables are upright and hanging over. Ugh...

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Made some more progress. Installed transformers and PS section. Made an error in layout though. Sucked. I had to move the PS transformer over some to make room for PS section I built on a board. I have the option of building another PS section with terminal strips or purchase another chassis and start over. I picked move the PS transformer over and have the hole and see if it is going to be a problem with prospect. Now I have an extra hole in the top that I wish was not there. If it is a problem with my prospect I will have to find another buyer or just add it to my collection. Start wiring it up tomorrow. Enough done for an old man today. This thread is really not about selling but rather to get more people into building their own amplifier. I hope members do not mind pictures of my daily work. 

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P1030995.JPG

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22 minutes ago, henry4841 said:

Made some more progress. I hope members do not mind pictures of my daily work. 

 

 This is awesome I enjoy the pictures and watching the build I'm sure others do as well. I really don't see the extra holes being an issue someone will happily buy this from you when it's complete thanks for posting!

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The PS transformer is closer to the output jacks than I like but that transformer is shielded and I can always use some shielded cable to connect the OPT to the output jacks if necessary. That an I can move the PS transformer a little more forward away from the jack. I could just call the extra hole a ventilation hole but it is covered by a pad for the PS transformer.  

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On 8/15/2022 at 8:31 AM, Curious_George said:

Shingles. I would like to have bought a steel/metal roof, but the price was a lot more than I was willing to spend. Plus, I had to save some money for my future in ground pool. 

 

I remember replacing the roof on my last home with shingles, because the Sun had badly damaged it, particularly on the South and Southwest sections.  At least I used white shingles, and installed a Whirlybird vent, which I thought was kind of cool.  Those two things, the white roof instead of black, and the very efficient vent, allowed the attic and the top floor of the house to be noticeably cooler in the summer.

 

Meanwhile, my mother had replaced the roofs on her last house and on her current house with steel, with no indication of ever needing to replace it.  She’s a vocal fan of steel roofs, while I live in a condo apartment, which has sheet roofing, but we’re not as free in some of the choices we make, which must be approved by a majority of the owners.

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2 hours ago, henry4841 said:

Now I have an extra hole in the top that I wish was not there.

 

They make nickel or chrome plated sink plugs/caps in smaller sizes.

You can cover the hole with one if so desired...you may have to enlarge the diameter of the hole to fit the cap being they only get so small. Or a dummy screw if you have clearance.

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