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Little Sweetie Forum amplifier project


henry4841
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2 hours ago, Racer X said:

 

Y'all know this is really the proper way....

Joke… Female waitress walks up to patrons and asks, where are ya’ll from? Female patron responds, where we come from, we don’t end our sentences with a preposition. Waitress says, where y’all from beyatch?

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13 hours ago, Curious_George said:

I've built directly heated triode amps with AC filaments (for simplicity and bare bones circuitry) and they sound good, but DC is really the only option. 

I built my 300B and 45 tube amps with DC for heat. But Maynard told me AC on those cathodes with that pot for adjusting out the hum works really well. He is probably right. Some even say AC heat sounds better. 

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Hey guys the reason I keep saying keep this thread on the high school maybe first year of college level is because this thread is going to be for the beginner electronic guy wanting to play with electronics. That and if you start talking circuits much above the ones in this schematic you are going to lose me. I use to hang out in the Pass thread on diyaudio just lurking mostly trying to learn something. You will find the big boys over there. The ones that made A's in high school without studying. Got a free ride to college and came out with a degree. They know those electronic formulas by heart and many can do the math in their head without needing one of those electronic calculators. I do not own one of them. Do not have a need for it. The emphasis on that thread is simple circuits much like this amplifier but SS. This amplifier has 3 circuits. I am going to start calling them by their electronic name instead of saying stages. Remember all you have to remember is you are learning a new language. The language of electronics. Then you can then talk the language with other electronic geeks. If you can understand these 3 simple circuits you will have a good foundation to build on. The PS circuit is about as simple as it is going to get and the lowest cost to build. It uses a bridge rectifier along with capacitors and resistors. Those old tube schematics of tube amplifiers used a lot of chokes, new word for some, just an inductor actually. They had to use them to filter out the ripple because they did not have good cheap capacitors to do the job back then to clean up the signal after the rectifier tube. Also they had to use a tube because it is all they had. With modern capacitors a resistor can take the job of the choke and does a more than adequate job. Some say a tube rectifier adds some flavor to the sound but that was not the intention in the beginning. It's job was to turn AC to DC which is what the amplifier tubes needed to work. Every part around the amplifier tube is just there to make the tube do a good job. Over there on the Pass thread the emphasis of those boys is to try and use as few parts in the circuit as possible saying less parts is going to sound better. They try to take a part out and make a circuit with a transistor that still works good. I do not visit there much anymore. At least not for the last few years. Some of those discussions get over my head pretty quick. It is fine with me if ya'll want to discuss more complicated circuits here among yourselves but chances are I am just going to glance over those discussions. You are going to have to talk to me about electronics like I have said, like a teenage boy or young adult. I do understand somewhat AM and FM circuits along with some understanding of the principles of FM stereo. Enough to do some repairs in those circuits. Those circuits are not used anymore though. Been replaced with chips doing a better job. There are not a lot of discrete audio transistors anymore either. Chips are doing that job mostly these days. There is no money making them anymore. In fact transistors are going to fade away much like tubes have in modern electronics. Just look inside a modern television vs one of those old tube televisions of old. Not much in there. To fix them most technicians just replace boards. At a copy machine place I last worked at some of the technicians there, making good money, did not want to solder anything. They just figured out which board was bad and got a new one. Cheaper that way. I never heard of one digging into a board trying to fix it. 

 

Well that's enough rambling for me this morning. Some of you other guys take over. Remember KISS for a beginner here though. That is what we are on this thread for. Try and talk where they can understand and learn. This amplifier of Maynard's is a good simple one to learn from. When I was trying to learn a little something about electronics in my teens and early 20's you started out with tubes then progressed to SS. 

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1 hour ago, henry4841 said:

I built my 300B and 45 tube amps with DC for heat. But Maynard told me AC on those cathodes with that pot for adjusting out the hum works really well. He is probably right. Some even say AC heat sounds better. 

A hum pot does work well for eliminating most of the hum, even with a horn bass enclsure. But, you still have 60Hz modulating your music signal no matter what. A 45 I can see using a hum pot because the filament is 2.5V, but with the 300B, a 5V filament, your hum will be greater. 

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2 minutes ago, Curious_George said:

A hum pot does work well for eliminating most of the hum, even with a horn bass enclsure. But, you still have 60Hz modulating your music signal no matter what. A 45 I can see using a hum pot because the filament is 2.5V, but with the 300B, a 5V filament, your hum will be greater. 

My 45 tube amp is dead quite. I can still hear a touch of hum with my ear directly on the speaker with my 300B but nothing to be concerned about a few feet away. I sure do not want to jump in there and try and eliminate such a tiny amount. Could make it worse fooling around. My 300B amplifier design has a lot of gain and I would rather the gain to be much less but it is what the schematic I built it from calls for. Sound though is fantastic. A SET 300B has a very distinct sound that I like. 

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1 hour ago, henry4841 said:

Hey guys the reason I keep saying keep this thread on the high school maybe first year of college level is because this thread is going to be for the beginner electronic guy wanting to play with electronics. 

 

Who's the beginner, you're the beginner !     *subscribed*

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If you like watching an old man building an amplifier real slow you have come to the right place. Did a little work this morning. Installed my star ground wire. If you take a piece of 14 gauge wire and put one end in a drill and the other in a vise and give it a few turns it will be straight and stiff. Wired up the filaments and put the virtual ground Maynard taught me how to do. That is the R12 and R13 resistors. The B on the schematic goes to ground. Helps eliminate hum. There is also R14 resistor Maynard puts in his builds to lower the voltage some. Calls for a .3ohm 3 watt one and all I had was a .4ohm 5 watt one. Should work fine. You want to give that resistor some breathing room. It is going to generate some heat. When it calls for 6.3V on the heaters that was years ago when the house voltage was 110V. These days it is 125V at my house. I put the heater wires high on my build. A guy called Uncle Doug on youtube does it this way. I like his work and have been doing it this way ever since. They are high and drop down to the heater pins. This is AC so you want to keep it away from the other components in the circuit. There are couple different ways to wire the heaters. I have probably done it all 3 ways in some of my builds. I chose to do it the way you see on this build. Wires will be tidied up after finishing the build. You may notice I put a P on the chassis next to the tube sockets that are pre and a Y on the output tubes. Helps me not to mess up. 

 

Almost forgot to mention, all the pins for these tubes are 2 and 7. Makes it easier to wire up. And if you see any big mistakes I have made let me know. We want Area51 to get a good home built amplifier. Kinda like a joint forum effort with me being the monkey putting it together. 

P1040002.JPG

P1040001.JPG

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To clarify, when a transformer or transformer section does not have a "center tap", you can make a "virtual ground center tap" by using two low value resistors (100 Ohm/0.5W) as indicated on the Little Sweetie schematic. This virtual ground center tap allows you to terminate the filament circuit to the positive end of C5, which is the cathode of the 6Y6GT. 

 

Although the schematic has a hand written note "B Ground". The filament circuit can go to either place, signal ground or positive side of C5. 

 

This "virtual ground" concept can only be used on low voltage transformer windings, not high voltage windings. 

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To help illustrate what you can do with terminal strips, I have uploaded a "Williamson Type Amp" circuit. Although not as clean looking as I would like, it is more of a working test bed. It is push-pull 1625's (807) about 25 watts RMS. Solid state power supply, AC filaments on everything and dead quiet. 

 

The ground bus is a combination of straight bus bar and star type topology. The power supply section is at the top of the picture, amp section mid-picture and below. 

Bottom Chassis View_01.jpg

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Regarding filament wiring, Is there an argument for parallel wiring as Henry shows vs. series (daisy chaining) the filaments together? I have seen both ways and pondered if series was better to minimize AC wiring lengths. Probably a matter of preference, but I'm enjoying following along and trying to learn.

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1 hour ago, henry4841 said:

We want Area51 to get a good home built amplifier. Kinda like a joint forum effort with me being the monkey putting it together. 

Message from Area51: Henry, I want to tell you how much I appreciate your efforts. I didn't do anything to deserve the privilege of eventually receiving the Little Sweetie amplifier (and a little electronics education) you're building. I just happened to hop on the forum right after you posted your intention to build a low cost, reliable SET "forum amp" and you were kind enough to exchange communications; granting me this good fortune. And now another stroke of good luck: being able to "watch" Little Sweetie being built. I'm a lucky monkey. Thank you. 

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2 minutes ago, Area 51 said:

Message from Area51: Henry, I want to tell you how much I appreciate your efforts. I didn't do anything to deserve the privilege of eventually receiving the Little Sweetie amplifier (and a little electronics education) you're building. I just happened to hop on the forum right after you posted your intention to build a low cost, reliable SET "forum amp" and you were kind enough to exchange communications; granting me this good fortune. And now another stroke of good luck: being able to "watch" Little Sweetie being built. I'm a lucky monkey. Thank you. 


Pre-congrats on your future amp…

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52 minutes ago, capo72 said:

Regarding filament wiring, Is there an argument for parallel wiring as Henry shows vs. series (daisy chaining) the filaments together? I have seen both ways and pondered if series was better to minimize AC wiring lengths. Probably a matter of preference, but I'm enjoying following along and trying to learn.

Both can work well. If you series wire, ensure you connect the output tube first (because this will usually be the tube that draws the most current, then subsequently wire other tubes in decending current draw. 

 

Also, ensure your wire gauge is adequate for the current draw. 

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1 hour ago, henry4841 said:

You are right. I put it in the wrong spot and not like the schematic. Good catch George. I will change that in the morning. This is what this thread is all about. Teach.

To the members following along, in a lot of cases, you can ground the filament circuit center tap or virtual center tap and be done. However, in some cases, the filament circuit can be "biased" by the cathode circuit as shown via C5 or a voltage divider. Elevating the filament circuit can help reduce hum in some amp circuits. Connecting to the cathode circuit was/is very popular. 

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7 hours ago, henry4841 said:

The PS circuit is about as simple as it is going to get and the lowest cost to build. It uses a bridge rectifier along with capacitors and resistors. Those old tube schematics of tube amplifiers used a lot of chokes, new word for some, just an inductor actually. They had to use them to filter out the ripple because they did not have good cheap capacitors to do the job back then to clean up the signal after the rectifier tube. Also they had to use a tube because it is all they had.

 

The resistor cannot take the place of a choke, the reason for using chokes (inductors) is to filter high current without losing a lot of voltage. The inductor value in Henry's will have a high impedance to the 120Hz ripple yet pass DC by. Think about a power supply with a DC load of 100mA, a 10k resistor will drop 1000 volts!!! Yet a choke with 10k impedance would be 13H and a DC resistance of maybe 500 ohms, so the choke will do the same filtering as the 10k resistor yet only drop 50v.

 

Also they did only have tube rectifiers which cannot handle large repetitive peak forward currents, the more capacitance you have in the power supply the more current is demanded to charge it up every cycle. With a tube rectifier you are limited on how much capacitance you can use.

 

Just wanted to clear some of that up.

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On 8/7/2022 at 9:55 AM, Edgar said:

I don't know if anyone here has tried any of these. I have a pair to use in a PP EL84 amp that I haven't built, yet, so I cannot comment about sound quality.

My Musical Power Supply output transformers have shipped out. I should have them Monday. That was a quick lead-time. I can't wait to test them. 

 

I ordered the OT10SE-HF (HiFi version).

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1 hour ago, capo72 said:

Regarding filament wiring, Is there an argument for parallel wiring as Henry shows vs. series (daisy chaining) the filaments together? I have seen both ways and pondered if series was better to minimize AC wiring lengths. Probably a matter of preference, but I'm enjoying following along and trying to learn.

 

If you run filaments in series they all have to be rated for same exact current draw. E.g. you cannot run 6.3v type preamp tubes and power amp tube filaments in series. If you have all filaments that require 150mA then you can run them all in series.

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