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Little Sweetie Forum amplifier project


henry4841
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As far as probing around inside a hot tube amplifier, it is best to understand a little about tube amplification before doing so. They do bite pretty hard. Much like those old CRT tv's. This is a good reason I like this design. No need for a beginner to be messing around inside this amplifier when it is plugged in. If you happen to have a problem you can unplug it and then work on it with a multimeter checking the few parts that make up the 3 circuits. First try and find which circuit the problem is in. That is the way technicians work on electronic gear. Then you are just working on a few parts that make up that circuit, most times around the active device, tube or transistor. When I was in my 20's I fixed a few car radios. Most of time all it needed was a transistor replaced. If that did not get it to working check the capacitors around that transistor will get it going again 95% time. With this amplifier if you have trouble finding what you did wrong there is always the shotgun method of repair. Replace all the little critters. Believe me I have done that myself with some board SS builds with many more parts than this amplifier. Sometime easier than spending a lot of time trying to find what I did wrong.

 

I can remember only one tube build I gave up on and it was built on one of those made for building PCB boards. Only one I have tried doing on a PCB. It was a SET 300B and the big reason I gave up was I was working with those expensive 300B tubes and did not want to take the chance of hurting one of them trying to diagnose what I did wrong. I removed all the parts and gave the board to a friend who built a SET 45 tube amplifier with it without any problem. I just took my tubes and transformers and built a point to point SET 300B amplifier from one of the many old schematics available online. Probably my favorite tube amplifier but I do not use it all that much. Reason, big and heavy, I do a lot of amp switching in and out. That and it those tubes generate a lot of heat running up your power bill. A smaller tube amplifier still sounds mighty good with a lot less juice. You start being picky saying one tube amplifier sounds so much better than another. I do not think I have ever heard a bad tube amplifier. 

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Tube gear is a lot easier to repair in my opinion. Modern solid state amps are usually direct coupled and when a transistor goes out, there is a daisy chain reaction and more transistors are damaged due to direct coupling. 

 

If a SS amp has a catastrophic failure, I'll replace all the transistors in that particular channel except for the input section but sometimes they get zapped too. 

 

 

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Tubes are very forgiving and much easier to start out making amps with vs SS. As mentioned it's often a daisy chain effect where the entire output stage transistors fail along with many of their components to make them work.

 

I still recommend a preliminary check of voltages before buttoning the amp up. That means clip the ground lead to your meter to ground, keep one hand in pocket and use the other hand to probe different areas in the amp to make sure you have the correct operating points.

 

B+ feeding the OPT should be ~250v

 

6Y6

Plate- 235v

Cathode- 30-35v  *depending on the tube some may draw more current than others so you get different readings.

Grid- 0v

 

6SJ7

Plate- 80v

Screen- 45v

Cathode- 1v

Grid- 0v

 

Once these voltages are confirmed then it should be okay to move forward.

 

For those that read here to learn, a small correction on the instructables definition of a tube and transistor taking a small current and making a bigger current is true for a bipolar junction transistor but not for a tube or Mosfet. A vacuum tube has transconductance which means it's a voltage controlled device, not current controlled. The grid doesn't require current like the base of a BJT. Transconductance just means that for a given input change of voltage we are given a specific output change in current. For example a 12AX7 has a transconductance of 1600uMhos, that means for every 1v change at input you get 1.6mA change through device. Transistors have beta or technically "hfe", which is strictly current gain. An "hfe" of 100 means with 100uA of current through the base to emitter forward biased diode you will get 10mA current from collector to emitter. Now there are some transistors that are very similar to certain tubes, pentodes to be specific and they are Mosfets. Both have transconductance, both have very similar output characteristics and both are voltage controlled current devices. Just wanted to clear up the different types of transistors and how they function compared to a vacuum tube.

 

This leads me to one huge thing one should take away from that. In that in the family of "tubes" they can behave very differently. So when someone says "tubes are more linear than transistors" that is not true because a Pentode is a tube and it has similar output curves to transistors. So triodes and Pentodes are vastly different in characteristics. In fact a triode is linear due to internal negative feedback. Simply the plate voltage also controls the current through the device, so as the plate swings down towards ground current decreases, as the plate swings up near supply voltage current increases. This is feedback. The engineers went in and added another grid to shield this mechanism which is why the output curves changed so drastically. As you'll notice looking at the plate curves for Pentodes they have straight lineas across horizontally not vertically like a triode. That horizontal line is plate voltage and it's horizontally flat because current doesn't change vs the plates potential. Looking at a triode as the plate curve goes more negative it gets moved down in current making the curves more vertical. So when you are in your next tube vs SS argument you can make the distinction of triode vs transistor because really a pentode behaves more like a transistor in regard to it's output characteristics. i.e. the collector voltage doesn't have any effect on collector to emitter current, it's controlled via base to emitter current only. This is also why you get a lot more power out of Pentodes vs Triodes, as the Pentode swings it's plate low there will be no reduction of current through the device.

 

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12 hours ago, henry4841 said:

There is no reason for you to be inside this amplifier when it is running 'hot' with this build. No adjustments need to be made. 

 

12 hours ago, henry4841 said:

I suppose some old man that has one foot in the grave may get killed

Yeah, that would be me so I'm happy to learn no adjustments need to be made.

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Hi guys. Yesterday morning I mentioned my best friend that always had to be right. Looking back it was kinda amusing in that if we disagreed about something early in the morning I knew what to expect the rest of the day. He was going to get on the net finding something to try to prove to me that he is right. If he was right I would tell him so. If not I would just not say anything. Not saying anything let him know I did not think he was right. Not saying anything sometime can say a lot. Actually was entertaining to break the monotony of the day looking back.  I sure miss him. Passed away 2 years ago.

 

Not sure how much I can get done today on the build. May have to take my GF to Dr. Nothing serious, got into some poison Ivey or oak playing in the dirt. She loves her flowers and vegetable garden. I rather play with electronic critters. I will see when I talk to her this morning. 

 

I would like to talk about some of the repair, build, videos I like this morning for those that like that kind of thing. For you strictly tube guys I like Uncle Doug. https://www.youtube.com/c/UncleDoug  (copy and paste). He is a very good teacher and knows his stuff. I enjoy watching him repair an old guitar amp. Guitar amps is what you are going to see being worked on but the technology is the same with tube HiFi stereo gear. The all around repair guy who really knows his stuff I like is https://www.youtube.com/c/12voltvids or perhaps this one https://www.youtube.com/c/JordanPier   They have a good electronic background but use a lot of common sense in their work. They only dive into those data sheets when they have to. For the more engineering type I like https://www.youtube.com/c/xraytonyb    He is an electronic engineer and like engineers love playing in the data sheets splitting hairs selecting parts. The other two see what they have in stock then looks at the data sheets to see if the transistor they have will work in the place of the bad one. Close enough they will say to get the broke gear working again and getting paid. I take the more common sense method. At least I try to. Like looking at the 2 different output transformers I have been talking about. The Edcor's I have always used in all my builds, other than some Chinese kits, except one vs the Hammonds Maynard uses. Like I said before I am a cheap diy'er. But still wanting to use good parts. The Edcors were cheaper than Hammonds a few years back. Now since I am having my first look at the Hammond 125cse's my common sense tells me they are going to be just as good if not better. About the same size, actually weigh slightly more and look well made. Guys we are just talking about wound up wire. Let's not get into my wire sounds better than your wire thing. There is no magic put into wire. The Edor's do look better with those blue end caps. Maynard feels much the same about coupling capacitors. A good cap from a good co. that does the job will work fine. I can really see what he means. I played around with coupling caps some years back and at the time I thought Orange Drop caps had a sound I liked best. Anyways that is my go to coupling cap. That is what I am using in this build. They are big though. Maynard uses a good polyester cap. Cheaper, smaller and does the job. Much like what Klipsch is using in their crossovers now. A common sense repairman part. Those Orange Drops are not expensive so they are as good as any. I am not one to throw a lot of money into one part expecting some big improvement in sound. The biggest contributor to what you hear is going to be the tube or transistor you are working with. All the other parts are just there to try and get the best out of that active device and make it work. Some designers like to think their designs sounds the best. I see it as splitting hairs. If you want to step up to really better, work with a different transistor or tube. 

 

Well enough rambling this morning. Everyone have a good day. 

 

Oh about to forget I dragged out the other two designs of Maynard's I have built yesterday to have a listen again and compare them to this one and the Zen I built. All 4 sound really good. The other two designs of Maynard are really flea watt ones but if you came to my house and I did not tell you anything about the numbers of any of the amps you would never know they are in the 1 watt and less range with my big horns. The only real difference I heard was in the bass. The other two flea watt designs of Maynard's did not have as strong a bass as the Sweetie or the Zen. Midrange in all 4 was very good with me liking the tone of the 6Y6 tube best. The bass in the Sweetie and Zen was a tie in my book. I have a number of one or two watt amplifiers I have built. I picked the Sweetie to do this project because for an inexpensive build I like this one the best. I could have just as easy chose to use another design. The midrange of the Sweetie will hold it's own against any of my builds. Just being curious yesterday I counted the number of working tube amplifiers I now have. I counted 11. So I do have the luxury of comparing the sound of different amplifiers that most of ya'll don't but to remind you they all are home built amplifiers.  

 

 

 

 

 

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22 hours ago, henry4841 said:

those tubes generate a lot of heat running up your power bill.

My 2A3 amps didn't seem to get very hot at all, especially compared to the ST-70 with four EL34 output tubes in it. We had a baby sitter one time, who lay an LP on the amp... C'est la vie.

 

Have you posted a pic of the finished product yet?

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1 hour ago, Marvel said:

My 2A3 amps didn't seem to get very hot at all, especially compared to the ST-70 with four EL34 output tubes in it.

 

 My Black Magic 25 with stock EL-34 tubes ran quite hot I was pretty disappointed after owning the 275 with KT-120's that ran luke warm at best. I ended up ordering 120's to replace the 34's I had to adjust the bias from almost to the top with the 34's to damn near the bottom for the 120's to get the same 100Ma reading. 

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1 hour ago, Marvel said:

My 2A3 amps didn't seem to get very hot at all, especially compared to the ST-70 with four EL34 output tubes in it. We had a baby sitter one time, who lay an LP on the amp... C'est la vie.

 

Have you posted a pic of the finished product yet?

Work in progress with pictures on this thread. Hope to finish this week. 

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5 hours ago, henry4841 said:

 

Not sure how much I can get done today on the build. May have to take my GF to Dr. Nothing serious, got into some poison Ivey or oak playing in the dirt. She loves her flowers and vegetable garden. I rather play with electronic critters. I will see when I talk to her this morning. 

Shingles

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I used a cheaper power transformer that called for on my 300B and it runs a little hot as well. It is a Hammond and they are very conservatively rated and will stand a little pushing from the specs. It is one that George Anderson recommends using with his 300B amplifier. From what little I know about the SET 300B amplifiers is if one has not heard a 300B amplifier with the Western Electric 300B's you do not know how good one of them can sound. I do know there are people that will pay thousands for a used pair of them. Tubedepot has a pair of NOS ones for sale at $9000.00. If I had deep pockets I would love to have some or at least hear a SET 300B with those WE tubes. Never going to happen but no harm dreaming. 

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15 minutes ago, henry4841 said:

Shingles

 

Shingles can be quite painful.

 

I wish your girlfriend a quick recovery and that the symptoms aren't severe.

 

Weird fact I never had Chicken Pox as a kid or adult. I wonder how many others in here also haven't had Chicken Pox. Or if they have got Shingles later in life? I have known several people later in life to develop Shingles from their previous bout with Chicken Pox early in their life. Weird how something can lay dormant for so long waiting for your immune system to become weakened then attack again decades later. Life is weird.

 

 

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Getting closer. Both circuits finished now to wire OPT's and speaker outs. With luck may fire up tomorrow. Best to sleep on it and check my work and wiring in the morning. Remember old and forgetful. If there are any following this build you can look at the pictures and see how I work building using the schematic. Sorta like building a retail kit with instructions if you can half way read a schematic. 

P1040005.JPG

P1040006.JPG

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The member CWelsh has graciously made a BOM (bill of materials) with part #'s at Mouser and Tubedepot for anyone that wants to challenge themselves and build this little amplifier. You guys with better eyes can look it over and see if there is anything I may have left off. 

 

I am a follower of Nelson Pass who tries to watch and read everything he has done in his life. The man is a genius in my book. I am sure there are others just as smart doing the same thing he does but I like his  theory and ideas best. Much like a simple SET tube amplifier. Three circuit two stage amplifiers. You only have to try and understand 3 circuits well instead of the many more in other amplifiers. That is what designers do. Put different circuits together to make something. They can take one circuit and play around with it and then add it to the other well known circuit they like. This little amplifier I like and am building has 3 very simple circuits if you study each circuit independently you can understand how adding them together can make an amplifier.  There are some of those more talented guys than myself mentioning adding a choke in this power supply. Probably help with better numbers but what Maynard designed works well and does it job sounding good to me with no hiss on my horns. One can certainly add a choke if they care too but wanting to keep this project simple and the least expensive I chose not to do so but to build Maynard's amplifier just like he designed it. My go to choke is the Triadmagnetics C-14X for those wanting to know the one I use when a design calls for a choke. Use to cost something like $15. I actually have two of them in stock. In my book Maynard is the man when it comes to tube amplifiers. He has probably forgot more than I ever knew about tube circuits. 

 

Watching some of Nelson's videos at the Burning Amp Festival every year there is sometime someone there that will call him an engineer. He is quick to point out that he is not an engineer but does not mention he got his degree at college in Physics if I am not mistaken. Electronics was just a hobby for him and his friends when he was in college. They built speakers and amplifiers selling them to fellow students. He has taken his hobby and built a successful money making business from it. Being close to my age he now spends his time at his ranch in California turning his two business' over to qualified partners with him just playing with circuits at his house for future products. I bet he gets a healthy check each month and does what he enjoys doing at home. 

 

I hope to finish this amplifier today or tomorrow and do some testing. At least that is my plan with my fingers crossed by the way. Been a few years since building a tube amplifier and being older does not help. I use to be able to build them and they work. CRS is getting worse every year now. CRS is can't remember and I will let you guess the next word. 

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8 hours ago, henry4841 said:

I thought some of ya'll might like to see an old mans cluttered play room. 

 

 

P1040007.JPG

I see an HP-334 series distortion analyzer up there in the top left corner of your shelf. I have 3 that I'm not sure what to do with. There were the item to have back in the day. 

 

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Here is an initial draft of a parts list for the Little Sweetie. @henry4841 and I are going back and forth checking details as time allows. I downloaded Henry's Mouser order form to produce the core of the list, so I think some of the quantities may reflect orders for "stock" rather than actual requirements for the project. We will get those details cleaned up before we publish a final version.

 

Please be encouraged to review the list for errors and omissions. I will welcome suggestions for edits and/or formatting that will make it better for new builders. I expect to do some clean up on descriptions as well.

 

Thanks in advance!

Little_Sweetie_Parts_List_V2.xlsx

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50 minutes ago, Curious_George said:

I see an HP-334 series distortion analyzer up there in the top left corner of your shelf. I have 3 that I'm not sure what to do with. There were the item to have back in the day. 

 

Bought it on Ebay as working of which when received did not. Seller reduced price and I replaced the caps and got it to working, somewhat. It works but does not output signal for display on scope. Anyway not needed these days with the Arta software which is all a hobbyist needs to look at the distortion profile on a computer. Used Arta pretty regularly years ago but not sure if I remember how now. 

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