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Little Sweetie Forum amplifier project


henry4841
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3 hours ago, henry4841 said:

I was going to ask you this morning when you first get them to tell me in the country boy way. How they look, feel, compare in weight, size to the Edcor 10 watt ones we have been using. But you have already gone to work on them. Good job. 

I just compared the MPS 10 watt OPT to the Edcor 10 watt OPT. The MPS is 2.1lb, the Edcor is 1.75lb, so the MPS is slightly bigger. Overall build quality is good, but not as clean as the Edcor. The MPS transformer varnish dip is not as clean as Edcor's process. This has no bearing on performance, but is strictly an aesthetic issue. Other than that, the end bells are natural finish, not painted. 

20220825_090457.jpg

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1 hour ago, Curious_George said:

I just compared the MPS 10 watt OPT to the Edcor 10 watt OPT. The MPS is 2.1lb, the Edcor is 1.75lb, so the MPS is slightly bigger. Overall build quality is good, but not as clean as the Edcor. The MPS transformer varnish dip is not as clean as Edcor's process. This has no bearing on performance, but is strictly an aesthetic issue. Other than that, the end bells are natural finish, not painted. 

20220825_090457.jpg

Look good to me. Bet they have to sound good up until their limit. Paint them black all over and there you go. 

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Did some tiding up and some checking with scope. Sine wave looks good at 50hz, 1Khz, 10Khz and 20kz. Here are some pictures of the underside after doing some tiding up. Mine will never look as good as say Maynard or Mike but I am proud of it. The other picture is the coin Area51 picked out. We decided to glue it over the hole with the date showing. Who knows years from now someone might see it in a thrift store and will think it was built in Mexico in 1953. 

🙂 

P1040015.JPG

P1040016.JPG

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1 hour ago, henry4841 said:

By the way that signal generator I talked about yesterday died on both channels. Put it with the Chinese scope that died a few months after warranty. Luckily I had another one of those $100 Chinese wonders I bought years ago to fall back on. 

 

A local audio friend asked me once what sort of function generator I had.

Some cheap Simpson 420 I replied, with a dodgy amplitude pot. 

Well, he replies back that you need something that puts out a real sine wave.

 

Nonetheless, I ended up with a HP 654A test oscillator.

Didn't think much of it, until I compared the two units on a scope.

 

The Simpson 420 has sine waves that have little peaks on the top, like they tried to make that triangle wave look like a sine, but not quite...

The Hewlett Packard puts out clean beautiful sine waves.

 

While for most, a Chinese function generator will be perfectly adequate. The Simpson 420 still works fine after cleaning the amplitude pot, and the square wave function works/looks good on a scope.

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Thanks Mike for the heads up on the HP 654A test oscillator. May keep my eye open for one. I have an HP 8657A radio frequency generator I use for radio work on tuners and those old receivers of the 70's. Love that thing. HP made great old equipment and probably still do. Pricey new though. 

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On 8/24/2022 at 4:59 AM, henry4841 said:

And Captain made a schematic with the voltages at different points in the build which I used yesterday morning finding the problems I had. I bet he has one of those electronic calculators I do not want to even touch.

 

Nope, no special spreadsheets or calculators, just the datasheets, paper and pencil tells all one needs to know about where the tube should be operating given the specific circuit it's attached to. Honestly after engineering amps for so long you learn to memorize specific tubes properties, mainly just need to know mu, plate impedance and transconductance.

 

Grounding is one of the most misunderstood concepts in amp building. Many have a "recipe" they use that works good and they stick with it. Glad you got it all straightened out and quiet.

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For people interested in breadboarding, having a DC power supply is the only way to easily work out circuits for me. I don't have a tube DC supply, I went SS that way it can be efficient and adjustable and fully regulated. I have two, one is a Miada regulator issued in pdf back in the 90's that uses an LM317 floating. Then I have another heavy duty one with large heatsinks and pass transistors. If I want to test a specific tube at a specific B+ I just dial it up and it's ready to go.

 

I work all my circuits out this way before even building the power supply.

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@henry4841 I think you would really enjoy making turret boards for your power supplies, it's actually quite easy. The kit to install them is cheap, I just purchase the epoxy boards in bulk and cut to my own size. The kit comes with the exact size drill bit and the install tool and swage tool. Basically you just pound them in the board as they are compression fit then swage the underside. They are very heavy duty and hold components nicely, they also dress the build up substantially vs the perf board.

 

Another thing to play with is PCB boards. For these simple designs you don't even need a computer to make the files. Just purchase double sided clad board and draw your circuit traces with a heavy duty permanent marker. Once you have all your traces covered with the marker just soak the board in Ferric Chloride and after it eats the remaining copper away clean the marker off and drill your holes. I once did it this way for easy circuits but now I just use software, for development I'll print the circuit out over glossy paper with Laser jet printer. Typically just bring the files to Staples since I don't have a nice printer. Then heat transfer the traces from glossy paper to clad board. Soak in the mixture and remove and voila a nice prototype board to see if it works before sending the gerber files out for production.

 

Just other fun ways to make boards for builds.

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7 hours ago, captainbeefheart said:

 I don't have a tube DC supply, I went SS that way it can be efficient and adjustable and fully regulated.

I have two of these. One is missing a tube I borrowed for another piece. I need to go through them and make sure they're good to use.

 

 

20220825_214723~2.jpg

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7 hours ago, captainbeefheart said:

now I just use software, for development I'll print the circuit out ...

What are you using? I've installed KiCad on my work pc (linux mint), but they have installers for other distros as well as mac os, windows, etc.

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1 hour ago, Marvel said:

What are you using? I've installed KiCad on my work pc (linux mint), but they have installers for other distros as well as mac os, windows, etc.

 

I use KiCad with Ubuntu for lots of my audio projects and like it. Easy to use and pretty flexible, lots of various footprints for parts in the library.

 

Lately I haven't been making PCB's for audio since tubes lend themselves so nicely to p2p wiring. l

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Morning guys. This Sweetie project is mostly finished now and I hate to see this thread drift into the sunset. Everyone has behaved and got along and I enjoyed having everyone that had something to say here. Perhaps you guys might want to change the title to "Little Sweetie Forum Amp Project and Beginners Builds" and keep it going. A moderator would have to change the title I believe. I guess the main thing I want to point out is if an uneducated redneck old man from Alabama can put one of these things together maybe you can too. It will take someone that does not mind working with your hands and average intelligence but most here have those qualities or they would not be lurking or participating in the discussion. George has found a cheaper OPT to use which from what I see would be perfectly fine for this build. Save $40 to $50 on the parts bill. Yesterday I started talking about what the hobby of electronics really is. The study of electron flow. The electronic word for that is current and the way it is measured is in amperes or amps. My personal Sweetie is running .045 amps or 45 milliamps. The abbreviation of 45ma. Those OPT's George found look good for 50ma and they weigh pretty much the same as the 10 watt Edcor's so popular on diy sites and are as large so they look good to me. Just right for a Sweetie build in my book. I do not get hung up on the sound of a part in an amplifier. There are there to do a job and if they can do the job good enough it should work fine. The output tube is where you want your money to go when building an amplifier. That is where the rubber meets the road. Having built all the tube amps I want I do not care to build another tube amp except starting out with one of those old tubes of yesterday. Most of us are not of the age when tubes ruled the electronic world. Tubes did so for 4 or more decades with the big companies making those tubes and they got it right. Especially audio tubes. The big name companies. The new makers of tubes these days are relatively small companies trying to learn how the old guys did it. For the popular hyped up Decware Zen being so in demand in my opinion to make it sound as good or better than this Sweetie one must get some of those old NOS EL84 tubes and take it to another level. That is going to raise the cost to around $1400. In my rough figures one can build this Sweetie with George's OPT's in the neighborhood of $350 in parts. Maybe a little more since I did have screws, nuts, wire, etc already in stock. There is nothing special in the circuit of the Zen. Like the old tubes the old designers got it right too. Not much has changed in tube design. There are now SS devices that can do the job tubes used to do. Like the power supply in this build. Maynard decided to upgrade the power supply to a newer and cheaper part the SS rectifier instead of a tube doing that job. It does just as good a job if not better. Gets the job done. If I build another amplifier it would have to be built around an old output tube. Your amp is never going to sound any better than the output tube you decide to work with. I found an old toroidal transformer I used perhaps 8 or 10 years with secondary voltages of dual 325V and dual 6.3V for the filaments in my box of transformers. Looks good for a EL34 family of tube build. Also wound up with some Hammond 125dse OPT's due to an error on my part ordering OPT's for this build so I have the expensive parts of a tube amp build already. The iron. Do not need another EL34 family of tube amp though. Already have two, George Anderson's SSE EL34 amp and an KT88 one. Maybe another Sweetie with premium parts, not that it will sound that much different than the one we have already built. Just because perhaps. This project was aimed at low cost with good parts type of project. I wanted to see how cheap I could build a small amplifier and still use good parts. This is what I chose and I think it sounds really good. 

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Just for some thought. Here is the schematic posted on the Decware site of the Zen amplifier 2020. I found some NOS RCA EL84 tubes for $120 each at Tubedepot. https://www.tubedepot.com/products/rca-6bq5-el84

 

Could possible be a another good beginner project. Tube rectifier but the circuits themselves look simpler than the Sweetie.   Steve seems to like simple circuits as I do. Gonna be more expensive though using NOS EL84 tubes like the Sweetie uses. I do not see a reference to ground resistor on the inputs or a cap. No cap on the cathodes of the tubes either. Probably not needed. Enough gain. I would want to add a reference to ground resistor and a cap on the input just because. Really simple build which is good in my book. 

 

UFOSCHEMATIC.png

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Just noticed on the schematic of the Zen Steve intentionally left of the values of the resistors. No problem for me because I happen to have an old schematic of the Zen showing values. Built one myself and have it here. At my house that is. That and some of the guys here are good at that kind of thing, finding the values.

 

This little amplifier is something we can carry one talking about keeping the thread interesting. How each one of us would build it even it we do not ever build it. 

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2 hours ago, henry4841 said:

In my rough figures one can build this Sweetie with George's OPT's in the neighborhood of $350 in parts.

The alternate output transformers that Henry is referring to are from: https://www.musicalpowersupplies.com/products/

 

Current pricing is (one) $44 plus shipping. These are nearly identical to the Edcor GXSE10 watt OPT's, but less expensive. 

 

The OT10SE-HF comes in 5k & 7k primary impedance. If you want another impedance, Matt (at MPS) can wind whatever you need. Just send him an email. MPS also winds transformers for Bottlehead products, so that should give you an idea of the quality you will be getting from MPS. Great products at a great price. 

 

Lead-time is very reasonable too (right now), I ordered 4 and Matt had to make them and I received them in 10 days. 

OT10SE_HF_DWG_USA (2).pdf

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Just some rambling thoughts this morning. If we put our heads together I bet we could design an amplifier around the EL84 tube as good if not better than the Decware Zen. His design is nothing unique or special. I would like to see it built with one of those Antek Toroidal transformers. Cheaper and shielded. We would have to have a design without using a center tap transformer. Also SS rectifier power supply. We could call it "Forum EL84 Amplifier". Be something to do and I would more than likely build it. Lets keep the cost down much like the Sweetie and not complicate it too much. Team effort will produce a fine sounding little amplifier. You younger guys are good with figuring and know how to use some of those simulating software programs. 

 

Any takers?

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