Jump to content

Little Sweetie Forum amplifier project


henry4841

Recommended Posts

For anyone reading and interested in Schade feedback, here is a great resource from O.H. Schade himself on beam power tubes. Inverse voltage feedback is what he calls it and gets going on page 43 in the pdf or 176 in the book.

 

https://worldradiohistory.com/BOOKSHELF-ARH/RCA-Books/RCA-Beam-Power-Tubes-O.H.-Schade-1938-48-pages.pdf

 

 

 

  • Like 1
Link to comment
Share on other sites

Below is a picture of the amplifiers I was comparing two nights ago. The Sweetie on top of my test SS amplifier setup and my EL-34 amp with the 6L6 tubes. I also used the Gold Lion KT-77's in the EL-34 as well to compare the sound character of each tube set I described in my last post. All three ways are really good sounding but different. I played one song on an LP all 3 ways to get an idea of the differences between the sound of the 3 sets of output tubes. Did my best to keep the sound level the same for each test. 

 

 

P1040050.JPG

  • Like 1
Link to comment
Share on other sites

I'm trying to play catch up and get work done while not push it too much. When I get done later I'll run some simulations of the Sweetie with varying amounts of plate to grid feedback and see what the results are for anyone wanting to experiment with the Sweetie.

 

All one needs to do is remove screen resistor from the plate and connect it to B+ that feeds the output transformer. Looking at the datasheet their curves are for Vg2=135 so it might be prudent to use a 47v zener in series with the screen. The reason for a Zener and not a resistor is with a resistor during signal when the screen pulls more current you'll get a sag across it squishing the plate curves causing transient compression of the power stage. The Zener keeps the voltage drop constant no matter the current pulled by the screen. 

 

If you want to get real fancy for optimal results use three 47v zeners in series making 141v string as a voltage regulator. With 3mA of screen current plus a bias for zeners of 20mA, a dropping resistor value of 4.7k should work well with a 250v B+, if you have less B+, say only 210v use 2.7k for the resistor. What happens is 3mA is pulled by the screen and the rest of the current 20mA will pass through the zeners. When the screen pulls more current the current through the zeners is reduced down by that much keeping voltage steady across the resistor. Simple voltage regulator for the screen, 3 diodes and one resistor.

 

For people just experimenting I'd order like 10 of the zeners and start out using them in series like the first method I described. If you like what you hear in pentode mode with plate to grid feedback and want to improve upon it then move on to the screen regulation circuit I proposed in the second method. With the first method the screen isn't really regulated as since the zeners are in series it will always just drop a specific amount in relation to B+, so if the rail moves so too does the screen in proportion. With the second regulated scheme if B+ shifts around the screen will not, it is regulated and stays at the voltage made by the zeners.

 

They are only $0.30 ea

 

https://www.digikey.com/en/products/detail/onsemi/1N4756A/977325

Link to comment
Share on other sites

13 hours ago, jjptkd said:

Curious how @Area 51 is doing with his new sweetie, any updates?

I have not heard anything more than what Area51 posted a few days ago when he agreed with me about this amplifier is all about the mids and vocals. When selling anything not hearing something after selling is usually a good sign. 🙂 I have not contacted him via PM because like everyone else there are more important things in life than a social media site. I believe he will say more about the amplifier in due time. I hope it is positive when he does. Maynard and I like the sound so I hope someone else does as well.  

Link to comment
Share on other sites

Morning guys. It is officially a month since I first started this thread and we have had 9K views. I would have never dreamed there would be so many views in just a months time about an old man building a tube amplifier. I just started this thread to have something to do being that it had been a few years since I built a tube amplifier and I do enjoy doing so. I am glad so many check in everyday to see what I have to say. That and the others contributing to the thread being so popular, especially Maynard. After all I am just building his amplifier design, it is his creation. 

 

For me an amplifier is all about the output power tube. Every other circuit works for the king of the circuit, the 6Y6 tube. It is what the other two circuits are there for, to make the power tube operate. The power supply circuit is self explanatory, the king needs a correct voltage at the right pins for it to operate correctly. Get a flow of electrons, think water flow, going inside the tube. There is electron flow going on in all the components of the amplifier, much like a river and it's tributaries. When you flip a switch you start the electron flow. The pre-tube, called pre-tube for a reason, works for the king as well. It has a job to do for the 6Y6 tube, increase the tiny signal to a point where the 6Y6 can use it. The 6Y6 cannot use the tiny signal as it comes into the amplifier, it needs a more powerful signal to get it started working. Maynard has mentioned one tube amplifier not needing a pre-tube, sometimes called a spud amplifier. I built one of those when I first started building tube amplifiers 10 years ago. Because it is easier to build. Just two circuits instead of three. Before then I had only built SS amplifiers using a ready made PCB board. Building a SS amplifier from a board is much easier than building point to point from a schematic. You stuff two boards, usually, and all you have to do is connect them together using wire and viola you have a working amplifier. That is if you stuffed the boards correctly. That involves putting component A in the holes on the board at point A. Component B in the correct holes on the board at hole B, etc and on and on. Just following directions not needing to know exactly why you are doing so. For me building a tube amplifier point to point is more challenging. On this next build I think I am going to build just one channel at a time instead of doing both at the same time. Doing both at the same time was the way I did it a few years ago but on the last build I made some mistakes and had to spend time finding where I made them and fixing them. Being rusty and too cocky I tried doing it the way I stopped doing it a few years back. Learned my lesson diagnosing the problems made during assembly not being careful and precise enough. Anyways back to the subject of the king of the amplifier the power output tube, the 6Y6 on this amplifier. The power stage is where the rubber meets the road and the main contributor to the sound you hear. The other two circuits contribute to the sound but supposedly in just a minor role. Optimally not contributing at all in the sound you hear. Anyways that is the way I look at it. Now at this time in his life this is what Nelson devotes his time to, operation of a certain power transistor he is working with. He had his pre guy, Wayne Colburn, build him a front end capable of working with multiple output devices. Those SS guys do not have the luxury we do of having really good active devices to work with, audio tubes. The making of a new audio transistor is practically non existent. Been that way for years. Nelson has to try and find a new transistor made for some other purpose that has the properties that will make a good audio signal. Most often made for switching applications. 

 

Well I think that is enough rambling this morning. My plans are to get some holes drilled in the Premium Sweetie plates this morning. There has been a post about the price I am going to ask for the Premium and I said wait till it is put in the garage section for that. I will say the parts count is approaching $500 and I would not build another case like I am building for someone else for less than $300 if that. That will give someone an idea of what the asking price will be. Just the chassis to build one of Nelson's SS clones is going to cost you $294.    https://diyaudiostore.com/collections/chassis/products/deluxe-4u-amplifier-chassis-steel?variant=39313326473289

  • Like 1
Link to comment
Share on other sites

Hey guys, I just found this guy on ebay who is selling chassis' to build your amplifier with. Good 100% feedback. Give it a look. Not custom built with hand cut dovetails but who cares if they are more concerned with sound than looks. Take a lot of work out of building a tube amplifier. My chassis 
I am building is approximately 16 1/2" X 11" X 3" 

 

https://www.ebay.com/sch/i.html?_dmd=2&_dkr=1&iconV2Request=true&_ssn=hma60&store_cat=8055728010&store_name=iagdiytubeaudioproducts&_oac=1

Link to comment
Share on other sites

29 minutes ago, henry4841 said:

Hey guys, I just found this guy on ebay who is selling chassis' to build your amplifier with. Good 100% feedback. Give it a look. Not custom built with hand cut dovetails but who cares if they are more concerned with sound than looks. Take a lot of work out of building a tube amplifier. My chassis 
I am building is approximately 16 1/2" X 11" X 3" 

 

https://www.ebay.com/sch/i.html?_dmd=2&_dkr=1&iconV2Request=true&_ssn=hma60&store_cat=8055728010&store_name=iagdiytubeaudioproducts&_oac=1

The Hammond chassis is available too. This is what I used on Rich's amp. Hawk USA has a good price and selection, fast shipping.

Hammond Chassis 1444.pdf

  • Like 1
Link to comment
Share on other sites

Guys I did not know whether to mention this I saw on Ebay being I did not want anyone to think I am recommending this guy but he is the one I just bought my signal generator from and I am happy with the buy. 100% feedback rating. Anyways he is selling some analog oscilloscopes for just a little money for what oscilloscopes are selling for these days. Perfectly adequate for audio work. Harder to get voltages and such, you have to learn how to do it on an analog scope vs the newer digital scopes that displays such along with the signal, but if you just want to look at the signal for problems it will work just fine. I would not want to be without an old analog scope in the playing I do. 

 

https://www.ebay.com/itm/265864241373?hash=item3de6be60dd:g:2LwAAOSwlOpi~865

Link to comment
Share on other sites

9 hours ago, henry4841 said:

Maynard, remind me again why and what purpose is C3 in the circuit?


I don’t have the schematic handy, but it should be the driver screen bypass cap.  Like the cathode bypass cap, it is used to put the screen at ground potential for the AC signal voltages.  That is the simple explanation.  
 

Maynard

  • Like 1
Link to comment
Share on other sites

There are different methodologies which builders use when constructing pairs of mono amps whether they are on one or two chassis.  Henry mentioned that he will build one channel at a time.  
 

Another method, which I use, is to construct both simultaneously by performing the same operation on each before moving on.  For example, I would install the input and output jacks on each before continuing.  Same for the ground bus, filament wiring, and so on.  Doing that allows for both units, or channels, to look identical in terms of lead dress, and component orientation.  My OCD necessitates having near perfect symmetry 😊.

 

When I worked at the power supply company I was surprised that a similar technique was employed when a large number of units was ordered.  For example, we would lay out 25 front panels on the bench, and then go from one to the next performing the same operation.  There too, it allowed for each one to look virtually identical to the next.  The boss of the section in which the 7 foot tall monsters were constructed had even worse OCD than me to the point that he insisted that the cable ties on the wiring harnesses be spaced exactly the same!  His units were true works of art.

 

Maynard

  • Like 3
Link to comment
Share on other sites

10 hours ago, henry4841 said:

Area51 posted a few days ago when he agreed with me about this amplifier is all about the mids and vocals.

Sorry I have been MIA recently. My water heater is the "gas station" type: short and squat so it can fit in the crawl space over the bathroom so when it failed and flooded it made a real mess everywhere. I have been busy and, as a result, distracted. I have had music on while I'm working; listening to Amazon Music streaming into Sweetie and filling the shop with my HIP Heresies. Can't really call it "critical listening".

I noticed right away the sweet sound of female vocals with Enya grabbing my attention and possibly indicating how the "Little Sweetie" got her name. Bonnie Raitt and Bonnie Bramlett got in there, too but I always like the Bonnies regardless of how their music is delivered. Everything else? That gets interesting. I mentioned previously that I wonder how much speaker selection and speaker placement is influencing my impressions - and the use of a pre amp with my living room Cornwalls. Other factors include my advanced age (old ears) and rock and roll history (blown out ears). I sometimes find myself complaining that it's difficult for me to hear and understand clearly the human voices in a television broadcast of a football game or other sports where the crowd or the marching band overwhelms the announcers. Does that indicate hearing loss or degradation of the quality of listening available to me? Probably.

I'm looking forward to some CD time although that will be on the Cornwalls. Turntable will be set up later so my Sweetie evaluation will continue with sources being CDs and streaming as I work to recover from the flood.

 

  • Like 1
Link to comment
Share on other sites

Sorry to hear about water heater. I think we all have been there in the past, at least I know I have. Luckily my old house has the water heaters at ground level.

 

Good to hear the Sweetie is working fine with no problems. I did my best to build it where it will not give any problems. One reason I like Maynard's designs because he is easy on the tubes running them for long life. For a tube amplifier the Sweetie runs rather cool. Should give years of service. The 6Y6 sound grows on you the more you hear it. Smooth and sweet sounding. If your speakers are somewhat bright the Sweetie may be just what you are looking for. 

Link to comment
Share on other sites

35 minutes ago, henry4841 said:

Sweetie is working fine with no problems.

Indeed. 

 

A couple questions:

 

Unlike some audio equipment Sweetie should be turned off when not in use, yes?

 

Is a warm up period of 10 - 15 minutes after powering up and before using recommended?

 

After turning off is there a "resting" period recommended before turning on again?

 

What should I use on the wood?

 

And, not particular to Sweetie but regarding tubes in general; Where can I find some sort of guide to tell me what tubes are acceptable replacements for tube "rolling" or experimenting in search of different sounds or tones or flavors or ... whatever you call it? How do you know what's interchangeable and what is not?

 

Link to comment
Share on other sites

Turn it off when not in use. 

I usually just turn it on and start using it. Warm up probably does help some.

No, if I decide to turn a tube amp back on I do not hesitate. 

A coat of wax will be all it will need to preserve the wood. 

Stick with 6Y6, and 6SJ7 tubes. I honestly do not think you are going to find any better 6SJ7 tubes than you already have. The tubes in it now should last a very long time. Those old tubes were made to last. 

  • Like 1
Link to comment
Share on other sites

3 minutes ago, henry4841 said:

Turn it off when not in use. 

I usually just turn it on and start using it. Warm up probably does help some.

No, if I decide to turn a tube amp back on I do not hesitate. 

A coat of wax will be all it will need to preserve the wood. 

Stick with 6Y6, and 6SJ7 tubes. I honestly do not think you are going to find any better 6SJ7 tubes than you already have. The tubes in it now should last a very long time. Those old tubes were made to last. 

 

Second all comments above.  My tube amps sound pretty nice just turned on and believe after 10 minutes or 3 songs they are pretty much fully warmed up.  Not much experience with the transistor amps these days, but they seem to take much, much longer to warm up.  Like hours.

 

Johnson's Floor Wax was all the maintenance I provided to my self stained LaScalas, very easy and durable.

  • Like 1
Link to comment
Share on other sites

33 minutes ago, henry4841 said:

Stick with 6Y6, and 6SJ7 tubes. I honestly do not think you are going to find any better 6SJ7 tubes than you already have. The tubes in it now should last a very long time. Those old tubes were made to last. 

Can do for Sweetie, thanks - I like the long life and affordability aspects but I was asking a more general question about tubes in general.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...