Jump to content

Little Sweetie Forum amplifier project


henry4841
 Share

Recommended Posts

One way of getting more power but keeping the low power detail of a triode output stage is what's known as "extended Class A".

 

It was first introduced in the 50's and used 807's in parallel, one triode strapped and one pentode wired. Since the triode strapped 807 requires much more bias voltage the pentode wired 807 is effectively at cutoff during idle and out of circuit.

 

Feedback within triodes:  The reason for poor power is when the plate swings low with a triode it decreases current through the tube. When the triode strapped 807 swings large signals the pentode wired one comes into circuit pulling the current up through the load effectively increasing the output power. So you have the same change in voltage but higher change of current.

 

The amp was introduced as a push pull amp but we can do a single ended version. I have made a push pull version before and they do sound amazing but I have never made a SE version of that circuit.


We don't have to use 807's either, many tubes work well with this arrangement and you can even use different types of tubes. I have one here with triode wired EL34's and pentode wired KT88's I am working on. The thing is a BEAST!!

 

If anyone wants the PDF of the circuit let me know.

Link to comment
Share on other sites

1 hour ago, captainbeefheart said:

As for output tubes, why not something more powerful like a 6BG6GA which is cheap and plentiful but also rated for 20 watts. Mind you that rating is for horizontal deflection duty and for audio applications is more a 30 watt tube like the 6L6GC. I use them a lot and they are fantastic tubes, plus the plate top cap looks neato.


Have you run older 6BG6s at much over 19 watts dissipation without issue?  If wanting to do that I recommend picking up some Phillips 6BG6GAs from Mike Marx at SND tube sales.  They are truly equivalent to the 6L6GC and will run at 30 watts dissipation all day without issue.

 

Maynard

  • Like 1
Link to comment
Share on other sites

6 minutes ago, captainbeefheart said:

One way of getting more power but keeping the low power detail of a triode output stage is what's known as "extended Class A".

 

It was first introduced in the 50's and used 807's in parallel, one triode strapped and one pentode wired. Since the triode strapped 807 requires much more bias voltage the pentode wired 807 is effectively at cutoff during idle and out of circuit.

For clarification, when Cappy says "pentode wired", he is referring to applying a voltage to the screen for a certain mode of operation. The 807 is a beam tetrode, not a pentode. 

 

PM me the PDF. 

Link to comment
Share on other sites

4 minutes ago, tube fanatic said:


Have you run older 6BG6s at much over 19 watts dissipation without issue?  If wanting to do that I recommend picking up some Phillips 6BG6GAs from Mike Marx at SND tube sales.  They are truly equivalent to the 6L6GC and will run at 30 watts dissipation all day without issue.

 

Maynard

1625's are fairly rugged too. Price keeps creaping up on them, but still reasonable. 

Link to comment
Share on other sites

1 minute ago, tube fanatic said:


Have you run older 6BG6s at much over 19 watts dissipation without issue?  If wanting to do that I recommend picking up some Phillips 6BG6GAs from Mike Marx at SND tube sales.  They are truly equivalent to the 6L6GC and will run at 30 watts dissipation all day without issue.

 

Maynard

 

No I haven't run the older 6BG6s at all, only the 6BG6GA which as you said will run at 30 watts all day without problems whatsoever, it's a very rugged and well built tube and great for audio. I actually re-wire old Heathkit W5m's to run the 6BG6GA instead of the very expensive KT66. Performance and sound is identical between the two.

 

Big fan of 6BG6GA!

Link to comment
Share on other sites

5 minutes ago, Curious_George said:

For clarification, when Cappy says "pentode wired", he is referring to applying a voltage to the screen for a certain mode of operation. The 807 is a beam tetrode, not a pentode. 

 

 

Yes since the beam forming plates are still at a DC reference they are still considered a "fifth" electrode, it's just not a screen grid. Some datasheets even call them beam pentodes.

 

When the patent was around they didn't use the word pentode.

 

Link to comment
Share on other sites

2 minutes ago, captainbeefheart said:

 

Yes since the beam forming plates are still at a DC reference they are still considered a "fifth" electrode, it's just not a screen grid. Some datasheets even call them beam pentodes.

Yep, I was just trying to keep it straight for the ones following along.

 

I think due to Philips/Mullards patent, nobody could mention a fifth element. 

  • Like 1
Link to comment
Share on other sites

Here is that monster extended Class A amplifier. Notice the massive power transformer that failed and I had to pull from the potted enclosure, that wasn't fun at all.

 

The owner isn't sure what he wants to do, rewind the power transformer or outfit a custom made one. Both very expensive endeavors. It's been here like a year and he still doesn't know what to do.

 

 

exta3.jpg

exta2.jpg

exta1.jpg

Link to comment
Share on other sites

19 minutes ago, Curious_George said:

What speakers and preamp do you have 51?

Currently I am privileged to oversee an embarrassment of riches including much of the Heritage line starting with HIP Heresies (sp?), old Fortes, Cornwall IIIs and my most recent and prized acquisition: a pair of Belles. All acquired prior to retirement and divorce so some may have to go in order to finance my audio habit (and pay the rent). Primary preamp is a Schiit Freya +. 

  • Like 3
Link to comment
Share on other sites

Morning guys. I bought one of these this morning to replace the generator that died this week. Wish me luck. Never know on Ebay but there are some good deals to be found there. Seller has a 100% feedback rating at least. They are over $300 new. 

https://www.ebay.com/itm/265848677670?hash=item3de5d0e526:g:rmYAAOSwUU1i~8x~&amdata=enc%3AAQAHAAAAoO%2FzyLYxtcRLYF9EYJ44wYRNZiD5uxPO44vdgby11u7zom8kITlzmr4%2FOo8hytfCShjB0tFdz7uJyCSl32iiwrseGLFbaBkxUsuXyej%2BE9mz29BRbQ2zVJU6yilMa3XJp77RycmJAkUqnW43tZViEM7CfjEo1rx280ERVo3zkRjx2yGf81XN%2F70IfKzPwmCQoL3Do32WQWFFM8z%2BdhcLh4g%3D|tkp%3ABk9SR4SmzezbYA

 

One may have noticed I have said many times this thread is for just the basics of electronics. Probably  as much as I know on the subject. This is a speaker forum and not diy but electronics and speakers go hand in hand. Some say what audiophiles are is those that start listening to equipment. At least the different offerings of equipment out there. I think the majority of visitors to this site know little if anything about electronics. Oh there are some very talented guys here but for the most part I think most are like I previously said. For those that care to know what little I have learned on the subject the last 50 years this may be where you want to visit. Now I have not played with electronics the whole 50 years. I took an interest in my 20's then family and earning a living strayed me away from it. Thirteen years ago I was forced into retirement due to major heart attack and started playing with electronics again to pass the time. Forgot most of what I learned in my 20's but remembered some of the basics and took a few online electronic courses to refresh my memory. It's fun and I like to do it. During the last 13 years I know I have built over 30 amplifiers possible 40 or more. Just playing and trying to learn. I was born and was raised when solid state was beginning to be the norm and being young thought that new would be better and did not buy or build any tube gear. But the last 10 or more years I got to wondering why would someone pay $25,000 for a SET 300B amplifier with only 5 watts. Outrageous was my thoughts. They are out there and by some not considered real expensive. What's the deal. The deal is quality sound over quantity that tubes deliver. I am talking Single ended triodes that are all class A. The purist way to amplify an audio signal.

 

You may have figured out that I love the vocals and tone of this Sweetie amplifier. There is talk of getting more power from it. Not hard to do actually. Just run it faster. What I mean is this. The 6Y6 power tube is rated for 12.5 watts. That is the dissipation of the tube and not what it puts out to the speakers. The norm for designers is to run tubes around 70% of their rated dissipation factor being for good performance and long life. Lets just for demonstration say we are going to run it at 8 or 9  watts to make it simple and stay away from math. Running the tube at 8 or 9 watts will produce 1 watt of amplifier power that goes to the speaker. Class A is wasteful. The way to get more watts out is to turn it up much like a car where it will go faster if you mash the accelerator and go say 100mph but if you steadily run you motor at 100mph you are going to shorten it's life. The motor will last much longer if you never exceed the interstate speed limit of 70mph. I know for a fact that most are only using a few milliwatts listening with Klipsch speakers. I said most. Klipsch speakers are excellent sounding with the benefit of playing real loud. Extremely loud for those that love that sorta thing. This is probably not going to be the amplifier for you. But if you listening at reasonable levels, most who visit my house say I listen too loud, you may want to consider this amplifier or the flea watt one of Decware. Both this one and the Zen of Decware are classified in my book as flea watt amplifiers. I have done some comparison of Deckert's schematic and this one of Maynard's and I much more like what Maynard designed than what Deckert is doing in the Zen. I am sure he knows this but wanting to keep it simple to build and still perform well he took out a few things. This is not to say the Zen is not well designed, it probably is from all the sales he is getting but he left out a lot that Maynard included in this Sweetie. I may in the future compare the two circuits and tell you what I see in the two schematics but this is enough rambling of an old man this morning.   

  • Like 1
Link to comment
Share on other sites

You technical guys did you see how Deckert is biasing the output tubes with just one resistor and cap on one cathode for both tubes? No cap on the grid of the input tube but using a resistor instead to limit the DC from the source, if there is any which is almost always the case. Also no reference to ground resistor on the control grid of the pre tube either. Getting away with it but I would rather see that resistor. In my opinion I would always use a cap from the pot to grid. The power supply circuit is pretty much the same as Maynard's Sweetie with 2 resistors and 3 caps. No voltage dropping resistor on the filaments and no virtual ground either. Just some differences I noticed. 

Link to comment
Share on other sites

19 minutes ago, henry4841 said:

You technical guys did you see how Deckert is biasing the output tubes with just one resistor and cap on one cathode for both tubes? No cap on the grid of the input tube but using a resistor instead to limit the DC from the source, if there is any which is almost always the case. Also no reference to ground resistor on the control grid of the pre tube either. Getting away with it but I would rather see that resistor. In my opinion I would always use a cap from the pot to grid. The power supply circuit is pretty much the same as Maynard's Sweetie with 2 resistors and 3 caps. No voltage dropping resistor on the filaments and no virtual ground either. Just some differences I noticed. 

Decware definitely is saving a few pennies by omitting a few very inexpensive components. The shared cathode resistor/cap is a surprise. Especially when the ultimate sound quality is touted with this amp. $0.75 more could have bought you separate channels for this.

 

The input tube grid if referenced to ground through the volume potentiometer, not a bad design. Definitely cost effective. Nothing wrong here unless the wiper arm opens and then you have no input reference to ground. Possible, but highly unlikely. 

 

Power supply could have used at least one choke for the money you pay, in my opinion. 

Link to comment
Share on other sites

5 minutes ago, Curious_George said:

 

The input tube grid if referenced to ground through the volume potentiometer, not a bad design. 

I thought that is what he is thinking too but just one extra step at 15 cents to put a 100K resistor there. No excuse though for not having a cap on the input grid. It needs to be there IMHO. He is getting away with not doing it so it must work. I am sure he knows exactly what he is doing. I cannot remember seeing the biasing with one resistor for both tubes though. Probably have but just do not remember seeing it done that way. 

Link to comment
Share on other sites

1 minute ago, henry4841 said:

I thought that is what he is thinking too but just one extra step at 15 cents to put a 100K resistor there. No excuse though for not having a cap on the input grid. It needs to be there IMHO. He is getting away with not doing it so it must work. I am sure he knows exactly what he is doing. 

A lot of modern amp gear does not have an input cap, because it is understood most modern preamp gear has an output capacitor. Eliminating the input cap, Decware can claim fewer components in the signal path, more ingenious marketing. 

  • Like 1
Link to comment
Share on other sites

Then there are those sources that do have some dc offset on the outputs. Many of the higher end sources  are direct coupled with no cap. What dc offset does on the control grid is throw the bias off to some degree that was designed in the circuit. If slight enough in real world terms you can get by with a little bit within reason within a certain tolerance. Simple amplifier designs like this one will be perfectly fine with 10% resistors or possible 20% tolerance. Very forgiving amplifier design. A cheap polyester cap on the input will prevent any dc interference on the control grid and I do not believe I have ever built a tube amplifier without using a cap on the input. Get by without one but why would one just to save a buck. Not to trash the Zen design or build quality because the man is very successful with that Zen and from all accounts it is very good sounding. 

 

George have you ever not used a cap on the input grid? 

Link to comment
Share on other sites

10 minutes ago, henry4841 said:

Then there are those sources that do have some dc offset on the outputs. Many of the higher end sources  are direct coupled with no cap. What dc offset does on the control grid is throw the bias off to some degree that was designed in the circuit. If slight enough in real world terms you can get by with a little bit within reason within a certain tolerance. Simple amplifier designs like this one will be perfectly fine with 10% resistors or possible 20% tolerance. Very forgiving amplifier design. A cheap polyester cap on the input will prevent any dc interference on the control grid and I do not believe I have ever built a tube amplifier without using a cap on the input. Get by without one but why would one just to save a buck. Not to trash the Zen design or build quality because the man is very successful with that Zen and from all accounts it is very good sounding. 

 

George have you ever not used a cap on the input grid? 

I typically do not use an input cap on amplifiers, tube or SS. When I redesigned my Phase Linear driver board for the 400/700, I did populate the PCB with the option to use back-to-back electrolytics to essentially emulate a non-polar input cap. 

 

I can't say I have owned or looked at all the equipment out there, but in my experience, almost all sources and preamps usually have no DC-offset, unless during the power up sequence, which is why you turn your source on first, then your preamp, then your amp. By contrast, you do the opposite when you power down your equipment. 

 

Granted a film cap is not that expensive to use, but you know how people feel about caps in the signal chain. I don't have a cap phobia, I just only use parts when they are needed. 

  • Like 1
Link to comment
Share on other sites

Hey guys. Yesterday talking about Deckerts Zen amplifier schematic I said I did not believe I have ever seen someone use just one cathode resistor and cap for both output tubes. I was wrong. Looking at the schematic of 2011 I posted of the Zen that I built my Zen from many years ago, I have built an amplifier that way, one cathode resistor for both output tubes. Probably just following directions, (directions is the schematic for diy'ers) not understanding what I was doing back then. Yes one can build an amplifier not knowing all that much of the circuits. Giving it some thought I believe it is a smart idea that could even be used in a stereo Sweetie build saving a few bucks. The only thing that one would need to change is using a 1 watt cathode resistor, R8, instead of a 1/2 watt one. I would like to get Maynard's opinion if he sees this post though. I am still going to use the input cap and resistor though, C2 and R3. Deckert and George are probably right and not really needed but that was not the way I learned about tube circuits and being old fashioned I will just throw away a few bucks there. Again I would like to hear Maynard's thoughts on the subject. As one can see I value Maynard's thoughts. Lot to be learned from old timers like Maynard and possible myself. My thought is the control grid of a tube is just like the water facet in your house in the way it controls water. The analogy of water flow and electron flow is a good way to understand what is going on in a tube. You turn the facet in your house one way and increase the flow and the other way to decrease it. The facet in a tube circuit is that control grid. To increase or decrease the electron flow in a tube circuit one changes the values of R3 and 4  in the 6SJ7 tube. The designer has chosen the values of those resistors by determining what is a good safe flow and uses the facet, control grid, to control that flow. If any DC gets on that control grid it is going to change what the designer built into the circuit. A slight difference in change it really is not going to matter that much since the tube has plenty of electron flow going through it anyways. But still it is going to change it some. The cathode in a tube is negative and the plate is positive. That is what is needed to get the electron flow going. The control grid, water valve, is less negative in relationship to the negative cathode and since negative repels negative it slows or can almost stop the electron flow in the tube. That is the reason old timers put a capacitor on the input. Capacitors let the AC through and stops DC. All this is just class room theory of how a tube works and not what actually happens in the field where work is being done. Deckert of Decware decided the cap is not actually needed and he is probably absolutely right. He has installed a resistor on the control grid which will reduce any DC from the source if there happens to be some which is a good idea if one does not want to use a capacitor there. Slightly cheaper way for sure. 

 

This thread is for beginners to learn what little I know so you guys with more electronic knowledge may want to just ignore some of the things I write about. 

 

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.

 Share

×
×
  • Create New...