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The long awaited "Little Sweetie" mono SETs

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Jason ("thesloth") originally proposed developing a number of low power diy tube amplifiers for the new forum section with two goals in mind: 1) easy to construct by those with enough experience to work with high, potentially lethal, voltages in a safe manner, and 2) to provide a level of sound quality, when used with Klipsch speakers, which rivals that of very costly amps.  I thought it was an excellent idea.  Of course, neither of us expected William ("wdecho") to bag the first entry award!!!  And now he is getting ready for yet another amp project!

In thinking about what I wanted to offer, a number of goals came to mind:

1) use of inexpensive, readily available, tubes which were originally designed for radio service, and which offer extreme long-term reliability.

2) a design which offers no compromises (to me, that means at least 1 watt/channel, which is more than enough for 90% of the users of both Heritage and Reference Series speakers with whom I'm acquainted), and very low distortion.

3) crosstalk limited to that of the source device to allow the speakers, which are capable of breathtaking imaging and a huge soundstage, to deliver the goods.

4) built like a tank with very cool operation, allowing use all day every day if desired.

5) low parts cost to be comparable with the imported offerings seen on Amazon and elsewhere.

6) ability to shape the high frequency response to suit individual taste and room characteristics.

The first two goals are easily met by more tubes than you can imagine.  I've always believed that the finest sounding audio tubes were developed for use in radio service.  So, it was a matter of choosing a driver and output tube which provide very low distortion.  The 6Y6, which was originally registered by Raytheon in July, 1937 is a very tough "battleship" of a tube with a 12.5 watt plate dissipation based on the extremely conservative design-center criteria.  It also happens to be very linear when used as a triode (since SETS are unsurpassed for creating an enormous soundstage and pinpoint imaging, there was no decision to be made about using that mode of operation).  It's only limitation is requiring a lot of drive to achieve full output (approx. 21 volts rms in this design), so a pentode driver was needed to effortlessly provide the tube with what it needs.  The latter function is filled by the 6SJ7, which can drive the output stage fully with only about 200 millivolts of input signal, while its distortion stays very low (approx. 1% thd per published specs).  In this design, 2nd harmonic distortion of the 6Y6 is only 2% at full output into the calculated 5k plate load (corresponding to a speaker impedance of 8 ohms when connected to the amp's 8 ohm tap).  When the plate load drops to 2.5k, which corresponds with the typical minimum impedance of most Klipsch speakers of 3.5-4 ohms, the distortion rises to only 5%!  Remember that the latter figure represents 1/400 of the total output power- quite insignificant.  Speaker impedances greater than 8 ohms will reduce the distortion to well below 2%.  Power output is approx. 1.3 watts at the onset of clipping into an 8 ohm resistive load.  I know many of you are laughing at this figure.  Well, don't!  Unless you have experienced just how loudly and cleanly even half that amount of power can drive the speakers in a majority of installations you will remain skeptical.  To eliminate soundstage reducing crosstalk issues (typically encountered when both channels share a common high voltage supply), these are constructed as individual mono amps.  That can be done on a single chassis, if desired.  Cool operation is no problem at all.  The specified power xfmr is used well below its rating, and only gets mildly warm after extended use.  Little heat is developed under the chassis as well.   Longevity of the 6Y6 should not be an issue at all since the quiescent plate dissipation is about 10 watts, a very comfortable margin of safety for the tube.  In addition, as noted on the schematic, filament voltage is going to be set not to exceed 6.3 under typical operating conditions.  Excessive filament voltage is a common cause of premature tube failure.  The 6SJ7 is unlikely to ever need replacement.  Parts cost for the amps, as constructed, is in the low $300 range depending on the source you choose.  Hammond output xfmrs tend to be a bit on the expensive side- going with Edcor can save some money.  I have never heard the latter; however William, and others, has reported that they sound wonderful.  Lastly, the amp incorporates the variable low pass filter which has become a standard feature in all of my designs.  R1, the 10k pot on the schematic, allows the user to shape the high frequency characteristics of the amps to individual taste.  I install it as an under-the-chassis control which is set once and then forgotten (until the amps are used with different speakers, or in a different room, etc.).  You can just as easily panel mount the control if you plan on using it regularly, or simply eliminate it along with C1 if you don't want that capability.  Lastly, the parts ratings are, in some cases, much higher than needed for this circuit.  Since the higher rated parts are often only a few cents more than those with a lower rating, there's no reason not to over-build.  Capacitors, in particular, last much longer if run well below their rated operating voltage and internal temperature.

The amps are totally silent with an ear at the front of the speaker.  The sound is silky smooth, definition (tested with vocal and choral music) is as real as if the performers were in the room, and bass output is very potent and approaches that of a well constructed SEP amp.  I can't understand why some claim that SETs provide anemic, mushy bass.  It isn't true.  As expected, the imaging is precise and the soundstage is positively huge with my RF-15s.  The amps will be tested with CWs as soon as I can get hold of  "the guy down the road" (a professional musician and music teacher who auditions all of my creations with very critical ears!).  From past experience we have found that when voiced for my speakers, we rarely have to change anything when used with CWs.  The same voicing usually applies to K-horns and LSs as well, based on my own experiences.

I've attached pics which should be useful for anyone who wants to use a similar format.  Most importantly, star grounding (bringing all grounds to a single chassis point), and keeping all AC wiring as far from the driver as possible is critical for keeping the amps hum free.  If you use a different layout, I suggest cutting a sheet of paper to the size of the chassis you will use and placing out the xfmrs, sockets, and so on to get the arrangement you desire.  Then, mark up the chassis and re-check the component placement a couple of times before drilling.

So, who's going to build a pair of these?  If you do, you are likely to question why you ever spent a large sum on an amplifier!  These could easily be the last amps you will want to own!  

If anyone notices something in the schematic which doesn't seem correct, please let me know asap!

 

DO NOT ATTEMPT TO CONSTRUCT THESE AMPS IF YOU ARE NOT SKILLED IN WORKING WITH POTENTIALLY LETHAL VOLTAGES! 

Maynard

post-40520-0-26480000-1445287805_thumb.j

post-40520-0-07680000-1445287825_thumb.j

post-40520-0-08460000-1445287871_thumb.j

Little Sweetie.pdf

post-40520-0-84480000-1445288313_thumb.j

post-40520-0-04340000-1445288380_thumb.j

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Very cool looking Maynard.  I'll bet they sound great.

 

So that's 300 for both amps, or one?

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So that's 300 for both amps, or one?

For the pair!  Depending on the type of sockets and other ancillary parts you use, it could be a bit more or less.  That figure doesn't include shipping costs, or tax.

 

Maynard

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those are actually very cool looking...

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I can't wait to read the article in greater detail. One neat feature I don't see very often is the "tone control" network at the input. Is the max settings -3db corner frequency around 7kHz? I am sure some of the old ears on here might not even notice hahahaha ;) 

 

Looks like a solid design and a beautiful layout! Great work Maynard.

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Ever thought about making an amp with XLR inputs for those who might use an active crossover?

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So that's 300 for both amps, or one?

For the pair!  Depending on the type of sockets and other ancillary parts you use, it could be a bit more or less.  That figure doesn't include shipping costs, or tax.

 

Maynard

 

 

Maynard,

 

I will take a pair if they are for sale. You can PM me and we can sort out things.

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Thanks guys!  Regarding the variable filter, you are quite correct "XX."  The corner freq. is about 7.24 kHz with the pot at full resistance and, indeed, I imagine there are some who would not notice the difference if it were fully engaged!  It certainly is visible on the 'scope though.  

Richard, I've never had any inquiries for an XLR as no one I know is using an active crossover or balanced cables.  It shouldn't be difficult to configure by anyone who needs that application.

Jim, I haven't thought about selling these but you will have first grabs on them if I do.  For now I need to hold onto them to use for demonstrations until the local guys get bored and bug me for something else to play with.  

Maynard

Edited by tube fanatic

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I imagine there are some who would not notice the difference if it were fully engaged!

 

 

That was a low blow to the old guys on here :P The high frequencies are the first you lose. The flip side to this is the people who can hear up high might be extra sensitive to them, especially with horn loaded speakers. I know when I have had my Heresy's hooked up to certain amps that the top end was a little to aggressive. 

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SET on the cheap...a nice way to go about it.

 

I'm still going to bread-board the parallel 6AQ5 SEP amplifier circuit. I've got all other projects out of the way...well, I wish that were true. It's more like shove all the other projects out of the way.

 

Nonetheless, I have a few bucks to order some parts. Do you order your parts from Mouser, Maynard?

 

I'll go ahead and buy some 6SF5, to keep the circuit in spec...other than the tube power supply that will be used.

I've read online where folks have ran the James 6123HS OPT over the max rated DC current of 90 milliamps, up to 120ma...Seems excessive.

I guess I'll just try it and see. If they saturate, I'll hear it.

 

What is the B+ voltage at the OPT and plates of the 6AQ5/6005 tubes? Your schematic doesn't have voltage notation.

 

Thanks!

Mike

Edited by mike stehr

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Beautiful looking work Maynard. Thanks for sharing it with us. This looks like it may be my first attempt at a build!

Stacy

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Hey "XX", don't forget that I'm one of those old guys on here (well, at least older than many and younger than some)!

 

Mike, send me a PM with the primary dcr of that James xfmr (I can't find the spec on it).  If it's too high it may not be suitable, but we'll figure something out.

 

And Stacy, thanks for the compliment!  Let me know when you're ready to build- you know where to find me!!!

 

Maynard

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Great layout, beautiful work. Where did you buy the chassis'? 

I buy all my parts, except sockets and tubes, from Mouser.  They are the most reliable and honest company I've ever done business with.  You couldn't find a better supplier:

 

http://www.mouser.com/ProductDetail/Hammond-Manufacturing/1444-972/?qs=sGAEpiMZZMsrGrAVj6eTvb4%252bPzjFYIIC%2feNnRI80QZ4%3d

 

Maynard

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Mouser's great!  So's Maynard... 

 

Flawless design and execution as usual Maynard!  

 

You're designing 'em faster than I can build 'em!  (Not complaining)  =]

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Most of the parts used for this project were pulled from my own stock (due to all of my other involvements I keep a lot on hand to minimize having to place frequent orders).  I simply don't have the time right now to go on Mouser's site to research a full bom.  That said, I'm not sure that using a kit approach to the diy amp projects is the best approach.  Two important facets of diy amp work are learning how to source parts, and how to lay out a chassis properly.  I'd rather see prospective builders ask individual questions which can then be answered in a way which could benefit others.  It would lead to informative discussions about things such as star grounding, chassis size and material, whether input jacks should be insulated from the chassis, providing ventilation for heat generating under-chassis components, and so on.  I've used that approach with many others (including Marty who did a couple of very creative builds after we discussed various aspects!).  It would also give the other builders on here a chance to present their methodology which, I'm sure, is different from mine.  Remember, these are intended to be "community" amp projects and there are enough tube guys on here to field any questions which come up.  Off the top of my head we have William, Fjd, new member "xx", the two Mikes (Stehr and BSE), and Erik.  Maybe I can even talk Jason ("thesloth") into returning!  What do the rest of you think about this?

Maynard

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Most of the parts used for this project were pulled from my own stock (due to all of my other involvements I keep a lot on hand to minimize having to place frequent orders).  I simply don't have the time right now to go on Mouser's site to research a full bom.  That said, I'm not sure that using a kit approach to the diy amp projects is the best approach.  Two important facets of diy amp work are learning how to source parts, and how to lay out a chassis properly.  I'd rather see prospective builders ask individual questions which can then be answered in a way which could benefit others.  It would lead to informative discussions about things such as star grounding, chassis size and material, whether input jacks should be insulated from the chassis, providing ventilation for heat generating under-chassis components, and so on.  I've used that approach with many others (including Marty who did a couple of very creative builds after we discussed various aspects!).  It would also give the other builders on here a chance to present their methodology which, I'm sure, is different from mine.  Remember, these are intended to be "community" amp projects and there are enough tube guys on here to field any questions which come up.  Off the top of my head we have William, Fjd, new member "xx", the two Mikes (Stehr and BSE), and Erik.  Maybe I can even talk Jason ("thesloth") into returning!  What do the rest of you think about this?

Maynard

I'm wanting to do this...

I built my current Kits with a rudimentary understanding of electronics, can follow direction and solder....

But I don't know a damn thing about how to trouble shoot...

Sure I can determine what tube went, but I can't determine if there is an underlying problem causing a tube to fail. I want to know more and I believe this Thread is going to be the catalyst

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I know it is a PIA making a BOM with the parts you ordered but it would truly help any first time builder to know the parts to order.

 

I think one way of learning how to follow a schematic is the ability to come up with a parts list by just reading the schematic.

 

Maynard shows the list of parts in a column next to the schematic diagram. Most resistors in older tube amp circuits are 1/2 watt, unless indicated otherwise. Capacitor values/voltages are listed, transformer brand/type are listed...even the type of diodes in the power supply.

 

It's just a matter of writing up a list of parts. I'm sure Mouser carries all the parts Maynard uses. I'm pretty sure Mouser sells Hammond iron, and probably bud boxes. (chassis')

I know it's a bit of a chore navigating the Mouser site, but that's life...good prices, and excellent shipping...

 

A kit amplifier is nice for a noob, just solder this to that, check the voltage...nothing in the way of a learning process. Which makes troubleshooting really confusing when thing are done incorrectly.

It's nice to know how some things work.

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Just to confirm what Mike said, everything used in this amp, except tube sockets and tubes, came from Mouser.  I even purchase my hardware from them as they sell both steel and nylon.  Their site is actually quite easy to use as the search function works very well. 

 

Maynard

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