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The long awaited "Little Sweetie" mono SETs


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  • 5 months later...
On 8/10/2020 at 6:03 AM, henry4841 said:

The little sweetie schematic was posted on diyaudio forum and I have built it. Sounds as good if not better than my SET 45 tube amplifier. Wonderful sounding little amplifier. 

Congrats.  Maynard is da man.

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  • 3 months later...

Hello, I just completed a pair of "Little Sweetie" mini-mono-blocks, based on Maynard's great design.

I  basically used the same Hammond iron, but added a choke to replace R10; the 180-2W resistor.

I also added a damper-diode tube in series with the red/yellow C/T wire, in the ground rail to provide a slow-turn-on.

I put a stopper-diode in the B+ rail to the blue wire of the output transformer.

It took me a very long 3 days to machine the chassis and wire the amps!

And upon first start-up, they sounded really nice, and are improving every day.

Thanks again to Maynard for sharing his wonderful designs and providing help to builders.

(I will try to upload photos when I figure out how to resize them below the 2MB limit.)

rgds, John N. Lumley

 

 

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On 11/22/2020 at 7:39 AM, tube fanatic said:

Your build looks very nice John!  Can you post pics of the underside?  If you have time, please add the pics to Henry’s picture thread in this section.

 

It would be informative for those following this thread to discuss your power supply mods and why you chose to implement them.

 

 

Maynard

 

Hello, I did not isolate the transformers from the chassis.

I use star washers to bite thru the varnish used in manufacturing, thereby grounding them.

I employed a 6W6GT 1/2 wave damper diode, right after the MUR4100E diodes.

This provides a nice and slow ramp-up of the B+, and helps filter out diode hash.

I also use UF4007 "stopper" diodes in series with both the B+ and the B++ rails.

These prevent interaction between the two stages, and improve the bass.

I used 47uF-470uF-47uF caps in the power supply, based on PSUD2 simulations.

I measure under 1mV-AC at the speaker terminals.

Overall, a very worthwhile project, proving again that the circuit design is paramount!

Expensive "audiophile" parts can't fix a bad design sound.

A schematic will follow; still having to reset my password every time I try to log-on again...

rgds, JNL.

 

IMG_2677.JPG

IMG_2676.JPG

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Ok, here's the 2) schematics in jpeg format.

I'm still very impressed with the sound of these little and affordable mini-mono-blocks!

It's all I really need to power my Tang-Band W8-1808 full-range drivers (93dB),

that are in a 2-way system, with a Tang-Band 25-1166SJ tweeter (93dB).

Hat's off to Maynard, again.

rgds, JNL.

 

scan0004.jpg

scan0003.jpg

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23 hours ago, mike stehr said:

The art deco flair in the schematic you've drawn is awesome.

 

I use a pair of 6AU4GTA half-wave damper diodes in full-wave for a 2A3 SET amp of mine.

 

Hi Mike,  great idea!

The TV damper-diodes give you a slow turn-on, and a low voltage drop.

The oil cap gives you low DCR, and a very long lifetime.   (if not forever)

As you know, that first cap right after the rectifier is the most stressed cap in the whole amplifier!

The Marantz model 2 amps used 2) 6AU4 then a 8uF motor-run oil cap.

rgds, JNL.

model2-schema.jpg

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22 hours ago, tube fanatic said:

You are quite the artist John.  Looking at the schematic made me feel good!

 

For the benefit of those following, can you explain why you chose to add the diodes in series with the opt and plate/screen supply of the driver?  
 

Maynard

 

 

 

Well, in a nutshell, the diodes prevent the driver tube voltage sagging, when the power tube draws heavy current.

 

If you Google "stopper diode", you find better explanations.

rgds, JNL.

Edited by John N. Lumley
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3 hours ago, John N. Lumley said:

Hi Mike,  great idea!

 

Actually, the idea was from a friend of mine who designed the circuit and coached the build for me.

The PS transformer is a Gramer television transformer, with a 10-12 Awg heater winding for the 6AU4.

A 36uf Mallory oil is the first capacitor in the supply, with a 4 Henrie choke, a 100uF Sprague Atom with another Mallory 36uF oil cap in parallel with the Sprague.

 

And I thought this audio amp power supply using 6AU4 for full-wave was just a bizzaro one-off...and it turns out Marantz did it with a commercial amplifier.

 

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  • 1 year later...

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So this is my 6Y6 amplifier, this is running in pentode operation.  This tube is a real gem in this mode of operation, and has a few things about it that make is a true outlier.

1) It's plate impedance make it so you can use a 2.5K Hz primary on the output transformer.  This is unique because this creates a lower winding ratio, which means you can put more windings on the same size core compared to other tubes that need a higher plate impedance that cause a higher winding ratio.

2) In pentode mode it makes about 4.5 watts of output power, this coupled with the low plate impedance and speaker above the range of 95dbs w/m make for a much solid base than is usually obtained by a single ended amplifier.

3) The 6Y6 doesn't need a great deal of voltage to drive it, less than 8 volts peak to peak, so you can operate a low Mu high Gm tube which gives a more laid back easy presentation to the music.  Where higher Mu tubes usually have a more "glassy", or "edgy" type of sound. 

4) A single ended amplifier based on the 6Y6, which has a maximum plate voltage of 200vdc can operate quite well at voltages even as low as 140vdc, which means you can make a power supply from a isolation transformer, but to get the decent wattage you need to have current in the range of 70ma per power tube, so the output transformers, and power supply need to be made to accommodate this, and higher Gm driver tubes feeding the 6Y6 will normally run higher that usual plate current as well.  Therefore designing a power supply that can feed as much as 200ma is a good idea.  The amplifier shown above is using a switch mode amplifier, that is very quiet.

5) Probably the most difficult thing about the 6Y6 is it requires more current on heaters, which made it unpopular with manufactures, that usually selected the 6V6, or 6F6 instead.

Best of all though is this tube sounds wonderful, much more balanced than most single ended power tubes in the same wattage range. 

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