Jump to content

The "Little Millie" SET spud amp!


tube fanatic
 Share

Recommended Posts

Within the audiophile community there is a subset of folks who have embraced a totally minimalist approach (similar to those who have given up conventional housing in favor of "tiny houses").  These enthusiasts typically listen in the near-field using the simplest possible minimum wattage tube amplification.  This project was requested by such a person who is currently using a pair of Dayton B-652 speakers with a Little Sweet Potato SEP  and uses a CD player as his source.  He will only use a spud amp as he wants a single signal tube per channel and no more.  His belief is that every gain stage in an amp degrades the sound (I don't disagree with that thinking).  For those unfamiliar with the spud concept, you can read about it here:

 

https://community.klipsch.com/index.php?/topic/160798-here-it-is-the-little-sweet-potato-sep-spud-amp/

 

His listening level is very low- typically in the 60-72 decibel range at a listening distance of 24-30 inches from the plane of the speakers.  We measured his average power need and it surprised us considering that the speakers are not overly sensitive (87 dB/2.83V/meter).  A mere few milliwatts (yes, you read that correctly) is all we came up with!  He decided that he wants to try a spud SET amp and asked me to come up with a workable solution.

 

The 50L6 (and its loctal equivalent, the 50A5) is one of the best sounding audio tubes I've ever experienced.  I can't imagine how many millions of these were in use during the days of the ubiquitous All American Five table radio.  They are readily available from virtually all tube sellers.  In this application, the tube is operating at an extremely linear part of its plate curves.  The result is vanishingly low second harmonic distortion.  Plate resistance is very low at approx. 800 ohms, and the amp is virtually immune to speaker impedance variations (assuming it doesn't go below 4 ohms).  Input sensitivity is roughly 3.25V (rms).  By using a CD player as the source, with the typical maximum output of 2V rms, it is impossible to make the amp clip.  You can literally turn the volume controls up all the way and let the musical selection determine the listening level.  Power output with 2V drive is approximately 50-60 milliwatts (depending on the tube tolerances)!!!  I know that some of you are shaking your heads in disbelief as you read this.   

 

To test the amp we turned the volume controls up all the way and played all kinds of music.  With some selections we were able to hit a peak of 81 db at the listening position which we both felt was too loud (we're old and our ears can't take it!).  Klipsch speakers, with their much higher sensitivity, will allow even higher playback levels.  The sound we experienced was, in our opinion, exquisite.  Regardless of the type of music, the sound was so enjoyable that we couldn't stop listening.  The depth and width of the sound stage was also outstanding.  And I'll add that even with the volume controls up all the way, the amp is dead silent.  

 

Like the Little Sweet Potato, this will make a terrific beginner's DIY project.  The power supply layout is totally non-critical and, if the amp is constructed with reasonable care, it should work without issues.  If you would like a copy of the schematic, send me a PM and I'll pass it along (it should be available in a few days).  You can use the picture below as a guide for parts placement, although other arrangements may be equally suitable.  

 

AS WITH OTHER TUBE AMPS, THIS ONE OPERATES ON VOLTAGES WHICH CAN KILL YOU!  DO NOT BUILD ONE IF YOU LACK THE EXPERIENCE TO WORK WITH LETHAL VOLTAGES IN A SAFE MANNER.

 

So, who is going to build this amp?  

 

 

Maynard

 

 

 

Little Millie 1.JPG

Little Millie underside.JPG

  • Like 5
Link to comment
Share on other sites

Now that is a simple circuit. I have been part of the less is more club, as part of my love of all things audio. Yes, I have some big speakers and amps, but I also have flea power amps and high efficiency speakers. There is a magic to the pairing. My lowest power amp build uses 6DN7 tubes that put out about 1-1/2 watts. They are magic with some Omega 7A speakers and even drive Khorns wonderfully.   

  • Like 1
Link to comment
Share on other sites

1 hour ago, tube fanatic said:

Absolutely!  In fact, depending on the impedance of the phones, you may be able to run them without using a resistor to change what the amp sees.  Which phones would you want to use?

 

 

Maynard

I currently use Klipsch Refernce One. I can't seem to find specs, I've had them for years. I'm sure they are easy to drive because you can use them with a phone or iPod. I'm planning to upgrade eventually and will probably look for something lower in impedance. Something from Focal or Sennheiser perhaps. I would love a copy of the schematic when it's available. Thank you for all you do here Maynard!

Link to comment
Share on other sites

17 hours ago, mike stehr said:

So you are driving the grid of 50L6 directly from a 2volt output of a CD player? (or whatever puts out 2 volts RMS)

External PS?

Indeed it is.

 

Nice to see you on here again Mike!  You are correct- the p/s is external (the same design as the Little Sweet Potato supply), and the grid of the 50L6 is directly driven.  I suggest that anyone contemplating building their own read the Little Sweet Potato thread above as it outlines the design philosophy and provides some technical details.  In the Little Millie, I'm using only a resistor for filament ballast as opposed to using a tube.  It was a design choice based on what the guy asked for.  The three power resistors dissipate a great deal of heat, so lots of ventilation is needed (or, alternatively, chassis mount resistors installed with heat conductive grease).  I was able to get the schematic drawn last night so here it is for those who are interested.  If anything doesn't look  correct, let me know!  And, if adjustment of the high frequency characteristics is not desired, R1/C1 and R2/C2 can be eliminated.

 

ONE MORE REMINDER ABOUT THE VOLTAGES USED IN THIS AMP- DON'T BUILD IT IF YOU LACK EXPERIENCE IN WORKING WITH LETHAL VOLTAGES SAFELY!

 

Maynard

LITTLE MILLIE PNG.png

  • Like 4
Link to comment
Share on other sites

Another great build and writeup Maynard.  If I didn't have powered Klipsch computer speakers I use for very near-field, I'd be building this.

 

Now what's it going to take to get you to design a really, really, really good phone pre with LOMC ?☺️

Link to comment
Share on other sites

6 hours ago, thebes said:

Now what's it going to take to get you to design a really, really, really good phone pre with LOMC ?☺️

 

 

No, I gave up on vinyl when CDs came out!  I sold my turntables and records in the late 80s and haven't looked back.  But, I do remember my days of having to get out the Discwasher kit to clean the records and then shoot them with the anti-static gun before playing with a slight degree of fondness...😁

 

Maynard

  • Like 3
Link to comment
Share on other sites

  • 3 months later...

New to the forum but have Klipsch RP-600m bookshelves and a powered sub. 

 

Very interested in flea watt SET and DIY, which led me here. 

 

I'm okay with a soldering iron, having rewired my crossovers, and I have played with high voltages before with my fender twin, but I'm not a pro at building from a schematic.

 

With four kids, budget is always a factor. 

 

So, given what I've shared, is the spud build too much? Is there support if I ran into trouble while building? 

 

I'd love to have a headphone output for my Sennheiser HD600s.

 

Currently powering speakers from source through Bifrost DAC with more than 2V output into an NAD 316BEE, which I would sell to partially fund this endeavor. 

 

Thanks for any direction you can provide. 

 

 

Link to comment
Share on other sites

More information is needed to know if this amp will supply sufficient power for your needs, such as listening level, room size, proximity to speakers when listening, type of music, etc.

 

As far as your phones, the high impedance would probably require using a matching transformer which is not a problem at all.  
 

Will be glad to help you with advice if you have any questions.

 

 

Maynard

Link to comment
Share on other sites

  • 1 year later...

Unless I’m wrong, It doesn’t seam anybody has tried to build this yet. So i’m gonna five it a go since i have many of the parts needed floating in “extras” boxes around here.

 

The only issue is i don’t need to build a Stereo unit, as i only need a mono-block amp for a mono bedroom rig. 

 

So to make a mono-block version, what change to the hearter resistor R15 to only feed one valve. Are there any other alterations I should make that I’m not noticing to the power supply?

 

thanks much,

Jake

Link to comment
Share on other sites

8 hours ago, Jazzmaster Jake said:

Unless I’m wrong, It doesn’t seam anybody has tried to build this yet. So i’m gonna five it a go since i have many of the parts needed floating in “extras” boxes around here.

 

The only issue is i don’t need to build a Stereo unit, as i only need a mono-block amp for a mono bedroom rig. 

 

So to make a mono-block version, what change to the hearter resistor R15 to only feed one valve. Are there any other alterations I should make that I’m not noticing to the power supply?

 

thanks much,

Jake


The easiest solution is to use the second 50L6 (or any other 50V/0.15A) tube as a filament ballast so you can leave R15 as specified to reduce under chassis heat.  With only one channel, plate and filament voltages will run a little higher since the p/s won’t be loaded down as much, but it will be ok.  If you happen to have an 820 ohm/10 watt resistor you can use that in place of R13/14.  Feel free to PM with other questions.  It may take me a few days to respond though.

 

I now use full time, single speaker, mono with a RB-51 series 2 near a room corner and the combination is pure bliss to my ears.

 

 

Maynard

Link to comment
Share on other sites

On 2/6/2020 at 9:56 AM, thebes said:

Another great build and writeup Maynard.  If I didn't have powered Klipsch computer speakers I use for very near-field, I'd be building this.

 

Now what's it going to take to get you to design a really, really, really good phone pre with LOMC ?☺️

Try this on for size Marty, I got mine a few days ago it sounds great

https://hagerman-audio-labs.myshopify.com/products/trumpet-mc-vacuum-tube-phonostage

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...