Amazing "Little Gem" flea-power SEP

[tube fanatic]

The local crowd is aware that I'm willing to design and prototype, at my own expense, just about any idea which I find appealing.  So, with that in mind, I was asked a number of months ago if I could come up with an amazing sounding flea-power amp for around $200 using all brand new parts, and subject to a number of other requirements besides price.  I'll list each requirement separately and explain the solution:

1) $200 price- Impossible!  The only way to achieve that would have been to seriously compromise on the quality on the output transformers, which I was not willing to do, and to use a design format which I would not have been happy with.  So, it came in at around $325 (that included the tax and shipping) which the guy said was acceptable.  I also went with my long standing "All American Five" amplifier circuit which greatly reduces the cost of the power transformer, and easily allows the power supply to be designed without the expense of a costly choke.

2) Very potent and full bottom end- This necessitated going with a SEP design to bring the power up a bit (to around 750 milliwatts/channel continuous @ 32 Hz into 8 ohms resistive), as compared with a SET which would have about half that amount of power, and a bit less bass potency.  The absolutely superb 500 Hz square wave shown below confirms the low frequency performance, and is something which will not be found with some of the similarly priced flea-power Chinese amps which are out there.  In fact, during the initial demo in which I played some bass heavy tracks, the guy accused me of having a hidden sub!  I consider that a compliment, particularly when he then called the amp a "little gem."

3) Totally noise/hum free operation- Achieved by using a separate power supply which brings extremely low ripple DC into the amplifier chassis.  By isolating the power transformer magnetic field from the open frame output transformers, and not having any powerline AC in the amplifier chassis, this was easy to achieve. With an ear against the speaker grille, one hears dead silence.  The music seems to emanate from total "blackness" (to use a term popularized by some of the audio reviewers out there!).  

4) Cheap tubes/huge tube life- All 5 tubes can be found for no more than $5 each, depending on the supplier.  Tube life should approach "forever" from a combination of the manual soft-start feature of the power supply (which prevents the normal filament turn-on current surge), running the tubes below their design-center ratings, and keeping filament voltages 5-10% below their specified values.  Between that, and having the tubes in open space to dissipate heat, the output tubes should easily provide 15-20,000 or more hours of use, and the voltage amp tubes far in excess of that.

5) Large soundstage/precise imaging- The amp is almost totally dual-mono as only the input filter cap of the power supply is shared.  Everything after is split which greatly reduces crosstalk that would shrink the soundstage.  Critical signal paths are kept extremely short.

6) Ability to adjust high frequency characteristics- I've been including my variable low-pass filter (aka "ear bleed filter") in all of the recently built amps.  This guy, however, didn't want it as an external adjustment figuring that he'd spend more time fussing with it than listening to the music.  So, instead, 25 turn trimmers were installed internally which allows the amp to be voiced on site and then left alone.  If different speakers are used in the future, it can always be readjusted accordingly.

7) Small size- The power supply is on a 7 X 7 chassis, and the amp on one that's 11 X 7.  It would be difficult to get much smaller without making construction a real pain!

If anyone is interested in the technical details of the amp, don't hesitate to drop me a PM with your direct email and I'll be glad to give you whatever info you want.  Likewise, if anyone wants one of these, let me know and I'll try to help you out.  Next up will be the matching SET which will run on the same power supply, and should cost no more than $250.  For less than $600 it will be possible to have the best of the 2 worlds of flea-power single ended amplifiers.  And don't let the extremely low power make you think that these will only play at a whisper.  In small to medium sized rooms, with Reference or Heritage series speakers, even the SET will allow very loud listening levels (to me that's in the mid to upper 80 db range- not enough to shatter windows, crack sheetrock, or burst eardrums, but still loud!).      

Maynard



 

[wvu80]

Very nice amp Mr. Tube Fan, very nice offer, and your amp project was timely for me.  I don't think I'm in the "serious interest" category right now, but I am in the "curious" stage, which means researching and learning about DIY amps.  The more I learn, the more I become aware of what I don't know.

 

There is a possibility my wife might ask me what I want for Christmas, and I've found it's always a good idea to have an intelligent answer. 

Edited November 11, 2014 by wvu80

[tube fanatic]

Very nice amp Mr. Tube Fan, very nice offer, and your amp project was timely for me.  I don't think I'm in the "serious interest" category right now, but I am in the "curious" stage, which means researching and learning about DIY amps.  The more I learn, the more I become aware of what I don't know.

 

There is a possibility my wife might ask me what I want for Christmas, and I've found it's always a good idea to have an intelligent answer. 

My wife stopped asking me what I want for Christmas a long time ago!!!  She now tells me what I'm going to get.  It is far less expensive that way!  Certainly don't hesitate to ask any questions about the DIY stuff.  In fact, as soon as I get a couple of free hours, I'll draw out and post the schematic for this amp in case anyone wants to buld their own. 

 

Maynard

[mustang guy]

Heck of a project!  Great work, and great design. It took me a few to figure out what the holes were, but now I see you are using them to keep the big resistors cool.  Isolating the PS was a good idea. Nothing but the sound goes in that amp. Are you going to perform testing to see how the numbers look?  Wouldn't it be cool if this turned out to be world class?

 

Way to go Maynard!

[mike stehr]

I like how when Maynard posts a amplifier project, it's always a different type circuit/topology. Single-ended circuits of course, but always a little different approach in topology. 

Yes, they are more than likely variations of amplifier topologies that have existed forever, (All American Five for example) but Maynard puts his design/spin on the circuits. And strives to use dirt cheap available tubes in the circuits.

 

I guess I admire the engineering/noodling involved that you do...you remind of the retired EE friend of mine that I visited when he lived up in these parts. He showed myself a little speaker design using minimal tools like a AC voltmeter and a function generator, how to get excellent bass using high efficiency woofers from one watt of tube power, how to come up with the proper tools for bread-boarding, like a tube DC power supply, and how to go about bread-boarding itself.

When I first met Dan, he had bread-board circuit of his own using a RCA 6EA7, a cheapie bridged Allied 1-1 10K inter-stage transformer, with a Marconi 45 output. He was using some sort of vintage Japanese output transformer. Can't recall what brand.

 

It was only one side, and the speaker he was using at the time was a 10" Whitely Stentorian full-range in a 3.5 foot volume cabinet ported at around 40Hz. I was hooked on single-ended tube amplifiers from then on...

He had scrounged most all the parts for a completed amplifier of this circuit using a TJ 2.5 volt 300B for output tubes. He was going to build it himself, but I acquired the project for $400...I sunk another $200 into the project to get it complete. (I've spent more on the amp, but it took $600 to initially get it going)

 

https://community.klipsch.com/index.php?/gallery/image/40-dht-2a3-amp/

 

So are you using rectified and filtered DC at 120 volts for the 5 tube filament string?

 

Mike

[tube fanatic]

Thanks for the comments Mustang and Mike!  As far as testing, there's not much to do.  With amps like this, I am mostly interested in the power down in the low frequency range since it tends to be more of a worst case representation, and simply look at the square wave appearance.  Most of the guys seem to like a somewhat rolled off top end, so I don't worry much about the power output being somewhat lower there.  As far as being world class, there's no chance of that!!!  Perhaps with a pair of Magnequest opts, and some further circuit tweaking it would be, but that would defeat the whole point of being inexpensive.  I tend to overdo the ventilation a bit, as you noticed, but with heat being the enemy of components I'd rather have more than less.  This unit, even after many hours of continuous use is barely warm.  And Mike, you are certainly correct about this being an arrangement which has been around forever.  In the 50s, companies like Lafayette and Allied marketed cheap "phono" amplifiers using 2 to 5 tubes with series connected filaments.  Unfortunately, they were plagued by 60 Hz hum issues, and had an electrically "hot" chassis depending on which way the plug was inserted into the wall (remember, this was in the pre-polarized or 3 wire plug days).  It's getting more difficult to find novel variants to use in these designs, but I keep trying!  The power supply provides about 161VDC under load.  The filament string requires 164V, and I drop an additional 5V which is how I keep all of the filaments running on the low side of what they require.  Ripple is on the order of 0.25% into the amp, and the filter resistors and additional electrolytics in there take it down to almost pure DC at the voltage amp.  Your 2A3 unit looks nice!  Did you or Dan paint the chassis yourself, or is it factory painted?  I wouldn't mind playing around with a painted chassis, but have read that there are adhesion issues unless the aluminum is first treated with zinc chromate to prevent chipping.  That said, the polished metal look was pretty popular in the 50s, so I can always claim that I'm just being retro!  

Maynard

[CECAA850]

I wouldn't mind playing around with a painted chassis, but have read that there are adhesion issues unless the aluminum is first treated with zinc chromate to prevent chipping.

 

Powder coating is really cheap to have done and creates a very nice, durable finish.

[JL Sargent]

You can powder coat something as small as a chassis yourself at home.

 

http://www.harborfreight.com/10-30-psi-powder-coating-system-94244.html

[thebes]

Having first-hand experience with your approach to flea amps, I can just imagine how good that must sound.

 

Your layout and soldering skills still floor me.

 

So what's the tube line-up? No need to answer publicly if you don't want a run on those tubes which appear to be nos Sylvania's.

[tube fanatic]

Thanks for the powder coating idea.  I'll have to check that out.  I have a friend who used to paint race cars, among others, so perhaps I can get him motivated to do it for me (not very easy with someone whose standard line is "I'm retired and only do what I want").  Marty, you know that my stuff isn't proprietary!    I'm hoping to have the schematic and parts list up by next week.  But, to answer you now, the voltage amp tubes are 18FY6s, output tubes 34GD5s (the 32ET5 will work just as well), and the filament ballast (used to sop up voltage) is a 60FX5.  As far as the wiring and layout goes, having a bit of OCD can be useful for some things!!!

 

Maynard

[mike stehr]

Your 2A3 unit looks nice! Did you or Dan paint the chassis yourself, or is it factory painted?

 

It's a metal bud box that I painted with Krylon semi-gloss black. It could have stood the zinc chromate treatment as it has chipped in quite a few places over the years.

That ol' 2A3 amp took a header to the floor in a burglary a bout 2 years back. Knocked out a couple of tubes, buckled the chassis a bit, busted a cap loose, and some more paint chipping as well.

 

Tested the tubes after the incident, they all test fine. Unbuckled the chassis, fixed the loose capacitor, fired it up, checked voltages, good to go.

I've been wanting to tear it down, re-paint the chassis, and make a wood base for the amp...eventually...

 

Interesting tube lineup in your amplifier project. Old radio and television tubes?

[erik2A3]

Maynard - what caught my attention was the similarity to the famous Grommes "Little Jewel" . The AA5, also a definite relic from the past! Very neat!

I confess to a double take when I read 164V filament string - and then remembered.

Cool little amp! Erik

Edited November 12, 2014 by erik2A3

[tube fanatic]

Mike, to answer your question, yes these are old radio tubes which came out around 1961 and were designed for "All American Five" service.  Rather than having filaments drawing the usual 150 ma, these draw only 100 ma.  Electrically, the 18FY6 is equivalent to the 6/12AV6, and the 34GD5 is equivalent to the 35B5/C5.  Being in the radio/TV service business, I developed a great appreciation for just how wonderful these tubes are sonically.  In fact, in their power range, the audio output tubes designed for the AA5 radio service are the best sounding tubes I've ever found when used single ended.  

Erik, you can't possibly be old enough to remember the "Little Jewel!"    That was a really terrific amp and Grommes sure sold lots of them.  Of course, it was a conventional design with a power xfmr and lots more output power.  It was one of many excellent budget amps of that era.  In using the AA5 design, a goal was to have only a single, low ripple, DC feed to power both the filaments and plate circuits.  That's where using a tube to ballast down the filament voltage comes in (in radios, a true ballast with nichrome filament was used or, in the earlier sets, a resistance line cord was employed).  Since the cathode of the ballast tube floats, heater/cathode voltage is of no concern, and it also allows the heater/cathode voltage of the first tube in the audio chain to stay below 100 which is the design-center maximum.  

Anyway, I got the schematic done and don't think there are any mistakes.  If anyone sees anything which doesn't make sense, please let me know immediately so I can make changes.  Hopefully, some will get motivated to give this amp a try (Marty???).  The circuit can be easily adapted for other AA5 tubes, so if anyone has some on hand instead of those specified, drop me a note and I can tell you what values to change.

Maynard  

little-gem-sep-3.pdf

[tube fanatic]

Erik, since you brought up Grommes, here's your chance, if you can find one of these!!!

 

http://www.grommesprecision.com/grommes/products/260a/260a.htm

 

 

Maynard

Edited November 13, 2014 by tube fanatic

[thebes]

Well Maynard now you've go me intrigued.  The transformer is cheap at a smidge over $20, but what about the output trannies?  Are you using Hammond or Edcor or what.  The mid-grade SE Edcors would run about $60 a pair, although could I use my 5K into 8ohm Edcor SE's be used?

 

Then, since the power supply is separate (aluminum meatloaf pan maybe) could I use a metal plate with an acrylic casement around it for the amplifier chassis without inducing outside noise?

 

Can your "tone" circuit be be built in?

 

Maybe this could be done for $200.  Maybe your just not chintzy enough spending all that wild money on Hammond cases.

 

I'm going to have to root around in my tube collection to see what I've got.

[Mallette]

Maynard - what caught my attention was the similarity to the famous Grommes "Little Jewel"

 

My first amplifier.  Powered my Frazier Monte Carlos nicely.  It was well used when I got it and eventually got too flakey to use.  Replaced it with a Dynaco SCA35.  I had no real knowledge at the time about anything except what sounded good.  How I managed such a string of excellent source and amplifier equipment still amazes me as it was shear luck.

 

Dave

[tube fanatic]

Well Maynard now you've go me intrigued.  The transformer is cheap at a smidge over $20, but what about the output trannies?  Are you using Hammond or Edcor or what.  The mid-grade SE Edcors would run about $60 a pair, although could I use my 5K into 8ohm Edcor SE's be used?

 

Then, since the power supply is separate (aluminum meatloaf pan maybe) could I use a metal plate with an acrylic casement around it for the amplifier chassis without inducing outside noise?

 

Can your "tone" circuit be be built in?

 

Maybe this could be done for $200.  Maybe your just not chintzy enough spending all that wild money on Hammond cases.

 

I'm going to have to root around in my tube collection to see what I've got.

Yes, the opts are Hammond 125CSEs which ran $93 for the pair from Mouser.  Unfortunately, your 5k Edcors are a bit too high in impedance to use here (but if you want to build a SET version of this, with around 300 mw/channel, I could run some curves for you which would let you use them).  The total for both chassis/bottom covers was about $38, so you could save some $$$ by going with the loaf pan and metal plate.  The latter should be fine as long as you star ground and keep the signal leads very short.  And as far as the high frequency filter, you could either employ it as an external tone control, have it as a "set it and forget it" internal adjustment as I did here, or set it up as a fixed value filter as I did in your cake pan amp.  Let me know what tubes you have around (I know you gave me a list last year, but I don't save emails) and let's see what we come up with!

 

Maynard

Edited November 14, 2014 by tube fanatic

[erik2A3]

Maynard:

In my 50s, so yeah, that Grommes amp was a bit before my time. It's among many I've studied, though, and was familiar before I ventured to this forum in 2002.

It's always cool IMO when the chassis is small enough to where internal wiring consists primarily of the leads of the passive parts, themselves -- as you've done here. Point-to-point is of course classic.

Great! Erik

[Brac]

Great work, really makes me wish I had gotten into tube audio back in 1985 when I finished High school with lots of collage credits in AC and DC circuit design and the like. 

[erik2A3]

Brac: never too late to get going on it!

Edited November 15, 2014 by erik2A3