Here it is! The "Little Sweet Potato" SEP spud amp!

[tube fanatic]

I can hear the question now: "what the heck is a spud amp?"  Well, based on some research, it is a fairly recent term which was coined to describe amps which use only a single tube per channel (some consider dual section tubes as being in the category even though they are really 2 tubes!), thereby making it a "one tuber."  Since a potato is a tuber and is often referred to as a spud...... you get the idea!

Spud amps are nothing new, however.  They were originally developed in the late fifties in response to a need for cheap stereo phonographs which needed the simplest possible amplifiers, using the fewest number of tubes and parts, and which could be directly driven by the high output cartridges which were in use at the time.  The tube manufacturers developed a number of types which could provide substantial (a relative term, of course) power when driven by such cartridges.  There was little concern for the 60 Hz hum which these units had since the speakers were usually small enough to avoid much output at such a low frequency.  In more recent years, spud amps have re-surfaced as beginners' projects in the DIY community.  If constructed with reasonable care, they usually work and can provide a nice introductory experience in tube amp building.  The circuits which I've seen still put the tube filaments in series across the AC powerline which makes hum-free operation nearly impossible.  Some of the circuits offered have kept the "hot chassis" design (the chassis being connected to one side of the AC power cord) which is too dangerous to even consider building.  The "Little Sweet Potato" takes spud amps to a higher level and offers amazingly excellent sound.

The amp runs everything, including the filaments, on extremely low ripple DC furnished by a separate power supply.  By removing the AC power sources from the immediate proximity of the amp, concerns about picking up hum are eliminated.  The power supply should be kept at least 1 foot away from the amp and can be hard wired to it if preferred.  An alternative is to use a suitable connector which allows the two units to be separated (I use Anderson Power Poles- they are excellent but a bit tricky to wire.  There are numerous online tutorials and Youtube videos showing how it's done).  If the amp is built carefully, it should be dead silent.  Since the B+ supply is split between the two channels after the input filter capacitor, it is effectively a dual-mono circuit.  There is no reason to consider building this amp as two separate mono units each having its own power supply.  

The circuit is the most simple I could come up with and uses a series filament string across the DC input.  Two output tubes, and a tube whose only function is to get rid of excess filament string voltage, comprise the whole affair.  A handful of parts, and the output transformers, are all that is needed.  Built as I have done it, parts cost is less than $200.  If less costly Edcor output transformers are used, and cake/loaf pans are used for chassis, about $50 or more can be saved.  

Power output is about 700 milliwatts/channel @ 1 kHz.  Due to the Hammond output transformers I used, power output is somewhat lower down around 30 Hz (but is still more than enough to provide excellent, solid, bass).  Since the amp requires about 2.1V rms of drive to achieve full output, it cannot be over-driven if a CD player (with typical output of 2V rms maximum) is used as the source.  This means that you can turn the volume control up all the way, if you wish, and get blown out of the room with no concerns about the amp clipping!  Some negative feedback, which is mostly active for the midrange and high frequencies, is used between the plate and control grid to reduce the inevitable distortion typical of single ended pentodes in response to high impedances reflected back from the speaker.  

As to sound, that was a big surprise given the very small Hammond output transformers (125BSE) used and the typical, very high, output impedance of single pentodes (resulting from the inability to use the usual negative feedback arrangement, which I normally employ, in this particular design).  It's simply wonderful!  In fact, if I didn't know what I was listening to, I'd have thought it was a SET.  The midrange has a "smooth as glass" characteristic, and the highs are clearly presented with no trace of harshness at all.  Bass, as mentioned above, is certainly not lacking.  If the amp is constructed with the variable, low pass, "ear bleed filter" shown in the schematic at the input, the high frequency response can be adjusted to personal taste very easily.  What's nice is that this amp will do justice to many of the Heritage or Reference series speakers in small to medium size rooms, within the limits of its power output of course.  I ran some A/B comparisons with the Little Sweeties (not very fair considering they are SETs) at a comparable listening level.  Yes the Sweeties, as expected, were "richer" sounding.  But, if I had a budget, I'm not sure I could justify the additional cost.  One last comment on just how quiet this amp is.  While I had my ear "glued" to the tweeter horn and rear port, I had my wife disconnect and re-connect one of the speaker leads at the amp (this was a blind test- I didn't know what she was doing at any time) and could hear absolutely no difference!!!  Listening to music emanate from total silence is a very enjoyable experience!  So, who's going to build one of these besides "?" (I'm not going to reveal which forum member is already gearing up to build one- I'll let it be his surprise!).  The attached pictures can be used as a guide for the layout (totally non-critical for the power supply).  I recommend building the amp and p/s with larger enclosures than the ones I used, especially if you have large fingers (mine are long and thin, and I still got lots of cuts from working in such close areas).

Now, the usual disclaimer:  THIS AMP RUNS ON VOLTAGES WHICH CAN KILL YOU!  DO NOT ATTEMPT CONSTRUCTION IF YOU ARE NOT EXPERIENCED IN WORKING WITH SUCH VOLTAGES IN A SAFE MANNER.  SEEK THE ASSISTANCE OF SOMEONE WHO IS KNOWLEDGEABLE!

Maynard    

 

One thing I forgot to mention- if you choose a different output transformer which has a lower primary DC resistance than the 125BSE (approx. 340 ohms), R12 and R13 will have to be changed to a value which produces approximately 118-121V between G2 and the cathode of the 50FK5s.

Little-Sweet-Potato-Spud-Amp-5.pdf

Edited December 10, 2015 by tube fanatic

[juniper]

Great topic Maynard, great info, and a wonderful story!!!!

[Jim]

Maynard,

 

I have to say this is almost like having a woman in her window get topless knowing you are watching. You can see but you can't touch. I am this close to you,but can't get my hands on one of these things you keep building,lol.

[tube fanatic]

William, you are correct about the 60FX5.  Its sole purpose is to drop 60V from the approximate 157-158V the p/s delivers under load (only in the filament string.  R12 and R13 are used to drop voltage for the B+ rails).  That results in the 50FK5s getting the correct filament voltage, and also keeps the latter's heater/cathode voltage under the maximum allowable 100.  A 600 ohm/15 watt resistor could be used instead, but that would add a great deal of heat under the chassis.

 

Jim, I just sent you an email............

 

Maynard

[CECAA850]

Yamtastic Maynard!

[Jim]

Jim, I just sent you an email............

 

Replied

[xxJPMxx]

Another beautiful layout Maynard!! 

 

I had a fun idea I would like to run by you. Instead of running the 60FX5 how about a cool looking 25 watt bulb? It should fit the 600 ohm requirement and will only burn at 6 watts so it won't be too bright, it should glow a dull orange like in the picture.

 

 

https://www.1000bulbs.com/product/96743/PLT-000601.html?utm_source=SmartFeedGoogleBase&utm_medium=Shopping&utm_term=PLT-000601&utm_content=Candelabra+Light+Bulbs&utm_campaign=SmartFeedGoogleBaseShopping&gclid=Cj0KEQiA4qSzBRCq1-iLhZ6Vsc0BEiQA1qt-zso1i3SpdnkCk8ccOInUsXJHJPrTOSHGMPzOW6N-WicaAq8a8P8HAQ

Edited December 10, 2015 by xxJPMxx

[tube fanatic]

JP, that's a cool looking bulb!  The idea would probably work but I question how it would tolerate the very high initial current flow through the cold filament string.  One way to deal with it would be to use a zener(s) across its filament to drop 60V and just let it run on the half wave pulses.  That's a trick we always use in the restoration of old radios having SS rectifiers to protect the #47 pilot lamps from flaring brightly until the tube filament resistance rises as they heat up.  The only concern I'd have is the possibility of adding noise to the B+ rail which may be audible.

 

Maynard 

[mike stehr]

I've got a old globe ballast tube buried away here somewhere...

 

Nice spud amp!

Edited December 10, 2015 by mike stehr

[Chrome Magnum]

You are a national treasure Maynard!  And so are your designs!  Thanks for making them as well as yourself available for us to build them... =]

 

Transistors and chips?!  I don't need no steenking transistors and chips!

[ljetson]

I have a power supply/chassis grounding question. I currently have a hot chassis version of this from the early 60s that I am experimenting with. If I were to modify it to the spud version, would I ground either of the steel chassis (power supply and/or amp) to the safety ground or do you not need to do that in this configuration? Can I use aluminum and not worry about grounding? Thanks, this looks fun!

[No.4]

Aluminum is conductive. I always make sure every exposed conductive part of the chassis is connected to safety earth. You can test the connections with a DMM to be sure. 

[tube fanatic]

15 hours ago, ljetson said:

I have a power supply/chassis grounding question. I currently have a hot chassis version of this from the early 60s that I am experimenting with. If I were to modify it to the spud version, would I ground either of the steel chassis (power supply and/or amp) to the safety ground or do you not need to do that in this configuration? Can I use aluminum and not worry about grounding? Thanks, this looks fun!

The simplest thing to do is to buy an isolation xfmr and just plug your 60s amp into it.  That will prevent the amp chassis from being "hot" regardless of which way it is plugged in (I'm assuming that it has a non-polarized power cord). 

 

You can build the above on aluminum chassis.  Use the pics as a guide.  The power cord safety ground connects to the p/s chassis.  When the amp is connected its chassis will be grounded to that of the p/s.

 

 

Maynard