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Old 7th July 2008, 03:50 AM   #1
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Default Spud anyone?

I had an email conversation with someone who was interested in a spud amp. What is a spud amp? The usual definition is an amp that uses one tube per channel. It comes from the phrase "one tuber" which was shortened to "tuber" which is another word for a potato, and so is "spud". Somehow the name stuck.

The emailer asked if I had tested any of the spud amps on the market. I explained that I rarely see other peoples amps any more since I just don't have time to do repairs or mods. Then he asked "Why doesn't Tubelab do a spud?" Well this question was asked two days before I had a three day weekend, so I decided to think about it.

There are a few spud amps on the market currently. Most either use a dual dissimilar triode like the 6DN7 or 6EM7 to deliver 1 to 1.5 WPC, or a single high gain pentode like the 6CL6 or 12GN7 to deliver 0.5 to 2 watts. A triode - pentode like the 6BM8 could be used as well.

I decided that if I was going to do a spud it would be different. First it must be simple, really simple. Second, it must be useful to me. I could use a small amp for my computer, but I need more than 1 or 2 watts. Third, it must be small. Fourth, it must be cheap. And last, it MUST sound good. For it to be a true spud, there can be only two tubes.

Now that I have defined the amp, can it be built? Time for some experiments. It was the fourth of July, so a little fireworks may be in order. I had been testing several different output tubes in a tired old Simple SE board to see if which tubes to keep, and which tubes to wholesale. I decided to use the same PC board to test some multi section tubes. How? Well we need to answer the question "Is it possible to build an amp entirely with clip leads?" I would have said NO before I did this!

Do you believe that this actually works, and sounds good too!
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Old 7th July 2008, 03:54 AM   #2
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Here is a photo of the whole setup. The scope shows input (bottom) and output (top) of one channel. The audio analyzer shows 1.6% distortion at 2 watts. Clipping begins at 3.5 watts in triode and 5.5 in UL with the "big" tubes (triode - pentode). I can plud a set of dual dissimilar triodes into the same sockets (with some component changes) for far less power.

I got through the whole day on July 4th and didn't explode anything. I did see some tubes glowing, and some that flat didn't work. But the "furball" that you see here just works! No tendency to oscillate at all, and it sounds good too. Now it I can only make it look good.
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Old 7th July 2008, 04:50 AM   #3
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Remember the design criteria, small, simple, useful, sounds good, low cost. I got the sound down right, which is usually the first thing to work on after you solve the "it doesn't work" stuff. I needed to design a PC board that was small. It had to be easy to build, Simpler than a Simple SE, and work as well as the "furball".

I spent most of Saturday in front of the computer working on the layout. By evening I had a prototype quality board ready to be made. The process to make a PC board involves some nasty chemicals that must be used in the dark, followed by cooking up a plate of acid on the stove. I waited till Sherri was asleap for that one. Finished this part late Saturday night.

Sunday afternoon I had to drill all the holes in the board, and then it was ready for "stuffing", the process of soldering in all of the parts.

The photo shows the completed board. It occupies less area than a music CD. The PC board is 4 inches by 6 inches.
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Old 7th July 2008, 05:50 AM   #4
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Excellent! Headphone amp possibly? I have to say hats off for managing all those clip leads without blowing something up!

-Justin
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Old 7th July 2008, 07:28 AM   #5
nitrate is offline nitrate  United Kingdom
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Congrats!!

I like your style, you build just like me

Leigh
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Old 7th July 2008, 01:25 PM   #6
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From a Japanese website, using two 6LU8. Here's how you should package that amp:

Click the image to open in full size.

The full link:

http://homepage3.nifty.com/softone/o...OFF070108t.htm
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Old 7th July 2008, 01:33 PM   #7
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This Spud amp reminds me of the Mighty Midget, by Pete Millet. That one also uses a compactron.

http://www.pmillett.com/midget.htm
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Old 7th July 2008, 02:13 PM   #8
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The board was populated with the same component values that were used in the "furball". I wired it up using the small 5K ohm Edocr OPT's in UL mode since they were more in line with the project goals. I also used an Allied 6K56VG transformer since I had one on my bench. It produces slightly more voltage than the power supply used with the "furball".

The amp worked on the first try with no tweaking. Tube dissipation was slightly over spec, but this was expected since I had planned for 300 volts of B+ and now had 320. No glow was visible in a dark room even after 4 hours of operation.

The amp measured quite good. Distortion was 3.05% at 5 watts, 0.88% at 1 watt. Clipping begins at 5.5 watts and 5% distortion is reached at 7.1 watts. Frequency response is 19Hz to 61 KHz at 1 watt. Power will be reduced somewhat when the tube is re-biased to a cooler operating point.

I listened to it for several hours last night. It sounds quite good. OPT saturation can be heard when playing techno or other bass heavy music at high volume. This can be fixed with larger OPT's, but that is not in line with the small and cheap goals. I plan to experiment a little further with this amp next weekend.

This was a three day diversion from my other projects. It was fun, and I didn't even blow anything up. I will stuff the entire amp into some sort of unique cabinet at a later date. Jose, the Kirin can is absolutely cool, but how to I put a flat PC board in a round can? The first step is to find a smaller power transformer.

This could become a new Tubelab PC board if there is enough interest. The board is smaller, so it would be slightly cheaper, depending on quantity (PC board houses charge a fixed artwork - tooling charge). My guess is that the whole amp could be built for under $200, not counting the enclosure. It is really a Simpler SE.

Is there any interest? Spud anyone? I will post the schematics once I actually have one. I kind of made this up as I went along.
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Old 7th July 2008, 02:15 PM   #9
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I believe the spud amp is one stage and not necesarily cheap.
The Gordon Rankin Simple 84 is an spud amp

http://db.audioasylum.com/cgi/m.mpl?...in&r=&session=

best regards Jaime
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Old 7th July 2008, 02:23 PM   #10
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Well, to paraphrase Madison Avenue:

"This Spud's for you"

SY is old enough to get it.
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