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Old 13th July 2008, 03:06 AM   #1
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Default Simple P-P 84 proto

I have been working on laying out a PC board for the big version of the Simple P-P. I have worked on it on and off for the past six months. I have come to the realization that two channels and a power supply will not fit on any reasonable sized PC board. Four octal output tubes with minimum spacing between them takes at least a 10 inch wide board, and that is ruling out fat bottled tubes like the KT88. I have started about 5 different designs, got frustrated and gave up. I think that the octal version of any type of Simple P-P will have to be mono blocks.

About two weeks ago I decided to put it on hold for a while and do something fun. I had a discussion with a friend about a "spud" amp a few days before a three day weekend. That resulted in a new design and a new PC board which was seen in another thread last weekend. I even got to crank up some tubes and make em glow. I had too much fun, so...

I took two more days off of work. There was the matter of the smaller version of the Simple P-P. I have two designs on my Tubelab 3 prototyping system. They both work, and are very similar except for the tube lineup. I took a guess and assumed that the EL84 version would be the most popular, and decided to lay out a PC board for it. Besides I have another reason for doing an EL84 board that I can't divulge yet.

By Friday I had a good PC board design. Friday night, I made a PC board. Today I drilled it, and populated it. This evening I hooked it up to my power supply, and plugged in the first 12AT7. It lit up and the voltages were about right. Then I attempted to plug in the second 12AT7. Do you see the problem here?
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Old 13th July 2008, 03:12 AM   #2
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Is that a little issue of Chinese hole tolerance? I actually had a 9 pin PC mount socket that came with solder in one of the pin cups... I've been enjoying PCB design lately too, but my EL84 PCB is still in development. I'm still getting a feel for how much space I can leave between traces without screwing myself.

-Paul
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Old 13th July 2008, 03:18 AM   #3
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Well I decided that the tube WAS going into that socket since it wasn't much fun to change the socket. Application of excessive force only resulted in a broken tube. After a few other dumb ideas didn't work, I resorted to a PC board drill bit in a Dremel tool set on max speed. Zipped right through the substance that the Chinese call ceramic. Second 12AT7 installed and working.

There was no B+ getting to the right channel OPT. It was an open trace in my low quality PC board.

Other than these two issues, the amp works very well. 15 WPC at 0.45% distortion. 16 watts however will get you 4%.
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Old 13th July 2008, 03:28 AM   #4
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Here the amp is still running on a bench type lab power supply. This lets me pick the voltage that the amp likes the best. I decide that 320 volts is where I want to be.

I adjusted the global negative feedback to 6 db. This seemed to still leave the music alive, while tightening up the bass. I attached speakers and a DVD player, and rocked the house!
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Old 13th July 2008, 03:32 AM   #5
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I take a guess as to what transformer will make 320 volts DC. In this case it is an Allied 6K56VG, which I hooked up. I put in a rectifier tube, and cranked it up. There is no choke in this amp, and there is no hum.

There will be some more testing, and more testing, and some serious tube rolling, before I can even dream of calling it a new product.
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Old 13th July 2008, 06:23 AM   #6
Vargas is offline Vargas  United States
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Nice job Tubelab! I always love reading about your work. For some reason when I hear P-P I keep thinking P-P "Sweep". Of course I wouldn't be simple anymore then would it. Cheers
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Old 13th July 2008, 06:52 AM   #7
Jeb-D. is offline Jeb-D.  United States
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It's looking good George! Is it a Williamson type topology?
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Old 13th July 2008, 08:30 AM   #8
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Hi George,

For your octal bottle spacing issues, have you thought of a modular design. rather than trying to fit it all one one board? Like a guitar amp...

Cheers!
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Old 13th July 2008, 01:04 PM   #9
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Quote:
have you thought of a modular design. rather than trying to fit it all one one board? Like a guitar amp...
That is where I am going. The plan is now a power supply board, and a single channel amplifier board. This takes away some of the "simple", but makes for a much better amp in the long run.

Quote:
It's looking good George! Is it a Williamson type topology?
It is similar. There are no driver tubes, since the extra gain is not needed with EL84's, and it needs to be "simple". There is a common cathode gain stage directly coupled to the split load phase splitter, which directly feeds the output tubes. I tried several different topologies, but this works as good as any, and is the "simplest". I prefer LTP splitters, but I couldn't make one work without an additional tube. This amp is currently running in UL mode. I will try pentode and triode mode ASAP. I also will "test" some 6CW5's.

Quote:
For some reason when I hear P-P I keep thinking P-P "Sweep". Of course I wouldn't be simple anymore then would it.
Not everything has to be "simple". I am working on a "universal driver board". I want a board that can drive up to 4 tubes with individual bias adjustments for each tube. It will be a fully differential design, and provide enough voltage swing and drive current for screen driven sweep tubes or 845's in A2. When you start thinking about big P-P amps you realize that the types of big tubes that are available varies a lot around the globe. Many sweep tubes that are common in the US don't exist elsewhere. Some big Russian tubes can't be found here, etc. The solution is to make a "universal" driver and power supply boards that can be used with off board mounted tubes. Back to the modular approach mentioned above.
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Old 13th July 2008, 01:07 PM   #10
SY is offline SY  United States
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Since it sounds like you're using the same topology and tubes, could your board be adapted to the Red Light District?
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