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Old 16th January 2007, 10:05 AM   #1
vlucian is offline vlucian  Romania
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Default starting PP - 6p3s

hi there guys! I'm trying to start my tube amp and I need some advices on steps I must follow in order to do the propert things. I'm not sure there is a step by step guide for beginners like me. What should I measure, what should I follow .. things like that. I'm using this schema:
http://www.arrakis.es/~igapop/6l6.htm

If I need to post more details I would do it happly.
thanks!
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Old 16th January 2007, 10:42 AM   #2
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Hi,

One thing I'd like to say, if you didn't know, is a 6P3S is more like a 6L6G (not GB or GC) in that it doesn't like highish voltages. It'll take abuse with current, but not voltage.

Cheers!
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Old 16th January 2007, 11:34 AM   #3
SY is offline SY  United States
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This schematic is another version of the Williamson amplifier (including the infamous design mistake- the low frequency stability will be marginal). The design is analyzed in detail in the book "Valve Amplifiers," and there are probably 10,000 discussions of every aspect of it here and at other tube forums. That's the key word you need- Williamson.

It would be worthwhile to try to find a copy of the David Hafler article in Audio where he builds the Williamson with an Ultralinear output stage like the one in this schematic. He even discusses fixing the LF problem. A little search engine work will pay off.
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Old 19th January 2007, 01:00 AM   #4
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I was reading up on that as best I could understand, and from what I gather from these discussions is that the Williamson design lacks in the pre-amp stage just before going to the final PP stage (regardless of whether it's ultralinear), is this correct? For example the 47k resistors, etc.
My question is, at what point in time were better designs/tweaks commonplace, and did this flaw mostly plague hi-fi amps while guitar amps did not care as much with the response?

On the sites I've found that have a great deal of schematics, such as these (and please suggest more cool sites like these you've run across):

http://www.triodeel.com/schindex.htm
http://tdsl.duncanamps.com/schematics.php
http://www.machmat.com/schema/index.htm

Are a lot of the older (late 50's / early 60's) amps prone to some performance problems if duplicated today without current knowledge?

And does the Willliamson flaw affect mostly the 6L6 family of designs, or is it across the board with just about any amp of similar design in that time frame?
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Old 19th January 2007, 01:17 AM   #5
SY is offline SY  United States
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The problem is one of LF stability. Reducing the coupling caps from the driver to the output stage by a factor of 5 or more will help that a LOT.
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Old 19th January 2007, 01:40 AM   #6
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As SY said, I would eliminate one RC network for better stability on low end.
Also, a constant bias of output tubes would be more efficient.
And I would decrease values of resistors in the global negative feedback at least ten times.

Check this one, you may like it more (I am listening currently very similar one that uses 6P3S tubes):
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File Type: jpg al1569a.jpg (81.3 KB, 600 views)
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Old 19th January 2007, 02:19 AM   #7
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unfortunately, that schematic is tiny. if you have to resize it to be able to host it on diyaudio's server, try imageshack.us, they do everything i need when i want to post pics on a forum.
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Old 19th January 2007, 02:30 AM   #8
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Thanks, I've checked out all the Altec Lansing designs as well, and have them saved on my hard drive.
I didn't originally post before on this thread, but
I was just curious for informational purposes as to problems with the original designs and how to "see" one that needed some adjustment.
I see that A-L did some original designs with offbeat tubes like the 811-A, etc.
I have a few projects to work on for now, but I think my very first or second post here was about a design using the 807, as they have a lot of power and in the 1625 version (different filament voltage) they are said to be in big supply (NOS military surplus), and cheap. So down the road, as I learn more I'd like to try a design based on these, since they are cool looking too with the plate cap.

I've seen a few DIY designs, but the only commercial one I see so far with a free schematic online is the Acrosound (TO-290 and TO-300 OT),
which is also a Williamson design, showing usage both with and without ultralinear. Any comments on this one would be appreciated too, as that's something I'd like to try maybe around Easter or so:

Schematic here, not sure if it's hi-fi or not.
Also the diagram shows KT-66 6L6 or 807
Does that mean that I could perhaps just take any circuit using 6L6GC's and substitute 807's or 1625's (not changing the component values) provided I rewire the socket and provide plate caps, since the 807's are rated at a much higher wattage and could breeze through it?

http://www.nijnkonijn.nl/machmat/sch...pp/acro206.gif
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Old 19th January 2007, 02:46 AM   #9
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807 actually is a repackaged 6l6 to use in transmitters on bombers.
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Old 19th January 2007, 02:53 AM   #10
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The 807's I believe need a 5 pin socket and are not cheap, and the 1625's need a special 7 pin socket as well, a bit unusual but the sockets are still available for a just a few bucks too.
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