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F5Turbo Illustrated Build Guide
F5Turbo Illustrated Build Guide
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Old 14th April 2014, 02:33 PM   #11
AndrewT is offline AndrewT  Scotland
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less on 8ohms than on 6ohms.
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Old 16th April 2014, 12:12 AM   #12
Smittys is offline Smittys
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Sweet build 6L6!!!!

I have F5 turbo v2 boards are they useable? Should I buy v3?
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Old 16th April 2014, 02:46 AM   #13
AudioSan is offline AudioSan  Norway
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regarding R25-28. this is depending on rail voltage and desired J-fet voltage.
if they all are 4.7K or all are 10K. the result will be the same. about 1/2 of the rail voltage.
calculation will be:
rail voltage/(R26+R28)*R28=cascode voltage.
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Old 18th April 2014, 02:46 AM   #14
6L6 is online now 6L6  United States
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F5Turbo Illustrated Build Guide
Smittys -

The v2 boards are completely fine. The silkscreen numbering is a bit confusing in places, and the Front-End PCB doesn't fit on anything of the UMS... but they are fine boards and will make a great amp.
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Old 19th April 2014, 06:01 PM   #15
toxic ingestion is offline toxic ingestion  United States
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Quote:
Originally Posted by AudioSan View Post
regarding R25-28. this is depending on rail voltage and desired J-fet voltage.
if they all are 4.7K or all are 10K. the result will be the same. about 1/2 of the rail voltage.
calculation will be:
rail voltage/(R26+R28)*R28=cascode voltage.
AudioSan,

What is the ideal or allowable range of voltages to the base of the cascoding transistors. That is, using a volt meter, what voltage would we like to see at the base of these devices after making in through the voltage divider and R29 or R30? I have read 9 volts, 15 volts, 17 volts. I'm sure there is a range, but I wasn't sure what is generally recommended. Thanks in advance.
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Old 20th April 2014, 04:01 PM   #16
BobEllis is offline BobEllis  United States
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I'd say at least 10 up to 30v where NP runs the V2. Going to 10V gives the jfets room to breathe and supposedly they sound better with higher Vds. Just watch the dissipation.
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Old 20th April 2014, 04:02 PM   #17
Zen Mod is offline Zen Mod  Serbia
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F5Turbo Illustrated Build Guide
12 to 15V , considering both dissipation and sweet spot
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Old 25th April 2014, 10:41 PM   #18
Smittys is offline Smittys
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6l6
Cool!! I'm in!!
So if I understand this correctly...for 100w class A I need 36 volts but I saw a antek trfo with 4x 35 volts is this enough? I will also cascade the jfets. Caps at 63v? I'm not sure about the bias resistors? Did I miss anything else?
Any suggestions would be appreciated!
Thanks
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Old 26th April 2014, 08:15 AM   #19
AndrewT is offline AndrewT  Scotland
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For 100W of ClassA, you need 5Apk into 8r0, or 7Apk into 4r0

ClassA is about current delivery, NOT voltage delivery.
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Old 26th April 2014, 11:17 AM   #20
BobEllis is offline BobEllis  United States
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Class A is mainly about current capability but you also need enough voltage to push that current into the load. Follow AndrewT's numbers for current and work backwards to the voltage peaks that you'll need. Add a few volts for the amp's inability to swing rail to rail. With typical CRC power supplies we build you can expect rail voltage to be somewhere around 1.2x the transformer's AC output.

You also need to consider the transformer's power rating. For class A operation the VA rating should be at least 2x the amp's power draw (rail to rail voltage x bias current). Better yet is 3x or more to minimize heating and chance of mechanical hum.

Assuming you will end up with roughly 40v rails 63v caps are fine. The usual rule of thumb around here is add 20% to the rail voltage and go to the next higher standard voltage rating.

Enjoy your build.
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