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Old 8th September 2013, 12:35 PM   #2281
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If building an F5T, the bias limit is determined by the diodes Vf. If you want to increase or lower bias, you must also change the source resistor values. If not using diodes, it is a non issue.
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Old 8th September 2013, 12:43 PM   #2282
emyeuoi is offline emyeuoi  Italy
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Buzz, I am using the diodes and the source resistor that was in your premium GB.
Based on that which is in your experience the maximun mV that I can reach during the bias?
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Old 8th September 2013, 01:27 PM   #2283
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I would max it out at between 380 and 400mV across the across the source resistors until you get the temperature performance stable. You can bump it a little after that, just be careful. This amp can get away from you faster than most if you have it biased too high and the diodes cut on. Highest I have had mine is 420mV.
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Old 8th September 2013, 01:31 PM   #2284
emyeuoi is offline emyeuoi  Italy
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Noted... I will not go more then 470 mV as mentioned on the FirstWatt article.
Thank for the advise
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Old 8th September 2013, 02:00 PM   #2285
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Quote:
Originally Posted by WalterW View Post
but when do you get your trafos? Then you should be nervous....
Just make sure the bias trimmers are in the right position before you start...I measured every potentiometer twice with the ohmmeter just to be sure..
But you are right it's always a very exciting moment switching the 230V... those big donuts can destroy everything...
I have mine set to the middle, hope this wont be a problem. I will be firing up with a variac, like I did with "regular" F-5. Is this sufficient sans light bulb tester? Or is this amp so twitchy the pots must be zeroed even with variac?

Thanks,
Russellc
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Old 8th September 2013, 02:08 PM   #2286
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IF you have a variac, you will be fine. Low and SLOW.
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Old 8th October 2013, 11:15 PM   #2287
SRMcGee is offline SRMcGee  United States
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Folks:

Can someone explain the purpose of Q1.1 and Q2.1 to me? I understood that cascoding helped to spare Q1 and Q2 from having to bear voltages beyond their meager capacity, which is why Q7 and Q8 are in the circuit. What do Q1.1 and Q2.1 add, and are they required in all implementations of the F5T?

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Scott
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Old 8th October 2013, 11:48 PM   #2288
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Q1.1 and Q2.1 is there so you can paralell 2 J-fet pairs to get higher Idss.
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Old 9th October 2013, 12:36 AM   #2289
6L6 is offline 6L6  United States
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In a build with lots of output fets there is a possibility to run out of oomph in the driver stage. The .1 Jfets allow paralleling for the reason AudioSan mentions, and also if you need more current to run lots of output mosfets.

Remember this design can scale up towards the ridiculous - especially in terms of how much heatsink you need...

Great meeting you at Burning Amp, It's always nice to attach a real face to a forum member.
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Old 9th October 2013, 12:02 PM   #2290
SRMcGee is offline SRMcGee  United States
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AudioSan and 6L6:

Thanks for the explanation!

6L6:

Not to go off-topic (very much), but I also enjoyed our conversation at Burning Amp and appreciated the time you took amidst the bustle to hang out. I'm already looking forward to next year....

Regards,
Scott
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