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F5 Bias Problem
F5 Bias Problem
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Old 5th March 2011, 05:01 PM   #21
ENSen is offline ENSen  Sweden
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F5 Bias Problem
The voltages over R1 and R2 are 52mV and 49mV -> Ids for the jfets are ~5mA.

5mA through 1.2kohm should give a Vgs~5.5V and not 750mV as I have. So the current must go another way. The only way I can see is the bipolar transistors. I will try to desolder them and see what happens.

Sorry maartentje for hijacking your thread but since the title is "F5 bias problem" I thought it was a good thread for my problem as well. If the correct way would be to start a new thread please let me know and I will to that.
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Old 5th March 2011, 05:49 PM   #22
Nelson Pass is offline Nelson Pass  United States
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F5 Bias Problem
Probably so.

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Old 5th March 2011, 09:45 PM   #23
ENSen is offline ENSen  Sweden
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F5 Bias Problem
Problem solved. I had mounted the bjts the wrong way. I checked it three times and still managed to install them the wrong.

Thank you very much Melon head and Mr. Pass for taking time helping me with this.

Hopefully I can hook them up to my speakers tomorrow and enjoy.
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Old 5th March 2011, 11:11 PM   #24
Melon Head is offline Melon Head  Australia
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It is good to see another problem solved.
Enjoy your amp
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Old 22nd March 2011, 11:46 PM   #25
6L6 is offline 6L6  United States
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F5 Bias Problem
I think I am having a similar problem -

With 0 offset, I can get only .1-.15v bias

With the bias at .6v, the offset is .3v

I also am running out of turns on the potentiometer...

Change the R parallel to the pots? (3.3k? more? less?) Or does this seem to be something else?
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Old 23rd March 2011, 12:17 AM   #26
Zen Mod is offline Zen Mod  Serbia
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F5 Bias Problem
change them to double in value

repeat biasing procedure from start
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Old 23rd March 2011, 12:24 AM   #27
Woody1911a1 is offline Woody1911a1  United States
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thanx zen mod
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Old 12th June 2011, 07:07 AM   #28
Chilli6565 is offline Chilli6565  Australia
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Smile R11 voltage run away

ok, my problem with biasing my F5 is that after adjusting p1 and p2 alternativley ( both pots set to zero ohms to start with ) the voltage over R11 climbs up to and is stable around 250mv.Above this voltage the voltage across R11 starts climbing by itself.

I just watch it climb to 0.6 volts then pull the plug...have had a couple of burn ups when i didnt get to cut off the juice quick enuff
I have replaced Q1 Q2 Q5 and Q6. I removed Q4 (IRFP240) and Q3(IRFP9240) and tested as best i could and I THINK they are ok.Still the problem persists.

Any help warmly recieved
" Digital is only ac anyway "
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Old 12th June 2011, 03:43 PM   #29
Russellc is offline Russellc  United States
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F5 Bias Problem
Originally Posted by Melon Head View Post
Just one more possibility. What is the Idss value of the Jfets you used?
I've wondered about that as well. Seems like no one had this problem with earlier builds, now some must increase or decrease the value of the resistor over the pots to get enough adjustment. The only variable that occurrs to me is the Idss values of the J-fets. I have sets from high 6's to mid 10's and have wondered which I should use in my current build, as I have already put the caddock standard value on the board. Since I have a range of J-fets to use, is there a certain value that would indicate which value of resistor is required for which J-fet Idss, or is this nothing to do with it?

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Old 17th June 2011, 05:52 PM   #30
mk57 is offline mk57  United States
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I finally gotmy F5 up and running with .60 bias and low offset I have one side working normal and one side with about 1/3 of the gain of the other. I have checked most everything bue can't seem to see anything. Ideas??
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