F5 Bias Problem

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ENSen

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Joined 2010
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.
 
E

ENSen

Member
Joined 2010
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.
 
6L6

6L6

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Joined 2010
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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 :eek:

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?
 
C

Chilli6565

Member
Joined 2008
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:eek:
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:)
 
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Russellc

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Joined 2003
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Melon Head said:
Just one more possibility. What is the Idss value of the Jfets you used?
Click to expand...

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?

Russellc
 
neoinc

neoinc

Member
Joined 2008
Hi,

If I got no problem bias R11 and R12 to 0.6V but has a output of 10mV to 20mV, will it create problem?

There isn't much heat from the mosfets, is it sign of something wrong?

I have let it play music and it seems works, but just do not feel the heat from the Conrad heatsinks.
 
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