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Old 8th June 2011, 04:29 AM   #11
6L6 is offline 6L6  United States
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You are having a fairly common problem as of late. Mine did the same thing.

You need to increase the value (more ohms) of the resistor in parallel with the bias pots. R3 and R4.

Look at this post and read everything carefully -- How to build the F5

FWIW, I was adding 2.2k resistors.

It will have the answers and a step-by-step procedure for you to follow. Although my amp uses the Peter Daniel boards, you will get the idea.

Good luck!
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Old 8th June 2011, 05:07 AM   #12
JZatopa is offline JZatopa  United States
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Thanks for the post 6L6 that is exactly what I was experiencing. I have been searching for days for someone having the same problem. What was your final resistance (~6.6k)? Do we know why this is happening lately?

If I just add extra resistors will it affect the sound? Should I order some higher ohm dale resistors and swap them out?
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Old 8th June 2011, 12:08 PM   #13
6L6 is offline 6L6  United States
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The best guess as to why it needed the extra resistance is
1 . Idss of used Jfets is small ,or
2. Ugs of used mosfets is unusually big , or
3 . both 1 & 2

Regardless, it's not that big of a deal, as it's easy to adjust. Don't get any special resistors, just add whatever you have laying around in series to what's there.

The bias voltage reading is only valid if there is zero offset. Add some bias, adjust the offset to zero, add some more, re-adjust the offset back to zero, and so on until you have the proper .6v and zero offset. If you can't get the bias to .6 with zero offset, add more resistance.

Also, Only one of my channels has 6.6k, the other would bias nicely at 4.4k . Add only what you need.

Last edited by 6L6; 8th June 2011 at 12:23 PM.
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Old 8th June 2011, 11:46 PM   #14
JZatopa is offline JZatopa  United States
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Ok so my last question is do I add equal resistance to both r3 and r4 or do I just add it to the side whose v drop (r3) is low.
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Old 9th June 2011, 12:05 AM   #15
6L6 is offline 6L6  United States
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Both R3 and R4.
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Old 10th June 2011, 03:09 AM   #16
JZatopa is offline JZatopa  United States
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So I went to radio shack and picked up some 1/2 watt 2.2k resistors and wired them up in series with R3 and R4. The good news is that I got one side to bias up to the right area. The bad news is that I shorted something on the other side and let the magic smoke out. Rather then just trying find the broken parts I am just going to order another kit from techDIY and a new board from the site. A little disappointing but at least I am on the right track.

On second thought I am going to just order another set of parts from techDIY and try to salvage the board. I am not sure how it will go but it will save me a little cash. Damn almost $50 worth of parts down the drain. Next time I will be more careful (I think I said that last time I broke something too).

Last edited by JZatopa; 10th June 2011 at 03:14 AM.
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Old 10th June 2011, 03:15 AM   #17
6L6 is offline 6L6  United States
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Where did the smoke come from? Do you have any obviously smoked components? Replacing a few pieces is not that hard.

Buy some wick and one of those spring-loaded solder sucker doohickies. You will need both.
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Old 10th June 2011, 03:48 AM   #18
JZatopa is offline JZatopa  United States
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R11 and R12 are what are smoked. I think I accidently shorted shorted R11 to R6. I am guessing I fried Q1 and or Q2. I don't mine ordering the extra parts, it will give me piece of mind that I can finally finish up this amp.
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Old 24th June 2011, 11:20 PM   #19
JZatopa is offline JZatopa  United States
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OK so I pulled the bad board and removed the outputs. They were shorted. I didn't test the other fets but I am sure that they probably are bad too.

I received my new parts and board, poplulated it and installed it the other night. I put extra resistors on R3 and R4 off the bat. The board gets close but if I get to .59V I have about .05V offset. I know I could take the board out again and add extra resistors but I was wondering if that is close enough.

What is better 0 offset with a lower bias or 50mv of offset with the proper bias?

I plan on using the old board with the lifted trace and the new board to make an F5 car amp. Now if I could just find someone to make a SMPS that will work for me.
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Old 24th June 2011, 11:32 PM   #20
Zen Mod is offline Zen Mod  Serbia
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bias schmias is just state of mind


it depends of you when is enough , even if more is always merrier

everything up to 100mV of offset , long term , is considered as OK , but 0mV is always worth chasing
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