Trouble getting bias on an F5

Pavo

Member
2019-07-23 5:00 am
So I’ve got my Aleph J singing and have moved on to my F5. I’m working with the diyaudiostore board, kit, psu, and chassis. I’m using the v3 schematic. I’ve tested the psu and everything checks out. Bulb tester is showing everything is ok. When I go to full power I’ve got near 0 volts (about 6mv) on all three meters across r11, r12, and offset. As I follow the procedure (one turn p1, one turn p2) I can get about 100-120mv bias across r11 and r12 after 4-5 cycles but then the offset (outputs) jumps up to about the same. Adjustment of p2 brings the offset back to zero but so does the bias across r11 and r12. I can’t seem to get any bias without significant offset variation. I assumed that I was having the same problem other people had with not enough resistance across r5 and r6 but have added 2 extra 2.21k resistors and have the same result. The only result appears that the .47 ohm r7 and r8 get hotter and hotter. You can see the results of the heat on one of them.

Any help would be appreciated. I’m pretty stuck here.
 

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Pavo

Member
2019-07-23 5:00 am
Yes, bulb tester removed. V3 schematic attached below. I did not test the idss values. I'm using the matched linear set from the diyaudiostore.com I'm 99.9% certain I have all the transistors in correctly.
 

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Pavo

Member
2019-07-23 5:00 am
Well the job is done.
You have successfully biased up your amp.

Thank you so much for your help.

1: Are you serious? I've just been confused on what resistors to measure across? (because now I really feel like an idiot). I know this is a learning site but jeeeesh....

2: Do you think I should try to go back to just the 2.21k resistors for R5,6? or stick with the hack job you see in these photos.