FirstWatt F6

[angrypat]

Help, just fired up f6, store boards, calvin fets, one side ok, other wont bias past .1v, how do find what i cooked? hopefully not too boned, never saw or smelled anything.

 

[Zen Mod]

exact schematic , what exact outputs ?

OK , it is long thread (original one), but we covered every possible ookup there already

 

[angrypat]

 

[angrypat]

r2 .1v, idont know to test fets, so far between the 2 boards nothing jumping out at me. will un mount and check solder joints

 

[Zen Mod]

so , IRFP in outputs ?

check voltages across zenners

write here

 

[angrypat]

z1 and z2 both 4.5v

 

[angrypat]

z1 z2 6v on other board

 

[Dennis Hui]

Pat,

I'm guessing the side that is not biasing up correctly has 4.5V across the zeners?
What are you using for R7 and R8 and what value zener are you using? What
are you rail voltages?

I'm guessing you may need to change to higher value zeners and decrease R7 and R8.

Cheers,
Dennis

 

[Zen Mod]

decrease R7 and R8 to 3K3

whenyou get proper 5V1 across zenners , if you can't still bias it properly , add green or red LED in series with zenners , opposite cathode orientation versus zenners

or replace zenners with 6V8

 

[angrypat]

yes, r7 and r8 are 10k, 25v rails, other board is same and biasing correctly.

 

[Nelson Pass]

Original circuit had Jfet outputs that only need a couple volts to bias.

Some Mosfets need as much as 6 volts from zener.

 

[angrypat]

left channel biasing correctly with 6v at zeners, same ingredients in right channel but its unhappy. i must have something wrong

 

[angrypat]

what should i see at q1 and q2, or are they untestable at this low bias?

 

[Zen Mod]

do what I wrote

it'll work then

 

[ColinA123]

Do these mods in red for a stable F6. (As Zenmod says,)

 

[angrypat]

Thank you all for the help, I know all of you have forgotten more than I know! I ordered the parts, 6v8 zeners and 3k3 resistors. This means 2 things I hate, waiting, and desoldering / rework........grrrrrrrrrrrr! I'll be back, have a good day, and be careful out there.

 

[angrypat]

She's alive! I made the suggested changes, fired up, set bias at .5v, offset at 0, let it cook, checked, looked good, and now she's making music! I only had a old cheapo tablet handy to run it off of for now, still sounds good though. Thank you Nelson Pass for the design, and thank you to the store, if not for boards and fets etc and 6L6's build guide and help from others I wouldn't have built it. Now, I need to build a pre amp! For tonight though, back to the music.

 

[Dennis Hui]

Congratulations!

 

[angrypat]

Now, if I could only get this silly grin off my face!

 

[ColinA123]

I know it's controversial to some people, but!
Try a "passive pre" my F6 works fine with nothing more than 10K stepped attenuator between source and amp. Certainly no lack of volume.

"Keep it simple!"