A guide to building the Pass F4 amplifier

[Rembulan]

@ Marra, what is that under Your Elco ?

 

[Marra]

@ Marra, what is that under Your Elco ?

There's a soft start board under them.

 

[qwertyl]

You can play with the source resistors... Change them to 10ohm and try again.

R3 and R4 changed to 10Ohm. Now I am getting about 6mA. There is definitely a difference in sound - before midrange was a bit hidden, now seems to be better.
I also changed R8 to 27k4. Do you think it is worth changing R10-15 to 150Ohm as well?

 

[Marra]

My F4 is just about completed and has the following readings:-

P/S +- 24.5vdc
Source resistor voltage 218mv both channels
Offset less than a mv but fluctuates slightly
Heatsinks are at 36C after 90 mins.

When I power down the output offset goes up to 225mv and slowly drops down.Is ok or do I have a problem?

 

[Zen Mod]

ok

 

[Marra]

Thanks ZM

 

[jdg123]

nervous shut down

Hello All,

Has anyone an explanation for the following:

Did a bulb test=OK
Removed bulb tester.
Took initial readings across 0R47 resistors (both channels) of about 350mV (0mV offset
Shut it down.
Turned down pots two turns each.

Then one channel stabilized at 300mV while the other just kept on dropping! I shut it down when the second channel hit 170mV because I thought something might be failing. Am I right to be nervous?

No smoke but strange behavior...

 

[Zen Mod]

....
Turned down pots two turns each.

.......

clarify ;

btw - P1 is for Iq , while P2 is for offset

nota bene that orientation of trimpots is important for CW vs. CCW behavior , meaning what will happen when you turn them in one or other direction

 

[6L6]

Disconnect suspect channel.
Adjust good channel until it's set properly.

Disconnect good channel.

Re-attach suspect channel.
Proceed with suspect channel

 

[jdg123]

Thanks guys.

This time I got a runaway bias that shot up to one volt across the single channel I attempted to bias.

And there was smoke.

So...

I tear down the whole thing and see what foolishness I concocted.

This was my third "First Watt" build and I'll be danged if I have a clue what when wrong. I'll report back in a few months...

 

[Zen Mod]

pictoooores , please

 

[jdg123]

I'll post after a few beers and a few days perspective.
I don't think pictures will say much...

 

[jdg123]

Beer and pictures

Well the beers did help...and hush my mouth, there may be a picture that helps.

Center MOSFET with the melty surface and writing burned off...

Must have just been a bad apple?

I can find no error in the build or wiring that would explain the readings and smoke

 

[Zen Mod]

either bad apple , or apple not squeezed enough ........ or apple squeezed at burr protruding mica

 

[BigE]

If Keratherm insulators are used, machining swarf can easily penetrate. That being said, keratherm is fantastic.

 

[jdg123]

I used mica and thermal grease, whiped down my mounting surface and applied the grease with my fingers to both sides of the mica to insure coverage and to look for impurities/metal. I don't think there was any contact between the MOSFET and the heat sink.

But it sure is what it looks like!

I'll inspect everything with a magnifier and report any findings.

Thanks for sticking with me.

 

[Zen Mod]

did you tighten them all properly ?

however - check rest of them and replace Dodo

take care of matching - if you don't have close one , replace that trio

 

[jdg123]

Yes, I exercised great caution in tightening them, reading the spec sheet for torque, experiencing what inch - pounds feel like by putting a screwdriver into a torque wrench, and tightening them a little at a time across all MOSFETs until they matched.

I have contacted h_a about a replacement ((numbers are still on the remaining MOSFETs) but understand I may have to buy a new trio.

I only have skills for checking MOSFETs with a diode checker; can you point me to an article for gaining more skills in this?

It took me 5 months to build so I am being patient. I can spend a little more time, no problem.

 

[Zen Mod]

go to Papa's FW site

Articles

Matching devices

first picture

 

[jdg123]

Thanks ZM.

You are a great help to folks in this forum.

I'll post any real progress (with pictures) as it applies.

Cheers!