F5 power amplifier

[kpsthakur]

RIP 2SK170....u were a darling....F5 biasing is a serious pain in the butt...if you increase bias slightly, you fry R7,R8, if you reduce slightly you fry the 10 ohm's and the JFET's...


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[AndrewT]

..............F5 biasing is a serious pain in the butt...if you increase bias slightly, you fry R7,R8, if you reduce slightly you fry the 10 ohm's and the JFET's... ......

That's not what Builders find.

 

[Steve Duffy]

I had reversed my MOSFETs in the bad board. I'll replace them, what are the chances the JFETs have survived I plan to replace the MOSFETs and see.

 

[kpsthakur]

I had reversed my MOSFETs in the bad board. I'll replace them, what are the chances the JFETs have survived I plan to replace the MOSFETs and see.

Test them with continuity on all 3 pins...if they all short then MOSFET's are fried...else u have a chance


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[kpsthakur]

That's not what Builders find.

I agree,Andrew, loosing my rare JFET & my less experience is the problem....found a way out...used a less powerful power supply with 4A rectifier to limit current to stabilize the bias around .40V on output resistors..then gradually moved to higher current PS, finally the actual PS that could provide ample juice...and was able bias it around .60V (about 1.2A in A class) for testing, now will build second channel similar fashion and bias full when I get that into a chassis


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[Steve Duffy]

Thanks kps the parts survived (but a few resistors were roasted) the amp is back together and biasing up nicely.

 

[Steve Duffy]

Biasing up the amp

Celebrating getting the amp together, thanks for all the help, cheers.

 

[kpsthakur]

Celebrating getting the amp together, thanks for all the help, cheers.

[emoji106]pure DIY


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[Soundhappy]

Celebrating getting the amp together, thanks for all the help, cheers.

Nice to know your amp work great.
Congratulations 🙂

 

[kpsthakur]

One channel working absolutely fine, second channel giving troubles...replaced JFET's, MOSFET's, trimmers ....but story remain's the same...Bias on output resistors tend to sync, but differ mostly by about .10V, and DC offset never turns 0, even after hours of playing....Surprisingly if I use lamp current limiter setup, the amp biases properly around 0.47V on both resistors, including DC offset to 0...


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[kpsthakur]

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[kpsthakur]

Last pic is while shorting the inputs during biasing


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[kpsthakur]

But still, DC offset never touches 0 or negative


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[nige2000]

Can we have a close up pic of the jfets please?

 

[kpsthakur]

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[nige2000]

Multi turn pots would provide finer adjustment too

 

[kpsthakur]

Tried them too, switched them back to single turn...[emoji19]


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[nige2000]

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Was thinking if they were genuine the tops looked a little different in the other pic

 

[kpsthakur]

They from DIYAUDIOSTORE matched class B specifically for F5 and series


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[AndrewT]

What are the Eline packaged transistors?