F5 Turbo Builders Thread

[buzzforb]

Andrew, are you saying you thought it wad p3? Seems unlikely based on description.

[anilva]

Originally Posted by AndrewT

I said a long time ago that the adjustment VR needs fixed resistors either side to limit the extent of adjustment. Seems folk either don't read or have a memory as bad as mine.

I agree with you. There seems to be a need for protection around how much you can play around P3 without severe damage.

[buzzforb]

I really dont think it was p3. I think it just points to a problem around the n channel jfet.

[pdhanwada]

f5 turbo manual states ""It differs mostly from the original by the inclusion of P3, which
aids in tweaking the symmetry of the circuit for the lowest possible distortion""

It is only for fixing the distortion.

[anilva]

Originally Posted by pdhanwada

f5 turbo manual states ""It differs mostly from the original by the inclusion of P3, which
aids in tweaking the symmetry of the circuit for the lowest possible distortion""

It is only for fixing the distortion.

That is exactly how I understood as well and hence was not careful with the tweaking. Little did I realize that it can blow the jFets if adjusted improperly. I have also noticed that it has impact on output offset and bias levels of the MOSFETs. I think I prefer now to leave it out completely or set it to middle of the trimmer range before soldering.

[buzzforb]

Exactly. I dont think it can throw the balance off baddly enough to cause a major fault, like was experienced by anilva.

[buzzforb]

Anilva, i dont beliebe this was your problem. You had an existing problem that was made worse by P3, but not caused by P3.

[sangram]

What happens if there is a short circuit from wiper to one of the jfet source pins? Would it not affect the DC gain (and therefore the current drawn through the jfet?).

The symmetry adjustment offered by P3 is (probably) to adjust for the difference in transconductance between P- and N- channel devices, and the only way it does that is to adjust the DC parameters around the jfet source. euvl and jackinnj had alluded to using slightly different values in the two halves of the circuit a year or so ago, P3 was a way to make it adjustable without having to desolder.

I guess my PCB and build will be redesigned to have a lower value of P3 with a couple of series resistors on each leg. Lower adjustment ratios but lower risk too.

[buzzforb]

Guys. pot centered, you have 9 ohms of degeneration for each jfet. Swinging it fully in either direction leaves you with 8 and 9.5. Not a significant difference. It could easily be adjusted out on the output. He had problems before adjusting the pot. THere was something wrong to begin with. Unless ZM or Nelson says otherwise. I would assume it was not the pot.

[a.wayne]

Fire up the 4 pr and show him buzz ....