F5 power amplifier

[mfrimu]

Hello,

I have read different instructions of bias settings....

1. Do I need to have a load (50W 8 Ohm resistor) or not?

2. All say set P1 and P2 to low - which side is low 1 or 2 (see attached image)

Pin 1
|
|_____
/ |
\ |
/ /___| Pin 2
\ \
/
|
|
Pin 3


Which side is low, I assume move trim towards 3 (pin 2-3 resistance zero)?

Should I bias with bulb in series first?

Thanks !!!!

[Marra]

Adjust the trimmer until you get the lowest resistance reading across the resistor that is in parallel;R5/6 if I remember correctly.Make a note of which way to turn them for when you are ready to set the bias.
You won't be able to set the bias with a bulb tester connected as it won't allow enough current to pass.Just make sure nothing smokes whilst the bulb is in circuit and as AndrewT says use it at each step.

[KatieandDad]

Do measure the pots for minimum resistance, I didn't and I paid the price.

[sangram]

1. No load is required. A multimeter permanently attached to the output is very helpful to ensure offset reading is always active.

2. Direction is less important than resistance. On different boards and with points flipped around, it can get very confusing. Basically the resistance across the pot needs to be zero. This places the output in cutoff. Safer to just measure the resistance across the drain resistor of the input jfet. It should be zero. The increase in this value is what increases the bias.

3. No, use the lightbulb to check everything is OK. With pots shorted and connected amplifier, light bulb should not light up. To set bias, remove light bulb and start turning one of the pots till you get an offset reading on the meter at the output. The rest of the bias procedure is quite easy to follow.

[mfrimu]

Thanks all for info. Basically Pot should be zero resistance...

[tinitus]

remember to adjust slowly, both pots, one by one, bit by bit

and watch DC all the time
there is no DC adjust pot
balance is achieved through the two bias pots

and remember it takes some time before it reaches working temperature, and everything is fully stable

[bcmbob]

Originally Posted by sangram

... With pots shorted and connected amplifier, light bulb should not light up. ..

Just a small point so new builders don't get scared off - the bulb usually lights up for a second while the caps charge. Then should dim to off. Fooled me first time

Also, remember those power supply caps retain a charge if not in circuit. It's best to discharge the PS before moving to the next step. A simple bulb across (+) & (-) will do the trick.

WORK SAFE!!!

[dieringe]

Originally Posted by bcmbob

Also, remember those power supply caps retain a charge if not in circuit. It's best to discharge the PS before moving to the next step. A simple bulb across (+) & (-) will do the trick.

I thought that's what the 2k2 resistor is for

[bcmbob]

It's been my experience that dissipation doesn't happen instantaneously with the 2K2. Only takes a few seconds to check and avoid surprises.

[sangram]

That is not a mistake.

The F5 uses opposite polarity outputs. The negative rail has the N-channel devices, reverse for the other side.

Check that the drain of the input jfet connected to the correct side. P channels jfet drain goes to negative rail.