F5 Turbo Builders Thread

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#4,208
Erlend Sæterdal said:
Hi can you tell me if bias adjustment turning down is clockwise or anti clockwise?
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This depends on the orientation that you installed your variable resistors (VR).

The variable resistor uses a three pin potentiometer.
Two leads are shorted together to create a two lead variable resistor.
If you short the middle pin to the pin under the adjustment screw you will find that the VR value increases when you turn in one particular direction.
If instead you short the middle pin to the pin not under the screw adjuster then the opposite direction is required to increase the VR value.

That shorting of the middle pin depends on how you insert the VR into the circuit board.

Did you analyse what direction of rotation YOU wanted before you assembled the PCB?
 
#4,209
If you have already installed them and are confused, then with the amp off measure the resistance across R5 and R6. For example if you are measuring across R5, the resistance across R5 will be almost 0 when the pot P1 is turned completely one way or the other. Knowing that, you can figure whether turning CW or ACW increases the resistance and then mark the pot just like 6L6 has done in his build guide.
First time start up with minimum resistance across R5 and R6. Use the bulb tester or variac.

Nash
 
#4,212
Ft5 v2 one channel out

I have a tiny, little problem. Been running the F5T v2 in a 3-way active setup crossed over at 125Hz (8 Ohms). It's the bass amp. Just started to get the setup sounding really good when I noticed something. One channel of the F5T was out. Hard to tell at that low a frequency. Was getting plenty of bass.

Nothing blew up, no burned smell. The bad channel heat sinks heat up just like the working channel. There is sound out of the broken channel, but it's really low, but undistorted.

My guess is that it lost the jfets or a single jfet. Jfets were not cascaded and the inside of the amp was very warm, but not hot. Didn't use heat sinks on the TO-92s. Voltage from the power supply is approximately 31.8v loaded.

I don't remember the Idss of the jfets, but I seem to remember them being 9-10mA and not closely matched. The article said to keep them around 8mA. maybe they were running a bit too hot and gave out.

Anyone concur or disagree with this hypothesis?
Had this same problem with a Mini-A a long time ago. Replacing the input fets worked.

Thanks for your input,

Vince
 
#4,214
vdi_nenna said:
I have a tiny, little problem. Been running the F5T v2 in a 3-way active setup crossed over at 125Hz (8 Ohms). It's the bass amp. Just started to get the setup sounding really good when I noticed something. One channel of the F5T was out. Hard to tell at that low a frequency. Was getting plenty of bass.

Nothing blew up, no burned smell. The bad channel heat sinks heat up just like the working channel. There is sound out of the broken channel, but it's really low, but undistorted.

My guess is that it lost the jfets or a single jfet. Jfets were not cascaded and the inside of the amp was very warm, but not hot. Didn't use heat sinks on the TO-92s. Voltage from the power supply is approximately 31.8v loaded.

I don't remember the Idss of the jfets, but I seem to remember them being 9-10mA and not closely matched. The article said to keep them around 8mA. maybe they were running a bit too hot and gave out.

Anyone concur or disagree with this hypothesis?
Had this same problem with a Mini-A a long time ago. Replacing the input fets worked.

Thanks for your input,

Vince
Click to expand...

Perhaps you could measure the voltage across R3 and R4 for both amps and compare to get an idea. Also have you checked the DC offset? If one of the Jfets goes then the offset will be off.

nash
 
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