F5 Turbo Builders Thread

6L6

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How much is the bias increasing?

What is your starting bias amount cold?

How hot are your heatsinks after 10min, 20min, 30min…, etc… ?

A 5U can run an F5Tv2 stereo nicely, so something else may be going on.

As always, please post a series of well-lit, in-focus photos.
 
The bias is slowly crawling up 1mv or so over a minute.

Heat sinks are hot to the touch but you can easily hold your hand on them. Sadly I lack a thermal camera. (really should get one)

I wouldn't be surprised if there is a mistake now. I know my thermistors are too far from the mosfets at minimum. I am attaching pictures. Also I know it's absolutely fugly inside the case. I really should have worked on my wiring skills, but I mostly wanted to see if I could build it.
 

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1. Yes. If your heatsink is too large for the bias you set, this will happen. Because of the way large heatsinks work with multiple devices, you get a temperature gradient between devices and since you are only sensing a single device, the other can begin to run away.

2. Yes. Requires modification of the schematic beyond the scope of the discussion, and the PCB is not suitable for the required changes. See the original F5 article for the thermistor connection. Use a small value cap + series resistor to damp the thermistor action at HF in parallel with the Th+R combination. Touch the thermistor to the heatsink instead of the MOSFET.

This is not an approved modification, but almost guarantees impossibility of bias creep. The penalty is longer warm-up time because of thermal latency, and more fiddly bias adjustment.


Would this happen too in a fan cooled setup that wasn't adjusting to temp. but just runs flat out all the time?
 
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To be clear, the effect of the thermal braking provided by the thermistor on the unsensed device is not zero, but it's less that on the primary device.

That said, my sole surviving F5T is on an open bench with fans blown towards a slightly inadequate heatsink. It does take a long time to warm up, the bias adjustment is more tricky, but I have no difference between the devices because I do not use the same method of compensating for temperature as the F5T schematic. I guess if I did, I would see a difference at least between the two devices. But the fan does force a more or less constant temperature on the heatsink, thus it might not actually exhibit a constantly rising difference.