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F5Turbo Illustrated Build Guide
F5Turbo Illustrated Build Guide
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Old 4th April 2019, 03:03 PM   #541
BigE is offline BigE  Canada
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Quote:
Originally Posted by sangram View Post
I may be wrong on this, but there isn't actually an official FW version of the F5 Turbo. It is an extension of the original F5 design, which Nelson revised on demand to provide more output power/current.

I strongly recommend a speaker protection circuit in a circuit like this. The diodes may cause runaway even at idle, and under load/heat they are capable of vaporising a voice coil in a millisecond.

Or, build with a dummy load and test against every possible adverse condition before connecting a real load.
I've blown up with speakers connected 3 times. All my mistake.

The speaker protection from the diyAudio store has worked flawlessly.

Highly recommended.
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Old 4th April 2019, 05:53 PM   #542
AudioSan is offline AudioSan  Norway
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i support this. One blown output is all thats needed to get full rail DC voltage to the speakers.
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Old 15th April 2019, 06:11 AM   #543
TboneAK is offline TboneAK  United States
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6L6,
Cooking my F5T v2 with mached toshiba grade B jfets, no casscode. I'm only able to get the bias up to .24v on both channels. This seems on the low side. You told nanchangbob to increase R5 and R6 and start over. So if I switch them out to 2.5K or 2.2K, that would almost double the resistance with the pots at full turns.

Don't have the inputs shorted, should I?

One thing is that both N channels took alot more turns than the P channels to keep the offset at 0vdc. Those two are nearly maxed out. I backed down a bit. Still cooking the amp. The bias is creeping up ever slowly. I'd like some fudge room with the pots.

I've read .35vdc is the target. NP said .4vdc for non fraidy-cats, or is that off limits for a noob.
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Last edited by TboneAK; 15th April 2019 at 06:22 AM. Reason: Added info
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Old 15th April 2019, 12:04 PM   #544
BigE is offline BigE  Canada
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.35 is the highest target.

0.4 is off limits or everyone.
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Old 15th April 2019, 12:40 PM   #545
6L6 is offline 6L6  United States
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F5Turbo Illustrated Build Guide
TboneAK - You are having a common experience, it's a function of the interactions between your Jfet Idss and Mosfet Vgs, and all you need to do is increase the parallel resistors.

Also, it's normal that one set of outputs (N or P) need more turns to get to the set bias value, as the Vgs of the N's are very, very rarely the same as the P's. That's why there's a adjustment pot on both phases. This also means that your bias will be slightly different on the N side compared to the P side with zero offset. This is normal.

Inputs shorted just helps things from wiggling around when measuring, it's not necessary

BigE - 0.4 is fine if you have tightly matched mosfet and your diodes don't start to conduct at idle bias. It's not "off limits".
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Old 15th April 2019, 02:39 PM   #546
TboneAK is offline TboneAK  United States
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6L6, BigE- Thanks!!
I’ll play is safe and set it at .35v. How I read it is basically is, higher bias = higher dissipation, out put, and lower distortion.

I’ll try shorting the inputs this next go around.

Quoting NP.
If you are competent, fearless and also own a fire extinguisher, you can find this point. Just run the amplifier into a reasonably low impedance until it gets good and hot – as hot as you plan to let it get - ever. Then adjust the bias to a point below where the idle current starts to really take off. You should find that this point is around 0.4 volts across the 1 ohm resistors. If you are a fraidy-cat, then just set it at 0.3 volts, and conservatively fuse the AC line.

After getting R5 R6 replaced to allow higher bias, and initial biasing, should I load the out puts with a bank of high watt resistors at 4ohms for a final adjustment as he suggests?

Thanks guys!
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Old 15th April 2019, 05:15 PM   #547
BigE is offline BigE  Canada
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Thanks 6L6. Maybe I'm a fraidy cat. Or I just have too many output devices on my heat sinks and can't bias above 0.32....

TboneAK, regardless of the value you choose to use, your heatsinks need to be big enough to dissipate the heat generated by the current flowing through the output devices. That is, the heat gain of the sinks has to be low enough to keep the operating current within reasonable limits. That is always a factor.

Please note that at the same bias voltage, hot MOSfets will conduct more current than cool ones. And their temperature will vary depending on the temperature of the room ( ambient temp ). So, if you bias in the winter in an unheated basement ( like I do ), then summer temperatures may bring operating conditions that your heatsinks cannot handle. The effect is that more current flows, which heats the sinks further, causing more current to flow, which heats the sinks further, and eventually, the mosfets are passing so much current, they blow up.

The rule of thumb is to bias so that the heatsinks are never more than 55 degrees C, year round.

This does not mean if you have big heatsinks you should bias above 0.4 V.
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Old 16th April 2019, 06:57 AM   #548
TboneAK is offline TboneAK  United States
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All is going well. Cooking at a shade over .3vdc. Went from 1K to 2k2 on R5 and R6. Much more responsive. It's not to that "I barely touched it" point. I'll go in small steps to .35vdc, and see how it goes.

I'm near the ocean (Gulf of Alaska) and it's very temperate and cool here. Not like the mid west where I grew up. Our house temps range 60-70F all year, no A/C needed. Outside temps rarely go much over 70F in summer, or under 10F in winter. 75F and up, the locals are running out of clothes to take off. Those of us white people give a whole new meaning to "White". It's not pretty, sun glasses are needed. It was sunny and 50F today, shorts weather!

That said, the closer I get it to running away lessons its stability if ambient temps are above my normal, or its put into a deferent amp rack/location. I shall stay on the Fraidy-cat side of .4vdc. I'm using the 5U chassis from the store. It huge.

Thank for all the help!!
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Old 17th April 2019, 05:53 AM   #549
TboneAK is offline TboneAK  United States
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I've been cooking it tonight. No load on the outputs. .38vdc, and 45C on the heatsinks. I can now see how ambient temp changes can effect the bias. Lift the cover off to make an adjustment, and the bias drops a hundredth or two. I have four DMM's hooked up. One for each off-set, and one each for the P-channels.

The transfo is humming now as well. I'm using the Antek 600VA unit. Normal?

I picked up some ballast resistors to make an 80w, 4.125ohm load bank. Should my next step be to back down the bias to .35, and hook up the load bank? I would think that the heatsink temps should rise, which will should cause the bias to rise as well. How fast does a run-away situation happen?
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Old 17th April 2019, 06:10 AM   #550
Zen Mod is offline Zen Mod  Serbia
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if you made it per prescribed schematic , having enough in heatsinking , nothing to worry about

Papa included those drain thermistors with cause ..... mount them properly and Iq will stay polite
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