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F6 Illustrated Build Guide
F6 Illustrated Build Guide
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Old 7th September 2017, 06:20 PM   #911
2 picoDumbs is online now 2 picoDumbs  Australia
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Increasing idle current through transistors.
To be safe don't exceed 1.5A.
You can decide later to join abfab. Now is probably not a good time. Enjoy your amp.
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Last edited by 2 picoDumbs; 7th September 2017 at 06:23 PM.
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Old 7th September 2017, 06:42 PM   #912
dabenza is online now dabenza  Spain
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Thanks, i wont be touching anything for the moment.

Cheers,
David.
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Old 7th September 2017, 07:16 PM   #913
ColinA123 is offline ColinA123  United Kingdom
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The rule of thumb for temp measurement
When the amp is stable, and up to temp
if the heatsinks feel hot, but you can keep your hand on them then fine.
If you can not keep your hand there they are too hot.

Many think that the F6 sounds better with the bias on the low side rather than pushing too far.
About 600mv is right for me.
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Old 7th September 2017, 07:34 PM   #914
dabenza is online now dabenza  Spain
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Quote:
Originally Posted by ColinA123 View Post
The rule of thumb for temp measurement
When the amp is stable, and up to temp
if the heatsinks feel hot, but you can keep your hand on them then fine.
If you can not keep your hand there they are too hot.

Many think that the F6 sounds better with the bias on the low side rather than pushing too far.
About 600mv is right for me.
I don't burn my hand. I can keep it there safely. But it feels hot. Ok where I live is pretty hot, so that doesn't help neither, but I feel I could warm up some sausagges on the disipants!
Well, I'm keeping it to 500 mV for the moment. I still don't understand whats the real benefit of going to 600mV (Is it more real volume?)

Cheers!
David.
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Old 7th September 2017, 07:37 PM   #915
2 picoDumbs is online now 2 picoDumbs  Australia
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Less distortion, better damping, slightly better bandwidth.
If you have 4 Ohm speakers more current is better, if you have highly efficient 8 ohm speakers then no need to increase the bias.
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Old 7th September 2017, 07:48 PM   #916
dabenza is online now dabenza  Spain
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8 Ohm, 89db. Not exactly very efficient, but they are ok. I don't need outrageus volumes.
Call me crazy, but after measuring again after ours swiched on, I started to play music with it. I started with the cheap speakers, in case. It feels detail, nice dynamics, open sound... so far so good. And I have the feeling that the sound has changed quite a bit after one hour playing... for the better, specially the bass.

Thank you all guys, you are amazing.

Cheers.
David.
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Old 7th September 2017, 08:54 PM   #917
Bare is offline Bare  Canada
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It's Nice :-)
You likely will discover , as I did that the F6 Is 'better' than the previously used source and one's speakers.
Bottlenecks are elsewhere than the Amp
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Old 25th September 2017, 06:51 PM   #918
costl is offline costl
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Thanks to everyone for immensely valuable build thread and comments, esp. 6L6, ZM, 2pD, and Russellc. I'm a newbie amp builder (several speakers build), on my first foray, and could not have gotten this far without them.

So, I just powered up my F6. Standard build, DIYstore boards (F6 and PSU), DIYstore F6 transistors and JFETS, 4U 300 case. Closely followed build guide (with ideas from F4, F5 guides). Only mod is the suggested 110R at R11,12. I am using the 5.1 Zeners, and 10K at R7,8, as per original schematic (ZM mod didn't sink in until too late, though I was aware of potential need to replace Z1,2). PSU measures 23.8 per rail.

Problem (as you probably guessed) is bias and offset. With pots at limits, I'm reading:
L: bias (voltage measured across .47R) is .21V cold (.23V warm) (~.48mA)
offset is ~.3V.
R: bias, is .28V cold, .3V warm (~.64mA)
offset is ~.3V.

(1) BIAS: My next planned move (as per threads, esp. at #2, #66) is ZM mod. So, (a) replace Zener with 6.8V, (b) replace R7 and R8 with 3.3K. Correct? Alternative suggestions?

(2) OFFSET: Will fixing the bias issues impact the offset problem? Should I still be able to zero offset even at this low bias? Any advice about where to look for the problem?

Also, in case it matters: (3) LED: I also had unlit blue diode on L channel. From threads, I'm guessing it was misoriented. Unfortunately, I destroyed it during removal. But will replace and be more careful checking this time before soldering.

(4) I ran the amp at this configuration for quite a while while trying to understand the problem. (a) Any danger in doing so? (b) What about connected to source and speakers? I tested it on some cheapies, and it sounded pretty decent. What's the damaged risked in doing so?

Many thanks in advance for any feedback.
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Old 25th September 2017, 07:15 PM   #919
Zen Mod is online now Zen Mod  Serbia
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F6 Illustrated Build Guide
go back with trimpots , change resistors and zenners , repeat setting procedure , enjoy
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Old 25th September 2017, 07:43 PM   #920
silasmellor is offline silasmellor  Denmark
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Hello fellow DIY'ers. I recently went back to my F6 and increased bias. Before it was running 0.52-0.53 V (1.13-ish amps), which i gathered from various forum posts was a bit low. Having also read some posts suggesting that higher bias improved the fit with low impedance speakers (mine are dynaudio x16s, 4 ohm 86db i believe), i decided to up it a bit, and managed to land around 0.61 V after an evening of tweaking and listening.

To be fairly honest i am not sure i notice much difference! In any case, i left it at that, the heatsinks measured a comfortable 50-ish. Now, it recently occurred to me that i had so far only measured temperature on the heatsinks using a small temp probe that came with my DMM - since it was metal and probably conductive i dared not touch any of the insides with it. So i got hold of a laser thermometer and discovered to my horror that the IRFP240s were toasting away at around 70 degrees once hot (i confirmed, very carefully, with the probe).

For now i will lower the bias again to get it below 65. That said, i wonder how common is it to be limited to such low bias? I am using the 4U chassis. The fets are mounted with thermal pads that i assumed was keratherm, but considering i got them from my local electronics shop i wonder if they are some other material that needs grease? Could this be a likely explanation?

Thanks in advance,
Silas
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