Many of the PCB traces are in the signal path , as well as many soldering joint , not very high end stuff
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Hello, guys, I need some help here adjusting bias. I don't know much about electronics and measurements, so please I beg you understand my misunderstanding. I have tested with the light bulb, and the bulb flashed for a sec and gently turned off. So far so good. I measured both channels of the PSU, and I get positive 25.3 Vdc and negative just the same in one channel, and 25.1 Vdc positive and negative on the other channel. I find it correct. I soldered the wires on the left channel to test. Now comes the problem I have to measure the 0.47 ohms "accross". So I measured touching both legs of the resistor. If that is correct, I get 0.000 volts, and I don't understand why. If I'm not measuring properly, can anyone explain it exactly how to do it? Just for fools. If I measure correctly, why could it be the 0 volts and how to find the problem?
Cheers.
David.
Cheers.
David.
When checking across the resistor for bias have you got your meter set to mv?
The bias voltage will appear as you (slowly) adjust P1, with P2 affecting the offset (which is measured across the output.)
If you adjust P1 all the way without the voltage showing you have a build problem.
Are Q1 and Q2 getting warm?
Everything is in the build guide.
The bias voltage will appear as you (slowly) adjust P1, with P2 affecting the offset (which is measured across the output.)
If you adjust P1 all the way without the voltage showing you have a build problem.
Are Q1 and Q2 getting warm?
Everything is in the build guide.
Ok, I have my meter set to mV, yes. Shall I move P1 while it is on? Q1 and Q2 don't get warm. Or do they take more than 30 seconds to feel some temperature? I want to kill myself.
I'm going to try. My Zeners have the value on the list from 6L6. God! I waited so long to get to this moment and... not working! It is really frustrating.
The amp should be on, and initially powered via a light bulb tester.
Make adjustments slowly and wait for everything to stabilize.
Concentrate on one side first, disconnect power to the other side.
Once you have got the amp to start working and warming up reduce the bias and start again without the light build.
If it will not bias at all then suspect a build or component fault.
Make adjustments slowly and wait for everything to stabilize.
Concentrate on one side first, disconnect power to the other side.
Once you have got the amp to start working and warming up reduce the bias and start again without the light build.
If it will not bias at all then suspect a build or component fault.
Ok, after a serious attack of anxiety, I relaxed, and today tried quietly. It was the first time I adjusted a pot. So, I follows what our friend said before and... yes, I started having volts! I adjusted and readjusted the 0,47R to measure +- 500mV and the offset on the terminals was 0 volts. I started over and everything looked stable, very sensitive pots by the way. Then I moved to the other channel. No problem neither. Easier than I could imagine, without nerves and calmly. It has been on for some couple of hours and everything is looking good.
But I feel the lelft channel is getting hotter than the right one. Should I worry? I will remesure and check the values are correct.
Sorry for the initial panic, and so many thanks for the help to all of you.
One question though. What does it mean to increase the bias? To which value can we get? What does it do? Can it break anything?
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.
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.
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.
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.
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.
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.
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
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
