Congrats
I very much appreciate the comment, but I'm not done yet...
Looking through my tools, and I just can't find my 3rd voltmeter. And I'm having a devil of a time getting it to bias using only two.
Having the 3rd meter will hopefully show the interactions between the 2 bias pots, because right now I'm getting more confused.
Start from zero on both trimmers; then turn each one half a turn withne meter across the resistor and one on the output. Once they start showing a reading its a case of back and forth between them and up and down as they interact quite intimately with one another and can swing from positive to negative millivolt readings on the output. I ended up with over 100mv to start with,
I very much appreciate the comment, but I'm not done yet...
Looking through my tools, and I just can't find my 3rd voltmeter. And I'm having a devil of a time getting it to bias using only two.
Having the 3rd meter will hopefully show the interactions between the 2 bias pots, because right now I'm getting more confused.
if you are still confused after reading war & peace which I wrote recently ......
put one voltmeter for output offset , second one across any source resistor
you don't need to know what's happening on other source resistor
current is going from + to - through two mosfets ..... so - when you have correct current through one source resistor and zero offset - current through other mosfet is where need to be
I had a similar experience, I was getting over 500mV on the outputs! Not to worry, no speaker hooked up, wont hurt a thing. Just keep bias low and dont soar it sky high. A variac helps. You'll find turning one pot one direction will cause the offset to be closer, but the bias goes down. (Or up, just seesaw back in forth.) Once you have bias low, (.40 or so) and the offset zeroed, you can turn one pot the direction that raises the bias, just a little, then you will see your offset just went up. Turning the other pot a certain direction will correct. Back and forth until bias is around .59.
With a variac, you can keep voltage a little low, then as you ramp up,( keep eye on bias) you can keep readjusting, and so forth. If you can fire up one channel at a time its also a lot easier
Russellc
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Not confused with your procedure, it honestly makes perfect sense to me.
Yep, I have done that.
The problem is that I can't seem to get the bias very high without having offset, like .2v or more. If the offset is zero, the bias current is very low, less than .2v
No, these are not Toshiba output devices.
Hey, all......
Question for you, on the "intimate relationship" between the two pots on the Amp PCB. As I got close to the magic .59 volts (say, any value above .50 volts) I found tweaking one pot (and measuring the voltage across, say, R11) had effectively the SAME EFFECT on the voltage across the other resisitor (R12 in my case). Conversely, tweaking the other pot did the same. Prior to testing, I had assumed changing one pot would only change the voltage for its one associated resistor.
In on case, during final tuning, I found myself "chasing" a 0.012v difference between R11 and R12 readings, and by tweaking either pot I could increase or decrease the R11 and R12 readings, but they continued to show that 0.012v difference. Since I had a low output offset (like, 0.1 mV), I attributed the 0.012v bias difference to tolerances in R11 and R12 values, and declared victory.
Have you seen that "intimate relationship" between the two pots, in which tweaking one effects both bias setting almost identically?
P.S. This is probalby a minor point, since the amp is performing magnificently--but I sure mulled over it, while I was doing the bias settings!
Question for you, on the "intimate relationship" between the two pots on the Amp PCB. As I got close to the magic .59 volts (say, any value above .50 volts) I found tweaking one pot (and measuring the voltage across, say, R11) had effectively the SAME EFFECT on the voltage across the other resisitor (R12 in my case). Conversely, tweaking the other pot did the same. Prior to testing, I had assumed changing one pot would only change the voltage for its one associated resistor.
In on case, during final tuning, I found myself "chasing" a 0.012v difference between R11 and R12 readings, and by tweaking either pot I could increase or decrease the R11 and R12 readings, but they continued to show that 0.012v difference. Since I had a low output offset (like, 0.1 mV), I attributed the 0.012v bias difference to tolerances in R11 and R12 values, and declared victory.
Have you seen that "intimate relationship" between the two pots, in which tweaking one effects both bias setting almost identically?
P.S. This is probalby a minor point, since the amp is performing magnificently--but I sure mulled over it, while I was doing the bias settings!
Yeah thats exactly what threw me to start with.I initially set one trimmer and then the other thinking I would have zero on the output; it came as a shock to find I'd got 100+ma! I was doing it with a DMM across both source resistors; once I was reminded to use one source resistor and the output the final setting up was a doddle.When I recheck things I'll have to check if there's any differance between settings across the sources. As somebody said this amp gets better with use.
observing one source resistor and output offset is way to go ;
chasing symmetry elsewhere in amp is nonsense ;
all you have to do , after initial bias setting is to use THD meter and tweak one source resistor , looking for lowest THD , trying to neutralize differences in xconductance of input Jfets , same as output mosfets
if you are in mood for that (chasing symmetry) - EUVL gave few tips ( additional source resistor for one input Jfet , needed Ids for that etc. ) ........
do not forget - current is flowing through feedback loop , too ......... so that's where is "missing" current through one output source resistor , if you compare them ......
chasing symmetry elsewhere in amp is nonsense ;
all you have to do , after initial bias setting is to use THD meter and tweak one source resistor , looking for lowest THD , trying to neutralize differences in xconductance of input Jfets , same as output mosfets
if you are in mood for that (chasing symmetry) - EUVL gave few tips ( additional source resistor for one input Jfet , needed Ids for that etc. ) ........
do not forget - current is flowing through feedback loop , too ......... so that's where is "missing" current through one output source resistor , if you compare them ......
Good points, ZM..... In fact, after "chasing" that 0.012v difference in the R11/R12 bias voltages, I elected to go for the zero DC offset. Considering just the tolerance of the various amp components, going for perfect bias tracking is futile/unnecessary.
I borrow the test equipment later this week for THD and IMD testing...... But since the real test is my ears, I'm pretty content right now.
I consider my outcome to be a perfect "42".....
I'll continue to nag Antek about my power transformer, though....
Thanks,
Ken Mod
I borrow the test equipment later this week for THD and IMD testing...... But since the real test is my ears, I'm pretty content right now.
I consider my outcome to be a perfect "42".....
I'll continue to nag Antek about my power transformer, though....
Thanks,
Ken Mod
Any reason not to use these to mount the mosfet's.
MAX08G Aavid Thermalloy Heatsinks
Also , I need to purchase some mica anyway so was considering using aluminum oxide pads from mouser. Any thoughts?
4180G Aavid Thermalloy Thermal Interface Products
MAX08G Aavid Thermalloy Heatsinks
Also , I need to purchase some mica anyway so was considering using aluminum oxide pads from mouser. Any thoughts?
4180G Aavid Thermalloy Thermal Interface Products
6L6....
Missing your posts in the thread for the last day or two (since we talked on the phone).
I hope that's a good sign--that you're busy being mesmerized by the sounds of your F5.
Is bias still an issue for you? Let me know if I can help......
Today I located some pretty nifty hardwood I hope to use for the sides of my amp. Will machine the top plate, probalby tomorrow. Only thing left are the ultra-bright LEDs for the VU meters, and the front panel. I've been playing my F5 "semi-naked" for the last few days. I'm humbled by the sound.
Ken
also known as Ken Mod, these days.....(!)
Missing your posts in the thread for the last day or two (since we talked on the phone).
I hope that's a good sign--that you're busy being mesmerized by the sounds of your F5.
Is bias still an issue for you? Let me know if I can help......
Today I located some pretty nifty hardwood I hope to use for the sides of my amp. Will machine the top plate, probalby tomorrow. Only thing left are the ultra-bright LEDs for the VU meters, and the front panel. I've been playing my F5 "semi-naked" for the last few days. I'm humbled by the sound.
Ken
also known as Ken Mod, these days.....(!)
