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Old 11th June 2012, 09:38 AM   #101
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On the good channel, the resistance over the pot seems to be set to about 60ohm.

On the bad channel, there is something strange, yes. The resistance seems to drift upwards! With pots dialed to max it ends up to stabilize around 444ohm on one and 500ohm on the other. But something makes it to drift.
What's wrong there?
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Old 11th June 2012, 10:33 AM   #102
juma is offline juma  Serbia
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Quote:
Originally Posted by Juuso View Post
... What's wrong there?
Your approach to the problem is wrong.
You measure what's convenient to you and not what is needed.
You were advised to measure the voltage drop accros JFETs' Source resistors. That would reveal the Id through JFETs and we would have a clue what's wrong or what to do next.
Also, I have an impression that you didn't read/understand the F5 article. You got to cover the basic stuff...
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Old 11th June 2012, 12:03 PM   #103
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No, this is not my profession. There are plenty of things I don't understand, no matter how many times I read them. To compensate me being a total idiot asking dumb questions in this area, there's other stuff in life which I master pretty nicely.

I measured the pots first, as suggested in here. It was the easy way out, if that would've told something. Did it? I don't know: not to me, except that channels behave differently in regard to resistance drifting with the power off.

I asked from you in the other thread about how to measure across R1 and R2 what you suggested. As you already pinpointed correctly, I'm an ignorant fool unable to understand what I'm reading when it comes to electoronics, but I rather ask than jump head first. With offset at zero (bias ~0.06V) or higher voltage dialed in - no matter the off-set?
Even tho I cannot access R1 and R2 directly, I can measure the voltage by attaching DMM on R5's / R6's leg and ground.

Do you -or anyone- have any comment on the resistance drifting? With power off that is. No, I don't understand this phenomena either.

Of course, if there are still boards and component kits available, I can order a new set, trash this one, and hope I'll get as lucky as I was with the first board, without understanding the stuff you master. Anyway, this has been a long project already now.
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Old 11th June 2012, 12:30 PM   #104
AndrewT is offline AndrewT  Scotland
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bad joints
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Old 11th June 2012, 12:52 PM   #105
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^Went thru the joints with my soldering iron after taking the board off the last time. It'd be fun actually if that was the case: I've always stated that anyone who can weld good, can certainly solder, but let's see...
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Old 11th June 2012, 12:53 PM   #106
AndrewT is offline AndrewT  Scotland
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If that doesn't solve it, then:
bad component.
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Old 11th June 2012, 01:26 PM   #107
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That's what I'm afraid of. That's why I measured the mosfet resistances, but those values tell nothing to me, except that none was zero.

Let's see: if it was a joint problem, it should be now revealed while I set up for the measurements juma suggested needed.
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Old 11th June 2012, 02:41 PM   #108
6L6 is offline 6L6  United States
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Begin by replacing the both the pot bypass resistors (R3, R4) with more resistance. If they are 2.2k now, go to about 4.4k or so. This will most likely fix the low bias on the 'good' channel, allowing the full bias amount, and hopefully a bit more.

Hopefully, on the 'bad' channel, you will be able to get the circuit to behave more normally... I doubt you will get the full bias, but you might be able to get .2 or something with zero offset. If this is the case, just continue to make the value of the bypass resistor (R3 or R4, depending on which channel) bigger (more ohms). My first F5, with TechDIY parts, has one pot with 6.6k in the bypass position! It biases fine and sounds great.

Some photos from my build thread where I lifted one of the bias resistors and added more in series --

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

Of course, you can just replace the whole resistor instead of adding a component or two, I just happened to have a bunch of 2.2k resistors on hand.
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Last edited by 6L6; 11th June 2012 at 02:47 PM.
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Old 11th June 2012, 02:41 PM   #109
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Ok, went up with the pots this time as per follows in order to do the measurement juma suggested last night:

mV
R11____0___0,2__1,0__2.5__3.0_______216__292__386_ _400
R12____0___0,0__0,0__0.4__1.1(dial)_213__290__384_ _399
offset_10___23___76__134__125_______209__212__228_ _229
R1________________________________________________ _75.7
R2________________________________________________ __2.9

Thus, poking with the soldering iron changed nothing.
Even tho I know nothing, I'd guess 75.7 vs 2.9 indicates some problem. Didn't see a reason to climb any further in voltage from there, since -per experience this far- offset does not change much from that.

What next?

Last edited by Juuso; 11th June 2012 at 02:53 PM. Reason: THE question added
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Old 11th June 2012, 02:49 PM   #110
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Quote:
Originally Posted by 6L6 View Post
Begin by replacing the both the pot bypass resistors (R3, R4) with more resistance. If they are 2.2k now, go to about 4.4k or so. This will most likely fix the low bias on the 'good' channel, allowing the full bias amount, and hopefully a bit more.
There is no any problem with the good channel. I just didn't want to dial it above 0.6V before both channels done.

Quote:
Originally Posted by 6L6 View Post
Hopefully, on the 'bad' channel, you will be able to get the circuit to behave more normally... I doubt you will get the full bias, but you might be able to get .2 or something with zero offset. If this is the case, just continue to make the value of the bypass resistor (R3 or R4, depending on which channel) bigger (more ohms). My first F5, with TechDIY parts, has one pot with 6.6k in the bypass position! It biases fine and sounds great.
Is the problem within mosfets? I.e. if I replace them with good ones, problem gone? How to know what's good and what's bad?
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