you could use the scope for this purpose. But tbh, you should own at least two scopes and ideally three when building amps. They do not have to be expensive at all.
Beware, although First Watt constructions are hard to break, the F5 (F5T even more so) topology perhaps has a modest but not irrelevant blow-up frequency. The devil is in the detail, and planning. For example, with proper planning, you would have had the nescessary amount of DMM’s, and would not have forgotten to dial down your pots before turning it on for the first time. All the info is here. Rush is another source of errors. I’d spend some time reading up, and order a few 10-20 dollar DMM’s, and then continue.
Beware, although First Watt constructions are hard to break, the F5 (F5T even more so) topology perhaps has a modest but not irrelevant blow-up frequency. The devil is in the detail, and planning. For example, with proper planning, you would have had the nescessary amount of DMM’s, and would not have forgotten to dial down your pots before turning it on for the first time. All the info is here. Rush is another source of errors. I’d spend some time reading up, and order a few 10-20 dollar DMM’s, and then continue.
One last question, Dennis, during this adjustments does a speaker have to be connected to the terminal?
Great advice, thanks very much once again for your time. When the thread is 100 pages, forgive my lazyness for going thru all.
It is in the first post, as stated several times. Enough of my time, good luck.
Dear Andy,
Thank you for your valuable opinion and for your suggestion!
It took me a bit longer to evaluate the second channel and to look back into the problem with the first one. However, I think that I found the problem.
With the second channel I was able to get 0.598 V and 0.602 V on MOSFET source resistors with 10mV bias offset. Which is, in principle, should be acceptable.
I then checked all the resistance values on the first channel, and find out that R2 is measured as appr. 15.8 kOhm on the board. I desoldered it, and it was all normal at 47.5 kOhm. I soldered it back in the board and checked resistance between several points on the board which having R2 involved - all gave non-correct values.
I started to suspect JFETs, I desoldered them and checked. I found that one of them is showing resistance between Gate and Source of 18.2 kOhm and 18.8kOhm depending on the polarity of probes.
I decided that it is a malfunction, which probably caused the uneven(unsymmetric) behavior of N and P parts of the circuit and lead to such a difference in currents. May be I handled it badly, or overheated while soldering...
I ordered new JFETs and will check the operation with them. They should arrive next week.
Maxim
Dear ItsAll. I followed the instructions for bias offset adjustment. While I go up to 0,4V accross R7 and R8, I see that my dim bulb tester go from dim to pretty bright. Is it normal? I am afraid of frying something inside.
^ What wattage bulb do you have in your DBT? Note, if you're in the USA, you'll commonly see a "equivalent" wattage rating for halogen bulbs. Be sure you note the actual wattage from the packaging.
See here for some great information about how they work and guidance for selecting the wattage based upon what you're testing.
https://antiqueradio.org/dimbulb.htm
If the DBT (before you started to bias the amp) went bright and then quickly dimmed with both amplifier boards connected, then that indicates that you don't have a 'dead short' to ground or anything drawing substantially higher current than normal unexpectedly.
tl;dr - it's likely normal.
If it began to brighten as you increased your bias current, then that is normal. Note... I would back off the bias to 0V25 (or so) across those resistors before disconnecting the dim bulb tester. You should not attempt to bias your amplifier fully with the DBT in place.
See here for some great information about how they work and guidance for selecting the wattage based upon what you're testing.
https://antiqueradio.org/dimbulb.htm
If the DBT (before you started to bias the amp) went bright and then quickly dimmed with both amplifier boards connected, then that indicates that you don't have a 'dead short' to ground or anything drawing substantially higher current than normal unexpectedly.
tl;dr - it's likely normal.
If it began to brighten as you increased your bias current, then that is normal. Note... I would back off the bias to 0V25 (or so) across those resistors before disconnecting the dim bulb tester. You should not attempt to bias your amplifier fully with the DBT in place.
Thank you so much for your quick reply. I was using a 42W halogen bulb. I didn’t have a 25W in hand. Your last sentence is extremely helpful also. I didn’t know about that.
bermek,
A reminder that you do not bias your amp with the DBT attached. Basically you use it at the initial power up to make sure nothing is amiss (large current draw) and you have P1/P2 in the minimal position. Then you remove it and begin the bias process by adjusting P1/P2 while monitoring the voltages across the source resistors and the DC offset.
A reminder that you do not bias your amp with the DBT attached. Basically you use it at the initial power up to make sure nothing is amiss (large current draw) and you have P1/P2 in the minimal position. Then you remove it and begin the bias process by adjusting P1/P2 while monitoring the voltages across the source resistors and the DC offset.
Well, this thread seems as good a place as any to post this… $100+shipping. PM if you want them. SOLD
Finally need this chassis for something else, and these boards have done such a good job that they really should find a new home.
More information and photos can be found in post 1 & 2 of this thread.
Finally need this chassis for something else, and these boards have done such a good job that they really should find a new home.
More information and photos can be found in post 1 & 2 of this thread.
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Hello Andy, I have similar problem. In one channel it is 0,600V and 0,650V. Is it ok? Thank you for an answer. Best regards. Valeri
Yes, it is ok, will work perfectly, and sound great .
Because of the differences in Vth (the turn-on threshold voltage) of the typical N channel Mosfet and the typical P channel Mosfet, the chances that the two halves of the circuit being identical bias at zero dc offset is actually quite low. And there is no reason to have it identical. As long as you can get close to zero offset, it’s all good.
Because of the differences in Vth (the turn-on threshold voltage) of the typical N channel Mosfet and the typical P channel Mosfet, the chances that the two halves of the circuit being identical bias at zero dc offset is actually quite low. And there is no reason to have it identical. As long as you can get close to zero offset, it’s all good.
Dear Dennis. I was able to adjust bias and DC offset. However, As I go above 0.4V bias, the 1.25A slow burn fuse wants to go off. (220V mains). What could be the reason? Should the fuse rating be higher?
Best regards.
I can't tell from the picture how many VA (watts) your transformer is rated for but you'll probably need 1.5A or greater (1.5A x 220V = 330VA or 330W). Do you have both amp channels connected to the PSU while you are doing this when the fuse blows? If you are able to get it to 0.4V over the mosfet source resistors (R11 and R12) without anything burning up, you're probably in good shape and just need to use a fuse with a higher rating.
Dear Twitchie. Thank you so much for clarification. Yes I have both channels connected. But I measured the 0.4V accross R7 or R8 as recommended. It should not matter I guess?
The toroidal I used should be rated around 350-400W. Maybe it is best I go for 2-2.5A fuse? Is slow burn fuse a must?
The toroidal I used should be rated around 350-400W. Maybe it is best I go for 2-2.5A fuse? Is slow burn fuse a must?
400vA/220v=1,8A fuse. I’d order a few of those and a couple of 2A and 2A2’s.
Make sure the bias/Iq measurements are made with DC offset zeroed/minimized.