Measures of the jfet:
DDM in ohm meter mode: 9,7 Ohm both ways
DDM in diode meter mode: 0,020 V both ways
DDM in ohm meter mode: 9,7 Ohm both ways
DDM in diode meter mode: 0,020 V both ways
Thanks Mooly.
Taking the resistors from the paired power amp as I don't have more left. I will test with one of the old jfet's (from the channel that was at that time working)
Taking the resistors from the paired power amp as I don't have more left. I will test with one of the old jfet's (from the channel that was at that time working)
Green light Mooly!
Wow, I've been sweating! I have no words to say you thank you.
I spent money and time on this but it was worth because I wanted to (at least) start trying to fix something. I don't even know if I really like this amp because I realise now it was sold to me while already not "fit" (to say the least). But it was really good for learning so many things.
Now bias regulates, no smoke, it goes out of protection in short time after powering it. I just need to let it on for a while and then I will adjust the bias with no bulb in series. On the service manual, setting the bias for 4mV while cold is written. Though it is very unstable when just switched on, then I'm thinking to wait some minutes before adjusting it. If I wait some 15 minutes while under bulb, it doesn't change wildly, some 1 mV only.
I wish I was able to be of any help for you but I see no way 😀
Thank youuuuuuuuuuuuu!
Wow, I've been sweating! I have no words to say you thank you.
I spent money and time on this but it was worth because I wanted to (at least) start trying to fix something. I don't even know if I really like this amp because I realise now it was sold to me while already not "fit" (to say the least). But it was really good for learning so many things.
Now bias regulates, no smoke, it goes out of protection in short time after powering it. I just need to let it on for a while and then I will adjust the bias with no bulb in series. On the service manual, setting the bias for 4mV while cold is written. Though it is very unstable when just switched on, then I'm thinking to wait some minutes before adjusting it. If I wait some 15 minutes while under bulb, it doesn't change wildly, some 1 mV only.
I wish I was able to be of any help for you but I see no way 😀
Thank youuuuuuuuuuuuu!
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👍 excellent. So it all working with just a resistor in place for the FET?
The bias will drift at first, that is normal until the whole area is all at the same temperature. The output FET's start to warm and bias increases and only when Q14 (in the sim) also warms will the bias stabilise. Don't overbias it, tweak the setting so it is correct when the amp is quite warm such as after playing quite loud for a while.
Ideally... it will perform better with the correct current regulator FET's but the resistor is a viable option. It would/should also be possible to make a simple current regulator alternative.
Three doodles... LED version would use a red led and the 1200 ohm would need tweaking.
The bias will drift at first, that is normal until the whole area is all at the same temperature. The output FET's start to warm and bias increases and only when Q14 (in the sim) also warms will the bias stabilise. Don't overbias it, tweak the setting so it is correct when the amp is quite warm such as after playing quite loud for a while.
Ideally... it will perform better with the correct current regulator FET's but the resistor is a viable option. It would/should also be possible to make a simple current regulator alternative.
Three doodles... LED version would use a red led and the 1200 ohm would need tweaking.
I meant I pulled the blown resistors from the power amp. I used one of the old jfets. I bought two because the measurements were suspect and I wanted them to be at least from the same period. Though, even the new pair had different resistance values. At this point, I can't wait and pay for another supply. It is more than 20€ shipping every time and those FETs are 6€ plus vat each.
Thanks for the clarification on the bias adjustment. I wonder why they say 4mV with the unit cold! Even because "cold" is relative. I.e. 30 seconds after switching on is pretty different from a minute and so on...
The alternative current regulator seems interesting!
Thanks for the clarification on the bias adjustment. I wonder why they say 4mV with the unit cold! Even because "cold" is relative. I.e. 30 seconds after switching on is pretty different from a minute and so on...
The alternative current regulator seems interesting!
I suppose 4ma cold guarantees at least some bias current which totally removes any audible distortion and from there on they let it do its own thing knowing (presumably by the design and heatsinking) that it will not rise excessively beyond that point.
Setting it hot could I suppose give zero bias for a short while if it was very cold.
Setting it hot could I suppose give zero bias for a short while if it was very cold.
Am I wrong thinking I should use a dummy load made out of an 8Ohm resistor and adjust the volume to get 2,828 V across the resistor itself to get 1W output? I found the sinewave but I do not have instruments.
I do not demand having it so precisely adjusted but I fear doing damages even because, just after switching the amp on, it jumps to 17 mV and I need to wait 8 minutes for the bias to decrease to 4 mV and stabilise. It sounds great, anyway, except for low volume listening, when the heatsink gets almost cold again
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Most DVM's will show a 1kHz sine accurately on the AC voltage range so you can set it that way. All you are doing with that is warming the amp up. You can just as easily listen to music for twenty minutes at a reasonable level. In many ways that could be better as you warm both channels together.
Don't get to hung up on setting this, you will never seem to get it correct as it will wander. Ambient temperature, taking the top off the amp, all these things affect it.
Start with the bias low when cold. Set both channels the same and you can use the 4mv value given. That is just 36 milliamps flowing in the output transistors and just 1.6 watts dissipation per output FET.
Now listen to the amp for twenty minutes with the top on. Get it nice and warm/hot.
Turn the music off and remeasure as quickly as you can. The reading will probably be higher than 4mv but falling fast as it cools. Reset to 4mv on both channels so it is at that value a couple of minutes after stopping the music.
The bias shooting high when absolutely cold isn't a problem and it will come down as the whole heatsink area stabilises in temperature.
The important points.
1/ You need some bias current when totally cold to stop audible distortion. It doesn't matter if that initial value is high.
2/ The bias should be at the recommended value when the amp is at a normal warm temperature.
If it doesn't sound great at low volume then check the bias under that condition. As long as you have a couple of millivolts across the resistor there is 'enough'. If it is near zero then you need to tweak it up a little in that cold state.
Don't get to hung up on setting this, you will never seem to get it correct as it will wander. Ambient temperature, taking the top off the amp, all these things affect it.
Start with the bias low when cold. Set both channels the same and you can use the 4mv value given. That is just 36 milliamps flowing in the output transistors and just 1.6 watts dissipation per output FET.
Now listen to the amp for twenty minutes with the top on. Get it nice and warm/hot.
Turn the music off and remeasure as quickly as you can. The reading will probably be higher than 4mv but falling fast as it cools. Reset to 4mv on both channels so it is at that value a couple of minutes after stopping the music.
The bias shooting high when absolutely cold isn't a problem and it will come down as the whole heatsink area stabilises in temperature.
The important points.
1/ You need some bias current when totally cold to stop audible distortion. It doesn't matter if that initial value is high.
2/ The bias should be at the recommended value when the amp is at a normal warm temperature.
If it doesn't sound great at low volume then check the bias under that condition. As long as you have a couple of millivolts across the resistor there is 'enough'. If it is near zero then you need to tweak it up a little in that cold state.
Thank you for the umpteenth time. I will do as you say.
In the meantime, a very Merry Christmas to you and the other members
In the meantime, a very Merry Christmas to you and the other members