80W Hybrid Audio Amplifier!

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#103
Tubes are JJ E88cc. Non symetrical imput couse dc offset, i know, but i cant match like single transistors 🙂
But it looks not so bad (with no dc servo):
 

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#109
The 1k2 output on the DC servo opamp divides the correction by the ratio 1k2:47r
That limits the correction applied to the valve -IN to ~550mV
The amplifier gain would need an output offset of <12Vdc for the servo to apply sufficient correction to bring the offset back to near zero.
Are there any circumstances where the output offset could exceed 12V?
 
#111
Now there are two things left that appear to be strange:

1st: Why are the rail voltages that different? Issues with your PSU?

2nd: The MJE350's collector current cannot be larger than it's emitter current. Measuring error?

Best regards!
 
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#114
Machina said:
Mesure from Gnd, +V 41,3/ -V 41,1.
That Mje350 on red text is original text what currents should be.
Can power transistors base be closed?
Click to expand...
When MJE350 is passing 11mA, it will be drawing a base current through the 100r.
Measure the Vrb and calculate the base current.
You can then calculate the hFE for that 11mA of Ie.

The 11mA creates a Voltage drop across the 360r. That voltage is the Vgs of the lower mosFET and that is what turns it on to pass 200mA. Check the datasheet and see how "normal" that is.

What are the 69mV and 474mV values at the output?

BTW,
that 11mA is only 11% higher than the specified 9.9mA on the sch.
That could be a good value or a not so good value, but it will not break the circuit operation. Keep the medium power transistor cool. @ 11mA and 138Vce it is dissipating >1.5W
A 20C/W sink will run hot, I would go for bigger than 20C/W
 
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#116
AndrewT said:
.................What are the 69mV and 474mV values at the output?
.............
Click to expand...

Machina said:
V drop on power transistors, base/emitter.
Click to expand...
the 474mVbe tells you that transistor has just started to turn on.
The 69mVbe tells you that transistor is no where near turning on.
That accounts for the 34uA of Ie. The output stage is still turned OFF.

You need to check that the two output transistors are not damaged.

Then start turning up the bias voltage to turn on the two output transistors.
Monitor the Vbias applied to the drivers and the Vbias applied to the outputs and the Vre applied to the emitter resistors. You need three or four DMMs so that you can see the changes as you adjust the current through the CCS.
Something is wrong !
what is the voltage across the driver drain resistors?
 
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#119
AndrewT said:
Then start turning up the bias voltage to turn on the two output transistors.
Monitor the Vbias applied to the drivers and the Vbias applied to the outputs and the Vre applied to the emitter resistors. You need three or four DMMs so that you can see the changes as you adjust the current through the CCS.
Something is wrong!
what is the voltage across the driver drain resistors?
Click to expand...
Nothing is wrong with this, output BJT doesn't conduct for low output powers and that is how they should work, it's a current dumping topology. Don't redesign already good design.

Only problem here is probably dc servo who can't apply sufficient correction voltage for this tube. And dc servo is not made according to schematic.
I built this amplifier myself, some earlier revision, and it work perfectly for years.


@Machina
Put 15V zener for dc servo supply, and 3k3 resistors. Measure voltage at output of dc servo op-amp when everything connected like it should be. If that doesn't help, try with some other tube.
 
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