Hi, i need help, help me see how much Ma is the static current of the four groups of rv7 and RV8 at the back of this Rogers ravensbourne amplifier? There are also two rv6. Is the voltage of rv6 5V? thank you,
You sent me a pm but for some reason its closed and not possible to reply to it.
I would need to see an alignment procedure to say for sure. RV6 looks like it will be used to set a symmetrical clipping point. The presets for adjusting quiescent current I wouldn't like to say without having one in front of me. You need to know the correct value of current to begin to adjust these.
I would need to see an alignment procedure to say for sure. RV6 looks like it will be used to set a symmetrical clipping point. The presets for adjusting quiescent current I wouldn't like to say without having one in front of me. You need to know the correct value of current to begin to adjust these.
The DC I tested on the fuse behind RV6 is 80mA. I have an attachment with a schematic diagram. Please have a look. Thank you.
I looked at the circuit but it didn't seem to give an alignment procedure.
RV6 alters the DC conditions of the first section of the power amp. Look at the circuit... that stage is AC coupled via C34 and C35 to the output stage. AC coupling means that RV6 has no effect on the bias current of the output transistors. The two halves of the circuit are separate as far as DC conditions go.
RV6 alters the DC conditions of the first section of the power amp. Look at the circuit... that stage is AC coupled via C34 and C35 to the output stage. AC coupling means that RV6 has no effect on the bias current of the output transistors. The two halves of the circuit are separate as far as DC conditions go.
I read (see link at bottom) that RV7 and RV8 should be adjusted to give 25 mV across R74 and R75.
I also read that RV6 is for setting up the dc conditions for the front end and phase splitter. "At a guess, you would adjust it to set the emitter and collector voltages of the phase splitter to around 1/4 and 3/4 of the supply voltage to the phase splitter."
Mooly may be able to make sense of the above.
https://www.vintage-radio.net/forum/showthread.php?t=67642&msclkid=45ebe909b50611ec95dcdb8b42b6d821
I also read that RV6 is for setting up the dc conditions for the front end and phase splitter. "At a guess, you would adjust it to set the emitter and collector voltages of the phase splitter to around 1/4 and 3/4 of the supply voltage to the phase splitter."
Mooly may be able to make sense of the above.
https://www.vintage-radio.net/forum/showthread.php?t=67642&msclkid=45ebe909b50611ec95dcdb8b42b6d821
They keep talking of DC offset at the speaker in that thread but the amp is AC coupled into two series connected reservoir caps (saves on speaker coupling caps). There will be a high offset with no load attached.
All I can think of unscientifically is that you adjust the presets to give both the required current and also a correct midpoint volts. So the two bias presets will interact. The big question is what is the required bias current. It is probably not high to prevent thermal runaway and also to limit heat generation. 50 milliamps does sound a reasonable ball park figure.
The amp IS AC coupled interstage but DC coupled to the load. The bias/offset is set by RV7 and RV8. Increasing bias of one
of the transistors must me matched by the other to maintain 0V offset. My gut guess is 50 mA is a bit high which would put
the static thermal output at 7 Watts - assuming the power supplies are +/- 35V. 7 Watts doesn't sound like much but with
the marginal heatsink in that unit it would run quite warm. If it hasn't been 'diddled' yet I would first measure the VALUE of
the emitter resistors (power off) to verify. Then with power on , no load, measure the voltage across EACH emitter resistor,
1 mA per mV. If it were mine I'd go between 5 and 10 mA and measure distorion to see if it's acceptable and increase bias
to lower the distortion. I would not expect very good numbers on that. I'd expect 0.5% distortion as good.
Hopefully the thermisters are still OK so the bias vs temperature doesn't go into runaway. Higher bias could aggravate it.
G²
I offerred up earlier that the emitter resistors were 1 ohm and should have 25 mV set across them.
That's very good. It's 25mV. There's nothing wrong. I'll send you a picture later to share with you
AC coupled to the load as well.
It is a single rail amp with the left and right speakers sharing an AC coupling on the speaker negative returns (the two reservoir caps). It saves on the cost of a cap.
Try to adjust the DC voltage at both ends of resistor r74 of rv7 and resistor R75 of RV8, 25mV. See the figure, hoping to help you adjust it later. I've seen another forum say the same adjustment before. Today, I also measured the DC voltage of the speaker under the condition of 4 Euro load speaker. Each box is about 6MV. What the forum said is 10mV. In fact, it's not available. 6MV should be a speaker without AC power, R74 and R75 are very ripe and hot. You should be careful when adjusting them, and they should be adjusted repeatedly. There are always changes. A picture is posted for your reference,
