No, that's not a preamplifier stage, but it is the first stage of the power amplifier.
It is also enclosed within an overall negative feedback loop, although the feedback
resistor seems to be missing, unless the amplifier is intended to have unity gain.
It is also enclosed within an overall negative feedback loop, although the feedback
resistor seems to be missing, unless the amplifier is intended to have unity gain.
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rayma you totally right, original design was using feedback loop, but after try differents values all produce a remarkable hum so no feedback loop installed, and I forget remove it the empty loop in the draw.
so that means that this circuit is only amp with an attenuator ? so can install the outside preamp to the actual amp inputs and just control the volume in the outside preamp ?
Ok, then the second stage is still direct coupled to the first stage.
To remove the first stage, you would have to use a DC voltage divider to properly
bias the second grid at 61VDC, and use a coupling capacitor to the source.
Or the second stage could be redesigned so that its grid could be at DC ground.
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Yes, then the amplifier's volume control should be left at maximum. This would be much easier,
and is what I would recommend that you do.
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If you mean use this to eliminate the first stage, it's better if you would just disconnect the first plate,
and instead connect a 20k resistor from the second grid to ground, to avoid redesigning the second stage. The second grid would go through a 10uF 200V rated coupling capacitor to the input.
Might be safer and definately better to mod it to something like in the attachment. You could add another 910R is series with the 30k to get the exasct same operating point as the original, but its certainly close enough as drawn.
HK
HK
Attachments
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0.01uF and 240k is -3dB at >60Hz, its probably 0.1uF or at least it should be for full range use.
Those 350V filter caps are working too close to their limit, as is the output cathode bypass.
You could try driving the amplifier as built, without your pre-amp, direct from your source. The input stage is direct coupled which is nice. Likely that the same cannot be said for what you are proposing (pre-amp / linestage 99% will be capacitor or transformer output).
If you didnt want to change the driver loadline you could keep the 20k plate load resistor and increase the 100R in the power supply to get ~130V at the plate, less distortion with the 30k but it will change the sound (which will change anyway with the removal of the input stage).
IME 5687 are notorious for H/K leakage, the tube will definitely be happier with cathode closer to heater potential.
Try it without the network across OPT secondary.
Those 350V filter caps are working too close to their limit, as is the output cathode bypass.
You could try driving the amplifier as built, without your pre-amp, direct from your source. The input stage is direct coupled which is nice. Likely that the same cannot be said for what you are proposing (pre-amp / linestage 99% will be capacitor or transformer output).
If you didnt want to change the driver loadline you could keep the 20k plate load resistor and increase the 100R in the power supply to get ~130V at the plate, less distortion with the 30k but it will change the sound (which will change anyway with the removal of the input stage).
IME 5687 are notorious for H/K leakage, the tube will definitely be happier with cathode closer to heater potential.
Try it without the network across OPT secondary.
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Thanks Hanze Khronye . I saw now that is not as simple as I was thinking, Due I also use my Digital direct on inputs as circuit is now, I will consider keep it as is now but will try without the network across the OPT sec. as you mention. I use my analog setup much less time than my digital. With the 240k it is .1uf installed not .01uf as circuit read.