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6M11 Compactron Preamp with Tilt Tone Control
6M11 Compactron Preamp with Tilt Tone Control
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Old 23rd February 2019, 03:02 AM   #21
VictoriaGuy is offline VictoriaGuy  Canada
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6M11 Compactron Preamp with Tilt Tone Control
PRR:
An earlier (simpler) version was published by SY- see pic.
It doesn't have the 1uF bypass caps or separate anode resistors, as in your schematic.
Only the 100R stopper is different from your suggestions.
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File Type: jpg mini-SY Line stage with tilt simple version.jpg (43.1 KB, 121 views)
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Old 23rd February 2019, 04:52 AM   #22
PRR is offline PRR  United States
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Quote:
Originally Posted by VictoriaGuy View Post
Just to be sure...
It's fine, however.....

TRY IT!! So it oscillates on the test-bench.... what harm happens?

With all respect to SY, it is more robust if the NFB does not share the same output damping resistor as the Outside World. In a Hi-Fi with 3-foot cables, it is probably no difference. Hang a 1,000 foot cable on there and there is considerable phase lag inside the audio band.

Yes, any amplifier will oscillate if layout is bad. I think you can wire this up tight and neat. One gotcha is the length of wires from the tube to the controls. As in many audio chores, you realy don't want long wires here.
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Old 23rd February 2019, 05:21 AM   #23
VictoriaGuy is offline VictoriaGuy  Canada
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6M11 Compactron Preamp with Tilt Tone Control
Quote:
Originally Posted by PRR View Post
It's fine, however.....

TRY IT!! So it oscillates on the test-bench.... what harm happens?
I did try it - it makes no difference that I can see whether I wire it with/without the changes in the bypass caps and anode resistors, or whether I move the NFB inside or 'outside' the last stopper.

There seems to be a very small amount of oscillation consistently 'widening' the scope traces - but I'm not sure if that's important. The oscillation I had previously seems to have been eliminated with the 220pF across the NFB resistor. I have some pretty long wires (antennas..) attached to the circuit now - power wires up to the bench supply and the leads from the audio generator to the input. Scope leads shouldn't be much of a problem, but they are hooked up to input and output as well. Question: Right now I just have the scope attached across the output.

Should I be loading the output with a resistor to mimic the input stage of a (tube) power amp?
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Last edited by VictoriaGuy; 23rd February 2019 at 05:24 AM.
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Old 25th February 2019, 01:03 PM   #24
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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Quote:
Originally Posted by PRR View Post
Yes. So can you.
No, I don't. There is no way to be sure that with a high gain stage plus 2 voltage followers there aren't some frequency at the gain is 1 or more and multiple of 360. So it is better to avoid it.
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Old 26th February 2019, 05:52 AM   #25
VictoriaGuy is offline VictoriaGuy  Canada
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6M11 Compactron Preamp with Tilt Tone Control
Quick update-
It's still on the breadboard but doesn't seem to have much bass getting through, even with the pot at full 'bass end' rotation. Anything below 200 Hz is half (-6 dB?) or worse
Lots of treble though - double the input voltage at the output (6 dB?).
I'll check the wiring again tomorrow.
I don't really understand how the tilt circuit works, so I have problems troubleshooting it.
And, it may be an amplifier problem and nothing to do with the tilt control at all, I suppose.
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Old 26th February 2019, 10:32 AM   #26
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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Try increasing C6 to 10F or even more. Leds are not good biasing methods. Tubes don't need them.
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Old 26th February 2019, 11:27 AM   #27
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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Also, a led gives about 2.5V which seems to be too high bias for a high triode.
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Old 26th February 2019, 03:33 PM   #28
VictoriaGuy is offline VictoriaGuy  Canada
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6M11 Compactron Preamp with Tilt Tone Control
Thanks, Osvaldo.
I'll get back to the electronics bench later today and report back.
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Old 26th February 2019, 03:53 PM   #29
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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Please, I don't want to be pejorative, that's not my intention; but do you know to bias a tube from the data sheet? There you can choose the best biasing point. I suggest to read anywhere in the web how to do it, and choose your best operating point, and forget led's, because excepting rare casualty, this element will not give the best bias for the tube. Also, they introduce noise in the circuit.
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Old 26th February 2019, 04:51 PM   #30
VictoriaGuy is offline VictoriaGuy  Canada
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6M11 Compactron Preamp with Tilt Tone Control
Quote:
Originally Posted by Osvaldo de Banfield View Post
Please, I don't want to be pejorative, that's not my intention; but do you know to bias a tube from the data sheet? There you can choose the best biasing point. I suggest to read anywhere in the web how to do it, and choose your best operating point, and forget led's, because excepting rare casualty, this element will not give the best bias for the tube. Also, they introduce noise in the circuit.
No offense taken.
Can you explain how a different bias point will affect the frequency response?
That's the problem I'm now having with this circuit.

It sounds like you only build circuits you design completely yourself?
I'm a beginner so I mostly build things from circuits that other people have designed.
In this circuit, I don't have a clear understanding on how the NFB and the 'tilt' tone filters interact.

I understand the concerns with noise and oscillation, which are certainly worse on a spread-out layout on a 'breadboard' (real board with terminal strips, not a plastic 'breadboard' with holes for wires). However, that's mostly under control for now.
I don't want to move to a final, 'tight' wiring scheme around the tube socket (which will help to control oscillation/noise, I'd hope) unless/until I get the circuit working with appropriate frequency response and gain.
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