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Old 4th January 2013, 11:38 AM   #11
12B4A is offline 12B4A  United States
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To go along with DF96's observation: I may have pressed some buttons wrong but it looks like the overall gain of each 12AX7 stage is ~.45? That seems like an end result that is opposite of what the 12AX7 is intended for signal amplification. Are you sure what you have drawn is correct?
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Old 4th January 2013, 12:34 PM   #12
DF96 is offline DF96  England
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To correct what I said earlier, if that really is the circuit then Miller effect will not be a problem as the stage gain is not high enough. If the 68k cathode resistors were bypassed then you would get lots of gain but only below around 4kHz.
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Old 4th January 2013, 01:18 PM   #13
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There's nothing in the circuit to give the reported frequency problem. There's a lot of wasted gain, for the purpose presumably of harmonic enhancement and sharpish roll-off.


Something's missing from the diagram, or the EF86 is faulty, assuming you measured the frequency response at its input.
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Old 4th January 2013, 01:39 PM   #14
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DF, cathode and g2 are AC coupled. What does that do the Bode plot!
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Old 4th January 2013, 02:03 PM   #15
DF96 is offline DF96  England
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Cathode and g2 should be AC coupled; that ensures that the EF86 acts as a pentode. It will only have a small effect on HF frequency response.

My guess is that the circuit is not as shown.
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Old 4th January 2013, 02:05 PM   #16
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My intention is to use a dual gang 100k pot to dial in some large bypass caps on the ECC83s cathodes as a gain control. So in the schematic the gain is the lowest it can be. I have not included the bypass caps because they are currently missing from my build.

The DC coupling is to avoid blocking distortion on bass heavy sounds and provide a flat response right down to DC. The preamp is intended to distort or be clean when it is desired.

The design as it is now is basically taken from Merlin Blencowes example circuits in his preamp design book (though it's a guitar/bass design book it does also give hifi examples for completeness). I didn't expect any resistor values to be completely wrong. Which ones were you talking about?

Also the EF86 is not faulty, I swapped it out for a known working one and the response was the same. :-(
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Old 4th January 2013, 02:39 PM   #17
Tesla88 is offline Tesla88  Italy
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Just to test , try lower blocking resistors on 12AX7 , 1M is way high although tha gain is low... anyway i don't get the sense of that circuit
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Old 4th January 2013, 02:56 PM   #18
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No joy, I've just AC coupled the EF86 to the ECC83 in case it was something to do with that. No change. Btw the 1M resistors are not grid stoppers, just part of a voltage divider. And the treble loss is in the EF86 stage, is perfectly flat in the rest of the preamp.

I also tied the screen diodes to ground instead of the cathode, no change.

Could it be something in the power supply maybe? I have to go out now but when I get back I'll complete the schematic.

If it makes more sense to you guys, I'm trying to make a sort of Culture Vulture-esque mic preamp for plenty of colour and grit with the gain turned up, but when the gain is backed off, clean enough for rock'n'roll. Hi-Fi is not my ultimate goal.

I'll report back in a few hours. Cheers guys.
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Old 4th January 2013, 05:46 PM   #19
DF96 is offline DF96  England
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Quote:
Originally Posted by Andyroid
If it makes more sense to you guys, I'm trying to make a sort of Culture Vulture-esque mic preamp for plenty of colour and grit with the gain turned up, but when the gain is backed off, clean enough for rock'n'roll. Hi-Fi is not my ultimate goal.
The gain control is in the wrong place for this; you need to move it earlier in the circuit. As it is you will simply be able to adjust how much output you get, with "colour and grit" determined solely by input signal level.

There is nothing in the circuit which would cause serious HF rolloff in the first stage. Therefore either this is not what you have built, or you are not measuring what you think you are measuring.
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Old 4th January 2013, 06:39 PM   #20
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If you see the roll off on the cathode and the tube is ok it is likely that the roll off is also present on the g1. If the signal on the socket is pin is ok, could it be a bad contact in the socket? Try to lift the tube a bit in the socket an measure directly on the tube pins for comparison. Also please let us know the DC voltages on all pins so the experts can judge the op.
And can please explain what your "audio probe" is?
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