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Old 29th August 2012, 01:54 PM   #11
alan73 is offline alan73  Australia
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Location: Sydney, Australia
Sorry, I was looking at the MiniAmp 2.0 schematic.I am puzzled! Is this a brand name amp or a DIY ? No doubt you have tried another 12 AX7 tube?Still think 0.33V input is 10 times too high, but if waveform is clean after the first stage, it suggests a fault in stage 2. What is the volts/div on the vertical axis of your waveforms? It is impossible to tell what P-P voltage swing you are getting.
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Old 29th August 2012, 03:36 PM   #12
DF96 is offline DF96  England
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If you are doing what you say you are doing (0.33V rms input) then you are grossly overloading the second stage. A typical ECC83 stage can only accept an input voltage of up to about 1V rms, but you may be putting as much as 10-15V into it. The result is that it is swinging between grid current and grid cutoff, so the interstage coupling cap will have a shift of voltage and so the 2nd stage will have a shift of bias. This is complicated by the tiny cathode decoupling capacitor, so instead of something approaching a square wave you get a sloping top.
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Old 30th August 2012, 03:17 PM   #13
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If you say its sounding harsh, you may want to look at the frequency shaping a bit more instead of the waveform. theres plently of bass cut, but not too much treble cut, put a low pass filter in by a 22pF cap from V1b grid to ground. that should round off those peaks a bit more too. Or, you could put a 22-47pF cap accross the plates of the PI.
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Old 30th August 2012, 06:07 PM   #14
jule is offline jule  Slovenia
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Join Date: Jul 2010
Thanks guys for great responses. You really know your stuff.

Today I was overhauling that amp whole day.

First I used shielded cables for signal path. Then I put another gain potentiometer(before phase inverter). Because I noticed that the phase inverter started to distort before preamp. Now I can adjust level of preamp distortion and PI distortion.

I corrected bias on power tube.

I also put in new tone stack (Baxandall 2 band + mid boost switch).

And oh my god now that amp sounds so heavenly! Oh my god I have never played an amp that sounded so good with guitar plugged in directly, without any other effects.

Thanks guys. What a great forum!


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Old 13th September 2012, 04:58 AM   #15
Struth is offline Struth  Canada
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Hi Guys

Are we assuming a Fender-style preamp, as there is no schematic posted?

The standard test signal for a guitar amp input is 70mVrms at 400Hz. Through a standard Fender preamp with a gain stage, EQ, volume and second stage, the output of the second stage can be made to distort mildly. This represents an 80Vrms signal!

The traditional flaw is the lack of grid-stops right at the tubes and one at all for the second stage.

Have fun
Kevin O'Connor
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