![]() ![]() The AC impedance is important when the output load (speaker) is being matched to the valve output impedance. As I said before, the DC resistance of my 125C is roughly 150 ohms per half-winding. The DC resistance is what matters when the quiescent dissipation is being adjusted, that is, when you’re setting the bias. ![]() I think you’re confusing the AC impedance with the DC resistance. Everyone who wants to tweak or even build an amp should be aware of the lethal voltages and should know about how to measure them and how to discharge the caps manually.Ĥ) The tonestack is very effective, if you find out you don’t like the control curve, simply try another potentiometer characteristic.ĥ) they create an artificial center tap for the heater supply, so they reduce hum.Ħ) Both are equally good circuit-wise, but especially for my tiny builds the toroidal transformer has the advantage of a lower exterior magnetic field and therefore interferes less with the other components on the chassis. So this resistor would be no advantage, it would be nothing but a false security presumption. So how does it sound? Well it sounds like a smooth, rockin’, warm, creamy, twangy … naaaa, no words can describe a sound better than hearing it for yourself!ġ) I won’t do that, you should always check the voltages before you work on an amp! Even if you’ve got such a bleeder resistor installed, you can’t assume that it’s working properly or that the time since you switched your amp off was enough for that resistor to discharge the caps. For the output-transformer I used a Hammond 125B and for the power-transformer I used a toroidal transformer with secondary: 200V 0.1A and 6.3V 1.5A which I found HERE. The preamp is inspired by the Vox Night Train (which seems to be inspired by the Blues Express variant of the Trainwreck Express). In addition to that the use of a phase inverter feeding the ECC99 over a self-splitting ECC99 poweramp delivers more output and therefore more clean headroom. The two 12AX7s not only allow you to compensate the gain-loss of the tone stack and to use a PI but also provide you with an overall different sound and overdrive behavior than an amp with just one 12AX7 like the MiniAmp Number 1. The second 12AX7 here serves as a third gain-stage and a cathodyne phase inverter which feeds the ECC99 triodes with their phase inverted signals. This one now got two 12AX7s in the preamp and provides a tone stack with treble, middle and bass plus a volume control. The MiniAmp 2.0 was build by me because I loved how the MiniAmp Number 1 came out and so I wanted to build another little amp just a little bit different soundwise and a bit more versatile. It is an approximately 1.5W-2W guitar amplifier with an ECC99 push-pull poweramp. This time I want to show you my new MiniAmp 2.0. ![]()
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