DIY amplifier lacks versatiliti, good/simple preamp anyone???

[iainpunk]

HI

i have been around on this forum for quite a while and have been building a bunch of working pedals (thanks for the help everyone)
last weekend, i build a little guitar amp with only one control, volume. i measured it at around 18W when at full gain
She sounds great and but it lacks a bit of punch and versatility (if you are known with the boss hyperfuzz FZ-2, its to much like fuzz 1 and not enough like fuzz 2, maybe to much mid-range?). i want to keep it simple, any of you know a simple two knob preamp?
preferably tone and volume(gain) controls.

the schematic i have at this moment is this:


the preamp jfet stage can be quite thicc sounding when using large caps like in this example.
the poweramp is class A inductor loaded. although its only supplied with 12V, the peak to peak output voltage can swing to almost 20V due to the inductor. (it is basically able to act like a boost converter.)

so the requirements for the preamp are:
1 or 2 knobs, volume(gain) and tone
transistor/jfet based preferably
a bit of a scoopy character

anyone have a good idea?

[PRR]

Electrical Engineering student-

What load impedance?

What bias current?

How big is the heatsink?

The base impedance of the 187 is about 200 Ohms. The maximum quasi-linear swing here is dozens of mV. Why do you feed it from a (potentially) high gain stage with an output impedance like 200K?

How is the bias stability? (I would expect idle current to rise with temperature, which makes it hotter, until blow-up, unless the heatsink is grossly oversize.)

Where do you get a 12H choke good for an Ampere or more? (Does it have to be 12H?)

[iainpunk]

Electrical Engineering student-

What load impedance?

What bias current?

How big is the heatsink?

The base impedance of the 187 is about 200 Ohms. The maximum quasi-linear swing here is dozens of mV. Why do you feed it from a (potentially) high gain stage with an output impedance like 200K?

How is the bias stability? (I would expect idle current to rise with temperature, which makes it hotter, until blow-up, unless the heatsink is grossly oversize.)

Where do you get a 12H choke good for an Ampere or more? (Does it have to be 12H?)

Load impedance: it seems to not do weird stuff wit everything from a 1 ohm folded horn sub cabinet up to a 16 ohm 4x12

The voltage over the inductor is about 7.5V, the esr is 3.9ohm, this gives a bias current of 1,92A

The heatsink is an old power supply heatsink. the 2n3055 which was originally on it handled 6A at anywhere from .6 to 12V so a power of 3 to 72 Watt, the transistor which is now on it only handles a mean current of 2 amp at 4,5volt, which is 9 watt of dissipation, the heat sink is overkill.

The bias stops changing after about 30 seconds after turn on, even when playing the amp for hours on end, it doesn´t change to much.

The input impedance of the power amp is actually quite high because its in a sziklai configuration. The base current of the ASZ18 is dictated by the collector current divided by the Hfe (which i measured at 49 when its hot), about 2 amps divided by 49 is 0,04, or 40mA is the input bias current for the ASZ18, then there is the AC187 with a Hfe of 163 when its cold, but its handling 40mA so it gets slightly warm, so the Hfe is higher in operation. But ill use 163 for the calculations, the real current will be lower... So 40mA/163 is 0,25. thats a quarter of a miliamp. The output impedance of the jfet stage is about 15kOhm acording to my measurements. 15k x 0,25m =3,6V, it needs a half wave of 3,6V unloaded (the loaded voltage is not important because its a current amplifier, we need to know the output current) to shut off the transistor. So it only starts to clip when the peak to peak voltage of the preamp is 7,2V. Unloaded the preamp can deliver 10-ish volt peak to peak. So yeah, it can overdrive the power amp with ease, thats what i build the amp for, but the drive is not really high gain.

the 12H choke is actually a auto transformer (center tap inductor in a full transformer core), to go from 230V to 120V able to output 1000W or more at the 120V side. 12H is what i measured the inductance to be. It came out of an old microwave, which was 1000W at the highest setting. There was also a 2nd transformer in the microwave, which took the 120V and made a 12V and a super high voltage output.

everything i used in this amp either came from a thrift store (microwave), cigar shop (the cigar box i mounted everything to) or an inheritance (i inherited a whole bunch of old electronic components from era´s long gone, from germanium power transistors to paper in oil capacitors and from a box of 18 metal can 741´s to more than 500grams of medium power<.5watt to 3watt> resistors sorted in old tea boxes and from unmarked transformers to vacuum tubes, EL50´s and others that have german WWII Wehrmacht markings on them <¨eigentum der Wehrmacht¨ and even an eagle holding a sw@stika>)

[Fancy Lime]

... and from unmarked transformers to vacuum tubes, EL50´s and others that have german WWII Wehrmacht markings on them <¨eigentum der Wehrmacht¨ and even an eagle holding a sw@stika>)

Don't tell me you built a time machine from those n@zi tubes and accidentally left a portal open! Although that would explain so much right now.

Andy

[iainpunk]

... and from unmarked transformers to vacuum tubes, EL50´s and others that have german WWII Wehrmacht markings on them <¨eigentum der Wehrmacht¨ and even an eagle holding a sw@stika>)

Don't tell me you built a time machine from those n@zi tubes and accidentally left a portal open! Although that would explain so much right now.

Andy

yeah, oops