Quote from: Ironsinthefire on July 21, 2017, 05:01:55 PMGiven the difficulty of finding appropriate P-channel FETs nowadays, would preceding the first stage with an inverting, unity gain buffer and switching the FET to an N-channel work?

It shouldn't be that hard to find P-channel JFETs.

(Refer to the schematic JFET_bender_7.gif) Without P-channels, use an N-ch on the first stage. Reverse diodes D1 to D5 except connect D5 to the +15 rail. Keep R3 and R4 the same and use negative bias on the Gate of N-ch Q1. Insert an inverting stage with a gain of one between the first stage and the tone stack. Use the stage after the tone stack as is.

Several designs like this are floating around.

This is a single that needs a heatsink:

http://www.diyaudio.com/forums/pass-labs/120445-pass-discrete-opamp-dip-8-package.html

Trying to attach a different schematic.

[attachment deleted by admin]

The schematic can be found here: http://www.prowessamplifiers.com/schematics/sunn/Coliseum_300_Bass.html

The compression chip is said to be SSM2012 or SSM2013.

The problem with using an integrator is that the output amplitude will change with frequency. What you want, starting with the oscillator is a circuit that generates square and triangle waves with fixed amplitude. A triangle can be generated by feeding current to a capacitor. When the capacitor charges to a preset amplitude, the current reverses polarity and ramps the cap in the other direction. The square wave is simply the control signal that controls current direction. Frequency is changed by controlling the current, or the capacitance.

To make a sine wave, feed the triangle to a differential pair of transistors which will round off the sharp peaks. Having a fixed amplitude triangle makes it a whole lot simpler.

There are several small electrolytic caps  in the power amp that may have old age issues and might affect tone in a negative way. Just looking at the cropped schematic posted I see C31, C36, C33 and C34. Many times replacing components in a somewhat random way can cause more problems that it solves. I suggest you replace them one at a time and pay special attention to polarity. Take a good closeup picture of the board before you replace the caps.

Quote from: tarahall on March 11, 2017, 09:28:38 PMAre the gate voltages for the 2n5462 (+3.3) & mpf102 (-2. the optimum for your circuit or are they just indicative of the voltages you needed to set for the specific devices you used?
I'm curious as we all know how varied the tolerances and ratings of JFETs of the same type can be.

Those Voltages are just to let you know what a typical might be. Each preamp needs to be adjusted for the JETs actually installed. The best way I found to adjust the pots is the "Gain Reduction" method. Set the pots to the zero Volt end, apply a signal and monitor the output of the stage with a scope or DVM. Adjust the pot for a 20% reduction of the signal level (first stage) or a 50% reduction (second stage).

Quote from: tarahall on March 11, 2017, 09:28:38 PM
I have tried adding your JFET idea to Rod Elliots's Project 27 preamp (mentioned elsewhere on these forums) and the results are certainly encouraging although I had to adjust the gain of the first 2 OPAmp stages upward. It made the preamp brighter with less mid-range wool and a crispness and bite one expects from a typical Fender front-end.

IIRC Rod's preamp used a Baxandall tone control network. The Fender type network will certainly sound different. I chose the gains of the two stages to mimic the headroom found in Fender amp and a distortion profile where distortion slowly rises and reaches about 10% before the opamps hit the rails.

Attached below is a layout I did and an updated schematic. I took you suggestion and added resistors in series with the pots to limit their span and make them easier to adjust.

As a general guideline I would say you want a JFET with an Idss roughly between 2mA and 10mA. The 2N3819 should work fine. If Idss is higher, the pot becomes very touchy, you might need a 10 turn trim pot. This all depends on the resistances in the feedback network. Lower impedances work with higher Idss parts. Just keep the ratio of the resistors the same.

Speaking in general terms, that inductor isolates any capacitive load from the amp. If the power amp has high open loop gain or marginal stability, (like most chip amps) a capacitive load might cause the amp to oscillate at an ultrasonic frequency. This usually causes the amp to self destruct in short order. Most amps have a Zobel across the output, a series RC network of something like 0.1uF and 10 Ohms. An ultrasonic oscillation usually causes the 10 Ohm resistor to go up in smoke just before the fuse blows

You might get lucky without the inductor, or you might end up with a smoldering mess.

Quote from: Vitrolin on February 02, 2017, 03:19:15 PM
manual states 130W in 8ohm 1%THD
at clipping

The chip has no way of knowing what the load is. It just reads Voltage at the output. The amp can put out more Voltage into 8 Ohms without clipping, but the chip just knows that it is more than the calibrated 0dB Voltage.

Chances are that if 0dB is 300W at 2 Ohms, it's 150W at 4 Ohms and only 75W at 8 Ohms. The power supplies sag quite a bit on these amps with a 2 Ohms load.

Looks like GB started a similar thread on DiyAudio and posted a couple of reply's.

http://www.diyaudio.com/forums/instruments-amps/301909-solid-state-version-vox-ac-30-guitar-amp.html

Post Edit: Schematic posted in the above link

I don't see any bypass capacitors on the layout. I would suggest 100uF or 220uF on each rail to ground.

B, E, and C connections of Q3 are correct. Emitter current comes from the output through R21 and Collector goes to the Base of Q4.

Are you sure about Q3? It would make a lot more sense if it was a PNP.