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Heathkit Signal Tracer to Gibsonette hybrid Guitar Amp Conversion

Discussion in 'Audio' started by JRUBIN, May 7, 2018.

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  1. JRUBIN

    JRUBIN

    95
    15
    Jul 17, 2015
    In this video I take my cosmetically damaged but fully functional Model T4 signal tracer and convert it into a guitar amplifier using inspiration from a Gibson model GA-8T schematic to produce a hybrid amplifier. The project starts off with planning and possibilities and then jumps into the progressive modifications to achieve the overall goal of the first iteration of the amp. A strategy is laid out across both schematics

    The wife officially starts the project with the cable cutting ceremony, the first order of business being the removal of the noise circuit. This is followed by the removal of the coil taps carrying high voltage DC to the front of the unit, no longer needed.

    In looking at the preamp side its obvious that nothing will be salvaged except for the heater circuits and the B+, so the best way becomes clipping everything out. Then all of the new components start to be assembled in the unit being mindful of the space. Also, a temporary input test point is also fabricated using the final components.

    The genrad 1650a is broken out for a cameo appearance for precise capacitor measurement. Then a temporary setup is made for a center tap tone value, so that the circuit can be completed without a tone knob.

    This is a nice time to test on the variac for turn up and test with the new pre amp side assembled.





    Continuing with the project after a trip to Radio Shack... yes, Radio shack, I was able to puck up a new 1M liner POT and a grounding amp jack. The Pot will need to be custom cut and chamfered to fit the unit. The Pot for the tone will go where the old power switch was, and the power switch will go where the old speaker switch was. So the first work is moving power off of the old location. After which it was decided the noise switch would be a better location and safer for power.

    The new potentiometer goes in for tone and looks like it came from the factory that way, the temp parts are removed and the knob is wired in for a more permanent installation. A great time to test the new tone control. To fully test a radio is added as an input.... tone knob works.... the jack gets installed next, also looks like a factory installation. Fitment is constantly checked against the magic eye tube.

    There is however much distortion in the preamp stage. and this has to be addressed, also the volume knob needs to be rewired. Time for troubleshooting. This lead me towards re-evaluating after installing the bridge rectifier. as it would also give me more voltage to the 12ax7 plates.... so that became a project of its own. The new voltage drop allowed for substantial plate voltage increases on the 12ax7.

    A test shows that the issue was cleared up after this. but it is important to determine at this point how much voltage a guitar pickup produces. With these voltages the function generator can be setup more appropriately for testing going forward.

    In looking for residual distortion coming in way too early, none was found in the 12ax7, however as we move towards the beam power tube the problem reveals itself.





    In this final part of the conversion series we look to see how this noise found on the beam power tube can be removed. It is quickly found that the plate voltage on that side has no real filtering besides the coil and one cap. The addition of a large filter capacitor has provided a significant reduction to the noise as demonstrated.

    Another test is done by way of an external speaker to determine if any of the distortion is caused by the speaker itself. The determination is that the speaker in the Heathkit is just fine. Dialing back the resistance of R7 would allow for limiting the signal into the beam power tube, saving the system and speaker from destruction.

    Following back to allow for more voltage improvements on the unit. This involved adding appropriate resistors to the unit. At this point everything is permanently mounted and some final additions are accomplished.

    Finally Pete at Cornerstone music does a critique of the amplifier for our enjoyment.

     
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