Converting a 1962 Cordovox Accordion Amp to a guitar amp…
A brief build summary:
- Tone Generator Cabinet gutted.
- Components parted out and scrapped.
- Tone Generator Cabinet used for build.
- Chosen for existing control panel cutout.
- Cabinet modified with 2×12 speaker baffle.
- Original Jensen C12R Speakers relocated.
- Chassis moved to tone generator cabinet.
- Circuit traced and redesigned. Schematic below.
- Amp rebuilt.
- Extraneous components removed.
- Multi-section can capacitors rebuilt .
- 30uf cap and 15k resistor from preamp filter section moved to chassis.
- This section originally located on tone generator board.
- Terminal strips added for mounting
- Control panel rewired
- Reversed mounting position.
- Fender-style tone stack built directly on pots.
- Input section modified for guitar.
- Preamp Box removed from tone generator board.
- Mounted next to control panel
- Preamp circuit built to new specs.
- Connector harness modified.
- Sections divided
- Split and routed to preamp and control panel.
- Connector Pin Connections:
A few notes:
Revisions to the topography of this amp are still ongoing. Although the amp truly sounds amazing, I don’t consider this work to be perfect or final.
During the process of converting this amp, I’ve attempted to retain as much of the original design as practical.
One of the unusual remaining features of the original Cordovox circuit is the negative feedback loop that returns to the second stage preamp tube instead of the phase inverter as is typical. As kudos to the original amp design and as an experiment, I’ve left it that way. It works and the amp sounds great.
After having rebuilt the power section true to the original Cordovox design I found amp voltages to run slightly high, but especially during the initial startup surge. Several factors contribute to the high transformer output, namely hotter modern voltages and reduced transformer loading. Much of the original load the power transformer had to support became extraneous to the conversion. Caps rated at 500v would be best suited to handle the inrush current surge at startup, but the addition of two CL-60 thermistors in series reduced the inrush current to acceptable levels for 450v caps and also provided a satisfying seven volt steady state reduction (2%) to the B+ voltage.
And if you find it useful…
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