Cordovox Accordion Amp Conversion with Schematics


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.
  • Amplifier
    • 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:

Circuit schematic:

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.

 

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Late 1960 Ampeg B-15N Schematics and Pics

Found with components dating to no later than November 1960, it could be one of the earliest B-15N’s produced.

We’ve nicknamed it “Chassis 24” after the hand- numbered chassis.

More information is with the schematics below.

 

Photos from the bench as she undergoes some long overdue service.

The circuit remains the same but heavily drifted and failed components have been replaced. Most carbon comp resistors measured 25 to 50 percent over their marked values. Surprisingly, the red Astron capacitors were within tolerance reading only slightly high on the meter.

Reslo Ribbon Microphone with XLR Connector Mod

Reslo Ribbon Microphone XLR Connector Mod.

Reslo microphones originally came with a now expensive and difficult to find proprietary connector.
I decided to update this mic to accept a standard xlr connection. The photo below shows the results of a couple of hours in the shop. The original threaded reslo connector was removed and ground down to accept a Neutrik XLR connector which was machined to fit. The Neutrik’s retaining insert was modified and screw holes were then tapped in the xlr assembly to secure it to the mic body. The microphone was then rewired. Addition of a ground circuit reduced the noise floor of this mic phenomenally. The transformer was left stock, although a replacement would certainly increase gain and lower the noise floor yet again. Finally, I installed a new ribbon.

Reslo XLR Mod

Hum-Free Premier 90 Reverberation Mods, Schematics and Impulse Response IR’s

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Premier 90 Reverberation Mods, Schematics and Impulse Response IR’s

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Download our impulse response IR’s for this Premier 90 Spring reverb unit here:
Premier 90 Spring Reverb Impulse Responses

My mid-60’s Premier 90 tube reverb unit came to me completely unusable due to the extreme amount of hum and noise it produced.  After extensive re-working of the circuit and complete replacement of all components, I’m excited to say that this unit is quiet enough for studio use.

Repairs began by removing the original two-prong cord, mounting  a fuse block next to the input transformer and adding a grounded power cord.  The antiquated can capacitor was removed  and two 5 lug terminal strips were securely attached to the existing reverb driver transformer bolts.  Soldered to these were the  replacement radial electrolytic caps.  Values were 47uf/450v, 47uf/450v, 22uf/450v and 100uf/100v.

In modifying the grounding layout for all of the components, the negative side of the electrolytics were  bussed to a single ground wire which then ran to another  terminal strip centrally located adjacent to the tube sockets. This strip would provide the location of my “star ground”.  All components were replaced and all grounds re-routed to this single point. It took many short lengths of 20 gauge hookup wire  securely bundled by cable ties to re-route the grounds.  1/2 watt metal film resistors replaced the noisy carbon comp resistors and the film caps were replaced with .01uf 400v Sprague Orange Drops.

Borrowing from the Fender 63 reverb schematics, I  isolated and elevated the star ground from the chassis using two 6A4 rectifiers and a 3 watt 15 ohm metal oxide resistor wired point-to-point on the lugs of the grounding strip.  Ideally, the only ground connections to chassis aside from the power cable would be through this diode and resistor network. Since it seemed to make little difference audibly, I left the input jacks and pilot lamp connected to the chassis.

It was mentioned in online forums that power to the  switched gain pot introduced hum but I found that after having  reworked the grounds it did not introduce any noise.

It was necessary to congest the chassis with yet another terminal strip dedicated to securely attaching the reverb send/return and output cabling.

Oddly enough, my ears found it unnecessary to replace the single diode rectifier although I suspect this will eventually happen.

By choice, I opted not to attach the foot switch back into the circuit.

I was fortunate enough to have several dozen of the appropriate tubes to sort through and found that proper tube selection seemed to make an audible difference that my tube tester was oblivious to.  I left the 6EU7 tube stock but considered rewiring the socket for the more readily available 12AX7.   The 6AQ5 was upgraded to a mil-spec variant, the 6005.  If the tubes are available it would be worthwhile to spend some time swapping.

My modifications to the Premier 90 Reverberation unit can be found on the schematics below. Thanks to Dave S. for the original drawing.

Premier 90 Reverb Schematic with mods.

Premier 90 Reverb Schematic with mods.

MTI Rotophaser with Footswitch Schematics and Images

The MTI Rotophaser is a rotating tremolo horn speaker system made in Italy in the 1970s. It is similar to the top half of a Leslie cabinet.

MTI Rotophaser Faceplate MTI Rotophaser Panel Gutshot

Intended to be driven by an external amplifier, a crossover network allows the Rotophaser to be paired with an external speaker cabinet.

MTI Rotophaser Crossover

An external footswitch is necessary to operate the unit.  It is entirely possible and perhaps preferable to eliminate the footswitch and relocate some or all of the switch functions to the cabinet.

MTI Rotophaser Footswitch Control PedalMTI Rotophaser Footswitch Control Pedal Gutshot

Two separate cables feed the footswitch. One cable distributes power (120vac) through the footswitch to control motor functions (slow, fast, off)  and the other controls  speaker routing.

Without a schematic, the wiring of the Rotophaser footswitch appears to be rather complex and mysterious.

I’ve drawn up two schematics for the footswitch portion of my MTI Rotophaser, bearing an inspection sticker dated early 1980.

MTI Rotophaser Footswitch Speaker CircuitMTI Rotophaser Footswitch Power Circuit

Your questions, comments or additions to this brief exploration of the MTI Rotophaser are most welcome.

Shure M67 Microphone Mixer/Preamp Mods


m67 Front

The Shure M67 is a vintage four channel microphone mixer commonly modified for use as a four channel preamp in recording studios. First manufactured in 1968, these became a standard in the broadcast industry. The M67 was used to record Woodstock and many a tv news program. Still readily available on the used market, they remain relatively affordable. With plenty of “vintage iron” to yield that classic 70’s tone, these units really shine as preamps.

The versatility of the M67 can be improved through a number of basic mods. The circuit layout makes it relatively simple to adapt this mixer into a preamp with four separate channels. Additional modifications to the gain staging of each channel make it  useful for either high or low volume sources.

There have been many internet forum discussions over the past few years as to how to modify these units. The mods I’ve chosen to do are largely inspired by these sources but especially the work of Rod Childers who has outlined his mods at Beyond Sanity Productions .

My mixer came to me modified with a balanced input circuit and the transformer’s line output was moved to the xlr. The transformer mic output was disconnected.

On channels 1-3,  I chose to alter the gain staging as per the Childers Mod and also eliminate the low cut filter. Going beyond the Childers mod, I opted to re-purpose the low cut filter switches to make each channel switchable between the master mix bus or an unbalanced direct 1/4″ output added for each channel.

Switching channels to the master mix bus dramatically increases the tonal options for each channel. This incorporates the gain boost of the master bus  and also the balanced master mix bus output transformer, both of which are bypassed by the Childers mod.

With low volume sources the gain stage modifications can be a bit too quiet.  As an option, I chose to leave channel four stock with its higher input gain stage unaltered. Channel Four also uses the master mix bus gain stage as a boost and utilizes the master mix balanced transformer as its output. The output can be summed with Channels 1-3 when they are switched in.  Leaving channel four’s circuitry unmodified expands the options available and simplifies the mod because it’s not necessary to drill extra holes in the chassis or extend any of the original wiring during the mod process.

I wound up with three different gain stages: Low gain 1/4″ out (Channels 1-3), Low gain through Master Preamp (Channels 1-3 switched) , and High gain through the Master Preamp (Channel 4).

To get yet a fourth gain option,  a channel’s gain stage could be left stock in a high gain configuration but then routed to a 1/4″ output,  bypassing the master bus preamp.

Shure M67 Switch Mod - Channel select between unbalanced low gain direct outputs or balanced high gain master mix busm67 012

I also opted to bypass and remove the tone oscillator circuit.  This requires that the board be lifted and jumpers added to replace the switch. I had to desolder a few additional leads to lift the board enough to work on the underside. Here’s the basic mod:

Jumper Red to White.

Jumper Orange to Brown

Jumper Green to Black with a 1.2k resistor. This is R6 on the Shure schematic.

Cut all other wires that run from the tone osc. switch to the board.

These units are reputed to be a bit noisy but  by replacing a few capacitors, tightly twisting all wire pairs, and adding ferrite chokes, I was able to reduce the noise  dramatically.

In the future I may look in to adding meter functionality to each channel’s direct output. Additionally, it’s been noted that changing volume pots to a 500k value may improve high end. Other possibilities include the addition of a -20db pad using the rear tone osc. switch and altering the values of the remaining low cut capacitor on ch. 4. I’m happy with mine the way it is.

VIEW OR DOWNLOAD A PDF GUIDE TO MODIFYING THE SHURE M67 BY CLICKING HERE

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