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.


And if you find it useful…

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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

Japanese Guitar

Japanese Guitar

Teisco Guitar

Mysterious 60’s Guitar

Research indicates it’s a somewhat rare Japanese Inter-Mark guitar. An import-export company, they didn’t actually manufacture guitars. It was probably made by Kawai/Teisco and rebranded under one of several names: Inter-Mark, Cipher, Pleasure, or Columbia. The Columbia Record Club offered Inter-Marks at one time, as did various departments stores. A setup and neck reset gave this guitar an amazingly low buzz-free action. Easy fingering on the 23 1/2″ scale neck and a great full bodied tone make this a definite player. Mahogany body, vintage sunburst styling, Eko style square pole pickups (Tone!), aluminum pickguard, cool switches, unusual chrome combination truss rod cover/nut “tray”, and a five bolt neck. Still have to brush off the dust, fix the strap button which somebody actually nailed on and dig up a tremolo arm and spring to fill out that classic sound, but she’s a keeper.

S Hawk Ltd.

S Hawk Ltd.

S Hawk Ltd. Booster and Tonal Expander

Unboxing the S Hawk Ltd. Booster and Tonal Expander

Favored by Rory Gallagher, and shamefully copied, the S. Hawk II Tonal Expander has become “the holy grail of boosters”.

Also pictured is the Hawk 1 Overdrive booster linear pre-amp/fuzz with volume control.

We’ve made a promise to never crack the case on these units, so don’t ask. Our friend Harry Kolbe, creator of the legendary S. Hawk, was kind enough to ship us the last few NIB Hawks in existence for use in an ongoing project. We’re ecstatic! The sound is– absolutely amazing and we’ve already tracked some astonishing vintage tones! Thank you Harry!

An Old Parlor Guitar

An Old Parlor Guitar

Here’s a guitar that is something of an enigma. Although identified as a 1920’s Harmony, this guitar bears many of the hallmarks of a little-known brand of parlor guitar–McKinney. Although construction style indicates Harmony manufacture, these guitars have also been attributed to Regal. McKinney may have been one of Regal or Harmony’s department store/catalog brands. To confuse matters more, it appears that there were at least two unrelated McKinney guitar manufacturers– one was possibly a remnant of the turn of the century McKinney Organ company while the other was a low quality offering of a Hawaiian style guitar school of the McKinney name. The hallmarks of these two guitars differ tremendously by comparison. Attributing a date has also been a bit uncertain. If the Geib and Schaeffer case is original to the guitar, it’s possible this guitar was made some time between 1929 and 1937. The guitar itself has been wonderfully refinished and an extremely unusual “suspended” bridge was added. Featuring an arrow straight 24 scale, 18 fret hard v-neck, figured quartered maple back and sides and clear spruce top. Even the nut, bridge, and strap buttons are wood. This guitar is a beauty. With extremely low action and near perfect intonation, she plays and sounds awesome. The maple lends a distinctly clear yet warmly brightened tone and is perfectly matched to the parlor-sized body.

50’s/60’s 50EH5 Tube Guitar Amp

I’ve been able to learn very little about this recent acquisition. I’d be glad to hear from anyone who may have some information regarding this vintage tube amp.

I’ve drawn up a schematic of this vintage 50eh5 tube amp HERE. The value of some components, such as the early ceramic capacitors remain inconclusive.

50EH5 amp front

Although some features bear a remote resemblance to several known amplifiers, I consider this vintage 50’s amp to be of a yet unknown maker. The amp is powered by a duo of vintage General 50eh5 tubes. A prized Hitachi 12AX7 and a GE 6211 serve in the preamp and tremolo stages. The circuitry appears to be completely stock and retains a hot chassis design. Unfused and ungrounded. For safety, future modifications will include the addition of a grounded isolation transformer and a rectifier bridge if it’s necessary to balance the transformer loading. The amp currently utilizes only a single blocking diode for rectification. Component choices of interest include obsolete dual capacitors and ancient color-coded ceramic capacitors sitting alongside the typical early wax capacitors and carbon comp resistors.

50EH5 amp back

The 6 1/2″ alnico Fisher speaker bears a date code of 014 which translates to week 14 of either 1950 or 1960. Although difficult to read due to their positioning, the potentiometers may bear a date code of 2, most likely meaning 1952 or 1962. To nail the date down on this amp, at this time I’m relying on the date stamped on the 35 mf octal cap– 8-51. Although it’s uncertain… from the information above, I’d like to say this amp was made in 1952.

The cabinet is made of plywood and is covered in both white and black “book-binding” cloth. Note the layout of the controls below. A switched potentiometer serves to regulate both bass and power. The volume control is labelled “Loudness”.

It’s quite possible this amp was once powered by a pair of 50C5 tubes. This would be consistent with my guess as to the amplifier’s date since, according to the Electron Tube Registration List, 50EH5 tubes, a replacement for the 50c5’s, came into production a bit later; in 1958 (source: Stadium was one such company that manufactured a series of similar amps.



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.

Dealing with Click Track Bleed

Cheap and Easy Tip of the Day:

A significant amount of click track bleed from the headphones can ruin a take. Use a  cheap pair of earbuds or in-ear monitors along with some ear muff hearing protectors. Any bleed will be completely isolated.

Great for recording acoustic instruments, especially singer/songwriters, where bleed normally a concern, this technique also provides exceptional isolation from extremely loud sources.

Since click track bleed is most noticeable with higher frequencies, try placing a filter on the click at about 1khz.

Curious Voodoo – The Subkick Microphone

The original intent of this article was to provide some sort of comparison between a simple DIY sub-kick, the more elaborate commercial designs and the over-blown DIY sub-kick designs found floating around the internet.  That didn’t quite happen.

Since the sub-kick has  become an indispensable tool in the studio, I was curious to compare my simplistic, cost-effective  approach with the more expensive alternatives.

Sub-kick transducers add tons of body to kick drums and are really effective on bass cabs and even floor toms. They’re a must-have for the studio.

Interestingly, I found that Yamaha does not make a frequency response chart available for their popular line of sub-kick microphones.  Repeated requests for such a chart have gone unanswered.  When Yamaha chooses to unveil their official frequency response charts, I’ll post them here.  In the meantime…

For comparison purposes, I chose the Yamaha NS10 as a substitute for the Yamaha sub-kick speaker.

I selected this studio monitor as an example because, with its proven track record,  it has often been touted as the classic studio sub-kick.  Pre-dating the NS10, Beatles engineer Geoff Emerick  first  explored  sub-frequency transducers (aka sub-kicks) while recording bass for “Rain” in 1966.

What sparse documentation Yamaha offers regarding their SKRM-100 sub-kick varies widely. In some spec sheets, Yamaha states their sub-kick’s  frequency response begins at 20hz.  Other  documentation states 100hz. Others claim 80 hz and yet others 50 hz.  So what’s the story? It’s not quite clear what the actual specifications are. Personally, I find it a bit odd that no frequency response chart is available. If  the response is similar to the NS10, the EFFECTIVE response would likely begin at the higher end of the sub-frequency range– possibly around 100hz. Although this is only marginally considered a sub-bass frequency, it does suitably fall in the range of most kick drums.


With the NS10, there’s a  significant 30db loss of gain below the 100hz mark which is officially considered to be the upper realm of bass sub-frequencies.

The next chart shows the frequency response of the studio standard AKG D112.  Despite the smaller diaphragm, it actually appears to handle sub-frequencies a bit better.


The diminished  low-end  response of the NS10  is probably due to the large amount of energy required to efficiently move the large mass of a speaker cone.  Overall low-end response will likely vary widely with the amount of air and mass (SPL) moved. Recording a heavy-footed drummer should show better results.

Setting all theory aside,  the screenshots below were taken from a random session and show actual response curves of a very simple DIY sub-kick with no processing.

In this case, a 6.5 ” Polk Audio mw6502  speaker from an old M7 monitor served as the sub-kick.  This is the same size as the speaker used by Yamaha with their sub-kick but with a somewhat better low end response than the larger NS10M.

As a sub-kick, the Polk performed admirably. It was  suspended directly from the drum lugs using elastic shock (bungee) cord. No resonant shell was used. The output was sent to a passive DI to provide a balanced signal and to prevent overloading of the preamp input.

Polk MW6502. A peak at about 75 hz…


The AKG D112 from the same session.  A strong peak  at just above 100 hz.


AKG D112

Together, the two mics complemented each other quite nicely and sat well in the mix.

Even without a shell and tuned head, harmonic resonances appear to be visible in the spectrum.

To me, these results demonstrate the fact that a sub-kick need not be expensive or elaborate to provide great results.

As one subkick manufacturer noted–“This mic is really nothing more than a different paint brush to paint a sonic landscape. [A subkick] actually doesn’t increase anything, it simply is a microphone with a different focus range than most traditional mics… to gather more of the frequency range… that traditional microphones are not designed to focus on. Reproduction of 25hz is generally not what [engineers] are looking for as much as capturing the total range of frequencies produced by the sound source. ”

So… why use a commercial sub-kick at all?

There seems to be a  lot of curious voodoo revolving around the need for expensive and elaborate sub-kick mics.

It’s not likely the Yamaha sub-kick can capture fundamental sub-frequencies with any better efficiency than the simple sub-kick setup illustrated above.  Mounting the speaker in a drum shell with an adequately tuned head could theoretically capture useful harmonics of the fundamental low end frequencies (but not the fundamental itself) thus enhancing the illusion of capturing a powerful fundamental sub-frequency. Pretty cool for psycho-acoustics, actually. Think analog aural exciter. Maybe. Other than that? A lot of it might be another form of psychology since they certainly look cool and, of course, the absence of any high end sure makes them sound more bassy.

My opinion? A simple arrangement with a properly selected speaker routed through a direct box may be all that is needed.