Maybe I thrive on clutter. One thing's for sure- a sterile environment stifles creativity, unless you're more of a modern copy-and-paste "look wut I made!" person. Okay, what were we talking about? Oh yeah -
The Conn Organ Amp On a Plank:
Originally this tube amplifier salvaged from a defunct ‘50s Conn organ was a power amp only, no preamp, and also had a sizable secondary power transformer that provided a 12VAC filament supply via an umbilical cable to the organ’s tone generator and preamplifier chassis. Before I removed that unneeded 12V supply transformer, leaving a big rectangular hole, this thing weighed a ton. Prior to circuit re-imagining, I used it for impromptu weight training in the workshop (fun fact: a big old power transformer makes a great as-found shot put); even now this amp is a weighty piglet. In the four now-empty circular holes on top were three capacitor cans and one of those interesting 0A3 radioactive voltage regulator tubes.
The Original Circuit:
After the audio line level input from the tone generator / preamp was a variable attenuator, a 25K pot with a locknut tightened screwdriver slot shaft. This was followed by a 7-pin 6C4 (like 1/2 a 12AU7) voltage amplifier, then into a 12AU7 wired as a paraphase inverter. The resultant phase-split signals powered a push/pull pair of 6L6G output current amplifiers. I forget what the B+ voltage after the 5U4G rectifier was, but it was very high, and there was a sizable wire wound resistor in line for voltage reduction and a bit of filtering, and the 0A3 regulator did its thing too - in whatever manner it does that, of which I have no clue. Doesn’t matter anyway, I removed all of the original circuitry and components except the main power and output transformers, and three tube sockets (7 pin, 9 pin, and the octal rectifier socket), which were still in fine shape 70 years after manufacture.
As Rewired:
The Conn’s new circuit is sort of based on a single input version of a 1950s Fender 5E5 Pro - which was like a much louder 5E3 Deluxe. Basically it’s a 1/4” shorting jack with a 1M to ground, through a 33K grid stopper into a 7 pin 6AV6 (which is 1/2 a 12AX7 plus two diodes, neither of which are connected here), into a volume potentiometer and a one-knob tone circuit pot (both 1M), then a 12AX7 cathodyne phase inverter section into a pair of cathode biased 6L6GC (or 6V6, 5881, plus modern Euro equivalents such as JJ 6L6 and the ubiquitous variants of Soviet 6P3S-E). The output tube sockets are wired up ‘60s Fender style, with 1.5K grid stopper and 470ohm screen grid load resistors on the dead pins. A very simple, efficient, and toneful circuit; all hail the visionary Saint Leo (Fender)!
My idea of fun, a true point to point scratch built tube amp wiring:
A 5AR4/GZ34 rectifier now produces even higher B+ potential than the previous 5U4, and I installed a paralleled pair of new 10W ceramic resistors (forgot the values, I bought a few to dial in the eventual B+) between the rectifier tube and the first filter capacitor to knock the voltage down. That cap is actually two giant 20uF capacitors in parallel ( = 40uF@600V) pulled from another project: a Dynaco Mark III that had been radically rewired by a genius amp guy, but something failed; I tried for years to troubleshoot it and eventually returned it to the original circuit (thanks Ron for your help).
After that, a Weber choke (mounted where one of the original capacitor cans was) and a couple more caps and resistors as per the usual Fender style Pi filter chain. The under load idling voltage at the output plates is close to 450VDC; I was shooting for that after being intrigued by a suggested circuit in an RCA tube manual for a pair of cathode biased EL34 putting out 40W with that speced anode voltage. I have no idea exactly how much power this boat anchor amp puts out, but it’s a damn lot, hurts my eardrums pushing a single Fender labeled Eminence Legend, and loud and sweet powering the compact Leslie cab’s vintage CTS 12”. Wouldn’t be surprised if it actually is around 40 real watts!
Misc Stuff:
The etching engraved brass Conn organ name plate isn’t original, it came in a bag of old ID plates and logo badges fortuitously found at a garage sale. Looks cool though; it’s attached with double sided tape. The very interesting pointer knobs are both one of a kind orphans; I call the black one (which is the tone control) "Dolphin Head" rather than chicken head, and the gray one (volume) is named "Fairchild" - I think it’s from a dead oscilloscope that a friend parted out.
The amp is mounted on a locally logged and milled tight-grained old growth Oregon alder board; it used to be an unpainted closet shelf in the utility room of a neighbor’s remodeled 1960s house. Ain’t no more of those trees around here anymore, logged out and never to be seen again - most of the hills in the Oregon Coast Range have been replanted in Douglas fir monoculture. Walking in those forests is a weird experience- without a biodiverse plant ecosystem the woods are eerily silent; very few birds, bugs or animal life of any kind. Compared to some other man made hellscapes, it’s a fairly benign eco dystopia, a cross between a normal temperate zone forest and a cornfield.
I sanded that alder plank smooth and put a coat of amber shellac on before mounting the amp on it. The two cream colored handles were luckily found at the Assistance League thrift store, usually mostly a higher end used clothing place. After putting the loaded amp chassis on the board a dowel was placed underneath and the plank rocked back and forth to find the balance point, and the handles were placed in that sweet spot. Four rubber feet (actually large cabinet door bumpers from the neighborhood True Value hardware store) were mounted to the bottom of the board, and that was that: Tube Amp On A Plank #2.
The Compact Leslie Cabinet:
This compact Doppler effect cabinet was custom made... but not for me! The creator of this beauty, Jim Hobbs, put it together for his own use, crafted from highest quality 13 ply birch, with the spinning rotor components from an ancient Model 25 Leslie. Or maybe it was a Model 125 (?) - I was so surprised, delighted, and infatuated with it when Jim gifted it to me that I wasn't paying strict attention. Another post dedicated to this cab will soon arrive here on the Night Lamp blog, but not today.
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A brief note re the photo at the top of this post: toward the left of the frame are two other amplifiers mounted on wooden boards. At the top of the audio mini-stack is Tube Amp On A Plank No. 3, a restored Stromberg Carlson stereo power amplifier that's sometimes hooked up to the JBLs and sometimes not. On the floor is my living room guitar amp: a Hammond M3 extension cabinet w/ an Altec 12" inside, atop which is a small Bogen paging / background music amplifier rewired as a variant of a mid '50s 5F2A circuit - Tube Amp On A Plank No. 4.
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Thanks to David for the ancient tube amplifier, Jim for the Leslie, and a big thank you to brother Jon-San for the vintage vacuum tubes.
Click on any photo to see larger, higher def images.
The photo of the circuit wiring was taken with a Lumix ZS-25 pocket camera; all the other images were shot yesterday with an Olympus OM-D E-M10 MkIII body w/ a Lumix 14-45mm lens. All editing done with the limited but intuitive and quick Preview app included with iMacs.
