Tube Amp On A Plank No. 2 + Compact Leslie Cabinet

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.

Thrift Store Fun: Vintage 1960 Norelco AD-4877M "Labor of Love"

Wednesday was a dental appointment day in neighboring Springfield, and afterwards we stopped by the St Vincent's thrift store on Q St. There were some interesting Asian ceramic pieces in stock that day, and my eye was caught by a pair of brightly painted home made speaker cabinets.

According to the staff, some idiot removed all the screws holding the back panels on, yet still couldn’t get them off, and then grabbed the burlap grill cloth of one cab and ripped it, just to see what the speaker looked like. Seeing a dual “whizzer” cone, he left in a huff, leaving a bunch of screws laying around.

No doubt due to their appearance, these cabs were priced low, so I took a gamble since I’ve had good luck in the past with brightly painted home brewed cabs built in a certain era. It was obvious to me what they were: a labor of love by a 1950s or ‘60s do-it-yourselfer home handyman with a limited tool selection: an electric jig saw, drill, screwdriver, caulking gun, pencil and probably a long ruler, were all that was needed to build up these cute yellow cabinets.

Mr Handyman also had a novel solution for framing the grill material; in spite of having few tools to work with, he was an intelligent and most likely a creative sort of person.

One indication that these speaker cabs were valued by the builder was a blue Allied Van Lines sticker on one, showing he thought enough of them to pay to transport them as part of a whole house move, most likely to another region of the country.

Nice heavyweight terminals:

They metered out as okay:

The jerko Vinnie’s customer couldn’t get the backs off because the entire cabinet was caulk sealed. Using a very small pry bar, it came apart easily, with a sharp snap and pop noise as the 60 year old caulking separated.

The cleats were screwed in place:

The builder had totally stuffed every cubic inch of the sealed non- ported cabinet with fibreglass batting. Although not done very often anymore, at the time it was sort of a popular concept in the DIY projects as seen in magazines like Popular Mechanics.

Down at the bottom of all that carcinogenic stew was this:

Cool. The magnet cans look amazing, and the 8" (nominal size) speaker drivers themselves are really heavy.

The cones are in overall excellent shape, except for a small bit of damage to one surround, caused by the Gorilla of St Vincent's. Which sounds like the title of some story by Edgar Allen Poe.

"What a maroon!”, as Bugs Bunny would say. (hint for those not familiar with classic Warner Bros cartoons: Bugs meant to say "moron")

Here’s the label:

An online pdf of a vintage Norelco hi-fi ad shows these AD 4877M drivers as costing $26 apiece back in the day, which is the equivalent of over $600 for the pair in today’s money - no wonder the builder / owner kept them through a long distance move.

In the pre-internet days I'd seen a magazine article mentioning Norelco and Philips Alnico magnet drivers as being well suited for use with flat panel "open baffle" speaker design. Checking online today, it’s hard to say if that was so - one of many things gone down the internet memory hole.

Well, let's see what these Norelco speakers sound like. I hooked up the cabs, one as-is sealed and chock full o’ fibreglass, and the other with no batting and the back removed to approximate an open baffle audio experience, and had a listen to some pieces on the local classical station, KWAX, and selected tracks from a few of my favorite test CDs:

Sarah Vaughan, "Sarah Vaughan" (1954);  Bill Evans Trio, "Sunday at the Village Vanguard" (1961); Bill Morrisey, "Songs of Mississippi John Hurt" (1999); Modern Jazz Quartet, "Django" (1953-1955); Beach Boys, "Pet Sounds" (1966); Stan Getz and Charlie Byrd, "Jazz Samba" (1962); Dread Zeppelin, "Un-Led-Ed" (1990) - If you haven't yet heard Dread Zeppelin, you're in for a unique listening experience: Led Zeppelin tunes done by an ultra tight rock group in a Reggae style, with vocals by an Elvis impersonator.

Also auditioned were a rare String Quartet in E minor by Giuseppe Verdi (Joe Green!), JS Bach's French Suites played by Andras Schiff, Samuel Barber's Symphony No 1, and one of my all time faves, "Graceful Ghost Rag" by William Bolcom.

Never mind the slightly comical looking cabinets - these are nothing less than the greatest sounding single driver speakers I’ve ever heard, and very efficient - at a guess I’d say at least 25% higher efficiency than the Wolverine 8” loaded (plus 12” woofer) Electro-Voice Leyton speakers I have in the living room. Truly astounding detail - I could hear every nuance of brushes caressing the snare skins on vintage jazz recordings, Sarah Vaughan at conversational volume level sounded like she was right there in the room, the separation of moving harmony lines in the Verdi string quartet was incredibly lifelike, and Carl Wilson's vocal on the Beach Boys Pet Sounds album cut “God Only Knows” was sweeter and more angelic than I’ve ever heard it.

Since Pet Sounds is a mono mix I panned between the two cabs to hear differences in the cabinet configuration, and found the open back cab to be a bit more efficient (louder), and the closed one to have a slightly better low end, but not by very much.

Uh oh, I feel another project coming on. Maybe build a pair of solid pine open back cabinets the same size as these yellow things (external dimensions 13.5" x 19" x 12", 1/2" plywood baffle). Or maybe just keep those funky 1950s labor-of-love cabs just the way they are, enjoy the wonderful sound, and who cares what they look like?

 

 

Tube Amp on a Plank # 1: RCA Victor RS-193 Single Ended Stereo Hi-Fi Amplifier

Summer before last, there was a big charity yard sale here in our neighborhood, proceeds of which benefited a local pets help society providing among other things, low-cost veterinary services. During this time in our nation's history when a sizable percentage of the population is morphing into a grievance driven neo-fascist authoritarian cult, rejecting anything related to the common good (and all you soulless gun-clutching lizard people know who you are), I'm thankful that our family lives in an old fashioned American town, now considered "socialist", where helping others still matters. Also parks, swimming pools, libraries, arts and music, summer activities for kids, senior centers, hiking and running trails, post offices, an Amtrak station, and dog play parks!

I helped others (and their pets) out by buying a few things: a pair of new Converse low tops, some Yakima rooftop rack bars, a few interesting books (Keith McCafferty's fly fishing detective murder mysteries), and an early 1960s RCA Victor stereo hi-fi console:

Don't know the model name or number of the console, but the amplifier section has an ink-stamped "RS-193D" on its top surface; see circuit description below. This console didn't have an AM-FM radio tuner, only a 3-speed phonograph record changer turntable, with a high output (near line level) ceramic element cartridge + stylus assembly. When I got the console home, I plugged it into power, let it warm up for a bit, put an LP on the platter, and turned the knob on the turntable to "Play". The stylus arm didn't automatically move over and plop the needle onto the record, as it should have done, but the platter spun, and the arm let me pick it up and place it on the outer LP grooves. Here comes the West Coast Dixieland sound of Jake Stock and his Abalone Stompers  - it works!

As expected, the musical tone could have been better, but it was okay, and surprisingly quiet and hum free. Next, as a test, I disconnected one side of the speaker-out leads, and hooked that up to a known good sounding speaker, and wow - that was a huge improvement over the stock built in drivers. It was all there - deep tight low end, detailed mids, and shimmery and almost magical sounding highs.

Since, 1) the record changer was basically not totally operational (and I wouldn't have trusted it with some of our vintage LPs anyway), 2) the console's speakers weren't exactly audiophile quality, and 3) as furniture, the console cabinet had seen better days, the next step was obvious. Rescuing the really great sounding tube stereo amp section and re-purposing it sounded like a fun project.

The amplifier was removed, and the console cabinet, with turntable, was sold on craigslist (really cheap) to someone looking for a vintage piece of furniture to refinish and use for storage - the full width hinged top lid was a great feature for that purpose.

Here's a quick rundown of the amp's basics:

- There are two high level (near line level) inputs into a dual section 6EU7 voltage amplifier / output driver tube, with its plates driving the input grids of a pair of 6BQ5 (EL84) output tubes
- Estimated output power: 3 to 5 watts per channel
- Controls consist of a dual section loudness potentiometer, dual passive bass and treble (also dual section) controls, and a balance pot
- 5Y3 rectifier tube
- Two separate output transformers

The next picture shows the amp innards, after I'd done a few minor changes:

In the above photo, note the flat brown "blob" in the upper left corner - that's a separate component board, encased in some sort of soft mastic-y material, that holds the loudness control sub-circuit. That's one of the keys to this amplifier's sound; not only a volume level control, the loudness circuit also has a variable level Fletcher-Munson EQ compensation, subtly boosting the lows and highs at lower volume settings.

A short list of the non-invasive "mods" performed on the amp:

- Added a power on/off switch (originally done at the turntable's "play" knob)
- Added an easy to access fuse holder
- Installed a larger gauge 3-conductor grounded AC cable, with strain relief
- Lifted one leg of the 6.3VAC tube heater loop from ground, and installed 2x 100 ohm resistors referencing both heater legs to chassis ground
- Removed the original hard wired audio input terminals, and replaced with a pair of standard RCA jacks
- Installed a low light-level LED power status lamp, hooked up to one leg of the heater supply; the added terminal strip at the bottom of picture just right of center, shows the voltage dropping resistor and IN4007 diode
- In place of the original control knobs, which had shafts long enough to reach through the 3/4" cabinet panel, four vintage made in Chicago Dakaware Davies style knobs were fitted

At some point in the past, some professionally done servicing had been done to the amplifier - note the pair of large orange .047uF Sprague coupling capacitors, and a replaced 220K plate load resistor. Nice to know this old stereo had been loved enough to be well taken care of.

A great part of doing this project is that since it was in fantastic shape and sounded good as is, no major repairs or circuit changes had to be done; in fact, the audio pathway in this tube amp is all original and unmodified. Also, no additional holes had to be drilled in the chassis, although some had to be enlarged to accommodate added parts, such as the fuse holder and power switch.

Here's a view of the back of the amp. The General Instruments multi-section can style capacitor is still in excellent condition after all these years - not even a bit of audible hum. The output transformers were made by Midwest Coil And Transformer, in January of 1962:

A top shot. Note the AC cable, stolen from an Apple MacBook Pro power supply:

The amplifier chassis was mounted on a piece of rustic finished old growth Oregon alder board, reclaimed from a house being remodeled in Eugene Oregon, originally built in 1964. This board, previously a part of a utility room shelf, had no finish on it. After cutting it to length, a light sanding got it down to clean wood, and two coats of amber shellac were applied, with a steel wool rub down between coats. The last step was polishing with a rag cut from an old favorite cotton flannel shirt.

The RCA Victor logo badge, with the classic style "His Master's Voice" dog and wind-up Victrola, was originally on the front of the console that this amp was removed from. The badge, made of plastic, has two round pins on its back, which fit into two holes I drilled into the front edge of the plank:

The black rubber feet are from an early 1960s McGohan tube paging amp, which received much larger new rubber feet.

Here's a picture of the nicely muted LED power indicator lamp in action:

The LED indicator lamp is a redundant feature - the four vacuum tubes are themselves a sure way to tell if this amp is on. But it was fun to install, and there was already a hole in that spot, so why not? At first, I put in a neon lamp, but yow - that was noisy; the LED is dead quiet.

One end of the chassis has the new RCA style audio input jacks. I had only black and red old fashioned Dymo label stock on hand, but they corresponded nicely with a vintage Switchcraft double-RCA cable with red and black coded plugs:

I like Dymo punch style labels, they're incredibly retro and vintagey looking, and certainly are era specific for this 1960s amp.

A shot of the other end of the chassis:

Close up - here you can see the bat lever of the miniature DPDT AC switch, the AC cable w/ strain relief, and new old stock fuse holder. Also, a good picture of the slot head brass wood screws and stainless trim washers I was lucky enough to find at our local "home town" True Value hardware store, only 10 blocks away:

The power transformer, made by the Foster Transformer company, also in January 1962:

Another front view of the amp. The RCA 5Y3 rectifier and RCA 6EU7 driver tube are likely original. The General Electric 6BQ5 output tubes might be original equipment, or possibly replaced when the amp was serviced a few years ago. Note also the interesting speaker output terminals; I had to fabricate a pair of dedicated speaker cables, with two "+" fittings, and only one "-" fitting.

Here's the back side again:

In its original location in the console, the amp's knobs were pointing up, and the tubes and power transformer were sideways. The pins on the vacuum tubes are probably a lot happier now. Actually, since the rescue and re-purpose, this old tube amplifier seems to sound happier overall.

Using an adapter cable, an iPhone was hooked up, with great results. In this picture, you can see the amp end of the speaker cable assembly:

The RCA RS-193 has been hooked up to a few different pairs of vintage speakers; here are three of them, with an Onkyo DX-1800 compact disc player as an audio source. First, a pair of late '70s JBL L19:

A pair of 1950s James B Lansing D123 12" full range in JBL cabinets, no tweeters:

Sure, both sets of JBls sounded as awesome as they always do with almost any amplifier, but the most relatively astounding match up was with a pair of 1980s Radio Shack Minimus 7 mini speaks. I could go on about how honest and realistic everything sounded with this system, but you might not believe that a 3 to 5 watt per channel amplifier and tiny speakers could reproduce music so well. Guess what? They do.

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The Circuit

Without going into a lengthy full analysis of the RS-193's circuit, a few key points are:

• The bass and treble controls are passive instead of active; that is, they are subtractive in their action, cut only, and since they don't boost bass and treble at their respective knee frequencies, it eliminates the possibility of any unwanted resonant peaks in the overall frequency response.

• Rather than "volume", there is a "loudness" control. Each channel's loudness potentiometer (1/2 of a double ganged 500K pot) is multi-tapped and connected to a sub-circuit between the driver amp and power output stages, which enables a variable EQ compensation effect (ie: Fletcher Munson curve) dependent upon the overall volume level.

• At roughly 56mA of plate current at idle through each of the 6BQ5/EL84 output tubes, this amp is firmly in Class A territory.

• With an 80uF electrolytic capacitor on the DC leg of the 5Y3 rectifier tube, the RS-193 has two to four times the primary B+ filter capacitance of most tube amplifiers with similar plate voltages, which helps to reduce hum and noise.

• Instead of two single-ended output transformers, each 6BQ5/EL84 is connected to the primaries of its own double-ended output transformer. The B+ voltage from the power supply is hooked up to the center tap, one primary leg is connected directly to the 6BQ5 plate, and the other leg, which acts as an additional filter choke, goes to the screen grid of each 6BQ5, through a dropping resistor (and also provides voltage to the plates of the 6EU7 driver tubes). This ingenious use of two double-primary output transformers, with their added AC ripple filtering, helps to keep the amplifier's noise floor down - a good thing in general, especially since single ended tube amps tend to have a bit more inherent noise than push-pull circuits.

All in all, a deceptively simple yet sophisticated, efficient, and tonally superior tube amplifier circuit design. This amp came along at the very end of the first tube audio era, just before transistors took over, and may well be the very last tube amplifier RCA ever designed and manufactured.

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Click or tap on any photo above to see larger, higher def images. All photographs taken with a Lumix ZS25 pocket travel zoom camera.