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 four chord "look wut I made!" person. Okay, what were we talking about? Oh yeah -

The Conn Organ Amplifier 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 unit 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 real 5881 and modern Euro equivalents such as JJ 6L6 and the ubiquitous variants of Soviet 6P3S-E including fake 5881). 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 - true point to point scratch built tube amp wiring (note that the tone circuit and another DC power supply filter stage were added after this pic was taken):

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 amplifier 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 the cab when Jim gifted it to me that I wasn't paying strict attention. Another post dedicated to this cabinet will soon arrive here on the Night Lamp blog, but not today.

*                  *                  *

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.

*                 *                 *

Thanks to David for the ancient tube amplifier chassis, Jim for the Leslie cabinet, 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, and the pic of the Oly camera was done with a 1st Gen iPhone SE. 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.

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.

*               *               *

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.

*               *               *

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

Peavey Pacer Tube Amp Conversion Project, Part 2

In Part 1 of the Peavey Pacer Tube Amp Conversion Project, we bought a really cheap Pacer, which the seller admitted didn't work very well. The main goal at the time was to rescue the very nice sounding Eminence / Carvin British Series 12" speaker that was in it. Here's how the Pacer looked when it first came home:

The Pacer's amp section was eventually successfully repaired. However, while it was busted, I decided to salvage the excellent quality light weight jointed pine cabinet, and turn the chassis into a simple tube amp. So even though it now worked well, it was still taken apart, and the working - and very much alive and loud - circuit components were sold, and the horrible looking and super heavy baffle board was given to a junk-loving neighbor:

Before it was cut smaller, I should have taken a photo of the replacement baffle, which came from an ancient (and dead) electronic organ. It was so old that the speaker cutout hole was larger than any made in the last 60 years at least, but luckily the mounting screw + T-nut diameter was almost the right size. After trimming the board to fit the Pacer cab, I shortened the left over pieces, to make a "rattle strip" frame on the front of the baffle, which was glued and stapled to the baffle:

Not shown is another smaller piece of leftover plywood attached at the upper left of the baffle to hold a logo badge.

Here's a picture of how much the new modern, larger T-nuts hung over the organ speaker baffle cutout - only three out of four tines took a grip into the plywood:

That old plywood is amazing - void free fir plies, extremely light weight and almost zero flex. Nobody makes plywood like that anymore:

A new machine screw + washer, one of four holding the speaker down onto the baffle:

This particular 12" speaker is a much used and well loved early '70s Altec Lansing 417-8C, re-coned badly by a place I won't mention:

When I first got the Altec, it was new, minty, not a scratch on it. If you ever wondered why many older guitar amp speakers are beat to sh*t, it's from heavy gigging during the waning days of the now bygone saloon and roadhouse era.

Often one-nighters, when the show was over the cables would get rolled up and shoved into the bottom of the amp, whatever effects pedals were also thrown in there, along with a beat up Unidyne 545 (later an SM57) vocal mic, and the amp (either a 1965 Fender Pro Reverb or a '64 Deluxe Reverb) would get humped out to the parking lot and tossed into the trunk. The wear on this Altec is from about 450- 500 gigs over a 5 year period:

My guitars looked almost as bad.

Here's what the magnet structure of an Altec 417 looks like under the sheet metal cover. What you see isn't the super powerful Alnico magnet, it's an iron basket surrounding the magnet and concentrates the magnetic field for higher efficiency, according to some scientific principles I don't pretend to know. "I only drive these race cars, ma'am, I don't design 'em!" (quote from A.J. Foyt):

Note the old blue Arrow stapler in the above picture - that's going to get used when the grill cloth gets attached to the baffle.

Here's the beautiful grill fabric, unrolled right after getting it in the mail from friend David in Massachusetts. This material also came from an ancient organ, but still in really nice shape:

I put that cloth under a heavy rug for about a week, and most but not all the kinky bends in the fabric settled down.

Sorry, didn't take photos of the grill cloth attachment process, which was an hour and a half sweaty wrestling contest between me, the material, the baffle, and the staple gun. After three long rounds, I finally won the match.

Check out the really ugly glue residue left behind after the original front control panel plate was removed:

Actually, I kept getting dissatisfied with the way the weave didn't want to lay straight, so the staples were pulled, and we started 'rasslin again. In the next picture, you can see a few of those staples. After getting the cloth on as straight and as tight as possible, a metal straightedge was placed on the back of the baffle, and the cloth was neatly trimmed with a new blade in a Stanley knife.

I was going to paint the baffle black prior to putting the cloth on, like most other guitar amp speaker boards, but the grill material looked best in front of the natural golden color of old fir. Note the "semi- floating" baffle attachment to the three cleats on the cabinet sides:

Here's a close up of a machine screw holding the Carvin/Eminence to the baffle, and a wood screw attaching the board to the cleats:

A still life portrait, with guitar, home brew amp and overdrive pedal, and ex-Pacer cab:

This upside down picture shows a possible component layout. The power transformer (also old ex organ amp), with 660VAC no-load on the secondaries, used to power 4x 6K6 output tubes, as well as 3x 6SN7 preamps + inverter and a 5U4 rectifier. It should (hope!) provide enough current to do a cathode biased 20 - 25W push pull 6L6GC circuit:

Penciling in the location of the tube sockets. It shouldn't be too hard to drill out their holes with a step bit, since the chassis is aluminum. The mounting holes for the power transformer will be reinforced with steel straps inside the lightweight metal chassis.

The awful dried up glue on the chassis front was bugging me, but removing it was a chore. 97% alcohol didn't faze it, and neither did GooGone or GoofOff or whatever that household solvent is. In the end, the job needed 3 grades of sandpaper, followed with steel wool, always sanding and scrubbing in the same longitudinal direction:

One end of the front of the chassis had gotten whanged in the past; some aggressive sanding made the ding almost gone. Hopefully whatever paint we end up using on the control panel will cover the remnants:

Removing the dried up zombie adhesive took a lot of time and effort - because of that, the rear panel plate will stay on, as is.

A view down at the power indicator lamp end of the front panel:

This last picture shows a possible control layout mock up. From left to right: two input jacks, volume, treble, middle, bass, presence, two as-yet unknown switches or controls (or maybe just nickle plated sheet metal plugs), and power-on lamp. Those vintage Davies maroon knobs go well with the color of the grill fabric:

The Saab logo thing, which fell off a 1975 Saab 99 Combi Coupe I used to have, is either a joke, or maybe a keeper.

Okay, that's all folks, until next time, when we start drilling attachment holes in the chassis for the transformers and tube sockets.

For the beginning of this project, see Part 1.

*               *               *

Click or tap on any photo above for larger, higher def images.

Testing Early '80s Malaysian and Japanese TI RC4558P Opamps With a Modded Tube Screamer

One sort of thing we have here at the Alley Gadget Shop is gadgets. Lots of them. Everything from a small, beautifully hand forged cable pulley made for an ancient French bicycle, to odd assorted guitar parts, obsolete Macintosh computers, and old tube amps and other vintage audio gear from the last century. Some work, most need repair, and a few are unfixable; and whatever is really dead often gets repurposed or dismantled for usable parts rather than just getting tossed.

Case in point was an interesting (and ugly) looking but totally defunct audio unit from the 1980s. Didn't work, no way it was ever going to be fixed, so we poked around for salvageable stuff. Besides a power cable, fuse holder, knobs and some assorted oddments, there were a few various integrated circuit chips, all in sockets. Most were 4558 type dual operational amplifier (opamp) ICs.

Texas Instruments RC4558P, made in Malaysia, 49th week of 1982:

Texas Instruments RC4558P, contract manufactured for TI by Japan Radio Corporation, 31st week of 1982:

Texas Instruments RC4558P, made in Malaysia, 1st week of 1983:

Other component manufacture date codes in this circuit, all from mid '82 to early '83, and a QC sticker on the chassis dated April 1983, correlates with the IC codes.

It might be a good idea to test those ICs, and see what shape they're in. But how?

Making a Temporary Test Rig

Hanging around the audio and art production space at the ol' Gadget Shop, is a mid-'90s Ibanez TS5 Soundtank series Tube Screamer, with TS808 circuit mods expertly done by David in Massachusetts. It's as good as any Screamer I've ever plugged into, tonally indistinguishable from the really great sounding 808 T.S. section in a stock early '80s Ibanez UE 300 multi-effector I happen to own and use.

When he did the mods, David installed a socket for the 4558 IC, and popped in a newer production TI RC4558P:

This makes it easy try out different ICs, and we took advantage of the socket to perform real time audio functionality testing on the three different sub-types of the Texas Instruments RC4558P that were salvaged.

To make a handy temporary test rig, the bottom plate and the control knobs were removed from the modded TS5 Tube Screamer, and the component board dropped smoothly away from the pedal body, allowing easy access to the IC socket:

An AC adapter was plugged in, the Screamer's output hooked up to an amp, and one by one we tested 24 RC4558P opamp ICs, including the one already in the TS5, while playing a variety of diverse sounding guitars:

We didn't bother reassembling the TS5 between each RC4558P test - just turned the amp volume down, shut off the AC strip the pedal power supply was plugged into, swapped the chip, and powered it all back on again. This way, testing impressions stayed fresh from chip to chip - it took longer to switch guitars than it did ICs.

Test Results, and a Cold Marguerita

Obviously, doing real time in-circuit testing of dual opamps, using an improvised and fairly inexpensive test rig, is by its nature non-scientific - we're not going to get any key parameter measurements like Vos, CMRR, Iq, etc. Our main goal was operational functionality testing - does a particular IC actually work, without any issues, in a typical example of a well known overdrive pedal circuit?

Main takeaways from today's testing:

1) With only one failure, all of the opamp ICs evaluated checked out okay, with the TS5's overdrive effect behaving itself in a predictable manner throughout the range of each of the three control pots.
2) All four TI RC4558P variants sounded fine in-circuit, with no fatal audio flaws: tonal and/or level degradation, glitches, or weird sonic artifacts.
3) There appeared to be very subtle audio differences between the four sub types of TI RC4558P we had, mainly in the overall amplification levels and possibly slightly differing low-end coloration. A couple times we went back and did multiple A/B comparisons between two particular ICs, just to make sure we weren't imagining those slight variations.

It might have been nice to have other brands, sub types and eras of 4558 chips, in order to compare them with the 1980s Texas Instruments ICs on hand. Possibly 4558s from other makers might have differing audio characteristics, and that's an idea for further testing in the future. But heck, forget about all that. It's late afternoon on a gorgeous and hot summer day - doesn't a nice cold marguerita sound like just the thing right now?

Yum, looks good...

*               *               *

If you're at all interested in obtaining one or more of the vintage 1980s Texas Instruments RC4558P tested in the above article, please consider going over to the right hand sidebar of this blog, find the orange rectangle, and click through to the Alley Gadget Shop's internet store at Reverb.

Note: as of July 2021, all of the vintage TI RC4558P ICs for sale had been sold. I hope everyone who acquired one is having fun with pedal projects!