Wednesday, April 26, 2017

Taking a Bandmaster to Dumbleland

As my experience inside tube amps increases, so does the wealth of knowledge I'm gaining from each project, and the past four months have been incredibly illuminating to say the least. Be it a ground-up build like the Tweeds, a circuit rebuild like the Princeton Reverb reissue, or this gem of a project that landed at my feet recently; partial restoration, partial conversion to a new fire-breathing beast...




Spotted on Reverb.com, this "silverface" Fender Bandmaster was listed as a '70s model with a nasty hum across both channels, located just across the river in Yonkers, NY; I snagged it for $400 with local pickup. Having no idea exactly what circuit this was, or if anything had been altered in it's past, I was mentally prepared for every scenario; it appeared to have a "drip-edge" aluminum frame around the grill cloth, which could make it a '68-'69, and potentially a better circuit for my needs, but who knows what the guts looked like, or how bad that hum was.


After an easy transaction, I arrived home and fired her up; nasty hum indeed, sounded like 60 cycles, likely from the power section of the circuit as it was independent of any control settings. A few quick strums told me that the individual channels sounded fine, and both EQ and tremolo controls were doing their jobs. Good news... I figured the filter caps and power tubes needed replacement, as well as the ancient two-prong power cable, so with everything else working, perhaps the hum would just disappear after those upgrades, fingers crossed...






The next step was inspecting the guts, and after clearing off the dust & cobwebs, the first thing I noticed were the power tubes: GE 6L6GCs, likely original, meaning this amp may have never been worked on before. Carefully removing the chassis, I then noticed the tube chart: this was an AB763 circuit, same as the last run of original Leo Fender Bandmasters prior to CBS taking over, and along the same lines as the great "blackface" Deluxe Reverb, Showman, and Twin Reverb circuits (double-checked the actual circuit to confirm this, as incorrect tube charts were a big part of CBS-era Fender conversion). Filled with blue molded Ajax coupling capacitors and some odd Y-shaped Whale "Common Negative" electrolytics in the preamp cathode bias positions (my first encounter with either component in person), I was feeling pretty stoked; this amp was a nearly 50-year-old virgin, and I was about to pop it's dusty old cherry.



The original circuit board, untouched by anyone other than spiders for nearly 50 years.


Immediately after placing an order for all new electrolytic capacitors throughout the amp, I began testing the resistors and coupling caps with a volt meter, replacing those that had drifted out of spec (carefully placing all removed components in a plastic bag). I removed the old 2-prong power cord, along with the ground switch and "death" capacitor (if this cap fails in an old amp, your chassis can become voltage-hot. No bueno, el shocko!), and replaced them with a modern 3-prong cable. I also began plotting some big changes at this point, including an expanded EQ and switchable Overdrive channel that could take this from stock Fender to fire-breathing tone machine (and back, if ever warranted).



The two Dumbles I've gotten to know; note the left amp has a dual-concentric pot for Gain & Trim, as well as Master volume, whereas the other has no visible trim or Master. The Accent controls negative feedback (presence), and here it exists as a pot (left) and simple switch (right).


I've been slightly obsessed with Alexander Dumble's designs for a little while now, mostly due to their arcane nature (who doesn't love a good mystery?); the guy has made less than 300 known amps, mostly built to unique specs and customized for individual players, fetching ridiculous sums on the vintage market. I've had the opportunity to play thru two similar yet different Dumble Overdrive Specials (no two are exactly alike), and can only describe them as being the absolute best Fender-style amps I've ever heard in person, with an unreal harmonic-rich overdrive that sounds as great on 1 as on 10 (fine, I only had them up to 4 or 5, but you get the idea). They have crazy amounts of gain on tap, but in a very refined, controlled way that somehow feels like it's about to explode at any moment but never does. I also love the tone of his modded Ultraphonix Fenders; they're a little nastier, rougher around the edges, less under control from the examples I've heard. With this in mind, I began seeking information. Knowledge has a way of demystifying things, and his Fender-based schematics weren't that intimidating (save for lots of local feedback and some fancy switches), so after a successful Princeton Reverb rebuild, my eyes were set on playing around with some of his ideas. By no means am I attempting to copy or clone an exact circuit, just trying to find where some of the mojo might lie.



In hindsight, probably should've used a color other than black...


Examining the Dumble links posted on Schematic Heaven, I noticed the 70's circuits were very similar to an AB763 Fender, sans tremolo, and comparing them to the Bandmaster layout, it made the most sense to sacrifice the farmost-right preamp stage of the Normal channel for the new Overdrive, while leaving the original Level pot in place but rewiring it to control the Vibrato preamp. This would keep all of my level/gain controls in one area on the faceplate, and all tone controls in another area. I sketched out the idea over the original layout (above), and waited for the parts to arrive.

In the Dumble OD circuit, there are level controls before, between, and after the two triode stages of the 12AX7 tube, traditionally labeled Trim, Gain, and Ratio, plus the original Level control that feeds the clean preamp into these cascaded stages. These 4 pots allow endless ways to balance the gain of these circuits and combine preamp stages to suit an individual's playing style; the Trim control is sometimes internal or on the rear of original Dumbles, but I wanted the ability to experiment with it easily, as I believe it's the coolest unique feature of this design.

A basic block diagram of our new signal flow:

Normal ch. Input 1 > Vibrato ch. preamp & EQ > DPDT toggle > OD stage (orig. Normal) > back to toggle > Tremolo & Phase Inverter > Output & Power


Using the 2nd input for a DPDT "Overdrive" switch, and Duct tape for a labelmaker.


For the tone stack, I added a Midrange control between the Treble and Bass, much like we did on the Princeton rebuild, but with a few slight component changes. Dumble used a larger 330pf capacitor to set the High-pass filter cutoff point slightly lower than a stock Fender; although I didn't have a 330pf silver mica capacitor on hand, I added a 120pf in parallel with a 250pf, giving me 370pf total on the Treble, and I love it! Gives the amp more teeth and bite than stock, can't imagine ever needing a Bright switch either. The new Mid control doesn't get quite as fat in this circuit as it did in my Princeton (where it was gradually bypassing the tone stack to ground), but has a narrower peak here that fits in very nicely just underneath our newly-sculpted Treble high-pass curve. The Bass is also much more effective and useful now compared to stock, incredibly tight sounding even at it's fullest range, and very interactive with the Mids.

There is an optional "Boost" switch in the schematic that essentially cuts out the tone stack from the circuit like the Princeton's Mid control, allowing the full range of frequencies to pass thru like a Tweed amp; I tried this, but reversed it. Just not what I want from this amp, especially with the new EQ curves working so well. Having played around with the "Deep" and "Rock/Jazz" switches on those original Dumbles, I decided to omit them here for the time being; my EQ is permanently in "Rock" mode, and the Bass control gives me all the depth I could imagine needing for now. The 820-ohm negative feedback resistor was also swapped for a Dumble-spec 3.9k, with no accent control for now; I'm digging it like this.

Also, like the Princeton, I doubled the ceramic disc capacitor values in the tremolo circuit from .01uF to .02uF to decrease the speed, but otherwise, that rest of it is stock. I may convert it from opto-sensor to tube bias at some point, but for now, I'm just happy it works.



Filter cap board on underside of chassis; all new TADs and 2-watt resistors, with an original Mallory removed on right.


Inspecting the Dumble schematics again, and paying close attention to the power filtering and voltages, I rebuilt the filter cap circuit with TAD electrolytics and resistor values per those Schematic Heaven files, and it worked a charm, with B+ voltages right on spec with those indicated. I firmly believe that THIS is the real Dumble secret, much more so than the preamp circuits; none of those would mean a damn thing without the tubes performing optimally, and his voltages seem to hit a real sweet spot in the electron vacuum. I'll be referencing these in future projects as well.



Oldies but goodies.


Speaking of tubes, I popped in a fresh set of JJ 6L6GCs, salvaged a lovely old RCA 7025 from the original set, and added a couple New-Old-Stock JAN/Phillips 12AX7WA (leftovers from a small going-out-of-business recording studio stash) to the preamp stages. I'm waiting on a matching JAN/Phillips 12AT7 for the phase inverter, and popped a current-production Electro-Harmonix in for now. May play around with a 12AY7 in the clean preamp stage (now V2), but the RCA is sounding really great; the JAN/Phillips has a great thick midrange in the OD stage (V1), and also for the tremolo (V3). It's fun to play around with these positions, as suggested by friend/amp guru Jamie Simpson of Booya! Amplifier Services.


It's also popular to replace the Bandmaster output transformer with a beefier model, either from a larger Fender or aftermarket; Dumble did this in his Ultraphonix Fenders, and some do it for more headroom and tighter bass in stock Bandmasters. I'm holding off on this for now, as I'm not sure I'll be playing this amp loud enough to really appreciate it. If the gigs get larger and I find myself cranking this past halfway often, then I'll reconsider it, and likely go with an Allen Amps TO40.



The completed circuit board, with Orange Drop capacitors used wherever an original Blue was replaced, Sprague or Nichicons on the cathode bias, and shielded cable used for most of the longer preamp stage runs.


After double-checking my overdrive channel and DPDT switch wiring, I crossed my fingers, plugged in my Tele, and fired it up... and I'll be damned, it worked and sounded great! That 60 hertz hum magically disappeared with the filter cap changeout, and the overdrive sounded fantastic even at very low 1am volume, but I'd have to wait until the next morning to really have fun with it.

And fun I had...


video




As my exuberance in that brief video may tell you, I'm incredibly happy with the results, and would like to think I captured some of the magic by way of the cascading gain staging, EQ curves, and most importantly, new B+ voltages. Until I get a custom faceplate made up, this is how she looks, and I'll be cranking her up at the Randall's Island Spring Festival with J.D. Patch & the Hell Dwellers on May 6. Come hang!


Removing the Fender logo was the final touch, along with a blue jewel on the lamp. Controls, from L to R: Input volume, Overdrive Gain, Ratio, & Trim, Treble, Middle, Bass, Tremolo Speed & Intensity. 



Wednesday, April 19, 2017

1957: Gibson Guitars, Korina wood, & Science-Fiction at it's finest.

A few months ago, I wrote about the convenient development of the fuzz-favorite Germanium transistor following a certain "weather balloon" crash near Roswell, New Mexico in 1947; Well, ten years later, a similar crash must have occurred near Kalamazoo, Michigan, because the R&D department at Gibson headquarters came up with some truly other-worldly ideas...




While the more traditional Gibson Guitar Company was struggling to compete in the same marketplace that Leo Fender was ruling with his Telecaster and Stratocaster models, President Ted McCarty patented some space-aged designs, supposedly influenced by science-fiction movies and the space race of the era; these prototypes were first seen at the summer of '57 NAMM show, and led to the introduction of the Flying V and Explorer in January of '58. Two of the other designs, unnamed at the time, wouldn't make it to the production line for decades; they surfaced in the early 1980s under the names Moderne and Futura.


The first appearance in a Gibson catalog of the Explorer & Flying V, 1958.


Original 1958 patents for what would become known as the Futura (left) and Moderne (right)

Made from slabs of beautiful white Korina wood (true name African Limba, although some early models also used the more standard Gibson mahogany) and featuring gold hardware with white or black pickguards and the new 1957 Patent-Applied-For "Humbucker" pickups, the world truly was not ready for these innovative designs, and wouldn't be for over a decade; Only 98 Flying Vs were produced from '58-'59, along with less than 50 Explorers, making them two of the rarest production-model guitars ever. Even rarer still, the Futura was limited to 5 or 6 units, with 3 known to still exist, but the original Moderne is the guitar equivalent of Bigfoot; possibly less than 3 made, with one rumored to still exist, although unverified (more on this later).




Early promo photos of Lonnie Mack (left) and Albert King (right) with their 1958 Flying Vs

The Flying V had a few early devotees, such as blues greats Albert King and Lonnie Mack, both of whom purchased their signature instruments in the first year of production. Mack was only 17 when he picked up Flying V #007 from Hughes Music in Norwood, Ohio after spotting one in a Gibson catalog; attracted to the Native American arrowhead shape, shop owner Glen Hughes drove to Kalamazoo, Michigan himself to pick up the instrument for Mack. Although Lonnie refinished the guitar in red and had Hughes install a Bigsby vibrato some time later (which Mack dubbed his "whammy bar", the first known instance of the term, after his 1963 single "Wham!"), it remained his favorite instrument for the rest of his career, and along with a Magnatone amp, helped define his unique sound.


Lonnie Mack with refinished '58 V and added Bigsby "whammy bar"


Albert King's original Flying V history is a bit more complicated, and greatly disputed; this writer won't claim to be the final word on the matter, but I've gathered as much information as possible and hope to present it all here for you to debate. Albert claimed to have purchased his first V new in 1958, attracted to it's unique look from a showman's perspective, and supposedly it was either stolen or lost in a craps game in the early '60s. Although no longer in production, Gibson assembled a Korina V replacement for King using leftover parts from the initial '58-'59 run, and stamped it with a '59 serial number; this became a favorite of King's for the next 18 years, until he sold it in 1981 to "Detroit" Frank DuMont for $2500. Some stories say that the '59 V was actually his original, and the sale was the result of a gambling debt to DuMont. Regardless, the guitar now resides in Steven Seagal's private collection, along with King's '67 Gibson V and the Dan Erlewine "Lucy" custom made for Albert (and spec'd from his '59). To add to the legend, actor Richard Gere claimed to have Albert's original '58 V, and sold it as such at a Christie's auction in 2011.


A couple shots of King with his '59-serial number V
Albert's 3 surviving Vs, the '59 (left), "Lucy" (center), and '67 (right).


60's rocker Dave Davies of the Kinks was a British Invasion proponent of the Korina Flying V, and helped push Gibson to reissue the instrument in 1967, albeit in a redesigned form. Another original '58 V was owned and played by Keith Richards briefly at the 1969 Hyde Park concert, although little else is known about this "arrow-shaped guitar", as Stones bassist Bill Wyman described it in his book; a rumor spread that it was actually King's original '58 V, although it was never confirmed to be true. That guitar was also stolen, along with many others, from Keith's Nelcotte villa in the south of France during the recording of Exile On Main Street. An original '58 V recently played by guitarist J.D. Simo was claimed to be Keith's Hyde Park guitar, but that is also unverified to the best of my knowledge and research; the whereabouts of the Richards' V is still unknown.


Keith Richards at Hyde Park (left), and Dave Davies playing his '58 V in an unconventional manner (right)




In 1980, perhaps the unlikeliest of Flying V aficionados arose in the form of Eddie Van Halen, then the hottest guitarist on the planet and enjoying his tremendous success with no shortage of incredible instruments at his disposal. The story goes that Dan Martin of the St. Charles Music Exchange near St. Louis, Missouri, acquired a mint-condition 1957 prototype Korina V from it's original owner under deceitful means; knowing he would receive a large sum for the rare instrument, he traded the older gentleman a nice but rather ordinary Gretsch guitar & Gibson amp for the museum piece, promising to keep it at home in his private collection. Well, he turned around and sold it to Eddie for a reported $10,000, beginning a long career of procuring instruments for the superstar. When the original owner discovered this, he was livid, but had no recourse. Van Halen used this '57 prototype Korina V in studio only, most notably for the recording of 1984's "Hot For Teacher". Eddie has stated in interviews that the '57 V is the prize of his personal collection and his favorite guitar.






Amos Arthur and his '58 Flying V

The last of our famous, or rather infamous, original Flying Vs is affectionately nicknamed Amos after Amos Arthur, the owner of Arthur's Music Store in Indianapolis, Indiana. Always appreciating the new and bold, Amos shared the same flare for showmanship that attracted Albert King to the V, and proudly displayed a '58 black pickguard model at his shop (one of only 10 originals with a black guard from the factory). It was sold a year later in 1959, and although the next 16 years are unknown, it resurfaced in 1975 at Norman's Rare Guitars in Van Nuys, California, where owner Norman Harris wouldn't let it leave. Residing in Norman's private stash for nearly 40 years, Amos was loaned out for the filming of This Is Spinal Tap in 1983 as a part of fictional guitarist Nigel Tufnel's collection, and was not seen again until guitarist extraordinaire and vintage aficionado Joe Bonamassa wrestled it away from Norman a few years ago. As one of Joe's favorite instruments, he has played it extensively on tour and in studio, but recently commissioned Gabriel Currie of Echopark Guitars to replicate it down to the finest detail so the original could be retired. As of this writing, Joe has received his Echopark Amos replica, and plans to begin using it on tour immediately.


Joe Bonamassa with Amos, almost 60 years later.







The original Explorer does not share the same prolific history as the Flying V, but with less than half as many units produced (19 shipped in '58, only 3 in '59, factory leftovers later assembled in '62-'63, 38 accounted for today and no more than 50 total manufactured), the Explorer stakes claim to the title of most expensive non-celebrity-owned electric guitar sold, with an original 1958 model fetching $1,100,000 at Denmark Street Guitars in London last year.


Clapton's original "modified" Korina Explorer

Although initially considered a bust, the Explorer had a resurgence in the 1970s, with Eric Clapton himself donning a '58 Korina, leading Gibson to reissue the model much like it did with the Flying V a decade earlier. Clapton's first Korina Explorer, seen on the '74-'75  tour, heard on the E.C. Was Here live album, and posed with for a MusicMan amplifier promo ad, had a chunk removed from the rear fin, and was reportedly sold to him as a one-off factory prototype from the '57-'59 era by Alex Music in New York City. Eric loved the guitar, but when he later discovered it was in fact a butchered original and not a rare prototype, he demanded the store take it back; they refused, and Clapton stopped playing the guitar altogether. It now resides in a Japanese collection.



Clapton with Explorer in Music Man ad (left) and triumphantly on stage (right)






Another unmodified original was purchased and used by Clapton in 1983, and sold at auction for his Crossroads charity in 1999.





Rick Derringer, Rick Nielsen of Cheap Trick, U2's The Edge, and Kirk Hammett of Metallica all have original Explorers in their collections (Nielsen actually has 2), but those were acquired much later on in the instruments' lives. Derringer's early '58 Explorer, seen often with him in the '70s, has the split-headstock design of the Futura prototype, which makes it about as rare as a Loch Ness Monster photograph with Bigfoot in the background. Still, if the Flying V was considered a commercial failure at the time, then the Explorer was the equivalent of a rocket self-destructing at launch, and disappeared from the Gibson catalog until 1976.



Rick Derringer with his ultra-rare split-headstock '58 Explorer



Explorer bass #001


Even rarer than the original Explorer guitar was the Explorer bass, of which only one has been proven to exist (along with rumors of one or two others). There's debate as to how or why it was created, possibly a special order, or a factory prototype that walked out the door, but it was built from a Korina guitar body with a mahogany neck, 3+1 headstock with banjo tuners (similar to what would appear on the Firebird model), and "Mudbucker" bass pickup. Regardless of it's origin, this serial number 001 bass ended up at Glen Hughes' music store in Ohio by 1962, where Lonnie Mack pressured his bassist Wayne Bullock to purchase it. The $300 price tag was steep at the time, but eventually, after a year of hanging on his wall, Mr. Hughes cut the young bassist a deal, and sold it to him for half price. Bullock played it for the next decade and refinished it first in green, then metallic blue, before selling it to Guitar Player columnist and guitar historian Robb Lawrence in 1973. Lawrence restored it to it's original look and got it into the hands of famed bassists John Entwistle, Jack Bruce, and Chris Squire, among others, before Explorer aficianado Rick Derringer saw the bass and had his manager purchase it. It changed hands many times since, hanging in a New York City storefront and even appearing on a Lou Reed track, but it's current whereabouts are unknown.



Lonnie Mack's band, circa '63, with original Explorer bass & Flying V


Saturday, March 18, 2017

Princeton Reverb Rebuild & More

For my second amplifier project, moving forward from my very successful "Two Stroke" 5F2A build, I wanted something that would be both fun and an even greater learning experience, without costing much out of pocket; I chose to rebuild my Fender Princeton Reverb Reissue (hereby referenced as "PRRI") to authentic 1965 specs, the AA1164 circuit to be precise (named after the month it was created, 11/64).


If it ain't broke...

Although the Fender reissues are solid amps in their own right, mine had fallen to 4th place in the arsenal (behind my Echopark Vibramatic 13, Groove Tubes Soul-O 50, and Two-Stroke build), hence, it became expendable for science. I would have settled for some fun modifications, but the PRRI's design is printed circuit board-based, with lots of ribbon connections and board-mounted pots; not fun for experimenting. The main bit of work involved here would be hand-soldering the eyelet board, much more involved than my previous build, and wiring it into the chassis, preserving the amp's transformers, tube sockets, and jacks (pots would need to be replaced).


The PRRI PCB (left), and my hand-wired AA1164 eyelet board (right)


After carefully removing the tubes and then the chassis itself, I measured a slight charge on the electrolytic power supply filter capacitors and discharged them for safety. Then, I started removing the PCB, while preserving as many off-board connections as possible. Next, the PCB strip attached to the the pots came out. I was able to keep the filament heater (6.3v) wiring intact across the preamp tube sockets, as well as the power transformer leads to the GZ34 rectifier tube socket. As for the rest. I left the wires attached to the sockets in hopes of reusing as many lines as possible.


In the meantime, I ordered up an AA1164 capacitor kit and eyelet board from Mojotone, as well as an assortment of carbon film resistors and set of appropriate CTS pots from Studio Sound Electronics. Once everything arrived, it took a Sunday afternoon to load up the board, and hot damn do those fat juicy Orange Drop caps look succulent; I could practically hear the difference in tone already.


After soldering up the components, I began wiring the board into the chassis, beginning at the input; I was appalled that Fender doesn't use any shielding on it's input wiring, as this is merely an extension of the guitar's cable until it hits that first 12AX7 triode stage. I also attached the input resistor directly to the tube's grid at pin 2, minimizing any chance for signal interference (more on this in a bit).

Another Fender gripe: the chassis was drilled for slightly smaller pots, not the standard size of the higher-quality CTS pieces I replaced them with. So, my 9/16" drill bit got a great workout on all six holes.



Input section, first tube stage & EQ w/Mid control


I included a few slight mods at the preamp stage, first lowering the size of the cathode bypass capacitors from 25uf down to 4.7uf, as suggested by friend and amp builder extraordinaire Jamie Simpson of Booya! Amplifier Services. This reduces the amount of low end content in the first two gain stages, tightening things up and eliminating a lot of excess flab at high volumes (Alexander Dumble also does this in many of his preamp designs & mods).


Input stage & EQ mods. FYI, the 7025 tube was a military-spec 12AX7 way back.


The next mod was done in between those triode stages at the tonestack; a popular one, the addition of a midrange pot, or "raw" control as some call it. The 6.8k resistor going to ground from the bass pot creates a midrange "scoop" that Fender Blackface amps are infamous for, and by adding a 25k or 50k pot before that resistor, you can dial the mids back in for a more tweed-like sound with more gain and thickness than stock (similar to the Vibrato channel of an AB763 Super Reverb). At it's full-on setting, the tonestack is essentially bypassed, and full signal is passing thru from the first triode stage to the second. Alternately, you could simply increase the 6.8k resistor until you find a value that you like, or put it on a switch to toggle back & forth with the original. Whether using a pot or switch, it can be fitted into the "Input 2" jack, or you could drill a hole in the rear panel. Since I've never once used the extra input, it was the most logical spot for mine. I prefer this simple addition to any other sort of gain-staging mods I've heard in the circuit thus far.



Mmmm creamy middle


Remember when I said we'd come back to that input resistor in a bit? Well, it's been a bit. On most dual-input Fender amps, each jack has a 68k resistor wired directly to the tip (hot) connector, then joined together and run to the first preamp tube triode stage. The funny thing is, when you only use one input, those resistors become paralleled, and result in a difference of 34k total resistance on the guitar input, allowing a subtle yet noticeable increase in high end clarity and sparkle. So, since we're losing the 2nd input altogether and only wiring up one, I'm going with a 33k input resistor (something found in some of Dumble's designs as well).


The next tweaks are in the reverb and tremolo sections, and include increasing the reverb output resistor from 470k to 1M in order to reduce the overall level returning to the circuit, and two component changes to slow down the speed & increase the depth of the trem; increasing the two ceramic grid resistors from .01uf to .02uf slows it down, and reducing the plate resistor to ground from 1M to 470k adds to the depth of the effect, both aspects I've taken issue with on the PRRI. Now you can get a slow, deep throb that wasn't possible on the PRRI, especially since the Depth control didn't do a whole lot til you got halfway around the dial.






After working thru all of this, the next dilemma was at the power supply section; the PRRI integrated it's filter caps into the main PCB, whereas the original AA1164 circuit didn't leave room on the eyelet board for the four large 20uf/500v electrolytics, and instead utilized a "cap can" that attached to the chassis much like a tube socket. I didn't have the tools to punch or drill such a large hole, so i decided to improvise and create a secondary eyelet board for the filter caps.





Another thing Jamie Simpson taught me is the importance of grounding the filter caps feeding the preamp tubes closer to that stage of the circuit than the others; in this case, points "A" & "B" on the leftmost caps feed the power tubes, while "D" at the far right feeds the preamp, so I've split off the ground so that the two caps on the right ground near the preamp section.

Also, when wiring up the power section, I discovered that Fender had done something odd on the PRRI: the fuse was wired AFTER the power switch, not before it, which goes against any schematic I'm familiar with. If that switch fails or shorts to ground while the amp is plugged in, fireworks could ensue, and not the fun kind. So, I reversed it, with the fuse coming immediately after the AC input on the "Hot" lead, before the power switch. Now it can do it's job and not burn down the house.


Getting there...


At this stage in the project, after making all of the necessary wire connections and checking for ground continuity, I decided to fire it up and check my voltages, first without any tubes (but plugged into an 8ohm load, always). Success! My purple pilot light lit up, and heater voltage as present throughout the circuit. Power transformer secondary leads measured 380V AC, a bit more than the 340V called for on the original 1964 Fender schematic (thanks to improved infrastructure over the past 50 years). After this test, I loaded up the tubes, and got some nice soft glow.



First power up, pretty lights!


I discovered a few things at this juncture: first, the DC voltage coming from pins 2 & 8 of the rectifier tube was reading 525 volts, extremely high compared to the 420V called for in the schematic. Also, there was no bias current whatsoever on pins 5 of the 6V6 power tubes. Turns out I wired the 1N4007 diode backward, a common mistake, but now my voltages were too LOW across the board; I also mistakenly had a 100k resistor in the spot where it should be a 27K, parallel with the capacitor to ground, bleeding off current. After these changes, I finally had negative bias voltage and accurate voltage readings throughout.

Upon plugging in, we have sound! Nice sound, too. But, a few issues arose... both the volume and reverb pots were shorting out at their null points, and so I began investigating my grounding scheme. Running the reverb pot to it's own ground point separate from the volume worked for that. As for the volume, I took a few ground connections from the back of that pot and sent them to the treble, including the input ground and new mid/raw pot. Everything sounded great, especially the new & improved tremolo circuit, but it was 1 a.m. so I couldn't crank til the morning...


video



...and what a morning it was. Holy sh*t, this thing rips! It overdrives much more gradually and smoother than stock, and the mid/raw control adds some serious body to the tone. I'm digging it right around noon, measuring about 12k (from the stock 6.8k). Here's a little demo, plugged into a 2x12 with Celestion H30s:





Just for fun, I tried a few more mods, first removing the negative feedback, which I did not care for... the treble gets a funny paper-y harshness to it, especially when overdriven.  Next, I added a 2nd volume (inspired by Dumble), using the extra speaker jack for mounting a mini-1MA pot. Originally tried it just before the 2nd triode of the V3 12AX7; did not care for it at all there, so I moved it to the next stage, just after the .02uf coupling cap and before the phase inverter triode, while also adding an extra .047uf coupling cap (probably too big but all I had on hand)... annnd BINGO. Sounds absolutely fantastic, allowing the first volume to be cranked up for some great overdrive. Without the extra coupling cap after the pot, it was noticeably scratchy when turned; the coupling cap blocks DC voltage from leaking stage to stage in the circuit, while allowing AC (alternating current) to pass, as in the guitar signal. I'll go back and experiment with different cap values in the future.



Master volume control on chassis rear panel






All done, back in the original cabinet with a fresh set of JJ tubes and a Celestion G10 Vintage speaker, ready to be gigged with tonight (I'll have my tweed as a backup, just in case). I might try the same WGS G10C that I've got in my tweed build, but for now, this one sounds pretty good.  Also, on my very last power tube bias reading, it landed in 19.65mA, the first year of production for this circuit; I found that to be a pretty damn cool coincidence!







Before (top), after (bottom).

Finished.