Fender Re-Issue Reverb Hacks

This article explains how to tweak the Fender Re-Issue Reverb units to sound more like the old vintage units.

Warning: Tube equipment contains very high voltages, often several hundred to over one-thousand volts! Do not attempt anything described here unless you have training and experience working on tube electronics. Also, doing what is suggested here WILL VOID YOUR WARRANTY. I suggest only modifying equipment which is out of warranty.

Ok, now that the warnings are out of the way, lets proceed.

First, we'll talk about tubes. The original Reverb used a 6K6GT as the driver tube. This tube is long out of production, but is similar and pin-compatible with the 6V6GT which is what Fender puts in the re-issue. The 6V6 is a more powerful tube, and expects a lower transformer impedance than the 6K6. Luckily Fender used a transformer which is essentially identical to the original (13K primary). So the first thing you want is a new old-stock 6K6GT. These tubes are luckily not an audiophile or common guitar amp tube. Changing this tube will lower the amount of drive to the reverb spring, widening the range of the "dwell" control. The drive will be a little smoother, softer without the tendency to plink so much.

The other two tubes are usually a Sovtek 12AX7WA and a Chinese 12AT7. Both are OK tubes, but you can improve these by changing them. I'd start with the 12AX7. Ideally, you would want to get a genuine NOS 7025. These are rare and expensive though. I've found that the 5751, especially GE are excellent in this application and many others. The 5751 is a shock-resistant military dual triode. They are usually completely free from microphonics. They have a gain of 70 vs. 100 for the 12AX7, and for military use are rated only for 280 volts. They sound very smooth and clean. Some can be noisy, but most are pretty good. The 5751 are readily available as well.

The Chinese 12AT7 isn't too bad a tube. But, again nothing beats good old USA (or English, German, Dutch) stuff. The ultimate 12AT7 are Mullard gold pins. The GE's sound good, and the Phillips ECG work well in this application too.

The other tweak requires opening the unit, and removing the tone control PCB. Behind the volume control pot is small 250pf ceramic capacitor. Change this to 390pf, I've tried as high as 470pf. Changing this cap will definitely change the character of tone. I suggest starting with a 390pf film or silver mica cap. See how you like it. To my ear, the sound becomes a bit richer and fuller. Be careful soldering in a new cap. They didn't leave a lot of room, and you don't want to short it to the pot supports.

While you have it taken apart, you might want to check the reverb pan. I've seen a number of these where the springs have come unhooked, or improperly hooked in transport. That's why you should always, always, lock the spring down when you move the unit. You also may want to try a 3-spring pan instead of the stock 2-spring. The 3-spring isn't the vintage sound, but it will make the fullest, lushest, most church-like reverb sound. I put one in my Deluxe Reverb amp, and I really like it.

What the heck is a Fender "Bright switch?"

The "bright" switch on every vintage Fender amp is a switch that puts a small value capacitor (100 picofarads in tweed amps, 120 picofarads in the blackfaces, and 47 in most Deluxes) across the volume control, from input to output. Since higher frequencies see a small capacitor as an almost dead short, the high frequency signal bypasses the volume control altogether. The lows and mids get attenuated by the volume control, but the very highs go around the volume control through the capacitor without much attenuation. We hear this as bright. For this reason, the control is more effective as the volume control is turned down and conversely, the "bright" switch will have hardly any effect when the volume is turned up. If the volume is all the way up, it will have no effect whatsoever.

Another Classic Fender: '65 Fender Black-Face Deluxe Reverb

It's small and simple to operate, sounds terrific for any kind of music, and gets a great overdrive limitation when it's cranked up. The pure tone of a single 12" Jensen shows why it's really one of the greatest guitar speakers ever made.
Perhaps the primary secret to the great sound of this amp, however, is the complement of 6V6 power tubes. Essentially, the 6V6 is a smaller version of the 6L6 with a similar impedance and sound; it puts out about 60% of the 6L6's power. This means it can be a clean amp for a small country club gig, or provide just the right level for smooth overdriven leads in a small rock club band. It's great to use a Superb Reverb for rhythm and switch to Deluxe Reverb for leads. The two amps are a perfect match.
Power: 20 watts into 8 ohms
Tubes: One 6V6 duet output, four 12AX&, two 12AT7 preamps, one 5Y3 rectifier.
Speaker: one 12" 8-ohm Jensen Special Design
Special features: Dual sensitivity inputs on dual channels that bass and treble controls; the second channel has reverb and vibrato that can be activated with a footswitch.

Fender Tweed Deluxe Amplifier

One of the most popular tweed Fender amplifiers ever made was the Tweed Deluxe. The chronological order, from beginning to end, is: 5B3, 5C3, 5D3, 5E3, 5F3, with each being different from the others.
The 5B3 and 5C3 are nearly identical, with both having a wide top panel on the front and three inputs. Two inputs were for the musical instrument channel and one input is for the microphone.
Typical of both amps were the metal 6SC7 preamp tubes and 6V6 output tubes. 6SC7 tubes have lower gain than a 12AX7 and use a socket that is the size of the output tube. While the 6SC7 is twin triode like the 12AX7, the tube has only one cathode.
The 5B3/5C3 amps have an unique grid leak bias first gain stage. This is one way to bias a preamp tube in which there is no cathode resistor, but instead a capacitor and large value resistor (5 meg) are in series with the grid signal circuit. Grid current leaks out through the input circuit causing a negative voltage to appear on the grid, thus biasing the tube.
Of course this arrangement has its disadvantages, the main one being that it cannot handle a very large input signal. Grid leak bias circuits are pretty much obsolete and have not been used since the mid 1950s. Be careful using a tube driver or any other high-gain device on these amps.
The5D3 Deluxe was introduced in 1955. This was nearly identical to the earlier models except that it used a self-balancing paraphase inverter, whereas the earlier models used a conventional style. Some of the 5D3 Deluxes have 6SC7 preamp tubes, but others had the chassis punched for the large octal size socket with an adapter covering the chassis hole and a 12AY7 and a 12AX& pin place of the two 6SC7s.
From 1958 to 1960, the 5F3 Deluxes were produced. This circuit was almost identical to the 5E3. Voltages were somewhat higher, giving more gain. Cosmetically, it looked the same. Both the 5E3 and 5F3 had more gain than all the previous model Deluxes.
All tweed Deluxes had solid pine cabinets with Jensen speakers. Some of them came stock with P12R Jensen and some had the P12Q, which sounded better. When buying an old Deluxe don't assume that the speaker is original just because it happens to be a Jensen. Jensen made many models of 12" speakers, some of which were no good. For instance, both the P12S and the P12T are Jensen but are non-original with hardly any power. The P12N is worth major dollars, if you can find one.
Triad made the transformers for all tweed Deluxes. If the output transformer has end-bells, it is non-original. Original transformers were wound with papger and had no end-bells.

From Desktop Reference of Hip Vintage Guitar Amps

Classic Fender: "Black Face" Twin Reverb

Power: 80 watts into 4 ohms.

In its day, it was referred to as the workhorse of the industry. The Fender Twin Reverb was favored by Jazz, country, and rock players from the moment it rolled off the production line. At its peak, the Twin was considered the ultimate power amp because of its power and compactness. Its open cabinet provided a broad dispersion pattern, perfect for filling up a club from a small stage.
Much has been made about the "pre-CBS" era of the Twin Reverb (and other Fender products), and in the opinion of many amp critics, there is something to said about the controversy.
When Columbia Records (a division of Columbia Broadcasting System) purchased Fender Musical Instruments in 1965, Leo Fender was removed from the management loop and a team of previously frustrated engineers quickly made several circuit changes to the amp. These changes, they said, "would make the amp more stable and extend tube life while eliminating most of that nasty distortion." The changes involved adding resistors and capacitors at several points in the circuit.
The result was that the post-CBS Fender models sounded constipated and tight, with none of the life and feel of the earlier ones. When amp sales fell dramatically, the sales force demanded a return to the pre-CBS design, which was quickly agreed to by Fender management. Though the lame Fenders were produced for just eight months, it took much longer for the retailers to sell them off, and the damage was done. Pre-CBS amps became immediately more desireable for musicians in the know. Because Fender replaced its traditional black-faced control panel with shiny silver plastic panel at the time of the changeover, it's easy to distinguish between pre and post-CBS amp.
The sliver-faced amps built between the CBS takeover and the later ones with a master volume and pull-distortion knob are very good-sounding amps, and a bargain for those who can't pay the inflated vintage prices of an old black-faced twin.

Who invented the guitar amp?

Contrary to popular wisdom, Leo Fender did not invent the guitar amp. His amps used basic circuits that could be found in the Radiotron Designer's Handbook, most of which were invented and patented by the Western Electric scientists working at Bell Labs. Vacuum tubes circuits were first used in audio amplifiers and radios rather than in musical instrument amplifiers.
By the time Leo entered the amp field, Rickenbacker, Gibson, National and other manufactuers had done much of the pioneering work.
Fender started making musical instruments amplifiers at his radio shop in Fullerton, California during WWII. Using surplus parts, the first few were custom built for professional musicians. After the war, he started commercial production with he and Doc Kaufman started building K & F lap steel guitars. At that time, many people saw the electric guitar and the amplifier as a single instrument, so Fender saw the two as interconnected.
Leo Fender had an immeasurable impact on the development of the electric guitar, but in his early years he contributed little to the development of the amplifier. It was his conclusion after several years that the limitation of the speakers and Alnicomagnet pickups available to him kept him from obtaining perfect electric guitar sound. After he developed the Telecaster, Stratocaster, and Jazzmaster pickups, he focussed on improving the tone circuits in his amps to make up for inherent pickup and speaker problems.
Tone controls were Leo Fender's foremost contribution to the field of amp design. When Fender amps started sporting treble, bass , middle and presence controls in 1957, the modern guitar amp had more or less arrived. As a bonus, Fender amps were rugged and roadworthy. Starting in '48, Fender built cabinets that were virtually indestructible. To test durability, Fender sent his amps on the road with bands like Bob Wills and his Texas Playboys. These Fenders were knocked around in buses over thousands of miles of highways and endured countless nights in smoky dance halls and bars. Leo saw the abuse his products got from working musicians, and he built his amps to take it.

One of the Greatest: '58 Fender Tweed Champ

This amp covered in luggage-style tweed is considered the cornerstone of all tube guitar amplifiers. First produced in 1953 for beginners, most of them were instead purchased by lap steel guitarists for either country or Hawaiian music. But the fact that they were cheap made them also attractive to young rock and rollers.
The Champ provided great distortion at a relatively low volume level, which suited the mikes and studios of the 50s and 60s..
Technically speaking, the Champ couldn't be simpler. One 12AX7 dual-triode tube acts as the first gain stage and also as the output driver to a single 6v6 output tube, which put out a 3.5 watts (!) RMS inot a 4 ohm load. This was the simplest example of how to use a tube from the RCA tube applications manual, a classic reference book supplied to amp manufacturers in the 40s and 50s. If you are one of those lucky people who have managed to get your hands on the Champ amplifier, do you need to change your 6v6 tubes every few years if you use it regularly. The 12AX7 shouldn't need replacement if its a good one.
If you have bought a Champ Fender from Ebay or Craigslist and you are disappointed with the quality, check to see if the speaker is a true 4 ohm model. Blown Champ speakers are often replaced with a more common 8 ohm speaker, resulting in a big loss of power and tone. Looking for more power and tighter sound? Try replacing the 5Y3 with a solid state rectifier. But stay with a tube rectifier for sweeter distortion.