1954 Fender Pro-Amp

In the early 1950’s you could buy almost nothing better made or more powerful than the Fender Pro-Amp. The only other amp in Fender’s lineup more powerful and with more features, was the Twin-Amp. Fender put the word “Pro” in the name for a reason, this was an amp marketed to professional lap steel, pedal steel, and electric guitar players, and they were about the only ones who could afford plunking down this much cash for a guitar amp. In 2024 money the Fender Pro would have sold for over $4000 new, so that puts it into perspective what these amps meant to their owners.

This particular amp is a transitional model that has features of both the 5C5 and the 5D5 circuits. It has 3 miniature 9-pin preamp tubes (V1-V2 = 12AY7, V3 = 12AX7), a pair of 6L6G (5881) power tubes putting out about 26 Watts RMS, and is rounded out by a 5U4G rectifier. Starting with the front end, the preamps of this Pro are still the very old-school Grid Leak Bias circuits taken from the world of radio engineering. A preamp using Grid Leak Bias produces more overdrive and has a brighter grittier tone than the more common Cathode Bias used almost exclusively for preamp circuits made after the late 1950’s. The tone control is right after the first gain stage and is a very simple treble cut style control, this doesn’t offer a ton of options but works surprisingly well for most situations. Next the signal goes to the phase inverter which in the 5C5 style is a Paraphase or “Split Load” circuit, this is also a very archaic design not seen in amps after the 50’s, again due to the tendency for early onset distortion and lack of balance to the power tubes. These things today are not a bad thing, guitarists love distortion and vintage laden tone, but in the 1950’s and through the 1970’s Leo Fender was all about making his amps the cleanest and the loudest that current technology could offer - this meant he quickly moved away from these circuits and never looked back. Finally the signal ends up at the Class AB Cathode Bias style power amp common to many Fender Tweed models, it puts out enough watts to keep up with a full band easily. This amp has a negative feedback loop and a standby switch - both of which are from the 5D5 schematic. The Pro originally comes with a single 15” speaker, which gives the tone fatter bass, clearer highs, and a noticeable mid scoop. Because of the old-school type circuits in this 5C5/5D5 Pro, it oozes a ton of vintage vibe, the kind that is not well captured by any of the re-issues or boutique takes of the Tweed lineage. It is a unique piece of history that sounds incredible and is one of the better models in Fender’s lineup, then or now. Notable users include: Buddy Holly, Richard Lloyd, and Joe Bonamassa.

This amp came in to my shop having already been partially restored from a previous repair years earlier. The competency of the older restoration, which was a basic electrolytic cap job, was quite decent, using good parts quality and technique. However, there were a few major issues left unaddressed, including some preventative maintenance. This one needed a cap job for all coupling units, new power/plate resistors, a new power transformer, cleaning, and new tubes. The amp already had a speaker swap done, using a baffle converter to put a 12” Eminence GA-SC69 Alnico in place of the original 15” unit. This was done well and kept in place per customer request. Somewhere in history the cab was painted black and the grill cloth was given some artistic flair. Usually I’m not the biggest fan of painting tweed cabs, but this one actually looks pretty cool, and the paint was done better than any other I’ve seen.

Amp was completely restored with the highest quality parts possible. All coupling caps were replaced with (Handmade in the USA) Jupiter Red Astron Tin Foil units to replace the original leaky and decades old out of spec caps present since 1954. These caps are exact reproductions of 1950's Red Astron brand units originally put in Tweed Fender's. The outside foil markings of the Jupiter’s were all aligned to the correct junctions of the circuit - which are the ones that present the lowest impedance. This is a very old-school technique that helps prevent noise and is almost unheard of today. Most modern signal caps do not even have Outside Foil markings. New 3 Watt Metal Film power dropping resistors were installed for better reliability and lower noise floor - the original units had heat damage and were out of spec. New 1.75 Watt Metal Film resistors made to Mil-Spec by Dale were used for the hard to find 250K value plate positions. The original plate resistors had drifted up to as much as 340K! The rest of the carbon comp resistors were all tested and found to be within spec (or close enough), they were kept in place along with the tone control coupling cap to preserve the unique vintage tone of the amp. When restoring an amp like this it’s an important thing to not sterilize the tone of the amp with all new parts, certain parts that are not in critical or dangerous circuit positions can be retained to preserve the “drift” that occurs over time and is impossible to replicate otherwise.

A new American made Mercury Magnetics FTWD-PRO-P Power Transformer was installed - this is the correct type of PT for this amp and is an authentic reproduction of an early 1950's Fender Pro #6516 transformer. The one previously in place was from an unknown tube amp (most likely made in the 1960's) and did not have the correct power/current ratings or secondary HT voltages. The thing heated up so hot if felt like a space heater, not a good sign in a PT. Also it's physical format was completely wrong being the lay-down horizontal type - the new PT is the correct stand-up vertical orientation, it puts out the right voltages, and stays cool under stress. Along with Hammond, Mercury Magnetics makes the finest reproductions of vintage iron.

The original tubes all tested good and but the power tubes were replaced due to them being the incorrect model for this amp. A new set of JJ 5881's were installed to replace the 6L6GC's previously in place - this was done because of the 6L6GC having a 30 watt plate dissipation rating compared to the 5881's 23 watt rating. Although these are mostly compatible tubes, early 50's Fender amps like this Pro have output transformers that are more safely operated with 5881 tubes. V1 was kept a 12AX7 per customer request - the circuit calls for the compatible 12AY7 instead. The customer wanted the “Microphone” channel to be differentiated from the “Instrument” channel with the higher gain 12AX7 for another tonal option. The final lineup was: V1 = SIEMENS 12AX7, V2 = RAYTHEON 12AY7, V3 = EHX 12AX7, V4/V5 = JJ 5881 Matched Pair, V5 = EHX 5U4GB. These tubes were picked for best tone and reliability. The 5881’s Cathode Bias was set to 93% Class AB Plate Dissipation with a B+ of 427VDC. The tube sockets were all cleaned with a De-Oxit treatment to prevent noise from corrosion. The pots were sprayed out and the amp was cleaned inside and out. New #6 1.25" back panel screws were installed to replace the worn out originals. The amp now functions like it should and sounds great.