Dating Fender Tube Amps by Serial Number, Part 3
by Greg Gagliano

Copyright 1999, 20th Century Guitar Magazine.

I know, I know. It’s been over a year since Part 2 of this series was printed, but that’s because I’ve been busy collecting data. For those readers who may have joined us recently, I am doing some Fender amp research along with Devin Riebe and Greg Huntington. Our research efforts are now in their fourth year (will it ever end?). Part 1 and Part 2 of this series can be found in the March 1997 and November 1997 issues of TCG, respectively.

Again, I would like to thank everyone who responded to our request for data as outlined in Parts 1 and 2.

The main focus of this article will be speakers found in Fender amps, but before we get into that topic let’s go over a few other areas first.

Early silver face amps: Part 2 went into this topic in some detail, but since that article was printed, I have been asked a lot of questions about this subject. There still seems to be some confusion about how to distinguish between a silverface Fender amp that has the desirable AB763 circuit and one that has the less-desirable AB568/AC568 circuit. Part of the confusion stems from the lack of any AB568 or AC568 tube charts. Fender never printed any since there were plenty of leftover AB763 tube charts available and these were used well into 1969.

Although the cosmetics changed, the circuits remained unaltered from the blackface circuits on the earliest silverface amps. The date of the change from the blackface circuit to the CBS silverface circuit was dependent on the model, but most of the amps that were modified received the circuit change by mid-year of 1968.

The cosmetics of the silverface amps during the transition between circuits was also in transition making it difficult to determine circuit type on cosmetics alone. The amps during this period could have the earliest style silverface grill cloth or the more familiar silverface grill cloth. All of the amps would have the aluminum grill trim and they may or may not have the thin black vertical lines on the control panel. One clue that can be used is the ink stamped chassis date code that is usually located in the chassis, but is sometimes found on the underside of the chassis behind the tubes. These read something like T020366 or F034267 where the "66" and "67" denote the year and the "03" and "42" denote week of the year.

Of course, the most foolproof way is to pull the chassis and look at the layout. In fact, on the 40-watt and 80-watt amps you can simply pull the chassis out about 2 inches and look for the big honking ceramic power resistors that are connected to ground from the cathode (pin 8) of the power tubes. If those resistors are there, the amp has the dreaded CBS silverface circuit. I am also confident that the serial number can be used as a rough guide for determining the circuit, but again, pulling the chassis is the only way to confirm.

Transitional circuits: I’ve been getting quite a few reports from amp geeks about circuits that don’t completely match the schematic for a particular model. Leo Fender was notorious for tweaking circuits and the results of some of his tinkering can be found on late examples of an amp prior to the switch to a new circuit. Also, the different component values could be due to a necessary substitution on the production line when a particular value was out of stock. Ran out of 100K ohm resistors? Stick a 90K in there… no one will notice or care (at least not until the mid to late 1990s).

These changes have been observed on tweed, brown/blonde, and blackface models. Often the differences are minor such as small changes in resistor or capacitor values. I had a ’63 Concert with the 6G12-A circuit but had a completely different (factory stock) bias supply circuit than shown on the schematic.

Perhaps the most surprising transitional circuit that has been reported to me (thanks Brian!) is for the early 1968 silverface Showman. Two examples are known to exist; one from the February-March period and one from the March-April period. These amps have AB763 tube charts and normally any silverface amp made prior to May 1968 will have the AB763 (a.k.a. blackface) circuit. After April 1968, most of the big Fender amps received the AC568 circuit, which is a semi-cathode biased design. However, these two Showman amps have a fully cathode biased design that is factory stock!!! There is also an unconfirmed report of an early '68 Twin Reverb with the cathode bias circuit. If you see one of these cathode biased amps, please let me know!

These amps do not have a bias trim pot. The wire from the two 220K ohm bias resistors is connected to the brass control panel ground plate. The only thing connected to the bias cap/diode is the tremolo circuit. The cathodes are tied together and connected to a single 165-ohm resistor and 80 mfd 150V bypass capacitor which are both grounded at the other end. This cathode set up is similar to, but simpler than, the AC568 circuit. Every other part of the circuit (power and preamp) is identical to the blackface AB763 schematic.

Perhaps these two Showman amps were field prototypes, experimental units, transitional between the AB763 and AC563 circuit, or just a plain bad idea. Bad because cathode biased amps run very hot especially those with four power tubes (witness the Vox AC-30). In addition, a cathode biased Showman would produce something around 50 watts of power instead of the 80-plus watts from a fixed (grid) bias Showman. According to the owner of one of the cathode biased amps, it runs very hot… so hot that the tolex melted under the chassis mounting strap near the power tubes! Also, the amp eats power tubes, does not have much headroom and breaks up early.

Oddities: One of Leo's experiments or "one off" custom amps surfaced recently. It's a circa 1955 tweed Tremolux (5E9) that has two factory stock Jensen Hi-Frequency tweeters with a passive crossover between the Jensen P12R and the tweeters. I wonder if Leo was influenced by Magnatone's use of tweeters?

A few export model brown/blonde amps have surfaced that have a 6-way voltage selector switch located in the chassis, instead of on the back of the chassis. This necessitates removing the chassis from the cab to change the voltage setting (which would only be a problem if you hopping from country to country with the amp). Interestingly, these amps have Triad power transformers while concurrently produced domestic models had Schumacher units.

CBS era quality control: We’ve previously discussed the "OA" and "OB" date codes mistakenly stamped on the tube charts of January 1966 and February 1966 amps, respectively. As well, we’ve discussed misspelling "Division of Colombia Records Division" instead of "Division of Columbia Records Division" on the rear chassis panel on some amps from late 1965 and early 1966.

These mistakes were merely cosmetic. There were quality lapses in the circuits themselves during the CBS years, some of which had serious consequences. Examples of these include ceramic caps used in parallel to achieve the correct value instead of a single cap, change to often inferior sounding "chocolate drop" caps, and incorrectly wired circuits.

A friend brought me a ’66 Princeton Reverb that was humming badly. I recognized the 60 cycle hum and thought perhaps the filter caps were shot, but it turned out the two 100-ohm ground resistors for the tube filaments were never installed at the factory!!! I’m surprised the Fender dealer didn’t send the amp back and even more surprised that somebody bought it with the way it was humming!

I also worked on a late ’67 silverface Deluxe Reverb (with the blackface circuit) with an inoperable tremolo. Boy, was I surprised to see that the tremolo circuit was wired incorrectly by the factory!!! Either the factory worker was asleep at the wheel that day or there was a new hire that was still learning how to assemble amplifier circuit boards. I tend to believe that the latter idea has some merit since Fender practically doubled its size after the CBS takeover. Bigger facilities meant more workers with little or no experience.

Speakers: Like many people, I was a bit disappointed that Teagle and Sprung’s Fender amp book did not have much info on speakers. So, I have compiled a list of speakers used in Fender amps and took some photos of some of them as well. The list, presented below, is based on our actual observations, but is not comprehensive. If you have a Fender amp with a factory stock speaker other than one shown here, please let us know and we’ll add it to the list!

JBL: JBL speakers were optional (at additional cost) for nearly all models from 1960 to about 1980. JBL D-series speakers had orange baskets and Fender by JBL labels in the 1970s. JBL D-series speakers can generally handle upwards of 60 watts each. A pair of JBL D-120Fs in a Twin Reverb are only seeing about 40-watts each (no sweat), but remember that no speaker likes to see square waveforms. So, driving the Twin with any amount of distortion lowers the power handling capacity of the speaker, which makes any speaker more susceptible to damage… even a high-wattage type like the JBL.

Jensen: Jensen was the prevalent stock speaker in Fender amps from 1946 through about 1961. As the story goes, Leo Fender wanted Jensen to make some changes to speakers and either the speaker couldn't (price constraints?) or wouldn't do so. That's when ol' Leo switched over to Oxford as the standard speaker (though Jensens were still used from time to time). Just conjecture, but the lack of orders from Fender from 1962 - 65 must have hurt Jensen's pocketbook so they hit up the new owners of Fender (CBS) for some business. These Jensens wear brown and gold Fender by Jensen label and were put into Fender amps beginning in late 1965 through about mid-1967. Some amp geeks don't like the way these Fender label Jensens sound, but let your ears be your guide. I think they sound just spiffy.

Jensen Vibranto LI and MI series speakers (alnico magnets) and Jensen EM-series speakers (ceramic magnets), while excellent, were not used by Fender. I have included them here because I get a lot of questions about them. They are were often sold as replacements for blown speakers which is probably one reason why the ended up in more than a few Fenders. The Vibranto LI series speakers had a lifetime warranty and it seems that Jensen went out of the musical instrument speaker business just in time to avoid the claims. All speakers can and will fail eventually (just like the hard disk on your computer)… remember that.

Jensen speaker models denote their approximate power handling capacity and magnet type. The actual power ratings have been published in several books so I'll discuss them in general terms here.

The R, S, and T suffixes denote a low power rating… good for Princetons and Champs, but the R is barely able to handle the power of a Deluxe. The Q and P suffixes denote a medium power rating. These are especially good for multi-speaker amps up to 40-watts since multiple speakers divide the amp's total output power between them. For this reason, the P10Q is the speaker to have in the 5F6-A Bassman. Note that it does not appear that Fender used the "P" rated speakers very often. The N and LL suffixes denote a high power rating, with "high power" being a relative term. The P12N, on a good day, can handle 20 watts. It's no wonder that 80-watt Twins easily shredded a pair of them. Note that Fender did not use the "L" rated speakers (but Ampeg and Leslie did).

Oxford: Oxford speakers codes work in a similar fashion, but it is the letter that denotes power handling. The higher the letter, the higher the power rating. I found an Oxford ad in a 1960s trade magazine with the peak power ratings of some speakers: K = 25 watts, L = 30 watts, M = 40 watts, and T = 45 watts (12" speaker) or 60 watts (15" speaker). It is important to note that these are peak power ratings, not RMS power. The RMS rating is more realistic and is usually about half of the peak rating so use that as a rough guide.

The "J" rated speakers are usually found on 12-watt Princetons. The "K" rated speakers are found in reverb and non-reverb Deluxes and in multi-speaker amps up to 40-watts such as the Tremolux and Concert. The "L" rated speakers are found in reverb and non-reverb Deluxes, some Tremolux amps, and multi-speaker amps like the blackface Concert, Super Reverb and Vibrolux Reverb. The "M" rated speakers had good service life in the piggyback Bassman and Bandmaster amps, but were easily blown in blonde Twins. The "T" rated speakers were standard in Twin Reverbs, but like the Jensen C12Ns, they often had a short service life.

Many amp geeks don't like Oxford speakers found in Fender amps from 1965 through the 1970s. The gap distance was increased in the Oxfords that Fender used later in the decade and this reduced their efficiency (and they were cheaper to make this way). Again, I say let your ears be your guide. I've heard many great sounding Fender amps with Oxfords. I will admit that I prefer Jensens, but I've never let an Oxford speaker sway my decision from owning a Fender amp. Additionally, the Oxfords from early '60s generally sound very good. According to noted vintage amp specialist Gregg Hopkins, these early Oxfords were constructed similarly to Jensens from that period with respect to materials and voice coil gap. That could explain why they sound good.

CTS: CTS (Chicago Telephone Supply!) speakers were used occasionally in Fender amps until the mid-1960s. These are good quality speakers that tonally lie between Jensens and Oxfords. The alnico 10-inch CTS speaker was the most prevalent speaker in Super Reverbs from the mid-1960s through the 1970s.

Utah: Fender didn't use Utah speakers very much until the 1970s. The Utah speakers from the '50s sound very good and I've heard a killer '66 Super Reverb that was equipped with factory original Utahs. Generally, the Utah speakers of the '70s weren't as great sounding as their predecessors, but again let your ears be your guide. If you like the way your '75 Twin Reverb sounds with its Utah speakers, just leave it alone and go right on playing. Utah went on to become Pyle of Radio Shack and car audio fame.

Eminence: Eminence has its roots in CTS (Mr. Gault left CTS to found Eminence) and many of the early Eminence designs are similar or identical to CTS speakers (good examples of the similarities can be found in mid-1970s Ampegs). Fender began using Eminence speakers as standard in nearly all of its tube amps beginning in the early 1980s. These are generally made to Fender's specifications and in some cases, such as the reissue '65 Twin Reverb, the speakers were designed to emulate the Jensen C12N speakers which were often found in the original '65 Twin.

Rola: Yet another speaker that Fender used in the mid to late 1970s was Rola.

So in technical terms, why don't the non-Jensen speakers from the mid-1960s through the 1970s sound as good as Jensen speakers? Speaker guru Ted Weber explains:

"Utah, CTS, Oxford, etc. simply copied the Jensen designs and started competing for Fender's business. As a result of the price wars, they had figure out how to make the speakers produceable with a very low reject rate as well as use less expensive parts, i.e. smaller magnets. So, they widened the gaps to make them easier to throw together on a fast assembly line. This lowered the energy, so the voice coils were shortened to compensate. The companies also needed to produce speakers with long term reliability, so they doped the surrounds. The end result is that with some of these speakers you get a relatively sensitive driver that sounds great at lower volumes, but falls apart when you push it -- flabby on the low end and/or harsh on the high end."

Replacement speakers: It is very common to find non-original speakers in Fender amps made up through about 1980. Because reconing wasn't a common option until the 1970s, players simply replaced the speakers if they blew up. In some cases, such as Altec and JBL, the factory would recone a speaker. Today, reconing is a very popular option for players to keep their amp's speakers original. Reconing must be done correctly and with the right parts so stick with a reputable reconing service that offers a warranty. In most cases, the reconed speaker will sound nearly as good or as good as the original. In some cases, the speaker will sound even better. The reconed Oxford 12K5 in my Deluxe Reverb sounds better than any original cone 12K5 I've heard.

There is a strong market for used speakers. Many times a player can find an original speaker to replace the non-original speaker. Another option is to install vintage style speakers. Jensen has reissued the C10Q, P10R and P12N and WeberVST makes many models of Jensen-style alnico and ceramic speakers.

One final note before you scope out the speaker chart -- there are exceptions to every rule and this especially applies to Fender! So, if you see a factory stock P12P in a tweed Deluxe, don't be overly surprised.
Bandmaster 5C7, 5D7 (wide panel) Jensen P15N
Bandmaster 5E7, 6G7 (3x10) Jensen P10R, P10Q, Oxford 12K5R-1
Bandmaster 6G7-A Oxford 12M6, Jensen C12N
Bandmaster AB763 and silverface Oxford 12T6, Jensen C12N, Utah 12"
Bandmaster Reverb Silverface Oxford, Utah 12"
Bantam Bass Silverface Yamaha trapezoidal
Bassman 5B6, 5C6 Jensen P15N
Bassman 5D6, 5E6-A, 5F6, 5F6-A Jensen P10R, P10Q
Bassman 6G6 , 6G6-A, 6G6-B Oxford 12M6; Jensen C12N
Bassman AA864, AA165 Oxford 12T6; Jensen C12N; Utah ceramic
Bassman Silverface Oxford 12T6; Utah; Rola
Bassman Ten Silverface CTS 10" ceramic
Champ 5C1, 5D1 Cleveland 6" alnico, Jensen P6T
Champ 5E1, 5F1 Oxford 8EV; Jensen P8T; CTS 8" alnico
Champ AA764, silverface Oxford 8EV
Concert 5G12, 6G12 Jensen P10R, P10Q
Concert 6G12-A Jensen P10R, P10Q, C10R; Oxford 10K5
Concert AB763 Oxford 10K5, 10L5; Utah V10LXC1
Deluxe 5B3, 5C3, 5D3 Jensen P12R
Deluxe 5E3 Jensen P12R, P12Q
Deluxe 6G3, AB763 Oxford 12K5-6
Harvard 5F10, 6G10 Jensen P10R
Musicmaster Bass Silverface CTS 12" ceramic; Oxford 126PJ4
Quad Reverb Silverface Oxford 12T6; Utah V12PC; Rola
Princeton 5C2, 5D2, 5F2, 5F2-A Jensen P8T; Oxford 8EV; Cleveland 8"
Princeton 6G2, AA964 Oxford 10J4; Jensen C10R
Princeton Silverface Oxford 10J4
Princeton Reverb AA1164 Oxford 10J4, 10L5; Jensen C10R, C10N
Princeton Reverb Silverface Oxford 10J4; CTS
Pro 5A5 Jensen F15N (field coil), P15N
Pro 5B5, 5C5, 5D5 Jensen P15N
Pro 5E5, 5E5-A Jensen P15N
Pro 6G5, 6G5-A Jensen P15N; Oxford 15M6
Pro AB763 Jensen C15P; CTS 15" ceramic
Pro Reverb AA165 Jensen C12N; Oxford 12L6
Pro Reverb Silverface Oxford 12L6, 12T6; Utah, Rola 12" ceramic
Showman 12 6G14, 6G14-A, AB763 JBL D120F with tone ring
Showman 6G14, 6G14-A, AB763 JBL D130F with tone ring
Dual Showman AB763 JBL D130F
Dual Showman Rev Silverface JBL D130F
Super 5B4, 5C4, 5D4, 5E4, 5E4-A Jensen P10R
Super 5F4 Jensen P10R, P10Q
Super 6G4, 6G4-A Jensen P10R, P10Q; Oxford 10K5
Super Reverb AA763, early AB763 Jensen C10R
Super Reverb AB763, silverface CTS 10" alnico or ceramic; Oxford 10L6; Rola 10" ceramic
Super Six Reverb Silverface Oxford 10L6; CTS 10" alnico
Tremolux 5E9-A Jensen P12R; P12Q
Tremolux 6G9, 6G9-A Jensen P10Q or Oxford 10K5R w/tone ring
Tremolux 6G9-B Oxford 10K5
Tremolux AB763 Oxford 10K5, 10L5; CTS 10" ceramic
Twin 5C8, 5D8, 5E8-A Jensen P12R; P12Q
Twin 5F8, 6G8 Jensen P12N
Twin 6G8, 6G8-A Jensen P12N; Oxford 12M6
Twin Reverb AB763 Jensen C12N; Oxford 12T6
Twin Reverb Silverface Oxford 12T6; EV; Gauss; Utah; Rola
Vibro Champ/Bronco All Oxford 8EV
Vibrolux 5E11, 5F11 Jensen P10R
Vibrolux 6G11, 6G11-A, AB763 Oxford 12L6, 12M6
Vibrolux Reverb AA864, AA964 Jensen C10Q; Oxford 10L5
Vibrolux Reverb Silverface Oxford 10L5; CTS 10" alnico
Vibrasonic 5G13, 6G13-A JBL D130, D130F
Vibrosonic Reverb Silverface JBL D130F, K130F; Gauss; EV
Vibroverb 6G16 Oxford 10K5
Vibroverb AA763, AB763 JBL D130F; Jensen C15N; CTS 15" ceramic

A New Internet Resource: Over the last 12 months or so, I've had several people ask me to put together some sort of Fender amp guide on my web site. A great idea, but I simply didn't (and still don't) have time for such a venture. However, Mark Ware thought it was a good idea, too, and he put together a very informative Fender Amp Field Guide.  The site is nicely done and includes descriptions, pictures, and schematics for Fender tube amps as well as other Fendercentric information.

Dating by Serial Number Update: The good news is that good progress is being made, but I am in fact, still desperate (if you could only see the pitiful look on my face right now) for more info, especially for tweed, brown, blonde and silverface amps. I have practically no information for some models like the Bassman 100 and other "non-popular" silverface amps. And keep those blackface amp data coming! Also, I need more information from reissue ’63 Vibroverb amps and Custom Vibrolux Reverb amps.

I'm still trying to make sense of the "mysterious" production numbers and this is one area where I don't have a lot of info… mainly because production numbers were not written on a lot of the tube charts. However, the next article in this series (Part 4) will discuss some preliminary production trends (oh boy!). Were 1,000 or 10,000 or 100,000 Super Reverbs made? Stay tuned…

Reminder: We are still in need of information about any and all Fender tube amps. Everything is confidential, we don’t make record of who owns what amp in the database. What we need is the following:

1) Model name

2) Model number on the tube chart

3) Date code letters on the tube chart

4) Production number on the tube chart

5) Speaker codes and model (if speaker is original)

6) Transformer codes (if the amp doesn’t have date codes on the tube chart)

7) Cosmetic features (flat/raised logo, tweed/tolex, blackface/silverface, rough/

smooth blond tolex, white/skirted knobs, TV-front/wide-panel, etc.)

So please... send your amp data to the author by e-mail to Thanks for your support and stay tuned for Part 4 of this series!

Special thanks to Gregg Hopkins at Vintage Amp Restoration, St. Louis, Missouri for allowing me to photograph the speakers from his inventory.

The author and his partners would like to thank those people who have sent us Fender amp information (and you know who you are dahlings).  Also thanks to the many dealers that we visit for allowing us to make notes about the amps in their shops and at their guitar show booths.

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