I know, I know. 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?
Again, I would like to thank everyone who responded to our request for data as outlined in Parts 1 and 2. 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. Part of the confusion stems from the lack of any AB or AC tube charts.
Fender never printed any since there were plenty of leftover AB tube charts available and these were used well into 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 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 T or F 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 watt and 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. 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 K ohm resistors? Stick a 90K in there… no one will notice or care at least not until the mid to late s. Often the differences are minor such as small changes in resistor or capacitor values. Perhaps the most surprising transitional circuit that has been reported to me thanks Brian! Two examples are known to exist; one from the February-March period and one from the March-April period.
These amps have AB tube charts and normally any silverface amp made prior to May will have the AB a. After April , most of the big Fender amps received the AC 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 K ohm bias resistors is connected to the brass control panel ground plate. The cathodes are tied together and connected to a single ohm resistor and 80 mfd V bypass capacitor which are both grounded at the other end.
This cathode set up is similar to, but simpler than, the AC circuit. Every other part of the circuit power and preamp is identical to the blackface AB schematic. Perhaps these two Showman amps were field prototypes, experimental units, transitional between the AB and AC 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 In addition, a cathode biased Showman would produce something around 50 watts of power instead of the 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. One of Leo's experiments or "one off" custom amps surfaced recently.
It's a circa 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? 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: 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.
I recognized the 60 cycle hum and thought perhaps the filter caps were shot, but it turned out the two ohm ground resistors for the tube filaments were never installed at the factory!!! 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. 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. JBL speakers were optional at additional cost for nearly all models from to about JBL D-series speakers can generally handle upwards of 60 watts each. A pair of JBL DFs in a Twin Reverb are only seeing about 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 was the prevalent stock speaker in Fender amps from through about As the story goes, Leo Fender wanted Jensen to make some changes to speakers and either the speaker couldn't price constraints? 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 - 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 through about mid 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. 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 watts since multiple speakers divide the amp's total output power between them.
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 watt Twins easily shredded a pair of them. Note that Fender did not use the "L" rated speakers but Ampeg and Leslie did. 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 s trade magazine with the peak power ratings of some speakers: 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 watt Princetons. The "K" rated speakers are found in reverb and non-reverb Deluxes and in multi-speaker amps up to 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.
Many amp geeks don't like Oxford speakers found in Fender amps from through the s. 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.