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How did CBS change the Fender Amps?

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  • How did CBS change the Fender Amps?

    Having serviced guitar amps for 9 years now, I kept hearing about CBS making design changes that negatively affected the tone of the Fender amps.

    Exactly what changes did they make, and in what ways did this alter the tone/frequency response of the amp?

    Looking at the diagrams for a silverface Deluxe Reverb A1172 and comparing it to the schematic of the AB763, the tone stacks appear to be identical. (The AA763 used a.033 where the AB763 uses a .047.)

    The coupling cap at the input of the phase splitter was increased from .001 in the AB763 to a .01 in the A1172, along with a compensating change in the grid resistors from 1M to 330K. (Without calculating it out, I would expect a slight boost in the low end here.)

    I also see a low pass filter at the grids of the output tubes, having a cutoff frequency of 88.5 KHz. The effect of these should be imperceptible at audio frequencies. I'm guessing they are for suppression of parasitic oscillations.

    Two filter caps were moved to the load side of the Standby switch.

    Aside from these, the two designs appear to be practically identical.

    So, why are the silverface units so maligned?

  • #2
    Originally posted by techineer View Post
    Exactly what changes did they make, and in what ways did this alter the tone/frequency response of the amp?

    So, why are the silverface units so maligned?
    The Deluxe that you have referred to along with the Princetons and Champs, probably had the fewest changes made to their circuits. The larger amps had major changes done to them that dramatically changed their tonal output. I guess that just because silverface Twins and Bassmans sounded bad, all silverface amps were bad.

    Additionally, the overall construction techniques and lead dress changed enough that they needed to add things like snubber caps to tame parasitic oscillations.

    Comment


    • #3
      Changes in the circuit design caused major changes in lead dress.

      In pre-CBS amps, the wires had straight runs from the board to the tube socket pins, and few, if any, crossed over. Post-CBS amps, however, had many wires crossing over each other, and were often wired in several layers.

      This simple wiring reduced cross-talk and minimized coupling between wires. Some believe that the straight or uncluttered signal path leads to a "cleaner" or "faster" sound.

      Other changes include coupling and electrolytic capacitors, although it could be argued that older amps had less expensive caps that sounded better for guitar.

      For instance, higher F electrolytic caps are now available at a much lower price, but they also "stiffen" the power supply and make the amp less responsive. Some musicians describe their old amps as "squishy".

      Better quality power sections also had a "negative" impact. In older amps, the current draw from the AC plug would vary as you played. Newer amps don't do this - the current stays constant.

      There's a ton of small differences like this that add up to a huge difference in an amps tone and feel.
      See the birth of a 2-watt tube guitar amp - the "Dyno Tweed"
      http://www.naturdoctor.com/Chapters/Amps/DynoTweed.html

      Comment


      • #4
        Fender was constantly changing circuits. Many well respected blackface amps had already been changed somewhat before the silverface cosmetics came along, but to sort of answer the question in short, most of the designs remained essentially the same until the early 70's. That's when Fender's engineers were told that they needed to add master volumes and boost circuits to the amps, and most importantly, boost the wattage higher and higher as this was the era of the "wattage wars". You got a hundred? WE'VE got 120! That's 20 better, huh! The master volumes and boosts were not the same types of circuits that we would use today, and just muck up the otherwise fine sounding pre-amp. As Fender tried to increase the power, they eventually ended up with really loud, honky, lifeless beasts that could only be reliably operated with the Sylvania STR's made at Fender's request.

        Another difference was in construction methods. CBS started pushing for cheaper production which led to the "rat's nest" wiring and the snubber caps to(not in all cases) prevent parasitic oscillation. Also, prior to the early 70's the grillcloth was stapled directly to the baffle, which was screwed to anchor blocks glued to the cabinet. Later silverfaces have a separate grillcloth face that sticks to the front of the baffle with velcro-like fasteners. these things get old and warp/fall off. We've all seen them. It looks cheap- and if it looks cheaply made, in your mind, the rest of the amp must be crappy, too.

        The end of the story is that even though there is a bit of crossover in the late 60's to early 70's where those amps are just as good as the earlier ones, Players of that time didn't have access to the kind of information that players today think of as being fairly basic. Without circuit knowledge, the only way for Joe Guitar Player to RELIABLY tell "good ones" from the "bad ones" with 100% accuracy, was by color. All those black ones are good. Silver, you might get lucky, might not.

        Thus, the legend is born and perpetuated.

        Comment


        • #5
          Definitely true, depending on what you heard or read.
          I heard that and the component orderings had ridden into some snags, and that a bunch of components that mattered were replaced by ones that...just don't care...whatever would make an amp function...because they're now what is on the shelf, got put in.
          And that early after the CBS change the parts lasted for a while until re-ordering occured...those amps were like the old ones...mostly.
          Hit or miss is easy to tell since I've tried the old BF Twins and some SF Twins, profound differences in the tonality and way the tone knobs worked...immediately recognizable if you've tried one, then the other.

          Comment


          • #6
            Popular even then were statements like 'made like they were in the good old days' and the like. I read that Leo Fender once said "our slogan should be, We don't make 'em like we used to...And we never did." Fender was notorious for building amps with whatever was on hand. Still, there was said to be a pride among the work force that helped maintain a standard of quality and workmanship.

            When CBS took over it was basically business as usual for awhile. Some of the early minor changes were standard responses to customer feedback. Leo might have made the same changes. But later on, as the big business mentality started to drift into production, faster techniques were used for assembly and less consideration was given to parts selection in favor of cost effectiveness. To compensate for this other design changes were implemented. Crutch and bandaid circuits that exist only to make a lesser amp function at all. Then there was the watt wars. Combined with the 'faster and cheaper is better' big business mentality some of those amps were a tone disaster.

            But I'm just telling the same story that's already been told. What it comes down to is that Leo's philosophy was more like 'Make the best product you can for the profit and people will buy it because it's good'. CBS was more about 'Build the cheapest product you can that will still get market price'. In some ways you'd think those two strategies land close to the same mark. But they don't.

            Chuck
            "Take two placebos, works twice as well." Enzo

            "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

            "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

            Comment


            • #7
              Vintage Fender Princeton Reverb Black/Silver

              I own a Silverface Princeton Reverb. As close as I can tell it was probably manufactured about 73 to 75. I bought it from a fellow bandmember in 1978 who played blues Harp through it and miked it into the PA. It sounded fantastic. He bought it used in the San Francisco bay area and used it constantly for about 3 years before I bought it. I used it for practice at home, weekly band practice and gigs until 1980. I played slide guitar on a Tele and used it onstage basically as a monitor. Between the both of us we kept it busy for about 5 years without a single failure of any kind. It never let us down at anytime or anyplace. As far as I know, it may still have the original fuse in it. It has the original Dust Cover, Oxford 10J4 Speaker, Tubes, Caps, Resistors. Everything looks and works perfectly. Since 1980, I've taken it out of my closet a few times and used it a few days at home, then mothballed it again.

              I recently decided to sell it. In trying to determine the manufacture date, I read everything I could find on the Internet for several days straight. Finally found and downloaded schematics and layout diagrams to compare to my Princeton (I'm an Electronic Technician by trade from back in the 1970's and am currently working as a Tech). I almost was convinced my chassis was a AA1164 circuit because it didn't come close to the other circuits I reviewed until I came upon the AB1270 circuit produced by CBS. Now everything I read started to make sense. Somewhere in my surfing I came across a post that said "no changes were made to the Princeton Reverbs". After carefully scrutinizing both designs, I did notice three differences. One being the option of using an alternate TR1 transformer (010020) on the CBS Silverface AB1270. Secondly, the AB1270 schematic specified a +/-10% tolerance on specified voltages instead of the +/-20% on the AA1164 Blackface. As you know, tighter tolerances to nominal plate voltages mean greater voltage swings before clipping occurs resulting in a little cleaner/higher power output to the speaker. And the last thing I noticed was the CBS AB1270 schematic increased the voltage rating on all Capacitors unless otherwise specified from 400volts to 600volts. Not surprising considering larger voltage swings, a little more power and rock/blues guitar players always pushing amps to the edge constantly for that distorted tone. I believe these changes by CBS led to a more reliable Amp with more consistancy between units. Manufacturers do not make any money on warranty repairs! That's probably why my Silverface Princeton Reverb I own never gave us any hint of a problem for the 5 years we used it. CBS upgraded and tweeked it for us.

              One of the things that initially threw me was the date codes on my Transformers! The Power Transformer TR1 had a date code of Nov 1963. The Speaker output TR2 Transformer's date code is July 1963 and the Reverb Transformer TR3's date code is March 1963. This is what led me initially to believe the Amp was an AA1164 circuit. After discovering the AB1270 schematic, I verified the Capacitors to be 600volt ratings, convincing me it was a AB1270 CBS circuit. The only reason for this I can think of is my chassis may have been started in 1964 or 65, then shelved before completion and later completed by CBS. My serial # is A16146. That's all I know! It's been a great Amp, and now it's time to go after 31 years in my possession. Anyone interested can email me, for additional info and photos. It's in excellent condition......It ain't gonna be cheap..........

              Comment


              • #8
                More likely your transformers were sitting in stock, having been bought by the thousands earlier.

                Look at the date codes on the pots.
                Education is what you're left with after you have forgotten what you have learned.

                Comment


                • #9
                  The numbers on my pots are as follows:

                  Volume: 021832 1MEG AUD-1377326
                  Treble: 021857 250K AUD2-35-13773
                  Bass: 021857 250K AUD2-35-13773
                  Reverb: 021873 100K LIN-1377323
                  Speed: 021865 3MEG-CCWA-1377310
                  Intensity: 032367 250K-A-1377451

                  Can you identify these?
                  Thanks.................phil

                  Comment


                  • #10
                    There were also cabinet design changes, going from finger jointed cab's w/ floating ply baffles to stapled corners with dadoes & glued particle board baffles.

                    But I'm not against SF Fenders, I've got several of them myself and BF models (too expensive). They make great platforms that with very modest tweaks make great sounding reliable amps.

                    Comment


                    • #11
                      'The numbers on my pots are as follows:

                      Volume: 021832 1MEG AUD-1377326
                      Treble: 021857 250K AUD2-35-13773
                      Bass: 021857 250K AUD2-35-13773
                      Reverb: 021873 100K LIN-1377323
                      Speed: 021865 3MEG-CCWA-1377310
                      Intensity: 032367 250K-A-1377451

                      Can you identify these?
                      Thanks.................phil'

                      They were made by CTS, mostly in 1973, intensity in 1974 week 51, so assume it's a 1975 amp.
                      See link for de-ciphering the codes
                      Speaker Codes, Amps, & More
                      Interesting that the bass and treble pots are the same, usually they are different tapers, with the treble being a 30% audio taper, bass a 10% audio taper - does the amount of bass seem to increase quite quickly as the control is turned up from 1?
                      What are the full eia codes on your transformers? Maybe they were from 1973 rather than 1963? They sometimes need a little interpretation. Peter.
                      Last edited by pdf64; 11-26-2009, 08:30 PM.
                      My band:- http://www.youtube.com/user/RedwingBand

                      Comment


                      • #12
                        It's been quite awHile since I used the Amp, I don't remember the sensitivity of the Bass pot. Both Bass and Treble pots appear to be audio tapers.

                        TRANSFORMER CODES:

                        POWER: AC10020 CSA 827-606-3-45
                        OUTPUT: 022913-606-3-29
                        REVERB: 022921-606309

                        SPEAKER CODE: 023010 465-330-10J4-5B
                        The grill cloth is blue sparkle without aluminum trim.

                        Also, the blue model name is: "PRINCETON REVERB"
                        not "PRINCETON REVERB AMP"
                        Last edited by ppumajr; 11-26-2009, 09:37 PM. Reason: forgot something

                        Comment


                        • #13
                          I'm not an expert on this topic, but I've looked at the issue in relation to Twin and Super Reverb amps.

                          From what I could find out, the big changes CBS made were in 1968, and, based on the circuit diagrams, it appears to me that the purpose of the changes was to make the amps more stable. For example, the maximum grid leak resistor for the 6L6 is supposed to be 100k in fixed bias according to the datasheets, but the Blackface amps used 220k. I have read that the circuit changes were related to the lead dress issue; keeping the original designs oscillation-free required exercising some care in lead routing.

                          However, based on negative reactions, CBS quickly reversed some of the changes they had introduced. I think CBS did stick with their decision to switch most tube rectified amps from GZ34s to cheaper and/or more readily available 5U4GBs, which have a bigger voltage drop than GZ34s and will affect how the power supply "sags" under high current draw.

                          Thus, it seems to me that the worst of the Silverface changes occurred around 1968, but, if you look, you'll see that these revisions were themselves partially revised in circuits published in 1969 and 1970.

                          Comment


                          • #14
                            'TRANSFORMER CODES:

                            POWER: AC10020 CSA 827-606-3-45
                            OUTPUT: 022913-606-3-29
                            REVERB: 022921-606309'

                            606 means they're Woodward-Schumacher (see prev link), the following '3' means 1973 in this context (of an SF Fender), and the '45', '29' and '09' are the week # of production.

                            'SPEAKER CODE: 023010 465-330-10J4-5B'
                            The '465' means in an Oxford, '330' means week # 30 of 1973.

                            The normal audio taper is 10%, meaning at 50% mechanical rotation (5 1/2 on the knob), electrically the wiper is at 10% of the total resistance, ie if it's a 100k pot, it will be 10k from the cold track end, 90k from the hot end.
                            A linear taper is a 50% taper, at 50% mechanical rotation, electrically it's halfway also.
                            Fender used a custom taper of about 30% for their Vol and Treble pots, ie halfway rotation was 1/3 electrically.
                            That's what the '2-35' code on those pots designates, I think.
                            My band:- http://www.youtube.com/user/RedwingBand

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