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  • A darker Fender Harvard?

    I'm currently building a Harvard amp and I'd like to try few mods.

    I found this reply on Gear Page: ''there were two different tweed harvards, iirc the transformers were different, because some of them sound more bright and clean, and some sound darker and meaner, the darker ones have the output tranny mounted on a bracket off the chassis.''

    So, I'm very interested to know what kind of circuit does make one Harvard sound darker than the other, it can't just be the tranny on a bracket.
    I would much appreciate if anyone could explain this in detail.
    Thanks!

  • #2
    Hi,
    which Harvard amp are you referring to?

    I'm asking this because the two variants I am aware of are completely different; the 6G10 is a class A single ended amp with cathode ( auto ) bias, while the 5F10 is a class AB1 push-pull amp with fixed ( external ) bias, so the circuits are completely different ( and they obviously sound different too ).

    Hope this helps

    Best regards

    Bob
    Hoc unum scio: me nihil scire.

    Comment


    • #3
      Sorry, it's 5F10. The 6G10 supposedly never existed...

      Comment


      • #4
        Originally posted by dinkotom View Post
        Sorry, it's 5F10. The 6G10 supposedly never existed...

        Regards

        Bob
        Attached Files
        Hoc unum scio: me nihil scire.

        Comment


        • #5
          Thanks, I have both schematics, but 6G10 was never produced, or so I'm told. I'm sure the original reply refers to two different versions of 5F10 (one with OT mounted on the bracket), so I'd like to know if there are some circuit differences between them. I know some Harvards were fitted with 6AT6, others with 6AV6 (more gain) for preamp, maybe this is what it's all about.

          Comment


          • #6
            Apart from the mu factor ( 70 vs. 100 ) 6AT6 and 6AV6 are very similar, and the inter-electrodes parasitic capacitances aren' all that different, so I think that there must be some other difference to justify the difference in sound you're talking about ( NFB with a small series cap to cut highs ? ) - I' m sorry I can' t tell more, I'm not into Fender Amps, as I prefer Voxes ( as you might have already guessed ).

            Best regards

            Bob
            Hoc unum scio: me nihil scire.

            Comment


            • #7
              You can beat me but the shown 6G10 schematic is exactly what I found as a schematic for the Princeton 5F2-A with the exception that there are voltages added to the 6G10 schematic.

              Comment


              • #8
                Originally posted by voxrules! View Post
                Apart from the mu factor ( 70 vs. 100 ) 6AT6 and 6AV6 are very similar, and the inter-electrodes parasitic capacitances aren' all that different, so I think that there must be some other difference to justify the difference in sound you're talking about ( NFB with a small series cap to cut highs ? ) - I' m sorry I can' t tell more, I'm not into Fender Amps, as I prefer Voxes ( as you might have already guessed ).

                Best regards

                Bob
                Thanks, adjustable NFB is one of the mods I'll try in this amp.

                Comment


                • #9
                  If you don't mind me asking, what are you using for power and output transformers?.

                  I have been thinking of a 5F10 myself.

                  Alby

                  Comment


                  • #10
                    Originally posted by Alby View Post
                    If you don't mind me asking, what are you using for power and output transformers?.

                    I have been thinking of a 5F10 myself.

                    Alby
                    OT is the same as for a 5E3 or BF Princeton I would say. If you want to get experimental, get them to wind the OT with secondary taps for either 8k or 6k6 load resistance with your choice of speaker.

                    PT is something like 280-0-280 at 120mA or so (to get the B+ of 305 with a 5Y3GT - however you might want to go to 285-0-285 because the B+ will drop a bit under load, but its not all that critical) with a 50V bias winding, and the heater winding would be around 2A (3A to be on the safe side) and the recto winding at 2A
                    Building a better world (one tube amp at a time)

                    "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

                    Comment


                    • #11
                      Without knowing all the lore and mystique around Fender and the Harvard amp I would guess that the "darker", "meaner" Harvard had lower plate voltages. This would cause it have less top and bottom end definition and earlier break up. But this is just speculation.

                      With the 6v6's drawing 40mA each at idle you would get about 300 to 305 plate volts with a 5y3 rectifier and a 285-0-285 PT. Thats as low as I would want to go personally. I have a 2 x 6v6 / 5y3 amp that only had 265 plate volts. It REALLY didn't "give it up" so to speak. I changed the rectifier to a 5v4 which brought the plate volts up to 305. It sounds sweet now. Instead of slapping you in the face like a Deluxe it just gives you a kiss

                      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

                      "If you're not interested in opinions and the experience of others, why even start a thread?
                      You can't just expect consent." Helmholtz

                      Comment


                      • #12
                        Originally posted by Alby View Post
                        If you don't mind me asking, what are you using for power and output transformers?.

                        I have been thinking of a 5F10 myself.

                        Alby
                        I must say I am happy as a clam to know a local tech who's been winding transformers from the late 50's, strictly old style, interleaved OT's, etc... He's done at least ten for me and many more for other guys, they never failed and sound great.
                        As tubeswell says, trafos for 5E3 or Princeton are nearest to Harvard's, so if you can't find someone to wind them for you to exact Harvard specs, these are the first that come to mind.
                        5F10 must be one of the sweetest and most soulful sounding amps. Ever since I've heard Booker T. & MG's I wished to have one.

                        Comment


                        • #13
                          Originally posted by Chuck H View Post
                          it just gives you a kiss Chuck
                          That's the best description of Harvard sound I've heard!

                          Comment


                          • #14
                            I do have a way to get transformers wound locally, probably free of a case of beer but I am saving that one for when I really need it.

                            How is yours coming along dinkotom?

                            I have two transformer sets on the way, both Hammond, 290AX and 290BX. With the AX I would have to build a negative bias supply off the supply voltage, no big deal but it would involve changing the chassis layout somewhat to get it all in there. I would do a custom board anyway so I can cross that bridge when I get to it. The 290BX has the bias tap but as far as available m.a. and supply voltages I would have to mod the power supply or rectifier tube to get the voltages down for the Harvard, I think the first filter stage showed just over 300 volts.

                            A 290AX spec. transformer with a bias supply tap would be ideal but I like the availability and price of the Hammond ones.

                            I will put more thought into this one once the transformers arrive. In the mean time if I go ahead and build a cab I would be kind of commited to do one wouldn't I.

                            Comment


                            • #15
                              Originally posted by Alby View Post
                              How is yours coming along dinkotom?
                              Bah...bad luck...I decided to build a chassis for it from a steel plate, ended up with a smashed thumb nail, burnt drill, semi functioning saw and other smaller obstacles. So, I'm back to square one, chassis, aluminium this time...
                              In the meantime I got sidetracked and became obsessed with building Magnatone vibrato (for the n-th time in last 15 years), and of course can't find correct varistors for it...
                              At least my revved up Champ is working fine!

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