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New build 5F1 & AA764 champ, 2 in 1

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  • #46
    Hi don. About my earlier remark, I think you might have looked too deeply at it. I was only chiding you for saying you got "no resistance" between two points, and I said I think you mean infinite resistance. meaning "no resistance" means zero ohms - a dead short. Just a terminology nitpick.

    As to voltages, it says right on MANY Fender schematics, including the AA764 Champ: voltages read +/-20%

    Something I harp on a lot: these are just guitar amps, not precision equipment. If your 360v B+ winds up 345 or 370, the amp will still work just fine.

    Spice is a wonderful tool, but you can also to to Duncanamps and download the free "tone stack calculator", which is already loaded with a variety of tone stacks, and in real time you can adjust the tone controls and watch the effect on the freq response. It also allows you to change the component values and see what that does. it is a delightful little thing.

    Big can caps full of 20uf sections are common. 20uf (or 22uf) individual caps are common. The only place to find high voltage 8uf and 16uf caps is in the nostalgia guitar amp suppliers catalogs, and you pay extra for that. They used those values back when because caps were not cheap then. They saved money with 8uf over larger caps.

    CAthode voltages? 1.4v instead of 1.8v, and 1.4v instead of 1.7v? Oh geez, I wouldn't even think about it. Close enough. If you have 200v at the plate per the drawing, and the plate resistor measures exactly 100k, and the B+ node for that was 330v also on the drawing, then there is 1.3ma flowing through the resistor. That means through the tube too, and of course thus the 1.5k cathode resistance. I get 1.95v across the cathode resistor. SO right there the schematic disagrees with itself.

    In the second stage, I can pretty much ignore the 40-some extra ohms in the cathode (it represents maybe 3% of the 1500 ohms), and see with the same 100k load from the same B+ they show different numbers. And of course the 19v the 6V6 generates across its cathode 470 ohms is totally dependent upon the individual tube. Bottom line is that the EXACT voltages on the schematic don't matter, and changing 1.5k resistors to 2k resistors to chase after it is wasted energy. Fender voltages are ball park figures only. When you see 1.7v on a cathode it is there to tell you not to expect zero volts and not to expect 8 volts. A volt or two is fine.

    Look at the 10k B+ resistor. It has 20v dropped across it on the drawing. That means 2ma through it. But adding the two sides of the 12AX7 circuit we get more than 2ma. SO again, the schematic disagrees with itself - if we take it literally.

    Schematic also notes 10% tolerance resistors. Older models were made with 20% resistors. That means a 100k resistor can be anywhere from 80k to 120k and be on spec. 10%? that still means 90k to 110k.

    Just my opinion, of course.
    Education is what you're left with after you have forgotten what you have learned.


    • #47
      Thanks Enzo, that's EXACTLY what I was thinking and I was going nuts because Ohm's law it's not an opinion, so I thought that either the voltages were just there as a guide or there was something up with the resistor values (or the current draw from the 12AX7).

      Also, thanks for confirming that the difference is not significant. For what is worth, when I changed a resistor to bring the B+ voltage down from 330V to 300V I couldn't really detect much difference in sound by ear (very un-scientific, I know) so I debated about mucking around the B+ and cathode voltages.

      The point is that I'm still waiting for several parts to come here from all over the world and I'm kinda bored, so I wanted to do some (small) tests and see their effects.

      Right now I have 300V B+ voltage, 200V on the plates and 1.385V and 1.42V on the cathodes, with 100k plate resistors, 1.5k cathode resistors bypassed by caps and 1M AC load.


      • #48
        Did you measure the cathode resistors? Did you try different tubes?
        Some chinese ECC83s have very high Gm, which could be a reason.
        - Own Opinions Only -


        • #49
          Cathode resistors where changed to film metal oxide 1% and measured 1.5K dead-on (those are 1W or 2W, can't remember).

          Unfortunately I don't have other 12AX7/ECC83 to test and there aren't any signs on the tube to find a datasheet. It's one of those no-name chinese tubes, just like the 6v6 power tube.

          But, again, I don't want to make it too much of a big deal as the amp sounds great as it is so I might just leave it alone and put better tubes in.

          About power supply and plate voltages, I'll have to update the schematic with the new readings, so I can give you all the information needed.

          Going from memory, 300V on B+ and ~200V on plates.
          Bear in mind we're speaking about a modded VHT 6 Special which has a SS rectifier.

          I'll post the updated schematic later with the various voltage readings.


          • #50
            Right now I have 300V B+ voltage, 200V on the plates and 1.385V and 1.42V on the cathodes, with 100k plate resistors, 1.5k cathode resistors bypassed by caps and 1M AC load.
            That in itself isn't completely consistent (assuming resistor values are exact): 1.42V across 1.5k means a plate plate current of 0.95mA and a voltage drop across the plate resistor of 95V. So with a B+ of 300V plate voltage should be 205V.

            But I am with Enzo, it really doesn't matter. Plate voltages between 190V and 210V mean close to center biasing and allow for a plate voltage swing of at least +/-90V.
            - Own Opinions Only -


            • #51
              Originally posted by Helmholtz View Post
              Some original Fender schematics show a note stating voltages within +/-20%. So your cathodes voltages should roughly be within spec.
              Cathode voltage depends on tube variation (Gm), plate voltage and actual resistor value.
              Back in the day, Tektronix made a special scope called the 570. Using it you could display the plate current curves of many tubes. Connections to the tube socket were made by short jumper wires. When you plugged a tube in, the very first thing you did was adjust the heater Voltage (with a small internal variac) because all the curves depended on it. So that 125V mains puts the heater Voltage in your old Fender to something near 6.8 to 7.0 VAC. It makes a big difference.
              WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
              REMEMBER: Everybody knows that smokin' ain't allowed in school !


              • #52
                Sorry for suddenly disappearing, life/work stuff took over.

                Here's where I stand now (still waiting for components from China to arrive ):

                As you can see, I've stripped it down to a very simple 5F1-like circuit.
                When the other components will arrive I'll change it back to the version with the AA764 tonestack and a switch to bypass it.

                For reference, the components underlined in red are those which have been changed/added from stock. The original amp is a VHT Special 6.

                In my last post I was wondering if I should change the cathode resistors because, when compared to the AA764 circuit which has 200V on the 12AX7 plates, I have only 1.38V and 1.46V instead of 1.8V and 1.7V.

                Still, the amp sounds great and so far I'm loving it so I'm not sure if this is a non-issue.


                • #53
                  Wow, time flies, almost 1 year, guess I have to do a little summary of how things went:

                  To put it short, after many tests, I decided to discard the idea of a 2-in-1 amp because everytime I went and played I found myself always sticking to the tone stack-less version, the one depicted in the above post.

                  So now I just have a 5F1 like amp but with much higher voltage on the plates of both the 6V6 and the 12AX7.

                  Although I'm pretty happy with how it has been sounding until now, whenever I cranked the amp up I've always found myself looking for a more smooth distortion (the clean tone is killer, though!), and, generally speaking, I want to try and adhere more closely to the specs Fender gave with its original 5F1, which brings the point of this post forth:

                  Other than changing the power transformer (which is great btw, VHT really used great quality power transformer and output transformer), how would you go lowering B+?

                  Installing a tube rectifier is out of the question: I'd have to drill a hole in the metal casing and I don't have the tools nor the will to do that.

                  I'm looking at the resistors on the power rail, namely R24 (330Ω), R26 (1KΩ) and R5 (which I already modified and upped to 25KΩ from the original 10KΩ) and was wondering if I could just put bigger resistors in their place and call it a day.

                  I'm aiming for ~150V on the plates of the 12AX7 and ~300V on the 6V6 as per original spec.


                  • #54
                    What is the voltage drop across R24?
                    - Own Opinions Only -


                    • #55
                      Sorry for the late reply, been a bit busy, tomorrow I'll open up the amp again and mark all the various voltages on the schematic.


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