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Fender Ramparte - Schematic

Discussion in 'Amp Tech Center' started by GERPUD, Feb 8, 2015.

  1. GERPUD

    GERPUD TDPRI Member

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    C4 is a tiny 0.47nF (blue,small like a disk cap).
    C19 is a little bit bigger and it is 1nF. I can see that one of it's pin is on the ground. Can't see the other pin, but it connect to one of the pot leg (with pot fully on or fully close). My meter would not always beep if it was on the wiper...
    Anyway...
    This cap is cutting weirdly the frequency response...

    I was also wondering what is the function of C20, C22, C15. They connect the output of pin 1 directly to input pin 2 (pin 1 throug R19 for C20).
    Is it a "negative feedback" for cancellation of the high frequency noise?
     
  2. Cleeve

    Cleeve Tele-Holic

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    Yes i do believe the c20 and c15 are for negative feedback at high frequencies, and along with r19 etc do some op-amp style gain setting so various tubes will still perform well with the gain at least partially controlled by feedback.
     
  3. GERPUD

    GERPUD TDPRI Member

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  4. GERPUD

    GERPUD TDPRI Member

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    The other part that does not seem standard is the group R33-R35-R34-C24-C23 that are connected to B+1 and to pin 8 of the power tube. Usually, I see only a cathode resistor and cap going to ground..
    What is this for?
     
  5. Cleeve

    Cleeve Tele-Holic

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    Your drawing is easier to follow than theirs!
    I can only speculate about the odd rig going from the B+ to the cathode via that pair of capacitors etc. My weak guess is that since the power supply is silicone rectified, the B+ at turn on will be fully up to peak voltage before the heater in the power tube heats the cathode and lets the cathode conduct and get positive relative to the grid. So maybe those two capacitors and that 6.8k resistor connected to the cathode yank the cathode positive the instant the power is applied to the cold tube or maybe it's some arc prevention scheme..

    Those caps and the resistor could be a path for what would be an internal arc in the amp if the speaker were disconnected or blown while the amp was cranked, making a path for the inductive zot to follow instead of lightning inside of the tube or between socket pins..

    Hell if I know!
     
  6. GERPUD

    GERPUD TDPRI Member

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    That's what I thought also!
    Woh...ok I'll leave them there....

    So the most expensive cap in the amp are used for that? (they are 1uF non-polarized cap, the biggest one in the amp...) How come they don't have that in their other amps?? Cheap amps must be made more hazard proof?

    At least, we can say that this amp is not a lazy cut and paste from a basic single ended schematic.... Indonisia seems to have hard working engineers...
     
    Last edited: Feb 11, 2015
  7. xxan1malxx

    xxan1malxx NEW MEMBER!

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    Hi, i reduce the hum by changing c16 to 47 uf, and rotating the ot 90°.
    Any mods to make the hot chanel less muddy???
     
  8. Cleeve

    Cleeve Tele-Holic

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    Remove C19
     
  9. GERPUD

    GERPUD TDPRI Member

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    Simply that? Why?
    That reduced the hum completely or just a little bit?
    I noticed that increasing C16 from 47uF to 94uF made a good difference, but did not solve everything. I was planning to move the OT from PT. I'll try only to rotate it. thanks!
     
  10. Cleeve

    Cleeve Tele-Holic

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    C19 as shown on the schematic is shunting treble signal to ground- the overall gain of the hot channel may be way hot with c19 gone, Worth a try to pull c19 to see how it sounds when the output of the first gain tube is not AC coupled to ground.
    If the Hot is too Hot with C19 gone, removing C2 will help, if still too wild, pull C3 as well!.
     
  11. GERPUD

    GERPUD TDPRI Member

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    I've tried adding a 47uf cap before and after R27.
    There is a hum reduction when added after R27, but the reduction is bigger before R27. So, a simple mod would be to add a 47uf cap in parallel with C15. This is more effective I think that replacing C16 (22uf) with 47uf and much more easier.

    I've tried removing C19 as suggested by Clint. Not an improvement... I sure got more treble, clearer sound but I lost too much bass. Maybe putting a smaller value, but I do not recommend removing it.

    Other observation: At low gain/low volume hot channel, the sound is compressed but no distortion. At the sound volume the distortion begins on the hot channel, it is about the same sound volume distortion begin on the cool channel. So we can't tell if the beginning distortion of the drive channel is the preamp drive or the power tube drive. We don't ever know if we get preamp distortion at all. It could be only compressed preamp with heavy power tube distortion. The master volume should be very helpful for that.
     
    Last edited: Feb 12, 2015
  12. GERPUD

    GERPUD TDPRI Member

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    Here what I just did:

    - Add a standby switch
    - Add a filtering stage before OT (44uF+4H choke)
    - Moved the OT from the PT (at least 6 in)

    Results:
    Major difference!! I don't recognize my amp!

    - I got rid of all the 120hz hum. I still have a little bit of white noise, but it is comparable to my hot rod deluxe. It is now a normal noise, not annoying any more
    - WAY MORE headroom. Distortion starts way later on the clean channel. I had it a little bit before 3, and now I can go clean over 3.
    - More and tighter bass. The stock amp had too little bass this is why I added a eq pedal. I don't think I would need it any more for that reason.
    - The hot channel does not sound the same now. It sounds more like "distortion" than "overdrive" now. Maybe I'm getting more preamp overdrive that power tube overdrive now. Much more "tight" overdrive.

    I think a part of the sound of the stock amp was made by the poor power supply. I feel that the transition from clean to distortion was mostly voltage sag.

    I'm pretty happy with the amp now, but I must say that the way the clean was breaking too soon was very pleasant to me. I play mostly blues and that sound was very interesting. Whatever, the Hum was too annoying to not make this mod.
     
    Last edited: Feb 13, 2015
  13. GERPUD

    GERPUD TDPRI Member

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    pictures
     

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  14. Abu Twangy

    Abu Twangy Friend of Leo's

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    Hope you had better luck than I did swapping speakers.

    I had planned to swap the stock speaker with the Jensen C12K from my DRRI. Two of the four speaker screws became loose in the baffle when I tried to remove the (glued down) nuts.

    I ended up cutting off the factory screws removing them and replacing them with T nuts/screws.

    My Ramparte sounds better with the Jensen--smoother rounder bass and highs, and my DRRI with the Ramparte speaker weighs a pound less.
     
  15. GERPUD

    GERPUD TDPRI Member

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    Let me guess: you had trouble with the two top bolt!
    I didn't have better luck. I've pull the stock bolt thought the ¾'' plywood. I've put plain 1'' wood screw instead...
    Any speaker will do better than the stock one. I which I put a 103db speaker instead so it would be easier to play with a band.
     
  16. GERPUD

    GERPUD TDPRI Member

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    I've added the master volume. (removing R24, and replacing it with a 100k pot). I'v used the connectors wires to make the link. There is probably better solution but that was the easiest for me.

    This is definitely an improvement for overdrive but also for clean.

    The stock clean volume can not be used as a clean gain. I can set it to be very clean, compressed or slight overdrive. Very interesting. I can now see that the best preamp setting for clean is a little above 3, it is compressed no distortion around 4-5, and slight overdrive over 5. Without the master volume, I had distortion beginning at 3, so it was power tube distortion

    For the overdrive channel, I can now say that stock amp is MAINLY power tube overdrive. I now need to be around 13 to get overdrive. The gain is not very hight even at 16. The sound is even muddier than before. This channel sounds like crap!
    I realised that it was due to a lack of high frequency and low frequency. I'v gave an other try Cleeve's idea to remove C19 and it is definitely an other improvement. Way more gain, less muddier. The channel is now usable. Need to find a way to increase bass.
    I was thinking to replace C20 with 0.022 or above be I would need advices.

    I may also like to add a tone stack, but I don't know it it would work, and if it would decrease the gain to much. I'd like to have your opinion on that too.

    By the way, I've added one coat of lacquer to protect the fabric (that seems to me very prone to stain and dirt). It is now a bit darker but not that much. Feels more solid and resistant now. Colour is not an improvement, but definitely durability.

    I'm also working on NFB loop. I've try removing C18 + NFB, but not a success. I've lost to much gain, I don't like the result. I'll try the original Fender Champ setting to see if it will be better.
     

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  17. Cleeve

    Cleeve Tele-Holic

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    I'm not certain how to boost bass, one must be careful making the coupling caps too large from stage to stage as too large a time constant can make annoying "handling noise" in the output when overdriven, making the sound of your hand touching the strings come through as bumps and low frequency garbage.

    As the feedback, perhaps a crude but effective trick I've seen on old designs would work here. Connect the power tube's cathode resistor and bypass capacitor to the speaker output rather than to ground. So on the schematic the "J31-2" should connect to the green speaker output from the output of the output transformer. If it squeals, reverse the black and green wires. With luck the DC current may actually cancel some of the standing current in the output transformer. The DC resistance is so low compared to the cathode resistor that practically zero DC is presented to the speaker.
    I'll dig around for a schematic showing this connection...
    Edit- Good ol' Magnatone!
    found a respectable example of this feedback connection-
    http://www.magnatoneamps.com/schematics/magnatone_410.pdf
     
  18. GERPUD

    GERPUD TDPRI Member

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    So you suggest negative feedback loop for power tube only?
    What I have tried, and what I've seen everywhere was a NFB from speaker tip to preamp tube #2 (like R12 on:)
    http://support.fender.com/schematics/guitar_amplifiers/57_Champ_schematic_Rev-B.pdf
    That's what I tried. Like I said, I had to remove C18 because nothing was happening. In this case, I needed to connect to the green wire because the black was creating more distortion and more high frequency.

    The drive channel really need some bass boost to be usable. Any ideas will be welcome!!
     
  19. Cleeve

    Cleeve Tele-Holic

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    Yes I suggest negative feedback for only the power tube, thus put c18 back to get the gain back up. The method pictured in the champ schematic is good too and may be better if the power tube only method is not enough feedback.

    I think for bass in the hot channel, perhaps it's C4 than needs to be bigger, say .022 like the cool channel uses, rather than the 470pf.
     
  20. GERPUD

    GERPUD TDPRI Member

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    Could you describe what this configuration should do on the sound/gain!!??
    I don't even know if we can call that NFB loop! No resistor, no impact on the preamp... Pretty interesting!

    I was maybe going to try adding 47 ohm resistor between ground and C18/R6 and connect the NFB loop there like:
    http://el34world.com/charts/Schematics/files/fender/Fender_champ_aa764_schem.pdf

    My main goal of adding the NFB loop would be to have a presence and resonance control since the amp has no tone stack. If you have an idea of how to add a tone stack without cutting too much the gain, that would be an other solution...

    Of course!! This cap can only be an error. It makes no sense. Pretty obvious now why to overdrive tone sucks!
    I'll put 0.022 in C4, and I'll also try 0.047 in C20 (to be the same than C7)
     
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