Rivera Fender Concert mods

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by doc, Feb 18, 2005.

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  1. BenH

    BenH Member

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    Hi Mike,
    Putting in two different ones, or rather a different one in each stage is ment to be a good idea to build up more complex harmonic overtones. I'd opt for the 1µ5 , then the 4µ7. But as I said, I've yet to try it.
    Ben
     
  2. TweeDLX

    TweeDLX Member

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    Ben,
    This is from Dan Torres' site:
    "Staggered response" involves changing the caps at the beginning of the preamp (first two stages) to higher values, then progressively lower values towards the end of the preamp - a still tighter, more pleasing bass response."
    I think this is what we're talking about.

    Mike :D
     
  3. BenH

    BenH Member

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    Mike,
    I just had a look in Dave Funks book to see what he put in his recomended gain stage, and he has a µ68 on the first and 1µ on the second. Kevin O'Connor on the other hand in his "standard" gain stage has a µ1 on the first followed by µ47. So I guess either way round can work.
    Please note that the two set-ups I just mentioned have very different plate and cathode resistor values than the set-up in our Concerts, so their -3dB point is going to be somewhere else. If I get around to it I will work them out and see what the equivelent values to put in the Concert would be.
    Ben.
     
  4. BenH

    BenH Member

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    Another cap stagering, if one was to move the tone stack to the end of the gain stage as donnyjaguar suggests, would be move the coppeling cap after V2A ( µ0047 ) to the frount of the 1 Meg. volume, and put µ01 in its place.
    I would also put an extra resistor on top of the volume poti, between 470k and 1 meg, because now that the loss factor of the tone-stack is no longer there, the input of V2A will clip much sooner, which is to be avoided if you don't want a scratchy drive tone.The 47k grid resistor could then optionally go. This leaves adquate drive reserve, and makes the volume poti easier to set.
     
  5. TweeDLX

    TweeDLX Member

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    Ben,
    When you say "on top of" the volume pot, are you meaning it's connected to the left and right connectors of the pot itself? Or possibly "in place of" the 47k grid resistor? Pardon my ignorance, but this is my first major "tweak" job. [​IMG]

    Mike
     
  6. BenH

    BenH Member

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    Mike,
    Look at donnyjaguar's revised schematic for moving the tonestack in the drive channel with his LINK back on page 7 post #95 so we're looking at the same thing.

    I would add a resistor of 470k ( or up to 1 Meg ) between the µ0047 cap coming from V1, and the 1 Meg volume poti in frount of V2A.

    The 47k coming from the wiper of the volume poti and leading to the LDR could now be removed, or rather, replaced with a wire. If in doubt, just leave it where it is.
    ( It should be noted that the LDR is omitted in the schematic for some reason ).

    Behind V2A, where you see the second µ0047in the schematic, this is the one you take and use at the frount. In its place you put the µ01 cap that isn't on this schematic, becauce it has been removed from between the junction of the 82k and 22k resistor on top of V4A, and the master volume poti, this is where the new wires leading back to the tone stack will now be placed.
    I hope this is clear now.
    Ben
     
  7. tlpruitt

    tlpruitt Member

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    What schematic are you referring to? That LDR is in the factory schematic I have. Its right below the 47k resistor.

    -Tim
     
  8. TweeDLX

    TweeDLX Member

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    Tim,
    There is a link on page 7 to Donnyjaguar's revised schematic for moving the tonestack. In DJ's schematic, the LDR is missing. Whether by accident or design, only the author can say... Perhaps Mr. Jaguar will chime in and clear things up for us.

    I think I understand what you've described, Ben. Looks like I'll be busy this weekend!

    Mike
     
  9. TweeDLX

    TweeDLX Member

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    Please tell me what I did wrong: I ran a wire from pin 1 (V1) to a .0047uF cap, followed by a 470K resistor to the right hand connector on the volume pot. Disconnected the .01uF from the Mstr. Vol. and jumpered from the 82Kresistor to the 250pf. Ran from the right hand connectoer on the MV to the wiper on Treble pot. I also replaced the .0047 cap by V2 with a .01uf cap as suggested. The end result was severe squealing whenever I turned up the Vol. pot. Obviously, something was wrong, somewhere! :crazy

    Mike
     
  10. BenH

    BenH Member

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    Hi Mike,

    Have you removed the wire running from the µ0012 cap (the mid poti switch is wired to the other end of it ) that leads to the junction of the 100K resistor and 250p cap that lead to the clean treble poti ?

    Have you removed the wire from the treble wiper to the volume cw lug ? (clockwise shorted to wiper. (left and right confusses me) )
    This is where the volume now has that additional 470k resistor ( I guess ).

    If all that is right, then you are probably have a cable dressing problem. Not unusual. The new wire running from pin 1 of V4A to the junction of the 250p. µ0012 and 100k resistor ( Where you have just removed a wire ) is carrying a strong ac signal, and therfore you must use a good quality shielded wire, with the shielding grounded at one end. It should also be kept up away from other sensitive parts. Try carfully moving the wire around a bit, and see if this effects the feedback tone or volume, if so then here is where your problem lies.

    I hope this will be of some help to you and solve the problem.
    Ben.
     
  11. TweeDLX

    TweeDLX Member

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    "Have you removed the wire running from the µ0012 cap (the mid poti switch is wired to the other end of it ) that leads to the junction of the 100K resistor and 250p cap that lead to the clean treble poti ?"

    I missed this! It's probably the culprit. Just to mention it, the lead from V4 is only one inch in length (Goes from just before the 82k to the 250 pf). Tough to move it much. The other lead (from V1 to the volume pot) is quite long, and may pose more of a problem. Thanks for all your help with this![​IMG]

    Mike
     
  12. BenH

    BenH Member

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    Hey, if it's only one inch long, I shouldn't worry about putting a sheath on it !
     
  13. TweeDLX

    TweeDLX Member

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    Just an update...
    Yesterday I once again moved the tonestack behind the volumes (ala DonnyJaguar), but the correct way, this time. :banana While it worked OK, it seems like it has way too much drive (pretty fuzzy at 4-5 on the gain knob with Master Vol. at 7-8 and Volume at 5-6) and a slightly "hollow" bass overtone even with bass down around 3. So I put it back to the way I had it before. I don't think this is where I want to go...:messedup

    Mike
     
  14. EBGB

    EBGB Member

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    I've been considering having my Rivera Concert modded.

    I am mostly concerned about the clean channel. The lead channel, if improved, is just a bonus.

    Is there any technical reason this amp can not be modded for blackface cleans? (And who can do it?)

    (BTW If the clean channel is affected, is the lead channel automatically also?)
    Thanks
     
  15. TweeDLX

    TweeDLX Member

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    Ethan,
    I'm pretty sure some others in this thread mentioned doing this mod. I haven't, but it doesn't seem like it's that far off to begin with. While the two channels are parallel through V1, you should be able to change the clean channel without affecting the other. One of the reasons I bought my Concert was the fact that it's hand-wired, and therefor easy to mod or fix. Any competent tech could do it.

    Good luck!

    Mike [​IMG]
     
  16. EBGB

    EBGB Member

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    My tech said the circuit is completely different from my 68 (blackfaced) Vibrolux--
    which is my reference for cleans.

    True?
     
  17. jay42

    jay42 Member

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    BF'ing the clean channel is mostly removing components. Any tech with a schematic can do it. It will not affect channel 2. I recommend BF'ing the bias supply and phase inverter also. The presence control was not a blackface element, so that requires a different pot and capacitor.
     
  18. TweeDLX

    TweeDLX Member

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    For those interested, here's what I finally wound up with in my Concert II. My goal was to make the OD channel more pleasing to my ear, reduce the flatulence, etc. Yesterday I made a number of changes which resulted in a vastly improved (as in: more useable) amp (to my ear).
    Here's what I did:
    Replaced the cap values in the tonestack from stock to the following-B=.047uF, M=.033uF, T=270pF; changed first stage coupling cap values to .0022uF; replace cathode bypass caps on V1 & V4 to 5uF & 2.2uF respectively. Replaced stock speakers with Weber 10F150's. Any thoughts, critiques or advice?

    Mike[​IMG]
     
  19. TweeDLX

    TweeDLX Member

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    OK, I couldn't stay away from it. After a few days of getting the new speakers broken in, it still needed "something". I put the tonestack back to stock values (using a Mallory cap for the bass) and opened it up some by changing the 5uF (1st cathode bypass cap) to a 10uf. Now this amp rocks! Dunpostin...

    Mike :horse
     
  20. mrcomputer

    mrcomputer Member

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    I was looking for more info regarding the PRII I'm getting later this week, when I chanced across this BB.

    I am the original owner of '83 Concert (F3 serial #) and I want to sweeten the lead channel a bit. I love the clean sound of my amp but I'm also very fond of tube-based distortion (vs. a pedal) and while my amp sounds smooth, articulate and sustainy with the master at 2 1/2 - 3, volume and gain at 6 - 7, I start losing that feeling the moment I roll the master beyond 4.

    The amp is pretty stock, I replaced the original speaker with an EVM12L quite a few years ago and I've re-tasked the reverb footswtch to activate the mid-boost on the lead channel (I never turn the reverb OFF - just run it about 2). Tubes are stock configuration - mostly Soviet-made.

    I also am very fond of effects-loops - one of the key reasons I bought this amp (25 years ago?!). While this is a Concert forum, any suggestions regarding a buffered FX loop on the PRII would be appreciated.

    I have some skills with a soldering iron and I can read a schematic, but I've prefered not re-inventing the wheel - so any help you may have is greatly appreciated.

    jbw
    (James B. Wood)
     
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