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Input wiring puzzlement: Ampeg Reverberocket R-12-R 1962

Discussion in 'Shock Brother's DIY Amps' started by chas.wahl, Nov 22, 2020.

  1. chas.wahl

    chas.wahl Tele-Meister

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    I have a new-found interest in this offbeat amplifier circuit, in its earliest of several variations. I'm busy investigating and learning more about how and why it's different from Fender circuits, but one thing has me stymied: how the inputs are wired.

    Ampeg-R-12-R-inputs-reduced.jpg

    There are three inputs, labeled guitar, accordion and mic on the control panel, but not identified that way on the schematic (and there's no layout that I've ever found, not for lack of searching). All of the inputs have a 47k grid resistor, but only the middle one (which I suspect may be the accordion input) has a switch on the tip, grounded through a 470k resistor. The grid leak resistor, 5.6 Meg, is not in the location I'm familiar with on Fender amps: from the tip of the Hi input so that it's ineffective when the Lo input is used. Instead, the Ampeg has the grid leak hung off the V1a grid, so that it's in the circuit for all three inputs.

    So my basic questions are these:
    1. Generally, what are the design principles of this arrangement?
    2. Is there any difference, impedance or otherwise, between the J1 and J3 inputs?
    3. What does the 470k resistor on J2's switch actually do, either when there's nothing plugged into it (switch closed), or when there is a plug there (presumably "nothing" in the latter case, except being lifted from the circuit).
    4. Is there any interactivity between the 3 inputs, when multiple ones are used.

    I guess what I'm seeking is an understanding on the level of Steve Luckey's excellent diagrams found over on EL34world.com:

    SLuckey-inputs-typ-Fender+Marshall-reduced.jpg
    Here are the complete set of the above, with additional Marshall and Vox special cases, taken from his "Amp_Scrapbook":
     

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    Last edited: Nov 22, 2020
  2. chas.wahl

    chas.wahl Tele-Meister

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    Oh, and by the way, I've been able to find not only the schematic (from September 1962, I believe) along with an Ampeg service manual (on TDPRI) that appear to be consistent/concurrent. The latter has all the component values, transformer specs, and test voltages and resistances. If anyone's interested, I can (re)post those. This amp has a paraphase PI, all octal tubeset, one-triode tremolo fed in between PI and push-pull 6V6 power tubes, and two-triode reverb with spring tank. It originally came with a Jensen C12R speaker. Cabinet roughly the same size as a narrow-panel Tweed Deluxe, maybe a couple inches taller.
     
    Last edited: Nov 22, 2020
  3. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    J1 and J3 are the same. No difference.
    The 470k when nothing is plugged in to J2 is in series with the 47k grid resistor of J2 which totals 517k. That 517k of resistance is in parallel with the 5M6 which equals 473k. When plugged into J1 or J3 the grid leak is around 473k.
    When J2 is plugged in, the grid leak is 5M6. This holds true for J1 and J3 when J2 has a plug inserted, so all the jacks are the same when J2 has a plug in.
     
  4. chas.wahl

    chas.wahl Tele-Meister

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    Thanks for that, appreciated. What would be the difference, for a guitar plugged in, between having a grid leak of 500k or one of 5.6 M (11 times more) -- would that change the gain (or signal throughput or noise level) in a noticeable way? Or somehow adjust for more than one type of source being plugged in? I guess what I'm asking is, is there a design intent for this configuration that I'm oblivious to?
     
  5. King Fan

    King Fan Poster Extraordinaire

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    I guess it begs the question: What were accordions putting out? I mean, besides “Lady of Spain.” :D
     
  6. chas.wahl

    chas.wahl Tele-Meister

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    I assume you mean the instrument's impedance. I don't really know what that would be for an accordion, and I suspect that the impedance for a mic would be different from that of a guitar, and maybe pretty variable for the sheer variety of microphones there are.
     
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  7. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    There possibly is. I have read the early ampeg design was based on acoustic basses with a piezo pickup. I will assume an accordion of the day might have a similar pickup. As you have mused the mic impedance can be all over the map.

    It is possible the 6SL7 was spec'd differently than a 12a_7 so a higher value grid leak may have been figured in. I don't think there is much difference between the two types of tubes though.

    For guitar use I would probably change up the inputs but it will work fine the way it is.
     
  8. Wally

    Wally Telefied Ad Free Member

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    What happens if you jump J2&J3 while plugged into J1? Hot input?
     
  9. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Once input J2 is used, the switch to the 470k is opened. A jumper between J2 and J3 would do nothing other than open that switch and the input on J1 would see the 47k stopper and 5M6 leak resistors.
     
  10. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    It could have been a little more interesting if the grid stoppers were not all the same.
     
  11. chas.wahl

    chas.wahl Tele-Meister

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    I'll let someone else with more knowledge & experience answer the question by @Wally with more authority, but I'm not sure what "hot input" means (sounds like it could be "not good"), and I don't see how it could be hot. Seems like jumpering J2 and J3 would simply join the two grid resistors there, but absent a ground (470k resistor to ground is thereby lifted) or a signal (just a jumper), I don't see how they themselves could provide any "hotness". If what you mean is that the J1 signal is seeing only a 47k grid stopper set off against a 5.6 M grid leak, then that's going to happen whenever something is plugged into J2, jumper or just a plug, to lift the 470k resistor and defeat the paralleling of that and the 5.6 M one.

    I think that @Lowerleftcoast makes the same observation I've just done, though I wrote it without seeing that, while he was posting.

    To his second comment, I go back to my puzzlement: presuming that the guitar and mic inputs are J1 and J3, why would they be, circuit-wise, exactly the same? Perhaps if the idea was to have two inputs allowing for two guitars plugged in, that might make sense. But I'm not sure how the interaction between two parallel sources would change things, either. Things seem to be converging on an impedance discussion, that could get complicated. I'm grateful for the explanation of what happens vis à vis the effective grid leaks -- it's a start.
     
    Last edited: Nov 22, 2020
  12. Wally

    Wally Telefied Ad Free Member

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    Would it not put the three 47k resistors in parallel and reduce the resistance to the input signal from the instrument?
    I have owned a Ric B9A with four inputs, and the the grid stoppers were put in parallel when one jumped the inputs that were not. The Univox 45 with three inputs acts the same way.
     
  13. chas.wahl

    chas.wahl Tele-Meister

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    I don't think so, unless either J2 or J3 tip were connected to J1 tip.
     
  14. Wally

    Wally Telefied Ad Free Member

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  15. chas.wahl

    chas.wahl Tele-Meister

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    Well, I don't see the same thing that happens with a Fender Lo and Hi input happening here, but I'm a self-confessed newbie. If what you're suggesting happens, though, the effective grid stopper for J1 would be 15.6k, rather than 34k for Fender Hi or 68k for Fender Lo, and the ratio of grid stopper to grid leak would be 15.6k/5.6 M, rather than, say 34k/1 M -- I think.
     
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  16. Wally

    Wally Telefied Ad Free Member

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    Chris, I had owned my little Ric B9A since maybe 1995?? I had never experimented with a cable across two of the three unused inputs when I had played through....great amp, imho. Solid copper chassis, OEM C12Q. There are not many of them around. Anyway, I stored a few amps for a young fellow. One of them was a U45. He tripped me to the trick of doing this thing. I tried it on my Ric....yessir! It was another notch up on the heat scale. It makes one wish there were an input signal ‘gain switch’ to a footpedal.
    The Ric B9A uses a 68K on each of four inputs. With three of them in parallel, the input resistance is down to approximately 22.7k
    Fun.
     
  17. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    @Wally
    Different animals...

    Ric B9A
     

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  18. elpico

    elpico Tele-Afflicted

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    "because it's simple, cheap, and works good enough for rock and roll" is the answer to most "why did they do this" guitar amp questions.

    Even a "high" impedance harp mic like a green bullet only asks for a 100Kohm or better input and this amp has 500k. Guitars and mics will both be happy with those inputs so why not? Then you also have the high impedance input for piezos like the guys already covered.
     
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  19. chas.wahl

    chas.wahl Tele-Meister

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    Thank you all. I appreciate the help and viewpoints.
     
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