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Thread: Help Double-Check My 5F1 Champ with 6BM8 One-Tube Reverb?

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    Help Double-Check My 5F1 Champ with 6BM8 One-Tube Reverb?

    Hi, all--

    I'm adding a 6BM8 one-tube reverb circuit taken from what I've been told is a modded JTM45 schematic* to my 5F1 Champ. I'm basically working on the famous Eric Barbour/Svetlana Fender/6BM8 one-tube-reverb mod--to my Champ. (So far, I have not found any complete schematic of Eric's mod, so that's why I'm using the modded JTM45 as a reference.)

    My schematic is (at this point in time) a pretty straightforward attempt to insert the reverb circuit just after the first preamp stage and just before the volume control, similar to the BillM Blues Jr. mod to the 1st Gen. reverb circuit.

    (I would prefer to insert the reverb later, just in front of the power amp, but I figured this would have lower noise and could sorta use the second preamp stage as a "mixer" stage.)

    Below is the complete schematic that I drew up. Please help me double-check it for errors. I should tell you: I'm not strong on theory. It's been years since I took those ol' college EE courses, and I don't know if I might have miswired anything or used the wrong component values. Please read the schematic carefully and help me spot any problems, no matter how small.

    I do have a few questions to go along with my schematic:

    1) I'm not sure about the way I inserted the reverb circuit to the signal path using the 500pF capacitor and 470K resistor. Some inserts I've seen just use a single 220K resistor. Other inserts use a 3.3M resistor with 50pF cap. I picked the values from the 6BM8 schematic.

    2) Are there any suggestions you'd make to improve the tone of the reverb section? I already trust the tone of the 5F1 circuit; it is the reverb circuit and its values I don't know anything about, except that it's fat and warm (thanks to Bob-I at ampgarage for this). I'm actually trying to emulate the ol' Fender blackface reverb, but with only one tube, because space is extremely constrained in the Champ.

    3) Is heat going to be an issue? I'm going to add quite a bit of circuitry to the tiny 5F1 chassis. It's going to be a "double-decker," with the reverb circuit on an eyelet board mounted over the main circuit and one extra tube. Since there isn't any ventilation in the 5F1 to begin with, I wonder if heat will be a big deal...

    4) Some of you might notice that I chose to use the C voltage (preamp, 250V) instead of the higher A voltage (called "V1/plate" in the JTM45 schem, 340V on the 5F1) as shown in the original modded JTM45 schematic. I figured it would be safer to do so, but it may be a mistake on my part. Please let me know if you think it will sound better if I switch back to the higher C voltage.

    And here is the schematic itself:

    Click image for larger version. 

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    Thanks very much for any help you guys can provide!


    * This is a link to the thread containing the original 6BM8 schematic:

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    Last edited by dchang0; 11-13-2010 at 08:21 AM.

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