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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:

    The Amp Garage :: Log in

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

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    You have no B+ voltage going to the first preamp triode or the 6BM8 (plates and transformer shown connected to ground). B++ going to the 6V6 screen is also connected to its grid. I'm guessing those are all typos in the schematic but you should clean them up before proceeding.

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    Thanks for the keen eye! I'll update the schematic and post it later. Am currently recovering from yesterday's crazed all-day Jschem session, LOL.... If you guys see anything else between now and when I post the updated schem, please let me know.

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    Wow, found a bunch of typos. The updated schem is here--I can't remove the one in the original post.

    This is Rev. 1.0d. To anyone following this thread, please use the latest/last schem possible, especially if building this for yourself.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    Last edited by dchang0; 11-14-2010 at 02:31 AM.

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    I would probably power the reverb driver and recovery from the 6v6 screen node or maybe even the plate node. Otherwise the reverb drive circuit (which creates watts, not voltage gain and must graw current) will be drawing current through that 22k resistor creating sag on the preamp. The voltage is quite low there anyway. I also might lose the 1M series resistor after the recovery stage and wire the 2M pot as a voltage divider. Otherwise you have no way of turning the reverb all the way down.

    The only circuit error that jump out at me is that you have no grid bias resistors on either section of the 6bm8

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    Ah, now that makes sense! I hadn't considered current draw at all. That would explain why the original 6BM8 schematic used the V1/"plate" voltage for the driver side of the reverb circuit...

    I ended up using your recommendation of the 6v6 screen node, labeled (B) in the diagram. Rev 1.0d is posted above.


    As for the 2M pot as a voltage divider--I don't get what you mean. The bottom leg of the pot is wired to ground (not readily apparent from the way I drew the schematic). The wiper goes back to the second gain stage of the parent 5F1 circuit. The top leg goes to the coupling cap (through the 1M resistor, which I may remove on your advice). So, if the wiper is turned all the way to the bottom leg, doesn't that put it at ground?

    I'll tackle the grid bias resistor thing next. Am not sure how large it should be and where to wire it. I went back and looked at two other 6BM8 grid-biased schems: one AX84 project, which has what appear to be 100K grid resistors on pin 3 (connected to more resistors on the path to ground through the bias circuit).

    http://www.ax84.com/media/ax84_m206.pdf

    I also went to look at the original Eric Barbour/Svetlana example schem, which has a 330K grid resistor on pin 3 to ground.

    Click image for larger version. 

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    And then there's this formula to calculate the value of the grid bias resistor:

    Nostalgia Air: Grid Bias Resistor Calculations

    Now, I have no idea of the theoretical values required by this formula, and I'd have to build the amp before I could make direct measurements, so I'm just going to guess that the value would have to be lower than the "balance resistor" of 470k (between the reverb inserts) but higher than the screen grid resistor of 330K at pin 7.

    I'll draw a schem later to confirm that I put the grid resistor in the right place, but let's wait until the reverb pot thing is straightened out.

    Thanks!

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    Last edited by dchang0; 11-14-2010 at 03:39 AM.

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    I think I'm going to have to simplify things quite a bit. Since the 5F1's chassis is so space-constrained, I literally can't fit more than a few caps and resistors in the second eyelet board. Those 25uF/500V caps are HUGE--too long to fit unless turned 90-deg to the rest of the components.

    So, I'm going to scrap this design, go work on a simplified version of Eric Barbour's 6BM8 circuit, then redraw the schematic with that design.

    Here's the new thread. Once we figure out the reverb circuit, I'll work on inserting it into the 5F1 again.

    http://music-electronics-forum.com/t22458/

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    Last edited by dchang0; 11-15-2010 at 05:19 AM.

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