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Add Reverb To Your Champ, Princeton, Whatever, The Easy Way.

Discussion in 'Shock Brother's DIY Amps' started by printer2, Apr 21, 2021.

  1. printer2

    printer2 Poster Extraordinaire

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    OK a little rough right now but just did it as a proof of concept and I am happy to say it will work. I will have to go over it a little, wires and stuff all over the place, I'll draw up a schematic of what I got. BUT I AM TOO EXCITED ABOUT IT I JUST HAD TO POST IT NOW!!! Sure, people have built spring reverb into Champs already but usually they are just splicing in Fender's circuit including the 3.3M mixing resistor and a gain stage to recover some level. But what if you wanted to add reverb to your basic Tweed or BF Champ, Tweed Blonde, any amp without adding more to the circuit, just ad a send and return? Pretty tall order, maybe we should just keep it to the Champ first.

    OK here is the concept. The Champ has two triode gain stages then the output tube (first day at school back from vacation kiddies). The volume control, with tone controls in the BF amps, are between the first and the second gain stage. Say we take the reverb signal off the volume pot and send it to the reverb, not too radical. But we need to return it back to the amp without attenuating the dry signal, leaves out using mixing resistors. We could have the dry signal leave the volume control and goes into the grid like normal, and return the wet signal into the cathode. Just like the Vibro Champ, the modulation signal enters the cathode.

    [​IMG]

    I need to go over the amp I used, it is a point-to-point and I am not sure what is going on with the negative feedback (told you I am too excited), but I took the reverb send off of the volume pot wiper and the return to the top of the cathode resistor. The mixing takes place in the triode without any mixing resistors, there is no back feeding of the signal as if you use too low resistor values if you do it with mixing resistors.

    I have been thinking about this for a while, just never made the time to check to see if it would work well enough. There are different routes to go with the reverb circuit, you can do a tube version but then you are pulling power from the power supply or need to make one up. Solid state reverb is more user friendly at this stage of the game. Two paths to take, one is to use a mid-impedance tank like Fender has in some of its amps.

    [​IMG]

    I was going to try this but thought about the power supply needed for the opamps. It would be best to have about the 30V across the IC to drive the tank. But if we used a 8 ohm tank we can use a chip amp along with a 12V supply, should work, I think. But I have two Fender mid-impedance tanks, wished I could get an 8 ohm tank cheap to try it out. Then it hit me, I have lot of spring reverbs. They are from Yamaha organs I salvaged some of the stuff when parting them out. Measured the tank and the driver side has a 3 ohm resistance across the coil. It does go up as you go up in frequency, I will just wave my magic wand and say it is an 8 ohm one.

    I bought some inexpensive amplifier boards from China, one is TDA2030 with heatsink and volume control. Being one of the inexpensive ones the terminal connectors are faulty (had to poke around with a small screwdriver to bend things so they work) the pot is flaky, I will pull it eventually. I think I bought it for $3-4, can't complain too much. But I did get it to work using a 12V Switched Mode Power Supply. I was thinking opamp recovery but I also thought a mu-amp will give enough gain. I also liked the fact that you can plop in a range of Fet's and not really worry too much about the device's characteristics. I had one breadboarded up (how convenient) from when I was playing around with it for a SS amp. I measured my gain to be 35X, would like more but we will try it out as is. The Yamaha reverb unit has a 100 ohm dc resistance on the pickup side, might be lower in voltage than other tanks, I have no idea.

    Won't go through the 'Why won't this work/troubleshooting' phase, had some scratching my head moments. But in the end I got it to work. I did have to add a Fet source follower behind the mu-amp to buffer the signal to drive the cathode. I do not have the most drippy of reverb but that is just a question of gain. I use 2N5458's in the circuit, doubt they are real ones. But that is all a question of tweaking the circuit. So with a 12V adapter, a chip amp and a recovery circuit all that needs to be modified in the amp (reverb level and footswitch in an enclosure in the back of the amp) is a shielded line soldered to the volume pot and one going to the cathode of the tube (and finding a hole to get the cables out of the chassis). Want the amp original again? Unsolder the cables and remove them, done. Timely ending to the first post, I have other things to attend to at the moment, see you later.
     
  2. kbold

    kbold Friend of Leo's

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    The easy way ...... with a pedal?
     
  3. bcorig

    bcorig Friend of Leo's Silver Supporter

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    Two words:
    Strymon Flint. But seriously, well done!!
     
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  4. printer2

    printer2 Poster Extraordinaire

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    The easy way would be not coming to the Shock Bothers forum I would think. Lot of amps in the stores. But some like the smell of rosin in the air.
     
  5. printer2

    printer2 Poster Extraordinaire

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    I just looked it up, I need to find a manual to see where it fits into the amp circuit. The tremolo is nice but I could live without. One thing about the SF pedal, $399 here. I can add reverb for peanuts compared to that. And your welcome.
     
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  6. jmp81sc

    jmp81sc Tele-Meister

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    Thanks for all of the work and sharing.

    I have an old Solid State Wurlitzer organ that has a reverb tank, I was considering making a stand alone reverb using the organ circuitry.
     
  7. bcorig

    bcorig Friend of Leo's Silver Supporter

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    I was trying to be ironic. Bad move on my part. I really admire the work you did. I should just have said that.
     
  8. Milspec

    Milspec Poster Extraordinaire

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    bath.jpg This is the easiest way.....
     
  9. printer2

    printer2 Poster Extraordinaire

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    Missed that. Can see that now. I heard of someone having a add-on or kit to add reverb and tremolo onto an amp, I thought it might be them. Was a little disappointed that it was just a pedal. A good pedal, I remembered afterward I saw the pedal before and was curious about it.
     
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  10. AlfaNovember

    AlfaNovember TDPRI Member

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    I like it! It strikes me that if you squint, the Fender 6G15 reverb is almost a second Champ with a reverb tank wedged in between the power pentode and a recovery stage. IMO using chip amps (and a reverb tank with the proper impedance for SS) is an excellent riff on the theme.

    Over on 'The Truth About Vintage Amps' podcast, Skip Simmons has been talking occasionaly about speaker-driven reverb, which uses two daisy-chained amps:

    https://vintageamps.libsyn.com/ep-61-less-than-zero

    Not that far different, really, just making do with what was available at the time.
     
  11. surfco

    surfco Tele-Afflicted

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    Zzzzzzz....
     
  12. bcorig

    bcorig Friend of Leo's Silver Supporter

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    I really like mine and was surprised how much I use the tremolo.
     
  13. printer2

    printer2 Poster Extraordinaire

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    There is nothing really groundbreaking with what I did, I am sure others have used the chip amps to do the same thing. The mu-amp, the pedal guys have used it in their BSIAB designs, where I came across it. The 12V SMPS adapters are cheap to get, found tons in thrift stores. You could even break the signal path anywhere in the amp where you want to insert the reverb and any signal loss can be made up with any SS technology you want to use. The only thing to see here is taking the signal at the triode grid and returning it to the cathode. You don't alter the signal path and the amp sounds exactly like it did before except for having reverb when you want it.

    And I will be adding the above reverb, with a higher impedance amp though, and tremolo to an amp I am building. The tremolo will be SS also, the Cathodyne phase splitter will also be SS. I will have one 12AX7 for the first gain stage and the second after the tone stack and volume control. With only one preamp tube I probably would not have enough gain to drive the amp much into clipping if a conventional 3.3M return resistor is used to isolate the send from the return. I have seen a couple of different tremolo circuits in SS I want to try with the amp, I would like to try combining the trem and verb into one and return them to the cathode.

    I can imagine a few ready made Champs could benefit from this method. The bigger amps may have more juice in the power supply to add some more parts to drive a tank, the Champ does not have the luxury. I need to try it with a switching amplifier for the reverb, with their small size and no need for a preamp I can see it and the recovery side built into the tank. Getting to the reverb level pot might be an issue for some, others not. I lucked out with my version of the Champ, I think the Yamaha spring unit will just fit into the bottom of the cabinet. It does not sound as good as a three spring unit but is fine for adding a little pace at the end of a note.
     
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  14. Bitsleftover

    Bitsleftover Tele-Holic

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    Hmmm.
    Interesting.
    Veeeeeeeeeeery interesting.
     
  15. printer2

    printer2 Poster Extraordinaire

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    This is roughly the circuit for the reverb. I am using it in a Champ styled amp but without the 49R resistor. I have a switch with a cap to bypass the cathode resistor or add NFB from the secondary. Seemed to work with the cap in place, should not, might be bad. I did not want to post a schematic until I knew what was going on but I had to disconnect it all as I did not have room to play with it. I am going to install the reverb into my Champish amp so I will post an as built schematic then. Just wanted to get something posted showing the idea of what is going on.

    [​IMG]
     
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