Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: sarkipinty on February 11, 2019, 03:32:10 pm
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Hello All,
I decided to build myself a Fender Champ, a very popular DIY project
in order to save costs I bought a small old transistor combo (namely Sidekick reverb 25) and took out the solid state amp and started building my little champ in it
as you probably know the sidekick reverb had a fairly nice reverb tank... got myself thinking... maybe I can add a reverb circuit to it
I found a guy doing a reverb mod on a champ here: http://www.offsetguitars.com/forums/viewtopic.php?t=35226 (http://www.offsetguitars.com/forums/viewtopic.php?t=35226)
with the final circuit looking something like this: http://www.offsetguitars.com/personal/sookwinder/cr/champreverb106.jpg (http://www.offsetguitars.com/personal/sookwinder/cr/champreverb106.jpg)
here is the problem:
after several hours of troubleshooting, I cannot find the problem with the circuit, sound is coming out but it seems like when is strum a chord, the sound immediately starts dying out (as if the power supply for the tube was struggling to keep up)
and there is no reverb effect no matter what i do with the pots
by trial and error I found that by disconnecting the 3.3M and 10pF cap from the grid of the 3rd ecc83 (see attached) the amp is working perfectly (if I max out the reverb it starts to get feedback, but that may be normal)
my question:
I don't quite understand the purpose of that component, it seems strange to me to send the signal from the "normal" circuit to the reverb part but before that immediately send it back to the 6V6 (via a gain stage?)
thanks in advance for the help!
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Follow the signal chain from the input til the power tube.
Where in that signal chain is the insertion point to the reverb? Where is the return point to the reverb?
Look at the schematics in reply #5 in this thread: http://el34world.com/Forum/index.php?topic=24315.0
And are you going to give up the vibrato in the VibroChamp to have the reverb?
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That reverb mod cannot possibly work as drawn in that schematic. The takeoff point and the return point are the same point! IOW, the entire reverb circuit is short circuited.
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Try this...
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Also, it looks like the tank from the Sidekick 25 Reverb is designed to be driven from a capacitor not a transformer.
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Thanks All for the feedback, let me experiment with it a little and I will come back with a conclusion
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The fix was very simple. Probably took more time to revise the schematic than it will to actually make the change in your chassis. Let us know how it turns out. There is another current thread talking about adding reverb to a Vibro Champ.
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@Sluckey I tried to separate the reverb input and reverb output as you draw it, but I get immediate loud screeching when the tubes warm up no matter if I have a guitar connected or all the pots all the way down
after some googling I also tried to shield the preamp tubes to see if it helps but no
coming back to my original post, the amp was working perfectly when I took the original schematic (with the reverb mod) but disconnected the 3.3M resistor and 10pF cap
so essentially my question is if it is safe to keep them disconnected
@tubenit I wish I had seen those schemas before I started, I would have built it differently, but in the end I have this weird 1,5 tube reverb circuit... and it seems to work (with some modification)