Mojotone Vibro Champ high pitched noise

Ian R. Taylor

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I finally got the Mojotone Vibro Champ actually working (with much help from people here) and can tell it will be a fun amp.

I'm getting high pitched noise, even with nothing plugged into the amp. If I put the volume around 7-8 it's just barely audible. It's loudest at 2-3 and again at 9-10.

The noise is very much controlled by the intensity knob - loudest when the intensity knob is full off, and when you turn the tremolo on you can hear some tremolo oscillation, even when nothing is plugged into the amp.

When I play music, I only use the tremolo with the intensity full on, and only sparsely with a momentary footswitch, I never will click it on and play a song with it. So if I run the intensity full (where I'd want it), variac the amp down to 90vac (where I like it and makes the noise less), the only problems are that I can't turn up louder than 8 or so and I can hear faint super high pitched noises that I know mics will pic up. Ultra high pitches wear you out if you sit by an amp for lots of hours.

Also, I don't doubt the amp will sound better after awhile on it's own. To me it sounded better after just a few hours today. Brand new tubes always sound a bit harsh. And I noticed an improvement just by removing the 12ax7 tube shields.

Initially I thought it was a tremolo issue because of how the intensity knob affects it, but when I remove V2, the tremolo tube, the oscillation went away but the noise, if anything, was louder.

D'tar suggested it may be a nfb vs pfb issue so I am looking into what that entails.

I will do some chopsticking and wonder if additional ground wires would help. So far moving the wires, especially around the pots didn't do much.

Everybody, Chopsticking, key of E, 1....2....a

I appreciate any comments or ideas anybody has. Thanks!
 
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D'tar

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Ian, you can simply disconnect the nfb feed from the output jack to the 2.7k Nfb resistor. If the oscillation/noise disappears then you know you have positive feedback rather than negative feedback. If the issue remains, at least you have eliminated one possibility.
 
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Ian R. Taylor

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Thanks, do you mean the wire with the red arrow pointing at it below? It goes to a 2.7k resistor.
nfb wire.jpg
 

Ian R. Taylor

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I just tried that and it sounds like a totally different amp - way, way more what I want. More Jimi and Jimmy! It's louder and there's much more harmonic magic going on.

With this mod it is much closer to the small Silvertone type amps, way more open.

I had earlier posted that the tremolo was no more but it turned out that one of the Electro-Harmonics 12AX7's that came with my kit is bad. I wonder if all my shenanigans with the bad wiring I had killed it?

The way I found out was by trying the EH tube in my old Silverface Champ, I'd just wanted to try it out, and when I did the volume was 1/4 normal. So I tried a working JJ tube and the volume went back to normal. When I put that JJ tube in the new kit amp's trem socket the tremolo worked, and without the noise issues. So it looks like I'm set as far as this stage!

I will wait 'til tomorrow to officially call this the end of Version 1.0. Well, it lasted a day, how long are v 1.0's supposed to last? If everything works tomorrow as it does tonight I will begin the next chapter - The Mods®. The amp is getting better folks! And this is just testing with the 8" ceramic that was in the SF, which never was a very happy speaker.

Mahalo!
 
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D'tar

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That's great you are finding some goodness in there! Did you reconnect the nfb to verify the phase is correct? A stock or smaller value nfb resistor in series with say a 10-25kl pot is enough to go from clean to mean and is a fairly common mod to such a circuit! Have fun and be safe!!!
 
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Ian R. Taylor

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So if I reattach the wire from the 2.7k resistor to the speaker jack's tip terminal, how do I verify that the phase is correct? Do I need an oscilloscope? I only have a multimeter. Would the amp be a bit quieter if the value of the 2.7k resistor was slightly higher?

And what is the purpose of doing that if it already sounds good - a possible improvement?

I don't play guitar but synth and any clean sounds I play on my synth go straight to a different amplifier/speaker setup, the sounds coming out of the Champ are only full blown distortion (I'd actually probably like more as long as it doesn't add noise) so would I benefit from adding a pot if I never want it clean?

Thanks!
 

D'tar

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If the high pitch squeal or anomolies return.... the phase is wrong.



Ps....

This one is for you!!!
 

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Ian R. Taylor

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Thanks for buying me lunch! I'll reattach the wire and see if it returns. A volume pot on it might be a good idea.

Just back from the leeward side - Oahu traffic on Friday - yipes!

I'll do some jamming tonight and if there's no issues I'll consider the intial build completed and look at the next chapter of Ian's Champ Amp Saga®, playing at fine theaters nowhere. This thread actually overlapped into that because lifting that wire is a keeper me thinks. But I might do some other tweaks that will blow it up even further so pots on the mods, even screwdriver type trimmers might be the way to go so it can be dialed in.

As always, appreciate the kokua...er...help :)
 

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