Weird ghost notes on my mojotone 18TMB

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Gayle Ellett
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Weird ghost notes on my mojotone 18TMB

Post by Gayle Ellett »

I am getting weird ghost notes on my mojotone 18TMB.
There isn't really any noise or hum, my amp is very quiet (noise-wise). But it sounds sorta like some low frequency (maybe 120 Hz that I can not directly hear) is blending with my played note, creating a new tone/pitch...lower then the actual note I am playing.

I hear it when I play a note around A 440. Way lower or higher ... then I don't hear it.

What is causing that?
Some sorta oscillation/tone from a filter or something?

Any advice would be appreciated!

I've tried different speakers, different guitars, different cables, etc...and no pedals are being used.

(Otherwise ... it is a great amp!! And I added the Marshall logo)
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Last edited by Gayle Ellett on Mon 09/25/23 1:23 pm, edited 1 time in total.
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Re: Weird ghost notes on my mojotone 18TMB

Post by Bieworm »

Sounds stupid, but, have you tried another speaker? A speaker inside another cab?
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Gayle Ellett
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Re: Weird ghost notes on my mojotone 18TMB

Post by Gayle Ellett »

Yes, I've tried different speakers, different guitars, different cables. No pedals, just straight in.
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Re: Weird ghost notes on my mojotone 18TMB

Post by Bieworm »

Could you post a sound sample to let us hear what you’re dealing with?
Ghost noting in an amp often has to do with insufficient filtering. You can sub in an extra cap at the 1st filter cap. Put another one in parallel to rule that one out. Could you give more detailed pics so we can see how things are soldered?
One thing to try is to swap the plate wires on the power tubes. While this is often the fix for an oscillation there, I’ve fixed strange noises doing that anyway
Also, do you have an extra EZ81? I assume those tubes are JJ, which are very problematic nowadays
In general we think the mojo layout is a disaster to work with. I see you soldered the grounds to the back of the pots, V1 grid stoppers at the jacks instead of the tube socket, you also have a lot of wires with too muck bare wire exposed.. that is pretty dangerous, but not the source of your problem (just an observation)
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JohnJankowski
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Re: Weird ghost notes on my mojotone 18TMB

Post by JohnJankowski »

Not standing next to you at the bench, it’s difficult to say. However, the 3 previous 18watt/1974-style builds I put together all achieved top performance by verifying the first node B+ was between 340-350. Also, adjusting the cathode bias resistor value to attain -12v bias.

This last build, I used solid state rectification and a 100 ohm, 10 watt sag resistor in series between the standby switch and the first B+ node.

Next, after verifying 350vdc, I used a bias meter pair plugged into the output tube sockets to read plate current and verify tube balance. I adjusted the cathode bias resistor for reasonable bias current. This resulted in changing that resistor from the suggested 180 ohms to 230 ohms, on my amp.

The amp sounds great. I’m quite pleased as has been everyone else that tried it.

You can use zener diodes as suggested on Josh’s signature line on this forum. Or, a sag resistor. The take away seems to be that these amps require that 340-350vdc and a follow up fine tweak of the cathode resistor value to get that 12v reading. Or in my case, 26-28ma plate current per tube.
Last edited by JohnJankowski on Tue 10/03/23 9:30 am, edited 1 time in total.
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Re: Weird ghost notes on my mojotone 18TMB

Post by JMPGuitars »

JohnJankowski wrote:
Mon 10/02/23 2:42 pm
weak of the cathode resistor value to get that 12v reading
That 12V reading is an average result, not the actual goal. The actual goal is 85% power tube dissipation.

Thanks,
Josh
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JohnJankowski
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Re: Weird ghost notes on my mojotone 18TMB

Post by JohnJankowski »

Ok, understood. My experience with my 18w builds has been that everything settles down very well with correct B+ and tube biasing.
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Re: Weird ghost notes on my mojotone 18TMB

Post by JMPGuitars »

Ghost notes are more commonly because of inadequate filtering. It being a mojotone kit, well, my opinion on those kits is known. 😉

The lead dress and the poor positioning of the filter cap can probably aren't helping. Bad ground schemes and poor leads dress only make the problem worse.
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