Mojotone 18-wat TMB passing signal when master volume and gain both off

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ed333
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Mojotone 18-wat TMB passing signal when master volume and gain both off

Post by ed333 »

Good morning, I've been pretty happy with my 18 watt build for a while...until now.

I recently changed the screen grid resistors from the stock 100R to 1K 3-watt metal films, as well as bumped the cathode bias resistor up to 200R.

When tightening all the hardware, I accidentally cracked the TMB input jack, so I replaced it with the same cliff-style jack as the original. For whatever reason now, the TMB channel passes a thin, weak sounding signal even when the master volume and channel gain are both fully off. What is causing this?

Furthermore, the "normal" channel is now unusable, because when nothing is plugged into the TMB channel there is a constant white noise or hiss/whooshing sound, no matter what level the TMB gain and MV are set. I've checked the grounds and they all seem to be connected. If I plug into either of the "normal" inputs, the noise floor is so high the amp is unusable.

Any suggestions or ideas on what I need to do here? Thanks in advance!

FWIW, this was my first amp kit build (completed in 2019), and I have since built (and restored/modified) other things that work just fine. The issues with this TMB are driving me crazy, though!!!
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JMPGuitars
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Re: Mojotone 18-wat TMB passing signal when master volume and gain both off

Post by JMPGuitars »

Post some high res photos of your build. Post voltages also if you can.

From your description, my guess is you have a bad solder connection somewhere causing the gremlin. A "highlighter test" might be your best bet to start troubleshooting.

Is the amp quiet with no guitar cable plugged in?

Thanks,
Josh
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ed333
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Re: Mojotone 18-wat TMB passing signal when master volume and gain both off

Post by ed333 »

The amp is not quiet with nothing plugged in, same issue as when trying to use the normal channel. Will post pics as soon as I am able, thank you!
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Re: Mojotone 18-wat TMB passing signal when master volume and gain both off

Post by JMPGuitars »

ed333 wrote:
Sun 11/27/22 11:54 am
The amp is not quiet with nothing plugged in, same issue as when trying to use the normal channel. Will post pics as soon as I am able, thank you!
Try and include some good photos of the input jacks. If the amp isn't quiet with nothing plugged in, then it's likely a mistake was made wiring the new jack.

Thanks,
Josh
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