jtm45 ppimv ringing/hum

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Canuck
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jtm45 ppimv ringing/hum

Post by Canuck » Wed Oct 14, 2020 3:20 pm

Have a jtm45 Kit I built years back. Through in a pair of el34s to give it a try. I did change the screen resistors and rebias. I put in a ppimv years ago as well. Yesterday after bring the amp home from a friends place I fired it up and noticed a ringing/hum/feedback type sound. Sorry really hard to describe it. It wasn’t loud but enough that it was obvious. Turning the master volume all the way up, or master volume bypassed in other words, got rid of the sound.
Has anyone come across anything like this? The amp plays fine and sounds good otherwise. Our little jam space is too small to just crank it up.

danman
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Re: jtm45 ppimv ringing/hum

Post by danman » Wed Oct 14, 2020 7:24 pm

Turning the ppimv pot to 10 does not bypass it from the circuit. When the pot is at 10 though, the full signal from the PI is allowed to pass through to the power tubes. It is true that the amp should sound "stock" when the ppimv is on 10 though.

I would start with trying a fresh preamp tube in each slot one at time and then a different pair of power tubes if you have any available. Tubes are always the easiest thing to check in a tube amp. After that I would open up the chassis and check the pot for any loose wiring or bad solder joints. It may also be a bad pot that is causing the issue but you need to eliminate one thing at a time before replacing the pot.

Canuck
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Re: jtm45 ppimv ringing/hum

Post by Canuck » Thu Oct 15, 2020 10:50 am

Fair enough on how the master volume works. Bypass is a poor choice in terms. With the master volume on 10 no hum/noise even with bright/normal volume cranked. As soon as I start turning the MV down the noise starts. It’s almost like something vibrating, even though there’s no noise being made. It builds up. It’s not loud though. I actually thought it was my furnace at first.
Wonder if I can record it an post it here?
I’ll run through all the tubes and see if any of them make a difference. I wonder if the PI would be a good place to start seeing as how the noise starts with the master volume.
Thanks for the help!

Canuck
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Re: jtm45 ppimv ringing/hum

Post by Canuck » Thu Oct 15, 2020 11:00 am

Tubes or something coming loose/breaking makes sense. It was fine for years. Now after bringing it home again it’s making noise. Something must have happened in transport.

danman
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Re: jtm45 ppimv ringing/hum

Post by danman » Thu Oct 15, 2020 7:17 pm

I'm betting there will be a bad solder joint somewhere in the MV pot wiring. Maybe it broke loose during transport. I would still check the simple things first though.

Canuck
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Re: jtm45 ppimv ringing/hum

Post by Canuck » Thu Oct 15, 2020 7:30 pm

I could totally live with that. Gonna try and crack it open tonight, get a bright light inside and have a look.

danman
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Re: jtm45 ppimv ringing/hum

Post by danman » Fri Oct 16, 2020 6:59 pm

Reflowing all of the joints would be a good idea even if they look ok.

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