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 Post subject: No sound on JTM45
PostPosted: Wed Jan 24, 2018 12:40 pm 
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Joined: Thu Nov 20, 2014 8:49 pm
Posts: 108
I finished up my amp to day. Everything seemed to be going as planned until I put the pre-amp tubes in. V1 wasn't glowing. It appears to be getting the same voltage as the others, but I wondered if it was just too dim to see. I was able to bias the amp, but get no sound. I tried swapping around V1 and V2, the original V1 tube does glow in the V2 position.

I looked over joints and the wiring diagram. There isn't anything obvious to me. I used a signal generator and tester. I did get a very loud pop out of the V2 output (pin 8) and wonder if this isn't where to look. But I ran out of time to troubleshoot today. Does anyone see anything obvious?

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 Post subject: Re: No sound on JTM45
PostPosted: Wed Jan 24, 2018 4:45 pm 
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Location: Truckee, CA
If the V1 heaters aren't glowing, you won't get signal through the amp. It's good news that you were able to bias it. That means that the problem is probably with V1.

Test your heater circuit. Remove all tubes, place the amp chassis with the tube sockets & transformers up, and turn on the amp (standby off is fine). Set your meter for AC voltage and read AC volts from V1, V2, and V3, sockets 4 and 9, and then sockets 5 and 9 (not reading to ground - one meter lead in socket 9, the other lead in socket 4). You should get around 6.3+ volts AC every time. If you don't get 6.3+ volts from V1, you have a bad connection in the green heater from V2 to V1. It's important that you are testing the tube sockets where the tube plugs in and not the solder terminal inside of the chassis. You could get voltage going to the solder joint on the solder terminal, but it's a bad joint that is not conducting to the socket. I've seen that multiple times.

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 Post subject: Re: No sound on JTM45
PostPosted: Thu Feb 01, 2018 12:46 pm 
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Joined: Thu Nov 20, 2014 8:49 pm
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Morgan wrote:
If the V1 heaters aren't glowing, you won't get signal through the amp. It's good news that you were able to bias it. That means that the problem is probably with V1.

Test your heater circuit. Remove all tubes, place the amp chassis with the tube sockets & transformers up, and turn on the amp (standby off is fine). Set your meter for AC voltage and read AC volts from V1, V2, and V3, sockets 4 and 9, and then sockets 5 and 9 (not reading to ground - one meter lead in socket 9, the other lead in socket 4). You should get around 6.3+ volts AC every time. If you don't get 6.3+ volts from V1, you have a bad connection in the green heater from V2 to V1. It's important that you are testing the tube sockets where the tube plugs in and not the solder terminal inside of the chassis. You could get voltage going to the solder joint on the solder terminal, but it's a bad joint that is not conducting to the socket. I've seen that multiple times.


This was exactly what was happening! My solder joints looked solid, but there was one on V2 that wasn't conducting to the socket. I noticed that only half of V2 was heating on second look.

I've got the amp up and running. It sounds pretty good, but the bass is pretty flubby. A professional amp builder that I've become friends with has given me some suggestions that I've already implemented to tighten up the bass, and a few more with parts on the way. I'll report back after completing them all.


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 Post subject: Re: No sound on JTM45
PostPosted: Thu Feb 01, 2018 12:57 pm 
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Nice job sussing that out!

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