Finished the build of a Weber 6M45P. Built Martin Manning's Deluxe Light bulb limiter and plugged it in. Light turns on, Bulb dims, looked good, Plugged the KT66 into the Bias probe, turned it on, took it off Standby and was unable to bias the amp, it starts up at 21 ma and if I move the bias pot, it will rise, sometimes ridiculously high (120ma) and then will return to 21ma. There's no way to turn it up with bias to 30-35 without it returning to 21ma.
Adding insult to injury the amp has no sound. The speaker will hum and will pop when I probe the grids on the 12AX7. Continuity checks out, wiring checked multiple times, Input jacks checked, Voltages have been taken of all grids cathodes and plates and nodes. (Ive attached my voltage readings) The schematic Weber provided doesn't come with Voltages (A glaring omission IMHO, but I'm wiser now) but from what I can tell the voltages do not seem out of whack. I did replace some parallel 40uf and 8uf filter caps with 47uf caps, just because it was crazy the number of caps Weber sent for a rather small board.
Lastly, I used the signal generator of an oscilloscope to send a sine wave thru the amp. I connected the scope to the grid of V1A (12ax7) and I get a normal sine wave. I turn on AC coupling and connect the scope to the Plate of VIA, I get a clipped sine wave. (photos attached). The clipped Sine wave continues through the amp. The master volume shows the same clipped sine wave. That can't be right?
I'm likely really over my head on this, and have made an appointment to see a professional tech later this month, which means I've got about 2 weeks to find the problem and finally get to the moment when it finally makes music.
If you're still here, thank you so much. You make this community great. I knew when I started this 2nd build that I would learn alot along the way. That is a gross understatement. I am attaching the Schematic, Layout, Voltage readings, a gut shot, and photos of the strange sine wave issue,
If youve got any suggestions... Thanks again,
Dan
JTM45 Build, looks alive but no sound,
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JTM45 Build, looks alive but no sound,
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Play it LOUD!
Re: JTM45 Build, looks alive but no sound,
Your scope is showing 2.48Vpp at V1A grid. That's way too hot! You need to lower your test signal to 100Vpp. Then the rest of your waveforms will look normal.dpb703 wrote: ↑Fri May 03, 2024 2:38 am Lastly, I used the signal generator of an oscilloscope to send a sine wave thru the amp. I connected the scope to the grid of V1A (12ax7) and I get a normal sine wave. I turn on AC coupling and connect the scope to the Plate of VIA, I get a clipped sine wave. (photos attached). The clipped Sine wave continues through the amp. The master volume shows the same clipped sine wave. That can't be right?
Re: JTM45 Build, looks alive but no sound,
if there is a scope, then is easy..
1. remove NFB wire
2. put 100mV signal on the input
3. trace to the output tubes pin 5
blind shot..i suspect there is no gnd wire on some tubes, missing
1. remove NFB wire
2. put 100mV signal on the input
3. trace to the output tubes pin 5
blind shot..i suspect there is no gnd wire on some tubes, missing
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Stevem
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Re: JTM45 Build, looks alive but no sound,
This might help.
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When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: JTM45 Build, looks alive but no sound,
Are you plugged into the main speaker jack and not the extension jack by accident? You must plug into the main jack for the amp to produce sound. Also make sure that you are not plugged into the limiter when biasing and trying to play through the amp.
Edit: Looking at the schematic, it appears that Weber does not use a shorting jack for the main jack. If this is true, you can plug into either jack and the amp should make sound.
Edit: Looking at the schematic, it appears that Weber does not use a shorting jack for the main jack. If this is true, you can plug into either jack and the amp should make sound.
Re: JTM45 Build, looks alive but no sound,
Looking over your voltages, something's definitely not right on V2. The cathode follower should have significantly more voltage on its plate than the preceding gain stage's plate since there is a plate load resistor feeding the preceding gain stage versus the cathode follower's plate being connected directly to the B+ rail. In other words, plate voltage should be much higher on V2B than V2A, whereas your voltage readings show a trivial difference in plate voltage between V2s plates. The bolded line in the voltage readings document you posted seems to indicate that you've also already identified the issue with the very elevated cathode voltage at the cathode follower. It's possible that whatever is causing these voltage abnormalities is also causing signal to be lost prior to reaching the phase inverter, which is why you're seeing signal amplification at one of V1's plates but hearing nothing out of the amp. Bepone's troubleshooting procedure would be excellent in determining if this is part of the issue.
As far as the bias issue, I wonder if you have a bad bias pot that results in an open circuit when not at the extreme end of its travel, resulting in runaway bias at anything other than maximum negative voltage/coolest bias. There could be something else causing the issue, but to determine if the issue is in the bias circuit I'd suggest removing your power tubes and tracking your bias voltage at V4 and V5's grids through the bias pot's sweep. If that looks good, I find it hard to believe that you'd have two dud KT66s, so I'd want to rule out an oscillation issue next.
As far as the bias issue, I wonder if you have a bad bias pot that results in an open circuit when not at the extreme end of its travel, resulting in runaway bias at anything other than maximum negative voltage/coolest bias. There could be something else causing the issue, but to determine if the issue is in the bias circuit I'd suggest removing your power tubes and tracking your bias voltage at V4 and V5's grids through the bias pot's sweep. If that looks good, I find it hard to believe that you'd have two dud KT66s, so I'd want to rule out an oscillation issue next.
Re: JTM45 Build, looks alive but no sound,
V1B cathode voltage of 5.5V is bad.
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Re: JTM45 Build, looks alive but no sound,
Really appreciate the info. Tomorrow the new Bias pot arrives. Hope that takes care of the bias.
Updates to follow…
Thanks,
Dan
Updates to follow…
Thanks,
Dan
Play it LOUD!