Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: valhala92 on June 24, 2014, 04:34:36 am
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I bought a '74 JMP Bass, it worked perfectly but after changing one of the power tube sockets (it was broken), 50 + 50 caps with F&T ones and installing 1 ohm 1 watt resistors between pins 1&8 and ground, the amp makes loud crackling and squealing sound (sometimes it produces normal sound but after 5 seconds, it will make very loud noises) and when I use Input II, it doesn't make any sound.
When I lower the bias to 12mv the amp becomes quieter (still makes noises) but when I increase it to 20mv or higher, it starts to making loud noises even with volumes on zero.
And when I pull phase inverter tube, the squealing will go away.
Here are the voltages:
V1 = Pin1: 147 / Pin2: 0 / Pin3: 0.993 / Pin4: 3.03 / Pin5: 3.03 / Pin6: 150.6 / Pin7: 0 / Pin8: 0.993 / Pin9: 3.01
V2 = Pin1: 130.5 / Pin2: 0 / Pin3: 0.731 / Pin4: 3.03 / Pin5: 3.03 / Pin6: 222 / Pin7: 131.2 / Pin8: 132.2 / Pin9: 3.01
V3 = Pin1: 176 / Pin2: 17.1 / Pin3: 27.2 / Pin4: 3.03 / Pin5: 3.03 / Pin6: 166.6 / Pin7: 17.89 / Pin8: 27.2 / Pin9: 3.01
V4 = Pin2: 3.01 / Pin3: 365 / Pin4: 364 / Pin5: -33.4 / Pin6: 366 / Pin7: 3.01
V5 = Pin2: 3.02 / Pin3: 365 / Pin4: 364 / Pin5: -33.4 / Pin6: 366 / Pin7: 3.02
And here is a sound clip of the noise:
http://picosong.com/Pjmi/ (http://picosong.com/Pjmi/)
I really appreciate it if you help me.
Thanks
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And when I pull phase inverter tube, the squealing will go away.
Have you, by accident, moved wires around while changing the tube socket?
Also, search for cold solder joints on the work you did.
Sometime ago I retubed a JCM800 KK signature. When I first measured cathode current it was shocking low, around 20mA's per side (both tubes that is, @ 430V on plates, 426V screen, quad of 6550). The amp is all PCB so no wires were moved around. After raising cathode current to 60mA's per side the amp started squealing and howling. Backed bias to 20mA per side and all is fine. It was the first time I saw this happening (12 years repairing amps). Still could not find a clue to why it happened.
If you get better luck, please, share your findings.
With Respect,
Best Regards
R.
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Maybe, 'cause there wasn't enough room for changing the socket.
I change tube the new tube socket with the old one, but it didn't solve the problem either.
If I connect pins 1 & 8 directly to ground, the squealing will go away with bias around 20-30mv, but when I set the bias about 40mv it will start again, and there will be drop in volume and distortion even in low volumes.
By the way, I checked solder joints from tube sockets with a multimeter, they were all connected.
Thanks.
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I see some nasty solder joints on the tube socket in the pic. Maybe the ground connection for the 1Ω resistor is nasty also?
I would temporarily remove that fuse holder to gain good access to that socket. Then redo all those nasty solder joints. If the solder joints on the cap cans look like the ones on the socket, I'd redo those as well.
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I changed the socket again yesterday and i checked the connections with a multimeter, they were ok.
Here is a picture of the caps:
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The 2 sockets appear to be positioned differently in the chassis?
Brad :think1:
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The new socket frame have a different angle, so it should be installed like this.
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After changing the power socket with a new one, and shielding OT primary wire, the noises went away.
Now I'm trying to fix the problems the amp had before, when I close the Input I volume all the way to 0, I can still here the sound of guitar, and when I hit the the standby switch to off it makes a popping sound.