Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: choosebronze on August 16, 2018, 12:34:19 pm
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Hey guys. I posted this same topic on TDPRI. I try not to double-post, but I'm freaking out a little, and you're all experts. So it can't hurt.
I have a Blues Deluxe RI. Never had any issues for 7 years. Hadn't played it for a couple months, but I reloaded the original GT tubes so I can sell it. And now it has no audio. I put the old tubes back in (including the other pair of 6L6's), and moved the 12AX7 positions around, same issue. I noticed V3 wasn't glowing it all. I touched it and it was ice cold. Same with all the 12AX7's I tried in V3. With standby off and the back panel off, there's a hum coming from the chassis but that's the only noise coming from anywhere in the amp.
I checked the voltages on V3, from the points that looked relevant on the schematic.
Pin 1: 3.27 V
Pin 3: .001 V
Pin 6: 3.26 V
Pin 8: .001 V
Pins 3 and 6 are low compared to the schematic. But obviously Pins 3 and 8 look unusably low. I think pins 3 and 8 are supposed to be measured as DC, and that's how I measured. But the number was the same in AC, too (might just be as close as my DMM will go to 0?). Try as I might (I've read Robinette's tutorial a half-dozen times), I don't fully grasp how tubes function. Should I test 4, 5, and 9 to make sure heater current is getting to V3?
Unfortunately that's as far as my knowledge takes me. I don't see any components that look brown or burned, or anything bulging. All the solder joints look fine to the naked eye. And really, all I've done since the last time it worked is change the tubes.
Any advice is greatly appreciated.
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A cold tube cannot make any sound. Set your meter to read 6.3 volts AC. Now connect one probe to V3 pin 4/5 and connect the other lead to V3 pin 9. You should have 6.3VAC. Since the tube is cold you probably don't have 6.3VAC. I suspect a bad solder connection to the tube pins, or a broken PCB trace or maybe a bad socket. You'll have to pull the tube board and do a close inspection. Probably be able to see the problem.
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Thank you Sluckey. I feel stupid. I measured between 4/5 and 9 and I got 6.4V. So then I figured if the voltage is on the pins, it's not getting to the tube for some reason. I looked at all the socket springs and they all looked straight and tightly spaced, but I re-tensioned them anyway and that fixed it. I've had bad pin connections cause noise before, but never seen it just fail to power a tube. At least that's solved. Thanks!