Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Woodrow F Call on March 25, 2025, 04:23:15 pm
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Ok, I got most of this amp sorted out. New Tank, killed some noise due to cracked solder joints, new input jack, hooked up the speaker wire.
Now the only thing I have left is some hum when I turn up the reverb. I've looked at the solder joints, reflowed the RCA cable joints on the board. It sounds like 60 cycle hum. I move the reverb tank closer to the speaker baffle and get less noise. I tried vertical, but it gets worse. I moved closer to the back of the amp and it gets worse.
Anything else I should look at? The noise isn't terrible until reverb is past 4 (which is just too much reverb anyways), but I'd like to get this dialed in as best as I can.
Original Post Below:
Hey everyone. I got a good deal on a USA made Blues Junior that wasn't working. Input jack was smashed and a speaker wire was not hooked up. Easy peasy. I put in a switchcraft jack and started testing it. No reverb.
I tap the tank.... I get the springy sound you'd expect. Looking at the schematic (attached), I think this tells me the problem is upstream of the reverb tank. I check for voltages at the test points TP21 and TP22. The note says values may vary with the tank. I get 20mV at TP21 and 0.0mV at TP22.
This leads me to think.... solder joint on one of the components or the chip is bad.
Am I missing something?
BTW, I don't have an oscilloscope to check other stuff. I need to get one just haven't gotten there yet.
I appreciate the help!
EDIT/UPDATE: I get infinite resistance on the input coil of the reverb tank..... I guess that's probably it?
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Try a new reverb pan
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I check for voltages at the test points TP21 and TP22. The note says values may vary with the tank. I get 20mV at TP21 and 0.0mV at TP22.
Voltages do not vary with the tank. Read note 2 again. You will not be looking for dcv at TP21 and TP22.
EDIT/UPDATE: I get infinite resistance on the input coil of the reverb tank..... I guess that's probably it?
Good suspect.
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Try a new reverb pan
Well, I just tried the one in my other tank... yup. Bad tank.
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I check for voltages at the test points TP21 and TP22. The note says values may vary with the tank. I get 20mV at TP21 and 0.0mV at TP22.
Voltages do not vary with the tank. Read note 2 again. You will not be looking for dcv at TP21 and TP22.
EDIT/UPDATE: I get infinite resistance on the input coil of the reverb tank..... I guess that's probably it?
Good suspect.
When I wrote that, I was meaning from tank to tank... is says from pan to pan. I figured that's why my input was 20mV and not 27mV per the drawing. I think my meter was set correctly.
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When I wrote that, I was meaning from tank to tank... is says from pan to pan. I figured that's why my input was 20mV and not 27mV per the drawing. I think my meter was set correctly.
Did you have a signal generator set to the proper level/frequency and connected to J1?
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I don't have a signal generator or a O-scope (well, one with the leads anyway).
I need to fix that.
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I don't have a signal generator or a O-scope (well, one with the leads anyway).
All those test points with AC voltages (except the 4 test points in the power supply) require you to connect a signal generator set for 10mVAC @ 1KHz to J1. Those test point voltages are meaningless without the sig gen.
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Thanks!
Update Question in OP.
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Rotate the tank 180 degrees in place. I often find jacks facing baffle to be quieter.
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Click the link to hear the hum. Can anyone verify if this is 60hz or 120hz?
I have tried a few other things:
Tank is correct.
I've moved the tank around, vertical even. No real change.
Ok, I shorted the RCA cables tip to sleeve. There was no noise.
Checked the power supply voltages on the IC pins (4 and 8 ).... One read 15.01VDC and the other was 15.00VDC.
I did not see any AC using AC or mV settings on my Fluke 117 for the IC chip power supply.