Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: rutledj on October 14, 2014, 11:36:55 am
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I have two issues that I'm trying to tackle
1. A constant low hum (not too loud but noticable). I'm assuming this is a ground loop but not sure since it gets worse when the reverb knob is increased (and no input signal). Playing, you don't notice it as much.
2. With no input you can here the vibrato oscillations even with the intensity at 0 and speed at its lowest setting. Is this normal for the older pt bias vibrato?
Thanks,
Rut
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It is not normal to hear the vibrato when off, but you may think it is because it is very common.
Take issue 1 first. Does this happen if you move the amp to somewhere else? Different electrical source? If so, pull tubes starting with V1 and put them back to see if you can locate the area in the amp of the hum origin.
If a tube pull stops it, you know the general area.
Is the hum constant like a vibration or does it pulse?
Has the amp always done this? Did you just get finished with the build?
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This is a new build. It is a constant hum. Tried relocating the amp and it made no difference. I have swapped tubes but didn't pull them one at a time as you suggested. I'll try this tonight.
Thx
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Which schematic did you use?
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I used the dual channel layout on this site (http://el34world.com/schematics.htm#Hoffman_AB763_Two_Channel_ (http://el34world.com/schematics.htm#Hoffman_AB763_Two_Channel_))
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in addition to isolating tubes, try isolating the reverb tank, does the hum increase/change when you touch the chassis or tank. use a chopstick and probe all your grounds and filament wires.
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1. A constant low hum (not too loud but noticable). I'm assuming this is a ground loop but not sure since it gets worse when the reverb knob is increased (and no input signal). Playing, you don't notice it as much.
Pull V4 (reverb recovery/mixer amp). Does the hum disappear?
2. With no input you can here the vibrato oscillations even with the intensity at 0 and speed at its lowest setting. Is this normal for the older pt bias vibrato?
You may not be able to totally silence the tremolo 'breathing' sound. Tweaking the bias may reduce it if it's too loud. Using a footswitch to disable the tremolo oscillator will totally silence it.
Change R68 and R69 to 10K/3W resistors for a Deluxe Reverb amp. Most of the other AB763 amps use the 1k and 4.7K for those resistors.
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I've had great luck in solving the hum problem. I separated the ground wire from the first two filter caps and the remaining three and ran the latter gnd wire to the preamp ground.
I also noticed that I mistakenly tied the B+ for the reverb xfrmr and oscillator to before the choke instead of after. This corrected the hum. I can still hear the oscillations if I turn the speed up and have nothing plugged in. I don't notice it much one I plug in the guitar.
I had already changed the power string resistors to 10k.
I now only have one problem. My tone controls don't seem to work on either channel. I can adjust them and hear more of a change in volume than a change in treble or bass. When the bass control is moved from 0 to about 8, the volume increases. After 8 it starts to decrease. I didn't use the mid pot as in the hoffman ckt, just treble and bass. I've double-checked the wiring against a regular deluxe reverb layout and it appears correct. I'll play with it more tonight.
Thanks for the help.
Rut
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"My tone controls don't seem to work on either channel."
Probably missing a ground.
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Yes and don't forget the tone stack (TS) tail R.
Brad :icon_biggrin: