Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: plexi50 on December 21, 2023, 06:44:17 pm
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I have some white noise as soon as i plug in a good guitar cable. Plugging into the guitar does not stop this. I was thinking a ground loop. But now i am thinking of changing the 68K grid resistor from the 1 Meg. Putting in a 10K & 100-250pf cap to ground off the grid input.Like the pic attached. And how would this effect the low impedance 68K input jack? Ive heard of these amps having all kinds of problems. Especially the weird PI plate resistors setup.Whatcha think?
I have already done much tube swapping. No change.
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I think it's more likely you have broken solder joints on the jack connection to the pcb. Or the plastic jack is otherwise physically damaged.
Fenders have successfully run on 68k high/low inputs for decades. No need to change that.
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Remember; Guitar cable without guitar; will always make noise with any amp .
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Remember; Guitar cable without guitar; will always make noise with any amp .
Yes i know that the cable alone will make noise until plugged into a guitar. But there is no difference when plugged in. I had been working on the PI resistors,screen and tube diodes. Will check the input jacks. I hate pulling these boards because of the thin ribbon cables that can break at times.
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Be careful with ribbon cable.
Failure to place or bend them correctly may result in noise. This could be the problem if you've been handling these cables.
You talk about hiss , what about guitar playing/ sound at the speaker ?
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You don't need to reinvent the wheel. Fender's input jack circuit has been around for over 60 years. Take the time to fix it right.
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Another option with this series of Fender amps is to remove the stock jacks. Replace them with chassis-mounted, shorting, Switchcraft jacks. Run wires from the jacks to the board.
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Another option with this series of Fender amps is to remove the stock jacks. Replace them with chassis-mounted, shorting, Switchcraft jacks. Run wires from the jacks to the board.
If the issue is the jacks , what we don't know.
The troubleshooting procedures are very complicated (if not unknown) if you are not a pro in this field.
It is easier to believe (wrongly) that a modification (a bandage) would solve the problem
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I'll check the input jacks solders at printed circuit
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The light buzz is there at idle and while playing. Moving away from the amp makes the hiss slightly go away. It just seems to be so sensitive anytime your guitar is in front of the amp. Take your hands off the strings and it can get worse. That's why i was thinking a grounding issue. Removing board now and checking the input jacks.
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Another issue I've had with these amps is ambient induced noise, especially fluorescent lighting, ceiling fans and light dimming / fan controls.
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Another issue I've had with these amps is ambient induced noise, especially fluorescent lighting, ceiling fans and light dimming / fan controls.
Yes this amp is very reactive to my lighting which is on the same outlet. Ive tried other outlets. No difference. Same AC source.
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The light buzz is there at idle and while playing. Moving away from the amp makes the hiss slightly go away. It just seems to be so sensitive anytime your guitar is in front of the amp. Take your hands off the strings and it can get worse. That's why i was thinking a grounding issue.
Many of my amps do this. Less with Humbuckers. Thinking it is normal.
Try this ground your skin to metal of your guitar using a jumper. You notice that the ground noise with hand off guitar is reduced because now they are at same potential and essentially eliminated that person to guitar ground loop.
Putting you guitar in front of your amp will create pickup feedback or interference. Just stand a few feet away when you play. Of course if you are rockin' turn it up and stand extra close, let those tubes get ringin'
I have LED lights in my area. These too create bad interference so I just pay in the dark :-) But I am looking for an AC LED hum reducer, would appreciate any ideas.
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Just put the new input jacks in yesterday afternoon. Still got that buzz but it goes away moving away from the amp.I got to say i :BangHead: these amps with the cheap input jacks. The new ones are much tighter when cable is plugged in.Got lucky and had no ribbon cable breakage. PLEASE BRING ME VINTAGE GEAR!!!