Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Sansteeth on March 28, 2025, 10:10:32 am
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Hello everyone,
I had an old Ampeg Gemini GV15 on the bench for a quiet Channel 1. It was open load resistor on the plate of V6 (6CG7) so that's fixed but it also makes one pop at start-up and a couple of pops when you turn it off. I'll be honest, the weird pops my customers complain about at turn-on and turn-off often leav me perplexed.
All filter caps have been changes for F+T's by a previous tech.
The popping noise:
https://drive.google.com/file/d/1MHljh_AqkObylWNoxIB6zLPCWLzOxGXz/view?usp=sharing (https://drive.google.com/file/d/1MHljh_AqkObylWNoxIB6zLPCWLzOxGXz/view?usp=sharing)
Let me know what you think
thanks!!
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Wow. Never heard anything like that! That's an interesting oscillation.
Is that V6 (6CG7) still good? Any idea what caused the open resistor?
I think it's safe to say that the problem resides in the HV circuit somewhere, unrelated to the power switch, since you're getting those thumps when turning manipulating the standby switch.
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Is the volume all the way off when you turn the amp on/off?
It's common for tube amps to thump (the speakers) 1 time when turned on/off, even with the volume all the way off. But I've never heard that sound when you turn the amp off. That's pretty strange. :dontknow:
The stand by switches contacts get burned over time from dcv arcing at turn on. Then they can make a popping sound.
Fix for that is a new switch.
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Thanks for your responses!
The 6CG7 is brand new, my guess is that the load resistor decided it was time to go. No burn marks or anything, it's an old amp, I'm not really surprised to see an open resistor and if there is no other signs, I don't investigate much further.
The popping noise appears when I switch the stand-by on and off or the power switch off without putting the amp on standby first.
Volume is all the way off when I turn the amp on or off. I agree sometimes they make one thumping sound or a quiet whine but this weird succession of pops, I don't know man...
Good to know for the switch, I'll keep that in mind
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The popping noise appears when I switch the stand-by on and off or the power switch off without putting the amp on standby first.
I think that may be a clue.
I agree sometimes they make one thumping sound or a quiet whine but this weird succession of pops,
Yeah, I don't recall ever hearing that before.
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So it seems like the succession of pops when I turn off the amp matches the speed of the tremolo/vibrato!
I really can't find any info on what the "after-beat" switch does, it is not the on/off switch for the vibrato/tremolo, it seems like the tremolo sounds more drastic and "clicky" when the "after-beat" is on.
But when it's off the popping sound is almost inaudible and when the "after-beat" switch is on, then I get the pops in the video.
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If the Echo Foot Sw is closed, does the thumping go away?
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No, thumping doesn't go away whether the echo footswitch or the vibrato/tremolo footswitch is on or off
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I was thinking the thumping was related to the original burned plate resistor, but the fact that it does not disappear when the signal from your new 6CG7 (V6) is shorted to ground suggests my thinking was incorrect. So probably not coming from V5/V6.
Don't have much other advice, other than trying to isolate the sound from Channel 2 (V4), the remainder of Channel 1 (V8), or the output section.
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Thanks everyone for your help, I told my customer about the thumping noise and he said he knows and couldn't care less. So I guess this will stay a mystery until it becomes a bigger problem and he brings it back to me :icon_biggrin: