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Old 18th September 2011, 10:30 PM   #1
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Default Watery sound in a guitar amp

Hi guys,
A while ago i built a 50 Watt 6L6GC Guitar amp. i havnt drawn up any schematics yet, but basically its a cross between the mesa boogie 2 ch rectifier and a soldano SLO (separate clean/drive channels, gain EQ and vol for each channel, crunch+bright switch on ch1) but every now and again i get a sound from the amp like water trickling through a pipe. i first thought it was a capacitor problem, checked them and one of them had been obliterated... but its still there.

Ive just finnished tidying up the heaters so they are elevated slightly and have artificial CT's i also recently rebuilt the rectifier and reservoir capacitor part. (standard bridge rectifier (no CT on HT transformer winding), no choke with standby switch) and its still there.

Anyone have a clue what it is? i have checked the tubes, and all are fine.
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Old 28th September 2011, 12:49 PM   #2
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Quote:
Originally Posted by razorrick1293 View Post
i first thought it was a capacitor problem, checked them and one of them had been obliterated... but its still there.
What do you mean by 'obilerated'? - why had this happened?, presumably replacing the capacitor without curing the cause of it won't help much.
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Old 28th September 2011, 09:10 PM   #3
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by obliterated i mean, HT voltage was higher than expected when bias was low during tube change. blasted the cap to pieces without me realising. changed the cap to 2 series 400v caps instead of a 500v one.
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Old 28th September 2011, 11:29 PM   #4
Enzo is offline Enzo  United States
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Elevated heaters, center taps, and B+ filters will cure hum, but not random noises.

My first suspect will always be tubes. After that, random noises are more often noisy resistors than noisy caps. Ay least in my experience.

Isolate the problem.

Is it common to either channel, or does it only occur when one channel is up? In other words turn channel 1 off, does it happen? Now turn channels 1 up and channel 2 off. Does it happen? And with both preamp channels at zero, does it still happen?

DO ANY of the controls have ANY effect on the noise? EQ controls change the tone of the noise? Any volume/gain controls affect its loudness? COntrols that affect it are after its source. COntrols that don;t are either before the source or not in its path.

Have you tried other tubes? Yours may be "fine," but that doesn;t mean they are not also intermittently noisy. Pull the power tubes and shake them. ANy with something loose inside? Pull each preamp tube and shove it back in to refresh the socket contact, any help? Wiggle each tube in its socket while the amp is running. ANy make noise when moved? That would mean socket losing contact.

Then start pulling tubes. Start at the input tube. ANy help? Now pull the next tube. Work your way through the amp pulling tubes until you find one that removes the noise when it is out. That will help localize the noise source.

Scope the standby switch. Its contacts could be shaky, and every now and then arc a little.
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Old 29th September 2011, 09:41 AM   #5
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Quote:
Originally Posted by razorrick1293 View Post
by obliterated i mean, HT voltage was higher than expected when bias was low during tube change. blasted the cap to pieces without me realising. changed the cap to 2 series 400v caps instead of a 500v one.
Right, so you cured the cause

Is the noise affected by the settings of the controls?.
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Old 29th September 2011, 03:39 PM   #6
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ok, the hum is almost gone now, heaters are elevated and have CTs, powertubes and preamps tubes are on separate windings, capcitors checked, i think the fault is in the power tubes, theres a small amount of microphony, nothing too terrible. but since i rewired the PI and bias and PA circuitry, it seems to have been tamed a fair bit. when i can get up some more cash il retube the whole thing with new tubes instead of searching around for used ones, deemed unusable for studios...

controls didnt affect the noise at all. also i suspect there was some feedback between the OPT and the 1st valve, i switched from a 12AX7S to a 12AX7EH. i think the smaller plate made a considerable difference.

in all i think this is a solved problem now. thank you for your input
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Old 29th September 2011, 11:34 PM   #7
tinitus is offline tinitus  Europe
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just had a partly split/broken 'jack-cable', with all sort of strange effects
nice soft one
and not very long lasting
might be wise to check it carefully
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Old 30th September 2011, 04:38 PM   #8
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checked all jacks and cables. i generally change/repair them as soon as they go.
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