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A friend of mine has an Ampeg SVT 4 bass head. He was telling me it has a hum. Constant low level hum. sounds like a 60hz hum.Do you guys think it could be from a microphonic tube?
That was my first guess.
Thanks
Edits: 08/06/09Follow Ups:
Of all things I had a microphonic 6L6GC ina Twin Reverb pickup some kinda hum (mahybe tranny buzz) and amplify it. I've heard others have this issue. Prolly more a problem in combo amps.
I replaced the offending tube & the buzz went away.
I had an (and read) about an issue with the first gen. ampeg gear (not the older SVT stuff, but the newer rack stuff) had a grounding problem. I tried both the first release of the SVP (preamp) as well as the SVT-2 and 3 amps. ALL had a hum issue of sorts. The later versions of the rack stuff from Ampeg got better, though...
"We ask our artists to be true. Actually, we demand that our artists be true. When poets, storytellers, & singers lie to us for money, our culture is diseased & in decline."
-Robert Fripp
I have never personally witnessed a tube causing 60cycle hum
I have witnessed and experienced a bad 1/4 input jack/solder connections
causing this
I have witnessed and experienced bass players with a bad instrument
cable going to the input
If he has nothing plugged into the head is it quiet?
Does this hum only occur if he runs a line out /xlr to a sound system?
Does he had a ground defeat switch?
I would suggest looking in other places other then the tubes for this
issue
However, its always a good idea to pull all of the tubes and them
tested and also reseating all the tubes is always helpful
When was the amp last re-capped and the diode rectifier bridges replaced? Could be some noise getting through old parts (maybe)?Ampeg SVT often have a pots in the bias circuit to adjust the bias or for hum control. These pots often get noisy. They are usually on chassis below volume/tone controls & inputs. Prolly good place to start. After checking the electrolytic caps' age.
Other places to quickly look are the jacks (need cleaning & check their gournd points). And the ground points to the chassis, for bad solder points. Worn out ground lugs, etc.
Are the lines from the input jacks to the circuit cracked, brittle, burnt? Solder look ok? Pull the tubes and clean the pins and sockets.
while you are cleaning and looking at the guts, I'd check the resistance of the resistors in the power section and be sure to replace with 1% tolerance, like Mills.
Edits: 08/08/09
Have you looked at the model this guy posted?
I googled that model he posted and it basically only has a couple
12ax7s on board for pre?
Re check the model and the schematic you posted is for an old 1970s
SVT
I just love the old SVT with 16 tens :)
Incredible setup
Sorry, not familiar with later model SVT's. Just the old 6146 and 6550 loaded head units. Have worked on 7027A Ampeg amps (very nice and loud). And the smaller 7591A Reverborockets. Ampeg seemed to change circuits as the amps rolled down the assembly line. I've seen 1962 R-rockets with solid state bridge rectifiers, GZ34's, 5Y3GT, 6V6GT or 7868 power tubes, all labeled RR 12RB's with the linked schematic pasted in the cab. :^)
I've been kicking around the idea of DIY a R12R clone with 6SN7 and 6SL7 front end/effects/tone. GZ34 rectifier. And 7591A power tubes. The 7591A in Class "A" cathode biased is quite a tone monster. I have the tubes and iron parts... one day.
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