Ampeg SVT4 pro hum

RE: Ampeg SVT4 pro hum, posted on September 1, 2009 at 07:41:58
FenderLover Audiophile Posts: 3309 Joined: July 31, 2007 Contributor Since: May 17, 2009	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.

RE: Ampeg SVT4 pro hum, posted on August 14, 2009 at 07:00:25

Dman
Industry Professional

Posts: 3842
Location: Southern Ontario
Joined: January 28, 2001

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...

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RE: Ampeg SVT4 pro hum, posted on August 7, 2009 at 11:00:27

Pro Sound Guy

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

RE: Ampeg SVT4 pro hum, posted on August 8, 2009 at 05:37:25

FenderLover
Audiophile

Posts: 3309
Joined: July 31, 2007
Contributor
Since: May 17, 2009

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.

http://www.schematicheaven.com/ampegamps/svtpoweramp6550a.pd

RE: Ampeg SVT4 pro hum, posted on August 10, 2009 at 12:57:15
Pro Sound Guy	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

RE: Ampeg SVT4 pro hum, posted on August 14, 2009 at 04:40:41

FenderLover
Audiophile

Posts: 3309
Joined: July 31, 2007
Contributor
Since: May 17, 2009

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.

http://www.prowessamplifiers.com/schematics/ampeg/Reverb_Roc

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Ampeg SVT4 pro hum

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