Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Toxophilite on February 27, 2015, 04:00:04 am
-
On my recent build I solved a hum coming from the preamp section by isolating the input jack from the chassis and running it's own ground to the star grounding point. I'm going to do some research on this myself.
On my previous AB763 deluxe lite conversion I didn't have to do this at all to achieve a nice quiet amp
-I was just curious if having to isolate the input is symptomatic of a grounding issue that could be resolved elsewhere in the amp?
-Or if it's just good general practice?, though I know fender almost never did it,
-Is this a ground loop issue and why is it necessary?
-Is there and advantage to isolating speaker jacks, line out jacks
-
My research indicates that there is no sure fire way to eliminate hum in 100% of all builds. Such things used to be referred to as the diabolical work of gremlins. Following proper grounding schemes increases the odds of success, but there is no 100% guarantee. Moreover there are different accepted grounding schemes, which contradict one another! Also the mere presence of a ground loop does not mandate that there must be hum. It's good that you know alternate methods to eliminate hum when it does arise.
-
+1 for JJasilli
K
-
For inputs, look at the 9 pin input jack that is used on modern amplifiers. The application can be tricky.
-
On my recent build I solved a hum coming from the preamp section by isolating the input jack from the chassis and running it's own ground to the star grounding point. ... I was just curious if having to isolate the input is symptomatic of a grounding issue that could be resolved elsewhere in the amp?
Without knowing anything else about the amp build, I'd suspect a ground loop/fault somewhere else in the amp. Your post implies this was an AB763-type build, and whos knows how many thousands of those left Fender with input jacks electrically connected to the chassis.
If you used shielded cable, and grounded both ends of the shield, that would be a loop (though large-area loops usually cause hum were small-area loops like this typically don't).
Or if it's just good general practice? ... why is it necessary?
There is only one time I needed isolated jacks: building an amp which had a ground-lift switch.
The switch in that amp disconnects circuit ground from the chassis, while keeping the ground wire of the power cord always bolted to the chassis (to trip the breaker in the event of a live chassis). Inside the chassis, every ground was connected to a ground buss isolated from the chassis, so that the switch was the single point of connection.
The lifted ground in that amp results in hum if it is the only grounded electrical device in use; it breaks a ground loop and kills hum if I'm playing through multiple amps/effects which each have their own grounded 3-wire cord.
And yes, there are other ways to eliminate ground loops in multiple-amp setups if you don't have a proper ground-lift on the amps/effects.
-
Aha!
I did some reading and moved things around a little
This is the AB763 deluxe build I'm just fine tuning but I wanted this subject to be about grounding and input jacks in general
i liked how isolating the input jack stopped the hum but it seemed that if i plugged any other thing grounded to earth into it..I would just get the hum again.
So I tried what I read somewhere (and what I believe is similar to what S. Luckey does and likely many people here do?)
-The earth is mechanically attached to one leg of the PT
-The star ground point is the original ground point for the amp which was the metal mounting surround of the rectifier socket, that's where the PT center taps go and many ground wires came off this.
I had 3 ground "nodes" radiating from this (though in close proximity)
1- goes to a terminal strip where ALL the filter cap grounds are attached
2- The other goes to 2nd terminal strip where(originally) the phase inverter, the tremelo grounds from the circuit board and the bias circuit grounds were attached
3- at the other the preamp cathodes were attached but I also thoughtlessly attached the ground from the tremelo depth and the 6V6 cathodes
The input jack was bolted right to the chassis grounding it there along with the preamp pots
It hummed a little so I isolated the input jack which stopped the hum
NOW though:
- I moved the 6V6 cathode grounds and the tremelo depth ground to the 2nd point with the phase inverter, bias circuit etc.
Then I UN-isolated the input jack (electrically attaching it to the chassis again)
and then detached point 1 from the star point and attached it to the input jack
Which made for a ridiculously quite amp.."Is it even on??"
And I learned something...Neat!
-
Whatever grounding method you use (star, buss, plane, etc), the golden rule is to connect the ground of any given circuit to the negative (ground) lead of the filter cap supplying that circuit. Make that connection as short and direct as possible, and everything else takes care of itself.
Not all physical embodiments of an amplifier follow this rule, but they might exhibit varying degrees of success (i.e., low/no hum).
-The star ground point is the original ground point for the amp which was the metal mounting surround of the rectifier socket, that's where the PT center taps go and many ground wires came off this.
This sounds like a conversion of an old amp, or using a very old socket.
If so, know that the original chassis/socket was not really a "star ground" circuit in all likelihood: a common approach was to use the chassis as a ground plane, and the extra lugs on the mounting portion of tube sockets as convenient attachment points for grounding to the chassis (instead of needing a huge iron to solder wires directly to the chassis, or using separate ring terminals on ground wires).
-
a common approach was to use the chassis as a ground plane
how many memories come to mind
(http://www.impextechnology.com/produtos/imagens/131_1.jpg)
Excuse me for the OT
Franco