Very low level Humm from speaker with the amp in Standby ???

[kagliostro]

A friend has build a custom amp

and on the speaker you can here a very low Humm when the amp is in standby

here you can see a photo

I was thinking that the Humm comes directly from the OT which windings act

like an "antenna" capturing the Humm from the other transformers or from

the filament wire

What do you think about ?

K

[John]

I think it's coming from all those knobs!! 

[Willabe]

I think it's coming from all those knobs!! 

Yes but only the green ones.


            Brad     

[TIMBO]

Hi K, I don't think its coming from the KNOBS 

What is the small transformer wedge between the cap cans, I'm thinking that its for the relay psu......

If it is can you post a schem.Thanks

[printer2]

The power transformer and output transformer should not be alighned in the same direction.

[kagliostro]

The small transformer is for 24v to supply some SS parts of the circuit (that I don't know in detail)

the 5v transformer for the relay is inside the chassis

My friend has build other amps (not humming at all) with the same PT - Choke - OT disposition

and, I repeat, the Humm in this amp is at a very low level

About schematic there isn't a complete and definitive schematic drawn, here attached there is the latest

version of what was draw, the amp is a JCM800 3channel with 3 separated EQ + gain stage

K

[1rebmem]

I just went through this.......a low level hum.
It was in fact induced from the PT to the OT.
I pulled all the tubes which included a recto tube so no B+.

Pull all your tubes and see if the hum is the same.
If so then it is probably inducing from the PT into the OT.

Basically it's like doing "the headphone trick".
You power the primary of the PT and leave the secondary unhooked.
Then your OT will have the speaker connected to the secondary and no connections to the primary.
Then listen for hum.

[kagliostro]

Good point

without the tubes there will be no heater current and if the humm is still there it easily comes from the OT (or the Choke ??? - can a choke give this effect ??? there there isn't AC ?)

K

[stratele52]

I wrote an answer earlier and it is not there ,

Hum on Standby ; 1- Standby witch is not at the right place on circuit
                         
                          2- Bias circuit is defective ,check  bias cap . If it is a fixed bias amp ( AB class )

[jjasilli]

Ditto to tube pulling & headphone trick.

1.  Tube pulling:  If the hum disappears on standby with the tubes pulled, I would re-install the power tubes and see if hum returns.  If yes, I would disconnect the power tube cathodes from ground*, to see if this affects hum.  If disconnecting the cathodes from ground defeats hum, then use that type of standby circuit.  Tubes can do weird things internally.  E.g., heater hum may be completing a weak circuit through the cathode to ground, without the strength of plate voltage to pull the electrons the other way.  In other words, I would take steps to absolutely rule-out tubes.  Also, there might be a fault in the standby circuit.  If the standby circuit opens the B+ supply, make sure there really is -0- plate voltage on standby.  

2.  Headphone trick.  The placement of the trannies may be causing them to induce hum in one another.  So 1rebmem's suggestion for diagnosis may not dispose with the issue, unless the 24V PT is disconnected or even temporarily removed from the chassis.  The placement of the OT & PT may also be an issue.

There is no absolution for hum.  It can happen to anyone on any build, including major manufacturers, no matter how many amps they've built.    At least your friend wasn't plagued with hum on his first builds!  Did he use the headphone trick in determining the placement of these trannies?  If not, he has invited the wrath of the gods!  

If all our suggestions don't work, then the slight hum you describe may be caused by the OT itself.  The ultimate solution for that is a potted tranny, but that's overkill for a guitar amp.


(*) unless the standby circuit already does this

[kagliostro]

Thanks Stratele52 & Jjasilli

account will be taken of your suggestions

Stratele52

there was, wrongly, present one other tread with the same argument

Jjasilli

as told the amp has a very low hum, that is more than acceptable

however my friend is going to try also a shield box around it

of course all the other suggestion received will be followed

K

[1rebmem]

In tracking down my minor hum problem (as I mentioned above) I discovered that a speaker connected to the OT secondary
works just as well (if not better) compared to headphones. When using a speaker you hear the hum level "real time" or exactly how it will sound with the amp built.

My induced hum was with a Tweed Deluxe 5E3 build using a Hoffman chassis, NOS Triad PT and a mojo OT.
With the pre-punched chassis the component placement was already set. But I had too much induced hum. Many others have built this amp with no induced issues. So why me?
The hum was there with ONLY 120vac input to the primary of the PT and a speaker connected to the OT.
So I happened to have some spare transformer clam shells to install on the open frame OT to provide shielding, the hum was the same, shielding didn't help (yes....well grounded).
When I laid the OT on it's side and adjusted it's positioning..........the hum was completely eliminated. I had a second identical OT and it hummed the same.
I don't know why I had this problem but I had to change the mounting orientation of the OT to get a quiet amp. It appears that this particular NOS Triad PT was at fault.

Here is the thread:  http://www.el34world.com/Forum/index.php?topic=16604.0

My humble suggestion is to prove if it's induced or not by disconnecting the PT secondary HT wires and taping them off.
Remove all the tubes then apply mains power to the PT and check your hum level. If you have hum then play with transformer orientation and see what happens.
I thought building a proven 5E3 layout wouldn't have hummed but it did......

[1rebmem]

Even though this thread had died, I came across some very interesting info on induced hum and thought I'd share it.

http://home.comcast.net/~machrone/bjr/bjrhum.htm

[kagliostro]

Many thanks 1Rebmem


I'm going to read the article with great interest


K

[John]

The slanted metal shield is interesting!