Hum from power section

[Colas LeGrippa]

Buenas noches amigos,


I have a guitar amplifier about to finish it's life in a demolition bin.


Here's my frustration : after replacing all the filtering caps by brand new ones, swapping the coupling caps to the power tubes twice, swapping the power tubes for known good ones, removing all  the preamp tubes to isolate the problem in the power section, I still get this problem. Before I start swapping the trannies, I post this thread hoping someone will  put me on the right track .  Will the Priest of the Rusty Resistor come to my rescue ?


When I turn on the  "G.D." amplifier with all of the.preamp tubes removed, it's dead quiet.
For 15-20 minutes. Then here comes my nightmare: all of a sudden, a hum comes out of my speakers. Just loud enough to keep me from using it. Something gets hot in there and shouts:  hummmmm hummmm, turn me off I'm overheating...


The pt is a toroidal tranny and stays cold as ice.  The audio transformer is strong enough for 4 tubes and is feeding only 2. The  choke is big enough.


Could it be one of the 2  1K 5W screen resistors that just can't  do it's job ?


I'm to the point of feeding the screens with UL taps to see if that would solve the problem.


Any tip, comment, point of view ?


Colas

[PRR]

Which knobs change this hum?

[Latole]

Wich amp ?
Any schematic ?


"Could it be one of the 2  1K 5W screen resistors that just can't  do it's job ?"

If resistor ohm reading is right or close , it is not the resistors.
________________________________

Any tip, comment, point of view ?

What scope see  ?

Try to reverse output xfrsm primary wires .

[Colas LeGrippa]

No knob changes the hum. There is only one volume and no matter what it's position, nothing changes. Even with the.preamp tubes pulled out, you think turning a knob would have an effect ?


The power section up to the phase inverter is based on a Marshall 2204. The trannies are different though. The power tranny is a toroidal from Avell Lindberg 360-0-360 + heaters taps,
the audio tranny is a Hammond (can t recall the no. , I am.not in my shop now ), the choke is a Hammond too.


It's a build of mine from scratch, aluminum chassis 1/8 th inch thick.


No old carbon resistors.


No ground loops. No, my neighbour isn't using a jack hammer nor other equipment. No fluorescent lighting and the 2 other amps I play every day doesn't hum at all.


Today, I will take the time to kick out the 2x 1k 5w to the screens and see what happens.


Meanwhile, I you get struck by a genius idea, lemme know.....


Thanx for your help

[Latole]

A build from a scratch.

You may begin by drawing a schematic no ?
Schematic as build is a must when you need to repair.

What about reading with a scope ?

No need a genius idea, just basic "tools."

[Colas LeGrippa]

Latole, see the Marshall 2204, it's identical.
Unfortunately , I don t have a scope.
Some resistors have a correct measurement but change underload and must be discarded.
I ll keep on investigating, I ll swap the audio tranny and the rest....I have no choice.
BTW, I think you live in Verdun no ? I am near Metropolitain boulevard and St-Denis
Come to my place with your scope , I ll pay you a beer....


Cheers


Colas

[shooter]

it's dead quiet. For 15-20 minutes.

so play 3 songs, power down, go play with the ladies, no problemo' 

[Colas LeGrippa]

Very good idea...but what about Linda ?

[Latole]

it's dead quiet. For 15-20 minutes.

Bad solder ?

[Colas LeGrippa]

I will triple check....but I don't think so.....but who knows ?????

[Latole]

Latole, see the Marshall 2204, it's identical.

Unfortunately , I don t have a scope.
Some resistors have a correct measurement but change underload and must be discarded.
I ll keep on investigating, I ll swap the audio tranny and the rest....I have no choice.
BTW, I think you live in Verdun no ? I am near Metropolitain boulevard and St-Denis
Come to my place with your scope , I ll pay you a beer....


Cheers


Colas

You work so hard with amps that this would be a very, very useful tool for you.
And you probably wouldn't be here on the forum, the amp would be repaired.

I live in Vaudreuil-Dorion.
Out of obligation, I occasionally go to Montreal, and every time I come back, I tell myself that I don't want to go there anymore. The traffic is infernal and getting worse. To help you at home with an oscilloscope, you have to be prepared to spend hours there.

Buy a scope , it less than $200 second hand

[Colas LeGrippa]

Thanx buddy