Newbie needs help! smoking power supply 70s Sound Big25 RT Italian amp

[fkitch]

Hi!
1st post other than the intro, and I'm really new into this fascinating world so... please, be patient.

I'm trying to bring back to live a late 60s / early 70s Italian amp, brand is 'Sound' and model 'Big 25 RT'. Very little info online, I read somewhere it's inspired on a Fender Deluxe Reverb. There's a circuit diagram although not 100% consistent it's quite accurate (attached below, apologies for all the notes, basically I note down everything I manage to figure out...)

After changing all electrolytic caps, cleaning pots and jacks, install 3 prong cord, these problems were solved:
   Weak and bad sound, hum, cracks/pops, red plating power tubes (2xEL84)
But this persisted:
   Power transformer getting hot, making 'rrrrr' noise, and smoke + the typical smell after a few minutes of operation.

Already tried this, always same symptoms:
   Different speaker
   Different power tubes
   Just clean channel tubes on, rest removed
   All tubes removed

I disconnected the power supply and tested it, gave these readings with a multimeter:
-Primary resistance (selector at 220v, measured at plug) 50 Ohm
-Secondary high voltage, 350v, (206v to center tap)
-Secondary filament 6.1v, (3v to center tap)
-no shorts between secondaries

I'm pretty sure I've left some basics checks undone out of simple ignorance, but...
Does it look like a faulty PS? If so, what would be a replacement?
Any other tips about the issue and the amp itself?
(chasis pics below are before changing all caps)
Thank you!

[Latole]

fkitch wrote:

I disconnected the power supply and tested it, gave these readings with a multimeter:
 
-Secondary filament 6.1v, (3v to center tap)
 
_______
6.1 AC volts no load !!  Look a very weak p.s
You should read at least 6.3 if not more.

How is Power Tubes bias ?   A very hot bias draw more P.S current.
EL86 is a 12 watts tubes, how many volts on the 120 ohms ? Is 120 ohms is good ?

Try PS with not tubes first ; read heater current il should be 6.1 ( there are no load)
Put preamp tubes only; how much do you read heater voltage ?

If not less than 6.1 vac. Plug Power tubes ;read heater AC voltage

Reports here , good luck

[sluckey]

Are you sure the smoke is coming from the power transformer (PT)?

Disconnect the two PT red wires from the two diodes and turn on power. Does the PT still get hot and smoke?

[Latole]

Original PDF schematic ;

https://www.freestompboxes.org/viewtopic.php?t=30169

[PRR]

Is the voltage selector set correctly for today's wall voltage?

Fresh battery in your voltmeter?

[fkitch]

Thanks so much for the answers!
I have to add there's also an intermittent noise, from the PT too, like frying pan noise or similar

Is the voltage selector set correctly for today's wall voltage?

Fresh battery in your voltmeter?

selector is at 220V, next one available is 280V, wall socket says I have 225V
Yep battery is fresh (thanks, would have never thought of that myself)

Are you sure the smoke is coming from the power transformer (PT)?

Disconnect the two PT red wires from the two diodes and turn on power. Does the PT still get hot and smoke?

100% sure smoke comes from the PT, it can be easily seen.
Yes, PT here hot and smoking as well with the red cables disconnected.

6.1 AC volts no load !!  Look a very weak p.s
You should read at least 6.3 if not more.

How is Power Tubes bias ?   A very hot bias draw more P.S current.
EL86 is a 12 watts tubes, how many volts on the 120 ohms ? Is 120 ohms is good ?

Try PS with not tubes first ; read heater current il should be 6.1 ( there are no load)
Put preamp tubes only; how much do you read heater voltage ?

If not less than 6.1 vac. Plug Power tubes ;read heater AC voltage

Reports here , good luck

Ok here it goes, 1st time I check bias...
Bias resistor 120,7 Ohm
Voltage drop across resistor 4,5V
Plate voltage 164V
Doing all the math I get very low ca. 3W plate dissipation, but I maybe wrong.

I read 2,3 vac at pins 4 or 5 with the el84s in or out. 2,5vac with all tubes removed

[Latole]

". I read 2,3 vac at pins 4 or 5 with the el84s in or out. 2,5vac with all tubes removed"

" 100% sure smoke comes from the PT, it can be easily seen.
   Yes, PT here hot and smoking as well with the red cables disconnected" 

-Fkitch
______

These tubes need 6.3 volts between pins 4 ant 5  or 3.1 volts from ground to 4 or 5

PT look bad

[tubeswell]

PT here hot and smoking as well with the red cables disconnected.

Your PT is toast

[fkitch]

Ok thank you!
What would be a correct replacement? I understand Hammond has many types of PTs, not sure what to look for, specially high voltage

[Latole]

First you need a PT with a 240 volt primary because that seems to be the voltage at your place.

Then, seeing the "low" voltage of the filter caps (350 volts) I don't think you need to go beyond 500 volts C.T. at the secondary.
Otherwise the Hammond 290BEX could do it I think.

Let's wait for Sluckey's answer, he always has the solution

[sluckey]

You need a standup style PT. I would use the Hammond 370JX. And wire the primary for 240V.



Don't cut the extra leads. Just snip the exposed ends, heat shrink the ends, and neatly tuck out of the way.

[Latole]

Sluckey you are right.

[fkitch]

You'll laugh but it took me a few seconds to realize 'standup style' was nothing electrical but the physical design of the PT!
Ok, thank you SO MUCH guys for your advice, will check local suppliers for that transformer.
I see, the three 5V wires won't be used but no need to cut them, just heat shrink
Thanks again
Saludos!