Sunn Solarus lots of hum

[kupervaser]

I changed that gold and silver cap to one new 22uF capacitor, but I still have the same loud hum.
The hum is present even when the guitar is not plugged in. So does anyone has any tips how to start searching for the source of this hum, without simply replacing all kind of parts? I have a scope and a signal generator, but I can not find any usable info on how to utilize this equipment to find hum, at least not yet. All info is welcome.

[loudthud]

First test: Do any of the volume, tone or reverb controls affect the amount of hum?

Since at last report you did not have a reverb tank, you need to short the connection at the reverb footswitch to ground. Those big caps on the reverb PCB will allow hum to get into the signal path if they are bad or missing. Have they been replaced?

Second test: When you flip the Standby switch to the down (no signal) position does the hum stop immediately, or does it fade out over a period of several seconds?

What kind of probes do you have for the oscilloscope? What is the voltage rating? What is the maximum voltage allowed on the input of your oscilloscope? Most tube amps have enough voltage to damage an oscilloscope if it is not used properly.

[kupervaser]

Thank you for the walk through.

Here are my findings.

First of all the hum gets louder when I turn the volume up.
Bu turning up the reverb knob hiss gets introduced to the overal sound. If the reverb is down all the way, no hiss.
Those broken capacitors on the reverb pcb have been swapped for new ones.
When the amp is on and I flipp the stand by switch to off, the hum is gone immediately.
When I pull out the first preamp tube (ECC83), hum is completely gone.
Also when I pull out the phase inverter the hum disappears.
I do own a reverb tank from another amplifier which I can connect, I however do not have a switch with correct connections. I might bridge it later to test.
I own a Tektronix 454 scope with probes which are X1 and X10 switchable.

Looking forward to your reply.

p.s. I also swapped 2 other electrolitics. Here is the new picture. The golden once on the reverb PCB are new and the 2 black capacitors are new as wel.


On this picture the blue 22uF capacitor is new.

[kupervaser]

Another small thing I just noticed on the picture. The 4ohm output connection on the OT (brown wire) is hanging loos. Is this ok or not? Because I see on pictures of other amps it is connected, like here:

[kupervaser]

I soldered the 4 Ohm tap of the transformer to the negative side of the 22uf (blue in my picture) capacitor.

Well, the amp works, but the sound is distorted, as if i am playing with a tube screamer in front of the amplifier, very nice sounding distortion, but not that loud. So to my understanding this is not right, so I desoldered the 4 Ohm tap and it is hanging loose again, the sound is again clean now and loud. Why is this 4 Ohm tap connected on other pictures of these amplifiers?

[loudthud]

Normally the 4 ohm tap is soldered to an otherwise unused lug on the terminal strip.

Look at the pictures in this thread: http://sunnforum.ampage.org/index.php/topic,4844.msg18880.html#msg18880

[kupervaser]

So when I remove the Ecc83 preamp tube the hum is completely gone. Does this mean there is a component around this tube which produces the hum? I tried several other preamp tubes, it makes no difference.

[loudthud]

The hum in your amp is probably caused by a bad ground. The way to tell is that the hum will stop immediately when the Standby switch is moved from the "Play" position to the "Standby" position (down). A slight hiss will continue to be heard for a couple of seconds.

One possible cause is corrosion between the input jacks and chassis and around the pop rivets that hold the terminal strips to the chassis. You can solder the terminal strips to the chassis if you have a really big soldering iron. The input jacks will have to be removed, clean around the threads, star washers and chassis holes, then re-install. Tighten the nut well.

Some of these amps have a problem caused by the common ground in the big Can Cap. The axial cap near the preamp is supposed to fix this, but there needs to be a resistor between the B+ from the Can Cap and the plus side of the axial cap. The axial cap needs to get it's ground on the preamp end of the chassis.

[kupervaser]

Ok, let me recheck the inputs first. Now I am reading the topic i want to clarify that it is not hiss I am hearing, there is no hiss at all, it is a hum or I am not sure something like a low zoom, sounds like a  sound a big power transformer would make, and it comes out of the speaker.

[BassHog]

I soldered the 4 Ohm tap of the transformer to the negative side of the 22uf (blue in my picture) capacitor.

Well, the amp works, but the sound is distorted, as if i am playing with a tube screamer in front of the amplifier, very nice sounding distortion, but not that loud. So to my understanding this is not right, so I desoldered the 4 Ohm tap and it is hanging loose again, the sound is again clean now and loud. Why is this 4 Ohm tap connected on other pictures of these amplifiers?

Soldering the 4 Ohm tap where you did directly connects it to ground, which is incorrect. it's normally soldered to the tab on the far left, which is connected to nothing.
So you can either leave it unconnected, or solder it to the far left tab. Both are the same as far as the amp is concerned.

As far as your hum, have you replaced the cap can yet? All of the Sunns I own including my Solarus have needed new cap cans due to humming. They all quiet right down and have no noticeable hum now.

One more thing, your Solarus is 100% positively an EL34 version. I have both versions and the 6550 version has much larger transformers. The pic you posted looks exactly like my el34 Solarus.

You can get the cap can here for the EL34 version, the voltages don't run as high for that version so this can will work great.
http://www.tubedepot.com/cp-ce-30-20x3-525v.html

The 6550 version requires a higher voltage cap can, which can be found but they are expensive.

[kupervaser]

Thank you, yes I am aware that mine is a EL34 model which is 40 watts. The transformers are smaller than the 6550 version.

I haven't replaced the can cap yet. I already have this capacitor at home:
http://store.triodestore.com/40uf50clammo.html

I think it would work too.

I didn't want just go on and replace all kind of parts. I wanted to see whether I can find the source of hum and than replace suspicious parts. This is why I asked how to debug such hum issues with the oscilloscope because I own one.

[BassHog]

As long as you don't have more than 500V for the B+ voltage in your amp that cap will work.
Any more than 500V and your pushing your luck.

[kupervaser]

So I have been looking at the power supply with the oscilloscope. I have checked for AC on each of the capacitors in the multicap.
Here are some results.



First capacitor after the standby switch:


Second capacitor (Point A on the schematic)


Third capacitor (point B on the schematic)


Fourth capacitor ( point C on the schematic)


In my opinion, this could be the hum I am hearing, can anyone support this?

[BassHog]

I would say yes it is.
You should not have that much noise especially at point A