Hum In woo6

[Claus-DK]

Hi Frans..

I received my brand new amplifier. Solderdude maybe you can help.
First open a big box with amp, admire it a little, then add the tubes and take it up in the living room, connect the CD playerput a CD in, turn on the CD, plug the headphones into the amp turn amp THEN COMES TO THE meanest growl AUDIO OUT OF headphones. Great is my disappointment.
The most strange thing is that when I turn off the amp it keeps humming as the music fades out.
here's a guy in England who have the same problem:
The facts:
• Amp running is on 240v 50Hz mains in the UK.
• I have tried different power sockets, a different location, running without an earth, running through a UPS.
• Hum is not affected by volume.
• Hum does not seem to be affected much by using different valves and swapping valves left to right (see list below).
• Hum still exists tabloid is no source connected (just power lead and the headphones).
• Hum is louder when on Hi impedance setting.
• During the start-up process, When slow start protection circuit switches two full power the hum increases.
• The capacitors look OK, I'm no expert, but do not the seem to be bulging.
• The power lead is nothing special but I can not see att making Any difference.

beste greetings claus, not me but the other Claus, just helped a bit translating..

[claus]

L1050590.jpg (226.03 KiB) Vist 11625 gange

Photo of inside wa6. i have tried a new rectifiertube, nothing changes.
mvh claus

[Solderdude]

Can you measure the DC voltage on the big cap between the trafo and the rectifier ?
Measure in DC setting and in AC setting across that cap.

be careful BIG sparks when you accidentally short !
Also measure the voltage across the 2 330uF caps (DC and AC)

[claus]

I do not have skills to do it but I will ask a friend if he would help me, I will post measurements
mvh claus
ps I use google translate so there is perhaps sometimes some strange words

[Solderdude]

Easiest points to measure are:

black pin meter on ground lug (with the red wires going to it)
red lead on pin 8 of the rectifier (= reservoir cap +)
Measure the DC component (DCV setting) and AC component (ACV setting)

also measure DC and AC on + of the 330uF caps AND also on the smaller caps (the outside of the 1k96 blue resistors, where the 2 resistors converge I think is the same as pin 8 (I expect a wire to run from pin 5 to pin 8)

check all the wires coming from the - of the reservoir caps going to the ground lug.

The amp is so simple by design it cannot be much else as the power supply.

This consists of: power transformer on top.
Going to the rectifier (pin 4 and 6)
heater is between pin 2 and 8 (about 5V AC over those pins)
Pin 8 is the cathode of the 2 'diodes' in the rectifier tube and is feeding the reservoir cap.
There should be a DC voltage on there with a small AC component (a few Volt max)
From that point the 2 gold Dale power resistors 'dissipate' the AC component (+ DC current) into heat and a 'cleaner' DC should be available from the 330uF caps on the sides.

I reckon the DC has too much AC component still over it.
When changing the tube didn't help the single cap (connected directly to pin8) might be defective (unlikely) or not connected properly (most likely)

Also check if both ground lugs near the tubes (where the ground is connected to the chassis) are properly connected.
They should be very close to 0 Ohm (or show the same value as when the test leads are connected)

I take it the hum is on both channels.

[claus]

First a big thank you for taking the time to familiarize yourself with my problem, it was ground lugs that was the problem. Now there is only a weak hum which I think is due to cheap tubes.
Here are the measurements that are made​​:
voltage 330UF 154V dc-35, etc. ac.
DCV setting 180V DC / 400 MV AC
330 uF caps DC 152.8 to 152.7. / AC 0815 -0830 AC.
rectifier pin 4-6 ok.
mellem pin 2-8 5.11 AC
2 diodes in rectifier tube 166 V DC / AC 310mv
My colleagues have tighten and soldered amplifier for me, so tomorrow's cake for them.
You should also have a large piece, so if you arrive in Denmark, you have something receivables.
But again, many thanks for the help.
mvh claus

[Solderdude]

You could lower the residual hum (if the hum doesn't change when touching the amp) by increasing the value of the smoothing/reservoir caps.
Every doubling of the capacitance should give a noticeable improvement.

You can do this by changing the caps or paralleling caps to the existing ones.
There seems to be some room in the amp for this.

[claus]

increase the value of the smoothing / reservoir caps.
you'll need to tell me where in the amplifier components is that you write about.
mvh claus

[Solderdude]

The 3 bigger black capacitors (330uF/200V) that are connected to the 'gold' power resistors.
Possibly also the 2 smaller ones where the blue resistors go to.

[claus]

Can capacitors have too high value, ex 330uF 200v switched to 990uf 200v. Will it affect the sound which value is used.
mvh claus

[Solderdude]

The higher capacitances will become too large to physically fit.
See what you can fit in there (simply look for voltage 200V> capacitors that are at least 105oC)
Theoretically the hum should be lower when the capacitance increases.

The sound might change subjectively (some say bass will improve and thus it probably will)
measurably the only change will be the hum voltage will be lower.

[claus]

I've put two 6SN7GTB SYLVANIA U.S. COIN BASE in amplifier, it took half of the last hum. I ordered a Mullard GZ 34 which I hope will take the last hum, otherwise i will solder new caps in the amplifier, I have found a place in Denmark where they sell Rubycon.
Have a nice weekend Claus

[Sorensiim]

Shouldn't the manufacturer fix things like this?

[claus]

Yes and he will probably also rep. it, but it costs 240 usd to send the amplifier to the usa and get it back again, and what's so wrong when it comes back in 2 months? I'd rather talk to Frans who understand these things and give a reasonable answer and a solution to the problem.
mvh claus

[Solderdude]

If the tubes used have a lower anode current the ripple on the power supply will also be smaller.
That explains the hum getting lower with other tubes.

It is a bit strange the amp has some hum.
On insensitive headphones the hum might be inaudible.
If you use higher efficiency headphones residual hum may be audible with this kind of designs.

a regulated power supply or an increase in capacitance may be a good mod in this case.

[claus]

when i use grado 325i and jvc dx 1000 there is more hum (but now very little and I'll have it removed) than sennheiser 6xx, but the strange thing is when ohms is set to 100-600, there is less hum than when it is set to 8-90
or have I misunderstood something.
mvh claus

[Solderdude]

On 100-600 Ohm there will be a higher output voltage (and thus also hum voltage) when NOT loaded with a headphone compared to the lower Ohmic setting.
One would expect the hum to be higher on the high Ohmic setting, at least on the HD6xx, as this doesn't load the amplifier as much.

For the 325 and DX1000 it is a different story as the output resistance and thus voltage division also will play a role.

The DX1000 is the most sensitive one followed by the HD6xx and 325 is least sensitive when we look at it POWER wise.
VOLTAGE wise the 325 is most sensitive closely followed by the DX1000 and lastly the HD6xx.
But tube amps aren't voltage providers with 0 Ohm output resistance they deliver output power (transformer coupled non feedback) when the output impedance is set for the correct range the load (headphone) is in.

So dependent on the output resistance of the amp (belonging to both settings) and the output voltage (of the hum) it will be hard to say which setting will give the loudest hum with which headphone.

It would be strange if the hum is louder on the low-impedance setting using a HD6xx though.

[claus]

I have soldered new and larger caps in amplifier (2 -220 uf and 3 - 470UF) now it sounds like a small machine gun.
on Monday i will bringe the amp. to a workshop.
mvh claus

[Solderdude]

Hi Claus,

That is very strange,
It seems some of the wiring inside is not quite correct.
a smoother DC should result in lower hum unless the grounding inside the amp is incorrect

[claus]

you're probably right. my problem is that I do not know anyone who has a occiloskob
so I can check wiring and ground.
mvh claus