HELP with Brunetti Wizard 6v6

[phil110567]

Hi there !

I have a Brunetti Wizard 6v6 amp.

It makes huge noises: Crackling, Popping and hissing at the same time.

THE noises are present even with volume at zero!

The amp can be played and I hear the verb.

No bad smells or fire or heat excess.

Every tubes were changed (power, preamp, rectifier, and the one for the 'verb) and it changed nothing with the noises.

The amount of noise may vary with time. Normally it is louder at beginning and it lowers a bit with time . Sometimes crackling and popping stop completly for a while (sometimes just after a log pop). It remains silent for a while and noises then suddenly are back.
However hissing is always present. Only crackling and popping vary.

Do you have some advice to give me ?
I have basic pratice skills like biasing (not this one). I have basic soldering skills (I can change a carbon comp resistor if needed). And I have good understanding of safety procedure.
I lack the knowledge about what to test and how, what to control. What I can try.

I lack qualified tech in my area...

I thank you by advance...

Philippe
 

[Latole]

Amp may need news filter caps.

Can you post pictures from inside the amp ?

This amp ?   http://www.brunetti.it/MANUALS/ENG/Wizard%206V6%20-%20ENG.pdf

[phil110567]

Hi Thanks ! yes this one ! it is not new born, second hand... Pictures are coming soon.

[phil110567]

Thanks!
Here's a picture:

[Latole]

Almost new amp.How old ? Look not a vintage one.

Poor contacts on solder or connectors, tubes socket may need to be cleaned

[phil110567]

It is from 2009 I think.

There are also some large blue caps on the other side of the chassis

[alfa75ba]

I don`t know if this could help, but a friend of mine has Brunetti Singleman combo, it is 2x6V6 class A, cathode biased amp, which had similar simptoms. Hiss, crackling when first turned on that would go away after few minutes, etc.

I looked into it and found cathode bias resistor to be first suspect. Every time I gently poked it with LONG wooden stick (other hand behind my back) the amp crackled. Poking everything around-nothing, so I replaced it. (it looked too small to me anyway- 2Watt max). I put I think 5 or 7 watt one, I do not remember, the same value of course. Might have pictures somewhere.

It worked fine for a while, but then started developing stronger problems,cutting out altogether, etc. It was the power tubes at the end, although there were almost new, and gold plated, and expensive, etc.

Maybe not the same thing with yours 

[Johntb]

That first pictures shows a lot of "newer" solder joints. Maybe some cold solder joints or"mods" and the one octocouple/vactrol on the right looks odd (melted)???

Just a thought.

Also dies it change with any specific pot movement?

You can try cleaning all pots too.

[phil110567]

Yes I see what you mean.

I had noticed that some solder don't look so professional. And I can see the black thing that lookd melted...

No, the pots don't seem to have an effect on the problem

[alfa75ba]

Just remembered that this Brunetti I worked on had kind of full- half power switch, and after the cathode bias resistor replacement was working fine until the owner started switching half power mode. Then it started acting funny again. He wrote to Brunetti and got advice to install new matched power tubes.
Later I tested one of the 6v6 had short between heater and cathode ( Pin 8).  Most probably the bad tube damaged cathode resistor.
You might check for short between heater wires pins 7 and 2 and power tube cathodes ( pin 8 connected together with blue wire).
Also check the power tube cathode bias resistor, the blue wire from pins 8 on both 6v6 together goes under the board to that resistor. In class A amp that resistor is very stressed thermically and can crack  fail.
Sorry if being too pushy, but this amps drove us nuts for weeks. And the symptoms were almost identical to yours.

[phil110567]

Hi there
First, thank you all  for interest, kind and precious advices.

I'll soon go to the repair shop to find a product suitable for cleaning contacts.

In the mean time, I took another picture focused on the power tubes parts.

Based on alpfa75ba input I have located the #8 pin of the tubes .
On the circuit #8 pin have black wire going from tubes to tubes. It also goes to the little component in orange metal casing which appears to be a resistor and also goes to blue condenser in metal casing. All these are highlighted in the picture with a little red line. Maybe it is the cathode follower thing (not sure).

So I should be able to test this resistor by controlling resitance : I should read 100 ohms. But there's also  the condenser in this part so should I drain the circuit first ?

THough the resistor seems to be very temperature-resistant maybe it is damaged. But what about the condenser ? Isn't it possibly faulty ?
Regards
Phil

[Latole]

As one member mentioned, several solders appear to have been touched up. It is possible that this amp has always had this problem and it may be the reason for the sale?

And maybe he saw a technician ... without success.

You should also check all the soldering under the printed circuit. You imagine the work that it will require: Disassemble the amplifier. I don't think you have the skills to repair it. Your diagnosis is not precise enough as if a competent technician had the amplifier in front of him.
Checking the resistance in printed circuits requires unsoldering one end for an accurate reading

[Latole]

Hi there
First, thank you all  for interest, kind and precious advices.

I'll soon go to the repair shop to find a product suitable for cleaning contacts.

In the mean time, I took another picture focused on the power tubes parts.

Based on alpfa75ba input I have located the #8 pin of the tubes .
On the circuit #8 pin have black wire going from tubes to tubes. It also goes to the little component in orange metal casing which appears to be a resistor and also goes to blue condenser in metal casing. All these are highlighted in the picture with a little red line. Maybe it is the cathode follower thing (not sure).

So I should be able to test this resistor by controlling resitance : I should read 100 ohms. But there's also  the condenser in this part so should I drain the circuit first ?

THough the resistor seems to be very temperature-resistant maybe it is damaged. But what about the condenser ? Isn't it possibly faulty ?
Regards
Phil

What you are talking about it is the cathode bias power resistor ( for all four 6V6) and their bypass cap.
You can read value on this resistor without remove this 'lytic cap.

Be carefull with the big filter cap, best is to discharge it before.

You write you know bias; Check if bias is ok and report.
Did you see 6V6 red plating ?

1- Power amp On
2 -Read plate voltage on one 6V6 pin 3. ; LETHAL VOLTAGE THERE ; if you are not sure or safe to read, don't do !
3- Read cathode voltage on one 6V6 ( or each side or the power resistor)
4- Report reading.


Circuit is like this picture showing only two 6V6 ;
How to calculate bias.
Let see bias resistor to know plate current;
Ohms law ; current :45 volts / 560 ohms = 0.75 ma ( 0.35 each tube if they are matched )

Bias ; Power : 340 Volts X0.75 ma = 11.9 watts

We know 6V6 GT tubes are 12 watts tubes. Bias is very high ( at 11.9 ) if amp work in Class A

Weber Bias Calculator ;

http://www.tedweber.com/webervst/tubes1/calcbias.htm

[alfa75ba]

Sorry, my bad. I thought there were 2x 6v6 for some reason.

However, the same applies. All four power tube pins pins 8 are connected together and go through that aluminium clad resistor to ground. Blue capacitor is parallel with that resistor.
Since they are connected to ground no voltage there when amp is off.
But still be careful when poking around.

[phil110567]

What you are talking about it is the cathode bias power resistor ( for all four 6V6) and their bypass cap.
You can read value on this resistor without remove this 'lytic cap.

Be carefull with the big filter cap, best is to discharge it before.

You write you know bias; Check if bias is ok and report.
Did you see 6V6 red plating ?

1- Power amp On
2 -Read plate voltage on one 6V6 pin 3. ; LETHAL VOLTAGE THERE ; if you are not sure or safe to read, don't do !
3- Read cathode voltage on one 6V6 ( or each side or the power resistor)
4- Report reading.


Circuit is like this picture showing only two 6V6 ;

I thank you very much

You're right. I'm very motivated to make it work properly but sure I don't have enough knowledge.

Yes, I meant "Cathode bias" not "cathode follower"

Regarding the bias, I thought it was only for Class AB amp...
I have a bias probe so I can give a try

For the rest, I'll try my best but It feels unsafe I stop

Kind regards

Phil

[Latole]

Check each tube with your bias probe and report.

IMO it is the first thing to do before going further.

This amp is not a amp to lean how to. It is a very hard to work on.

[phil110567]

Biasing info.

The bias information vary a lot

The value is between 43mA and 44 mA for 3 tubes. I've noticed that the fourth is often in the 45 mA
It varies a lot so I give some average.

Philippe

[Latole]

Biasing info.

The bias information vary a lot

The value is between 43mA and 44 mA for 3 tubes. I've noticed that the fourth is often in the 45 mA
It varies a lot so I give some average.

Philippe

Tubes mA are well matched.

mA wthout knowing plate voltage does not help and do not say bias is good

If plate voltage is, let say 340 volts X 44 mA = 15 watts for a 12 watts tube !
Weber bias calculator say do not go more the 90%
Try to reduce mA to 90 % tubes power dissipation on each tube to see if you hear crackle

I see in my shop a vintage Vox ( EL84 ) same bias ( 115 % ) ; very sweet sound.

[phil110567]

SO I take my meter and courage and go measure the DC voltage between the pin #3 and chassis, is it correct ?

[shooter]

the math is missing something;
refer to reply 12;

should be (340vdc- 45vdc) * .035 = ~ 10.4W

[phil110567]

I read 335volts on pin3 but it is varying also

[Latole]

I read 335volts on pin3 but it is varying also

Someting may be wrong, voltage must not varying. How many volts ?

It could be you issue ; bad solder in power supply under the printed circuit

[Latole]

the math is missing something;
refer to reply 12;

should be (340vdc- 45vdc) * .035 = ~ 10.4W

No .
http://www.tedweber.com/webervst/tubes1/calcbias.htm

[shooter]

plate voltage on most schematics is referenced to ground, NOT, cathode.
so you subtract cathode from plate.  IF that was already done, it would be good practice to note that on schematic otherwise you complicate a simple math problem.

[Latole]

plate voltage on most schematics is referenced to ground, NOT, cathode.
so you subtract cathode from plate.  IF that was already done, it would be good practice to note that on schematic otherwise you complicate a simple math problem.

I may return to school 

[66Strat]

plate voltage on most schematics is referenced to ground, NOT, cathode.
so you subtract cathode from plate.  IF that was already done, it would be good practice to note that on schematic otherwise you complicate a simple math problem.

I may return to school 

The Weber bias calculator, in the link that you provided, calculates plate dissipation using plate to cathode voltage.