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Under the skirts on a Velleman 4040

arcorob

arcorob

Addicted Member
If you haveever wondered what this puppy looks like inside...wonder no more. The person who did the build did a nice job...I just wish whomever was in here didn't treat some of teh small cover screws like BOLTS on a motor mount. I had to use a drill out tool on two of them...all good though..

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Someone changed the resistors on the right side outputs...left side is original (first pic here) and right side has the change..perhaps blew one ?

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20 Watts is much too high for cathode resistance, I think fusibility should be one of the main factors here. As it stands, a fault could cause these resistors to burn the pcb and/or damage other parts of the circuit. In normal use 2 Watt resistors would be much more than adequate such as the Tyco FRN200S range which are fusible and more importantly, flameproof. Looking at the photographs, the original white block resistors I have met before, they are often found open circuit so perhaps are fusible and flameproof. But replacing 5 Watts with 20 Watts is I think a bad idea, let alone the damage to the layout aesthetics.
http://docs-europe.electrocomponents.com/webdocs/0027/0900766b8002759f.pdf
 
Don't like the oversized cathode resistor with exposed bare wires. (last picture). This is likely from a previous lousy (emergency?) repair job.
Cathode resistors must be 1W MOX non-flammable type which protect the circuit if anything goes wrong. (tube failure) Don't believe me ? Just look at the pictures below who shows what could happen to a Velleman (factory wired !) amp. This one was damaged beyond repair and the only surviving part was the fuse !
 
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Yeah...I am finding some inconstancies

For example, those 5w 10ohm resistors I pointed out as original ? Turns out they are not...they are too high also...they are supposed to be 1 way according to the manual...

According to the seller this amp was assembled by technician Darrell Powell of "Digital Phase" fame. Have no clue who that is.

Would really like to know why the mods were made are they are not the type of things that make the unit safer or sound better...

The mystery continues...LOL
 
give clarity

Tubologic said:
Don't like the oversized cathode resistor with exposed bare wires. (last picture). This is likely from a previous lousy (emergency?) repair job.
Cathode resistors must be 1W MOX non-flammable type which protect the circuit if anything goes wrong. (tube failure) Don't believe me ? Just look at the pictures below who shows what could happen to a Velleman (factory wired !) amp. This one was damaged beyond repair and the only surviving part was the fuse !
Click to expand...

Are you talking about those 5w 10 ohm resistors (look like a sugar cube with red writing ?) Remember I am a newbie so ya have to point out things :D
 
Yes, the white body ceramic wirewound cathode resistors which are not original. You should replace all the 8 of them with 10 ohms/1 watt non-flammable (a.k.a "flameproof") film resistors. Leave about 5mm space between the resistors and the printed circuit when you mount them. In the event of a tube failure these resistors will open the circuit (like a fuse) without burning a hole in the PCB or starting a fire. Theses cathode resistors also serves as a current sampling shunts for the bias indicator and should be of 5% (or better) tolerance.
 
Thank makes sense

Tubologic said:
Yes, the white body ceramic wirewound cathode resistors which are not original. You should replace all the 8 of them with 10 ohms/1 watt non-flammable (a.k.a "flameproof") film resistors. Leave about 5mm space between the resistors and the printed circuit when you mount them. In the event of a tube failure these resistors will open the circuit (like a fuse) without burning a hole in the PCB or starting a fire. Theses cathode resistors also serves as a current sampling shunts for the bias indicator and should be of 5% (or better) tolerance.
Click to expand...

I just wonder...if the original builder was popping the 1w resistors (like a fuse) and upped them to prevent the pop...but put the amp at risk...would there be any other reason, other changes that warranted the move up ?
 
Maybe it's just me or old school thinking but hardwiring deserves a place around power handling tubes period. I've worked on and seen time and time again the effects just from the heat generated in these areas what long term damage can occur. Tube pins get hot and will draw heat away from the tube, through the socket and onto the board even under normal operation.

And if you charr and blow up components you have now created carbon paths that can even be etched into the substrate.

Quote: I just wonder...if the original builder was popping the 1w resistors (like a fuse) and upped them to prevent the pop...but put the amp at risk...would there be any other reason, other changes that warranted the move up ?

Answer: No.
 
Reply

marty59 said:
Maybe it's just me or old school thinking but hardwiring deserves a place around power handling tubes period. I've worked on and seen time and time again the effects just from the heat generated in these areas what long term damage can occur. Tube pins get hot and will draw heat away from the tube, through the socket and onto the board even under normal operation.

And if you charr and blow up components you have now created carbon paths that can even be etched into the substrate.

Quote: I just wonder...if the original builder was popping the 1w resistors (like a fuse) and upped them to prevent the pop...but put the amp at risk...would there be any other reason, other changes that warranted the move up ?

Answer: No.
Click to expand...

Reply: Thanks :D
 
arcorob said:
I just wonder...if the original builder was popping the 1w resistors (like a fuse) and upped them to prevent the pop...but put the amp at risk...would there be any other reason, other changes that warranted the move up ?
Click to expand...

I would say he was blowing them. Look at the first pic where the lower left 5W resistor is. The board is burned beneath it. Mine had problems too when I got it. Burn mark on the pc where one of these resistors lies. This is a common issue with these.
 
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I would mount the replacements as high off the board as practical to try to avoid burning the board,possibly use the same style as the 5 watters as they just open on overload.
there must be some kind of design flaw in the amp,or it pushes the tubes too hard,as this seems to be a common problem.
 
Good eyes

hatrack71 said:
I would say he was blowing them. Look at the first pic where the lower left 5W resistor is. The board is burned beneath it. Mine had problems too when I got it. Burn mark on the pc where one of these resistors lies. This is a common issue with these.
Click to expand...

I had NOT even noticed that...hmmm..

So I wonder. The amp is stable now. I know I have to change the right side one, but perhaps leave the 5w 10ohms ? The danger is if a tube red plates right ?

I just don;t want to be popping resistors if 1watt is just too low - sort of like a fuse that blows too easy...but then again, I am not that knowledgable. what is the test to see whats flowing there ?
 
audiodon said:
I suspect that the underlying theory is that better the resistor pops than that the transformer goes.
Click to expand...

Trust us on this - much better that a $0.50 resistor blows than a $100 output transformer (or, worse yet, an unobtanium output transformer...I'm still trying to find a replacement for a Fisher EL84 amplifier). Sand and wire are cheap, iron, not so much.

-D
 
Other than from defective tubes, it is not normal for cathode resistors to simply blow out, let alone on apparently multiple examples of a given unit. Therefore, there are likely design issues that are encouraging arc events to occur. With that thought in mind then, here are a couple of observations to possibly help solve the problem for all users of these amps:

1. The 180 ohm screen resistors appear to still be in tact, or if changed, it does not appear that the originals ever burned out requiring replacement, indicating that the source of the damaging cathode current is most likely from the plate in this (rather rare) instance. The screen resistors are hardly placed at an optimum position (very close to the screen terminal), but in any event appear to be doing their job.

2. Long OPT primary leads and traces on the circuit boards are always problematic relative to output stage stability -- particularly when a push-pull-parallel design is involved. Therefore, I would recommend a little circuit board surgery to help prevent such occurrences in the future:

As close to the plate pins as possible (pin #3, and preferably right at the pin), break the trace leading to this pin. Then, solder a 22 ohm 2 watt resistor across the broken trace -- that is, from pin #3 to the side of the trace leading to the OPT. This effectively places the resistor between the plate connection of the tube, and the plate connection of the OPT right at the tube, and will need to be done at each of the 8 output tubes in the unit.

Finally, the cathode resistors are correctly sized at a 1 watt rating, since the dissipation through these resistors is just under .5 watt at full sustained power output. With proper output stage stability then, unless a catastrophic event in a tube takes them out, 1 watt cathode resistors should last for the life of the unit. Installing the individual plate resistors will go a long way to ensuring that stability exists.

I hope this helps!

Dave
 
Lol

dcgillespie said:
Other than from defective tubes, it is not normal for cathode resistors to simply blow out, let alone on apparently multiple examples of a given unit. Therefore, there are likely design issues that are encouraging arc events to occur. With that thought in mind then, here are a couple of observations to possibly help solve the problem for all users of these amps:

1. The 180 ohm screen resistors appear to still be in tact, or if changed, it does not appear that the originals ever burned out requiring replacement, indicating that the source of the damaging cathode current is most likely from the plate in this (rather rare) instance. The screen resistors are hardly placed at an optimum position (very close to the screen terminal), but in any event appear to be doing their job.

2. Long OPT primary leads and traces on the circuit boards are always problematic relative to output stage stability -- particularly when a push-pull-parallel design is involved. Therefore, I would recommend a little circuit board surgery to help prevent such occurrences in the future:

As close to the plate pins as possible (pin #3, and preferably right at the pin), break the trace leading to this pin. Then, solder a 22 ohm 2 watt resistor across the broken trace -- that is, from pin #3 to the side of the trace leading to the OPT. This effectively places the resistor between the plate connection of the tube, and the plate connection of the OPT right at the tube, and will need to be done at each of the 8 output tubes in the unit.

Finally, the cathode resistors are correctly sized at a 1 watt rating, since the dissipation through these resistors is just under .5 watt at full sustained power output. With proper output stage stability then, unless a catastrophic event in a tube takes them out, 1 watt cathode resistors should last for the life of the unit. Installing the individual plate resistors will go a long way to ensuring that stability exists.

I hope this helps!

Dave
Click to expand...

You will have to talk to my tech..he speaks tube amp...lol

What I do gather is I better get some 1watt 10 ohm resistor asap...anyone recommend where I get this type (the sugar cubes as I call them)...In the old days, I called such thing ballast resistors...
 
So let me ask the team

If I turned on the amp and checked both sides of the resistor for DC voltage, would I be able to tell what is going through there now ? What would I measure, voltage ? Give me a cluse as I am willing to see would it POP the 1w 10 ohm ? Thanks
 
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