Trouble installing VVR - Help please

[clear and blue]

Hi,

I recently bought a VVR kit for cathode biased amps from Dana (after converting my Laney VC15 to cathode bias).

Having just installed the kit, I get no response from the VVR pot. The amp sounds unaffected to me and I have been scratching my head to think what I could have done wrong in installing the kit that leaves the VVR useless while still allowing the amp to run as normal.

As the Laney is very pokey inside and PCB based, I interupted the B+ at the HT fuse (100mA) for the B+ in and B+ out. Is there a problem with this?

The instructions were not very clear about how the Mosfet should be connected (i.e. which pins get soldered to which leads). I attached the Mosfet so that the front is pointing upwards/the same direction as the other components on the board. Could this be the problem?

The Mosfet is mounted on a heatsink from a cannabalised SS amp, using the provided insulator. With limited work area/tools, I did the best I could regarding anti-static in that I kept touching metal to discharge and only handled the Mosfet with small pliers which were also routinely touched to metal. Could a fried Mosfet cause these symptoms?

Any help will be appreciated.

Cheers.

[Enzo]

Maybe everyone else knows, but I don't, what is a VVR kit? Can you provide a link to it so we can familiarize ourselves with it? Can you provide a schematic of the amp circuit you are installing it in, or perhaps the original amp schematic with mod notes?

[clear and blue]

Thank you for your reply.

VVR Kits by Hall Amplification. VVR is a form of power scaling - controlling B+ to adjust output volume.

"VVR mod" is a quick sketch over the schematics to show what I have done. I didn't include it in the sketch but I grounded the VVR kit to the output transformer.

I cannot remember whether I installed the kit before or after the HT fuse but surely this should not matter?

[Steve Conner]

If you got the drain and source pins on the FET mixed up, it could explain your symptoms.

If you put the FET in backwards, then you mixed up the gate and drain pins. This will explain the symptoms and also destroy the MOSFET, so you'll need another one.

There's no ground wire shown in your schematic, but I guess the VVR kit needs one to work. Did you ground it?

[clear and blue]

Steve,

I grounded the VVR board to the ground connection on the OT secondary as it was closest.

I put the FET in the same way as it appeared to be on the Hall Amps website - when looking down on the board you are looking at the front of the FET with the heatsink underneath.

So a FET put in backwards would still pass B+ through even though it has been destroyed?

Thanks for your reply

[Enzo]

ANy part that shorts out will pass anything applied to it. A shorted part becomes a "wire."

[Steve Conner]

So a FET put in backwards would still pass B+ through even though it has been destroyed?

Yes, FETs often fail shorting all three pins together. Especially when the failure was induced by stuffing 300 volts into the gate pin, as you'll have done if you put it in backwards.

[clear and blue]

ahh! thanks for your help Steve and Enzo! I will give it a go the other way round with another MOSFET.

Steve, could you please advise a suitable replacement for the NTE 2973 (in the UK) as I must admit to knowing nothing about MOSFETS. looking on Maplins under Semiconductors>Voltage Regulators, the LM337 seems to be the closest although I don't think it would handle the voltage or the current even. Any ideas?

[clear and blue]

I would imagine that I could have easily shorted the MOSFET through either excessive solder heat or maybe ESD so I will check with Dana the correct orientation of the FET.

[Steve Conner]

The large MOSFET in the picture? Try an IRFP460 as the replacement. You won't get it from Maplin, try Farnell, RS or CPC.

But the picture is very clear and it's hard to see how you could have got it backwards. And these large MOSFETs are pretty resistant to soldering heat and ESD. I casually handle them without ESD protection all the time. So let's first of all verify that you have actually blown it.
MOSFET testing
http://www.repairfaq.org/sam/semitest.htm#stmos

Another thing that would explain your symptoms is if you got the "in" and "out" wires mixed up. Or maybe you made some other mistake assembling the kit.

[clear and blue]

Thank you for the advice. I will let you know what I find.

[clear and blue]

I followed your instructions and it turned out the MOSFET was ok - I had just got the B+ in and out the wrong way round like you suggested.

Thank you very much for the help! The VVR works a treat now.

Cheers

[Enzo]

And just for future reference:

Steve, could you please advise a suitable replacement for the NTE 2973 (in the UK) as I must admit to knowing nothing about MOSFETS. looking on Maplins under Semiconductors>Voltage Regulators, the LM337 seems to be the closest although I don't think it would handle the voltage or the current even.

You need to differentiate between the part and the application. Your MOSFET is a transistor, not a voltage regulator. ANd that LM337 is a voltage regulator, not a transistor.. The CIRCUIT is a voltage regulator, but the PART is a transistor in your VVR. The same MOSFET could be used in a voltage regulator circuit or an amplifier circuit, or a power supply switching circuit and so on. SO it remains a transistor regardless of the use it is put to. The point being that to search, we look for the transistor, not "voltage regulator." Unless we are specifically looking for voltage regulator ICs.

[clear and blue]

Thanks for clearing that up Enzo. I was a bit unsure when I was seeing Vin and Vout on the legs instead of gate,source and drain. I guess I need to expand my knowledge on from 1930's technology. Looking into an amp, the diodes, resistors and capacitors are easy to identify but transistors and the like look the same to me.

Actually, the name - Field Effect Transistor - should have given it away really. .

Thanks again for taking the time to help an electronic numpty

[Enzo]

Let me tell you something. I learned tube electronics starting about 1954 and on through the 1960s. I fiddled a litle with some very basic transistor circuits early on, but never really internalized transistors. I found myself on a service bench about 1974, and had to learn fast how digital circuits worked. I was working for an amusement company servicing pinball, jukebox and the then new video games - Pong!

SO I went from tubes to TTL logic. I got pretty good at TTL/CMOS logic. And our pinballs were still all mechanical relays and stepper units, and I will go head to head with anyone reading relay logic circuits. Then microprocessors happened. I had to learn them, the arcade industry likes to adopt new things. It was only at this point, sicne I had to also service the video monitors and jukebox amplifiers, that I really started to learn transistor circuits.

But in all this experience, I never really learned FETs. Oh conceptually, no sweat. But specifically and making it intuitive? Nah. Over time I learned about JFETs, they are pretty simple, but to this day I can never remember whether it is an N-channel or P-channel, and thus what polarity voltage to its gate turns it off. I have to stop, look, and figure it out.

And now that MOSFETs are more and more common, I have to admit they are not internalized to me, I don;t see them intuitively. Are they beyond me for learning? NO, of course not, but I just never have. MY expertise is in troubleshooting, not the electronics per se. There is a lot I don;t have to know to be a good troubleshooter. Just as I don;t need to know Thiel parameters to replace a blown speaker. I look at the MOSFET symbol, I have to check whether it is N or P channel. Subtle variations in type, I don;t know them. One of these days I will study up on it, but just as you don;t need to understand space charge to diagnose and replace a bad 6L6, I manage to get by with that gaping hole in my resume.

SO never feel bad for not knowing something, or for making mistakes, as there will always be TONS you and I don;t know. And as soon as we learn something, it will be revealed to us that there is yet more we need to learn. I been at the electronics thing for 55 years now, and I still learn things EVERY day.