[shuteye]

I bought a new power cord with a Furutech 20A connector for my AR single chassis tube amp.  The amp (VS115) has the fuse holder located right above the inlet jack.  To my chagrin, the body of the Furutech connector butted up against the protruding fuse holder cap, not allowing the last 3-4 mm of penetration.  The Furutech connector was covered with heat shrink.  I decided to cut a groove at the end of the connector body of the cord to allow room for the fuse holder.  When I did this, I exposed the two binding screws that bind the hot leads on the cable.  So I wrapped this with electrical tape, pressing it in to the groove I had created. 

I plugged the cord into the wall - so far so good, so I unplugged it, then plugged the IEC into the amp - it seated all the way.  I then plugged the cord in to our PS Audio Premier Power Plant and began to listen.  Big grin - the sound was very good, much better than the factory cord, but .... later in the evening when we turned on our television, which was plugged into a Panamax, totally seperate from the Premier, 4 diodes on the left channel circuit board of the amp exploded.  Damn they were loud!  And then burnt electrical smell stunk up the room.  The amp sits immediately below and behind the TV.  We were listening to music and the TV was on mute, not channeled into the amp.  The Premier and Panamax were plugged into the same 30A dedicated circuit wall outlet.  The modified power cord showed no signs of arcing. 

I know I am an idiot - I ruined an expensive connector and damaged my amp -  and I feel like s**t, but what in actuality happened?  Any ideas?

Thanks ...... Jon

[Niteshade]

You had a defective diode(s). That's all. I'd say the rest was coincidence. One defective diode could set off the others when the junction fused. Hopefully you'll be all right once they are replaced.

[jimbop]

Perhaps the plugging/unplugging of the power cord created (or magnified) a fault in the amp's wiring or circuit board connections.

[shuteye]

I should also add that the fuse did not blow and that we heard a small pop over the speakers the first time we turned the TV on (no volume, just checking on a program listing).  Prior to this, we have never had any pops in the system, ever.  The pop was of small magnitude, no worries about damage, and none noted as music kept playing.  It was only later that I decided to see if the pop was still present, turned the TV on again and firecrackers happened.  Had me running around like a mad man trying to get everything turned off.  We just got a new set of Vienna Acoustic speakers - thank God they didn't get damaged!

The exposed retaining screws on the hot leads of the power cord connector are only 1/4 inch apart and my mod removed the plastic wall between them.  After the incident, when I pulled the plug out of the amp, the tape had lifted from the groove and from the end of the plug you could glimpse the retaining screws under the tape, though they were still covered.  But again, no signs of arcing anywhere surrounding the power cord, connector or amp plug in. 

Does this added info change anything?  I would be more at ease about the situation knowing I wasn't the cause.   

[jqp]

The TV did it! I would play Frank Zappa's My Guitar Wants to Kill Your Mama and glare at the TV.



It sounds to me like you either wiggled something around in the wall receptacle or your modification somehow caused a reaction causing a ground fault?

[shuteye]

Didn't wiggle anything, can't see how drilling out a corner of the connector opposite the ground could have caused a ground fault.  But I'm just a learner here, educate me. 

If I put on Frank Zappa, my old lady would kill me!

[jqp]

Didn't wiggle anything, can't see how drilling out a corner of the connector opposite the ground could have caused a ground fault.  But I'm just a learner here, educate me. 

If I put on Frank Zappa, my old lady would kill me!

Drilling plastic would not be the culprit but something likely changed in all of that plugging and unplugging - probably in the plug receptacle or the wall receptacle. Because the TV made the popping noise it seems to be in the wall. I recently watched Sherlock Holmes...

I am not one to educate you on this except that I recently got a nasty ground hum which was fixed (temporarily) by unplugging one of my amps from the wall receptacle and firmly repluging it in - obviously I need to replace my receptacle and will soon have an electrician do some rewiring behind the wall. Are you confident that your wiring and receptacles are 100%? They can work themselves loose you know.

[shuteye]

Regarding plugging, unplugging ... the only action the system itself saw was unplugging the factory cord from the amp and Premier Power Plant, then reconnecting with the modified cord.  The Premier is plugged into a 30A dedicated circuit, and that outlet was not disturbed.  When I plugged the modified cord into the wall, it was a regular outlet just to make sure all was well with the mod before using it to connect the amp and Premier.

Niteshade, considering there was an initial small turn-on type pop from the TV before the second catastrophic incident, does this change your take on the failed diode at all? 

BTW, the amp is on its way back to AR.  I disclosed everything in the authorization form that must be included.  I wonder what their service techs opinion will be? 

[shuteye]

The amp is back at AR and Chris in their service department has been very helpful.  Only the left channel was damaged, and the exploding bits were resistors, not diodes.  Output tubes were also damaged.  He is convinced DC offset from one of our components is the culprit.  I have a Fluke multimeter on order and will begin checking components.  Am I experienced in this?  No.  Am I capable of learning? Yes.  So here is a question, can front end components such as a transport or DAC be responsible for sufficient DC offset to damage a hybrid amp?  According to Chris, DC offset of 10mV or less is normal.  Can a transport or DAC be capable of more than this?  Also, I have read that stress from DC offset can be accumulative, gradually baking a component such as a resistor until it fails.  Is there agreement on this?  Thanks for any sharing of knowledge.

Jon 

[mgalusha]

Many devices are capable of generating a DC voltage on the output if something fails. In a CD/DAC the output stage is often built from op-amps, which have a bi-polar power supply. If for some reason one half of the supply should fail or partially fail and it's direct coupled then significant DC could appear on the output. If a servo is used to control the offset and it fails, same type of result.

If the device has a single ended output and uses a coupling capacitor, they can sometimes get leaky and allow some DC to get through. I once looked at a preamp for a friend that was causing his Parasound JC-1's to shut down from DC. It would swing to +/- 125mV very slowly, slowly enough to watch on a meter and cause the amps to think it was DC and go into protection mode.

On a tube amp excessive DC on the input, provided it's not using an input capacitor, will cause the bias on the input tube to change, causing it to draw more or less current, depending on the polarity of the DC.

A possible scenario...

Say the input stage is using a 6SN7 biased at 8mA, not an unrealistic figure. This might mean a grid voltage of -4V or more commonly the grid is at 0 and the cathode might have a 500ohm resistor and be sitting at 4V, effectively making the grid negative in relation to ground the cathode. Now say the upstream component developed a problem and put +10V on the grid, this would cause the tube to turn on as hard as possible and draw whatever current was available from the power supply or was limited by the plate resistor. Say the circuit had a 15k plate resistor, 300V B+ and was normally dropping 150V at 8mA, the plate resistor would normally have to dissipate 1.2watts. If the tube turned on fully and the power supply was up to it, the 15k resistor would allow 20mA of current and would be dropping nearly 300V, which is 6 Watts. If a 5W resistor had been used, which would be reasonable for a 1.2W normal dissipation, it would not last long at 6W and poof, toasted resistor.

The amp is back at AR and Chris in their service department has been very helpful.  Only the left channel was damaged, and the exploding bits were resistors, not diodes.  Output tubes were also damaged.  He is convinced DC offset from one of our components is the culprit.  I have a Fluke multimeter on order and will begin checking components.  Am I experienced in this?  No.  Am I capable of learning? Yes.  So here is a question, can front end components such as a transport or DAC be responsible for sufficient DC offset to damage a hybrid amp?  According to Chris, DC offset of 10mV or less is normal.  Can a transport or DAC be capable of more than this?  Also, I have read that stress from DC offset can be accumulative, gradually baking a component such as a resistor until it fails.  Is there agreement on this?  Thanks for any sharing of knowledge.

Jon

edit: fixed typo

[shuteye]

Thanks so much for taking the time to respond, Mike.  I've printed out your response so I can refer to it as I do some research.  It also convinced me that I want to start learning more about elecronics and tube gear design.  I'm a 57 year old physical therapist (post surgical hand/upper extremity), and have been into audio gear at a modest level all my life.  My father was an early audiophile back in the 60's and got me started with his used gear.  I took a physics course in college, but that is the extent of my education at this point.  If you, or anyone reading this, have any books you can recommend that explain amp and preamp design for the dummy (to get me started), I would appreciate it.  BTW, I added your website to my favorites.
Peace,
Jon

[mgalusha]

Jon,

A good place to start is with Bruce Rozenblit's Beginner's Guide To Tube Audio Design

It starts with the fundamentals and is well written.

For a more thorough treatment I suggest Morgan Jones - Valve Amplifiers. This is a superb book and goes into much greater depth than Bruce's book but requires a bit more knowledge to start with.

mike

[shuteye]

Thanks, Mike.  I'll order them today.  Our amp is on its way back to us, but is not going into the system until I get to the bottom of what happened.  In the mean time, my old workhorse Quicksilver Mono 90's are chugging away.