DON’T DO THIS!!

[nevetslab]

Having waited all week for a new matched pair of J/J 6V6GT power tubes to arrive to finish the repairs on a Fender Deluxe Reverb, where one of the previous tubes arc’d inside, and toasted the 470 ohm screen resistor (with foil damage). The tubes arrived and I went ahead and installed them, biased them up. All seemed ok. I had pulled the chassis back out of the cabinet, to keep it out of harms way, with it now sitting on my lab jacks on the bench. I finally plugged in the speaker, it leaning up against my adjacent bench a few feet away. Switched out of S/B, to be greeted with loud obnoxious hum and noise, while the power tubes were now pulling 35mA on one side, 70mA on the other, and quickly shut it down.

It took me a few moments to discover I had forgotten to plug the three control panel ribbons back into the motherboard, which had been lifted up to replace the screen resistors last week. Dumb! OK….try this again. I had already removed the choke, having found no change in ripple on the output side of it from this initial speaker-connected power-up. Choke was, of course fine, as was the filter cap board on the bottom of the chassis. So now, all back in place and wired in, the amp sounds quiet, both tubes were balanced and running 24mA ea/9.6W @ 410VDC on the supply.

So, I put the chassis back into the cabinet, and re-assembled the cabinet. Began final test, only to now discover one or both of the new power tubes was very microphonic, to the point of tapping on them sent the amp into loud growling with an attitude!! I really didn’t want to pull the cabinet apart again, since the chassis won’t come out due to the baffle being oversized, So, I took the handle off and turned the cabinet upside down, facing the test gear and allowed me to have the power tubes facing me. Tapping again on the power tubes, I found it was only one causing it. I swapped the tubes, and it remained on the same socket that had the failed screen resistor and arc’d power tube.

OK. Tube socket issue. Ceramic base, forked terminals. What do I have to try and clean the terminals with? Found one of my Pace stiff round bristle brushes, 0.100 in dia. Cut off the tapered end, so I had all of that brush’s dia to shove into the socket. So, I discharged the supplies with my 100 ohm/50W probe, and went ahead, dipping the brush into Caig DeOxit and scrubbed each terminal. Then, using a larger nylon round brush, I scrubbed the terminals more with the same cleaner, and finally using a tiny Wiha flat-blade screwdriver, I lightly increased the tension of the forks.

I went and plugged in the first tube into the cleaned socket, and saw to my horror, the tube began glowing, and now seeing the 5AR4 that had been glowing all the while, amp on running in Standby!!!! Oooooohhhhhhh ya gotta be kidding me!! I plugged in the other power tube, looked up at the ceiling, thanking HIM for my not being lit up, having been doing all this with only one hand in contact with the tools going down the throat of the tube socket, while no other part of my body touching anything to complete a circuit.

Went and fetched a cup of coffee, came back and powered it back on (out of S/B). Now, no more loud growling as I pounded on the tubes. Shut it down, turned the cabinet right side up, powered back up and plugged in my B & K 1027 Sine/Random generator, set it for 10Hz BW for the white noise modulation and dialed up the maximum resonance where all of the cabinet was in vibration, and let it run (85Hz) as I sat down to write up invoice, then emailed that to my client. Success on restoring the tube socket. And bigger success not landing me on my ass or worse!! ALWAYS UNPLUG THESE THINGS!!! Thought I had done that.

[Randall]

Rough day, or as they say, some days you eat the bear, some days the bear eats you. Glad you didn't get hurt in the process.

[Chuck H]

Lot's of things can contribute to mistakes like that. Not the least of which for me is familiarity with a situation and becoming cavalier. I've been reworking a design, lots of live testing, and the open chassis has been on my bench for months now. Reworking it off and on between other projects. A few days ago I came dangerously close to touching the HV end of a resistor lead when connecting an alligator clip and I failed to put the amp in standby. When I realized what I was doing I pulled back and thought "Huh... That could have gone a lot worse." Incidents like that, if they don't go horribly wrong, reinforce lessons we all know (some learned the hard way). I hope we all remember to follow safe protocols and keep risk assessment peripherally engaged at all times.

[Tone Meister]

Your posts are always über informative, even when pointing out the mundane. If the novice (me) doesn't get sidetracked by the fact that the pro failed to pull the AC cord, there is plenty of solid info in your initial post.

Just want you to know that I appreciate your posts.

[Dave H]

I've done it a few times. You change a component or connection in an amp then go to switch it back on only to discover there's no need because it's already on! That's when the icy chill goes down your spine thinking what might have been.

[Chuck H]

I smell smoke. Is somebody cookin'? Oh crap! That's ME!

I was just working outside on a building in a popular tourist district. I was using one of those kerosene blast heaters that look and sound like a jet engine to warm and dry off surfaces. I had to put out cones to keep people from walking directly in front of the thing. Because the work area took up the entire sidewalk some people, naturally, walked around the cones and through the work area so I had to put up caution tape. One guy actually ducked under the caution tape and walked through the work area rather than go around in the road. Fortunately no one was charred. But this is an opportunity to mention that it's even worse to jeopardize a customer (or a by walker).

There was a post here some years back where a buuteek guy was selling amps with PT's that used those solder tabs near the top rather than leads from the bottom. No effort was made to conceal the solder joints. So there, inches from the power tubes, were exposed HV nodes. Someone on the thread said "I hope no one HERE built that." It was unanimously agreed that it was a bad design

About the worst I've done (WRT a customer amp anyway) was to send out a mod that lacked a tube retainer on a less than tight socket. The amp took a tumble off his back seat when he slammed on the brakes for some reason and the power tube fell out. He then ham handedly busted off the center key reinstalling it. As luck would have it he thought better about what could happen at that point if he turned it on and called me. I got the amp back, otherwise undamaged, and installed a retainer and a new tube. But if he hadn't called, but instead decided to turn the thing on I suppose it could have gone any of eight different ways! Some potentially ending in fire.

[Jazz P Bass]

When I first started repairing amps, my Father in Law brought me a Fisher 800B.

I had failed to notice that the On-Off switch was broken in the On position.

Bzzzzt.

To this day, I will always have a volt meter hooked up to the power supply when repairing an amp.

Always.

[Gnobuddy]

Unlike most of you here, I only began working with valve electronics a few years ago. Horror stories I found on the Internet, similar to the ones you've all been telling in this thread, convinced me to look for information on safety. Surely there had to be something better than our grandfather's "safety" techniques such as keeping one hand in a pocket? Electricians routinely work on high voltages today, so how do they keep from killing themselves?

Well, for starters, they wear special electrician's insulating gloves on both hands. So I immediately got myself a pair of class 0 Salisbury gloves. Class 00 would probably have been sufficient for any voltage I was going to encounter ( https://www.grainger.com/content/qt-...inspection-262 ), but I happened to get the class 0 pair on deep discount.

Obviously it takes more than a pair of gloves to keep you safe when working with high-voltage circuitry. Safety has to be a mindset. For instance, you still have to make sure the circuit you're working on is properly supported and won't fall into your lap while you're poking and prodding it. And you still need to protect your eyes from a possible shower of sparks.

But the gloves are a huge first step towards safety. Electricians are required to wear them when working on high voltage. We should be doing the same thing.

-Gnobuddy

[pdf64]

There's that old craftman's phrase intended to save wasting materials 'measure twice, cut once', ie double check the dimension before you start cutting.
Perhaps we too could apply it, checking and re-checking that there's no voltage on the mains and secondary circuits before we go diving into the chassis in front of us, or indeed double checking that the amp's power cable really is unplugged from the mains before taking the chassis out of the cab (cue cold chill moment when realised)

[Chuck H]

Perhaps we too could apply it, checking and re-checking that there's no voltage on the mains and secondary circuits before we go diving into the chassis in front of us, or indeed double checking that the amp's power cable really is unplugged from the mains before taking the chassis out of the cab

My bench has a dedicated amp plug on a conduit box. A three way toggle allows me to turn off the AC, send it to the amp or run it through the light bulb limiter. It's mounted under the bench so I just have to reach down and flip the switch. No grabbing a cord and pulling. What I haven't done is hook up a volt meter to it, ala post #7. I have a few cheap meters floating around and I could just hook one up to it. But I have another thought. That LCD screen doesn't exactly scream "PAY ATTENTION!" so I think I'll make a red LED circuit and mount it on the wall directly in front of me with a sign that says "AC IS ON".

[Gnobuddy]

I think I'll make a red LED circuit and mount it on the wall directly in front of me with a sign that says "AC IS ON".

That sounds like a good idea!

On the Aussie Guitar Gearheads forum, new DIY builders are encouraged to wire a neon-bulb indicator across the B+ supply before they ever power their new amp on. If the neon is lit, the amplifier is dangerous.

I think it's a great idea, but NE2 neon bulbs are starting to get scarce - either they have gone out of fashion, or it has something to do with ROHS regulations.

Tinkering with some 3mm and 5 mm LEDs a year or so ago, I realized that the brightness rating (usually in mcd or millicandela) is also an efficiency rating, because they all have about the same maximum current. You can get LEDs with mcd ratings from 50 to 20,000, which means there is an enormous range in efficiency.

I found that LEDs with mcd ratings of 3000 or higher will usually light up brightly at a current of 1 mA (ten times lower than what we used to consider normal LED current). The 20,000 mcd ones are bright enough to use as indicators at currents as low as 0.1 mA.

This means we now have LEDs that draw currents in the same range as the old NE2 neon bulb indicators did. They make excellent warning lights for valve B+ supply rails. Say you have a fairly typical 400 volts in a 6V6 amp; you can use a 390k or 470k resistor in series with an LED with an mcd rating of 3000 or higher, and it should light up brightly enough for the job. Power dissipation in a 390k resistor wired to 400 volts is under half a watt, so you can use a 1 watt resistor.

As a bonus, the LED and resistor act as a bleeder resistor for the filter caps, safely discharging them a few tens of seconds after you unplug AC power to the transformer.

I have been using LED warning indicators like this in my DIY valve guitar amp power supplies for a couple of years.

The fact that these high-brightness LEDs will light up brightly at 1 mA current opens up another interesting use: put one in series with the anode of a typical preamp triode (half a 12AX7), and it will light up when the tube is operating. That simultaneously tells you that there are dangerous voltages present, and also that the triode is wired correctly and working!

I have one DIY preamp that uses two small-signal triode-pentode valves. I wired up a green LED in series with the anode of each triode, and an amber one in series with the anode of each pentode. Those 4 LEDs acted as useful tell-tales as I was tinkering with the preamp, and I could tell at a glance if I had mis-biased any of the four individual devices, and whether the misbehaving one was a triode or a pentode.

-Gnobuddy

[nevetslab]

As long as you take your blinders off, you can see the sign. In my case, as reported....RIGHT THERE IN FRONT OF ME WAS THE LIT-UP TUBE RECTIFIER, EFFECTIVELY SAYING 'POWER IS ON' ! How I failed to even notice it, I guess, states where my attention was...the task at hand, and relying on standard practices of unplugging the unit in the first place.

As to using gloves, like most of us who do this daily, they really are not suitable when you need all of your dexterity to handle small parts in the course of servicing. Being attentive to all safety precautions is normally sufficient. I couldn't have dealt with the tools used in the procedure with gloves on.

[Gnobuddy]

How I failed to even notice it, I guess, states where my attention was...the task at hand

To err is human, right? It happens to all of us, we either get careless, or we get tunnel vision. The best we can do is set up procedures so that any single mistake by itself isn't fatal.

Multiple mistakes still can be fatal, and that is what most reconstructions of fatal aeroplane crashes reveal - that more than one mistake had been made,and it was the compounded effect that caused the fatality.

As to using gloves, like most of us who do this daily, they really are not suitable when you need all of your dexterity

I have the same problem with the gloves, and my compromise has been to always put on the gloves until I have verified with my DMM that there are no voltages present in the circuit. Once I'm 100% sure there is no dangerous voltage anywhere, I take off the gloves.

If I have to work on a live chassis, I wear the gloves, awkward or not. Better awkward than dead, I figure.

Being attentive to all safety precautions is normally sufficient.

And this is always necessary, it's the net of safety precautions that will hopefully catch us when we make one mistake, which will hopefully be non-fatal.

-Gnobuddy

[bob p]

Unlike most of you here, I only began working with valve electronics a few years ago. Horror stories I found on the Internet, similar to the ones you've all been telling in this thread, convinced me to look for information on safety. Surely there had to be something better than our grandfather's "safety" techniques such as keeping one hand in a pocket? Electricians routinely work on high voltages today, so how do they keep from killing themselves?

Well, for starters, they wear special electrician's insulating gloves on both hands. So I immediately got myself a pair of class 0 Salisbury gloves. Class 00 would probably have been sufficient for any voltage I was going to encounter ( https://www.grainger.com/content/qt-...inspection-262 ), but I happened to get the class 0 pair on deep discount.

Obviously it takes more than a pair of gloves to keep you safe when working with high-voltage circuitry. Safety has to be a mindset. For instance, you still have to make sure the circuit you're working on is properly supported and won't fall into your lap while you're poking and prodding it. And you still need to protect your eyes from a possible shower of sparks.

But the gloves are a huge first step towards safety. Electricians are required to wear them when working on high voltage. We should be doing the same thing.

-Gnobuddy

When it comes to safety clothing, I like a rubber apron. And I always wear non-conductive rubber soled shoes. I don't want to absent mindedly lean into something or have something that's electrified fall into me.

I remember reading a horror story in the local paper about a properly outfitted utility lineman who was wearing all of the right gear, and got electrocuted when a live wire contacted his groin. He was electrocuted, but survived. He lost his penis. That case went to court for some reason, and the utility company lost. No to say that the lineman won his case ... everyone lost on that one.

[bob p]

My bench has a dedicated amp plug on a conduit box. A three way toggle allows me to turn off the AC, send it to the amp or run it through the light bulb limiter. It's mounted under the bench so I just have to reach down and flip the switch. No grabbing a cord and pulling. What I haven't done is hook up a volt meter to it, ala post #7. I have a few cheap meters floating around and I could just hook one up to it. But I have another thought. That LCD screen doesn't exactly scream "PAY ATTENTION!" so I think I'll make a red LED circuit and mount it on the wall directly in front of me with a sign that says "AC IS ON".

Interestingly, I find SS gear to be more dangerous to work on than tube gear; although tube gear has higher voltages it's always pretty easy to tell when a tube circuit is energized -- the tubes give you a constant visual reminder. That's not the case with a SS circuit -- it's pretty easy for those to be energized without any outward physical signs to remind you.

To that end, I have a setup that's a bit like Chuck's -- I have a switched outlet that's dedicated for the UUT. It also has a pilot lamp, so that whenever the circuit is energized, the pilot lamp is on. In the optimal scenario, that warning light should be pulsing, and bright enough that it'll cast a shadow on the workpiece so that it'll catch my attention, if only out of the corner of my eye. I've often thought about adding an audible beep tone or a rotating light that's powered by energizing the outlet to the UUT so that I have an audible and a visual reminder when things are hot. But some of that just seems too geeky. Maybe I just need to keep one hand in my back pocket and behave as if everything is always hot.