Testpoints

sloober

sloober

Super Member
I wanted to add 10Ω resistors to my Altec 345a so I could check current through my really old RCA 6CA7 tubes. Another AKer has the same amp and had done this using testpoints mounted to the chassis. It took me a while but I found them from Mouser and they are great. Only wish I ordered more. They fit in 1/4 inch holes so I could easily use the air slots in the chassis without any modification. They make a perfect place to mount the resistors and are easily wired to the tubes. Mouser # 565-3542-0 Black - 565-3542-2 red.
John
Oh by the way, Mouser calls them "pin tip jack Gold Plated"

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Yup, from Pomona Electronics. Mine came from Johnson, which aren't as fancy as those.

Do you know why the single digit color codes are easy to remember? They designed them to directly correspond to the color/value band resistor code table.

0 is black, 5 is green, etc.

. . Falcon
 
sloober said:
Duh, that makes sense!

Now for setting the current through the 6CA7 tubes... .30-.40mA?

John
Click to expand...

30mA to 40mA accidentally running through your hand feels SO much better than 400V+ that slams into your arm like someone wound up and hit it with a creosote-soaked telephone pole, eh?

I remember one of my college professors was working on a big military signal tracer in the electronics lab with a pair of super magnifying glasses one day, and it turned out that the chassis was hot. His lower lip brushed up against the chassis when he turned his head back towards the unit after looking away for a moment.

WHAM!

He was slammed to the floor pretty fast, and was alright after a few minutes.

The funny part about this incident is that we were listening to 'Chain Lightning' on a Steely Dan cassette tape when it happened.

Everytime I hear that song these days on an oldies radio station (which isn't too often), I remember that day. I'm fairly confident that if he's still alive, so does he! :D

. . Falcon
 
:lmao:

You know this amp has a similar story attached to it. I had a ham radio guy help me with it back in the 80s. He was 80+ but would still climb 50 feet into a tree to fix his antenna. He had the 345a turned on and was checking voltages etc. As I watched over his shoulder. All of a sudden he jumped back like a 15 year old and nearly knocked me over.
"nothing like a good shock to remind you your still alive" was all he said.
I was just glad it didn't give him a heart attack!
John
 
The only bummer about having the test points in those areas, is they are right next to a 6CA7 and a 5U4. You gotta be careful, or you'll get your hand burnt.

I need to come up with some lengthy probes/tips, to avoid touching the tubes.
It's still better than having to flip the amp over.

The larger oval punchout makes a good place to park a potentiometer with a 3/8" shaft.

Still working on my negative bias circuit.......

I remember visiting my audio buddy retired EE friend when he lived up here, he would be testing something and get zapped with lower voltage DC.
He would just get this weird look on his face....kind of a strained "stuck" look......

I ask him if he is ok....and he was usually fine.....like he was used to it or something.....
Once he was unglued from the circuit, he kind looked like someone who just took a hit of crack. I've been zapped, and know the feeling....it is bit of a rush.

Mike
 
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Yeah they are close to the tubes, but there were obstructions in other locations and I wanted to keep them close to hook up the resistors. My probes are very easy to get in there, although they will get hot if left there. Still better than drilling holes. I am using the stock bias pot but am going to get a new one as it is not too clean. I am surprised my tubes still are pretty close. I tubes read .343Ma/.338-.318/.312

John
 
I've had a few "shockers" over the years. Sometimes you forget just how fatal 600-700 volts fused at 3-4A could be, when it only takes ~50mA to stop your heart! (by the time you blow the 3A fuse through your body you are probably done for- see diagram below)

Actually the WORST shock I've ever received (at least by it's physical effect) was not a piece of audio electronics, but a distributorless ignition coil pack capable of 60,000 volts! I was setting the cam sensor timing on my Buick Grand National , turning the engine slowly with breaker bar while I watched the DVOM to go from 5V to 0V. But I had my other hand resting on the top of the coil pack and the instant the voltmeter went to zero it was "lights out"

I literally blacked-out for a few seconds and woke up sitting on the shop floor about 8 feet away! LOL Then I had a bad case of the shakes and was a bit goofy for the rest of the day. (my wife still refers to the day as the day of my "shock treatment" which apparently mellowed me out)

I went to the doctor the next day and he said would be OK but said even though there was probably not all that much current, that type of high voltage and discharged so quickly COULD put me into v-fib (especially as I had pretty much given it a path up one arm and down the other)

That was about 15 years ago but it prompted me to to take some training, read up on electrical safety practices and shore up my protocols and techniques to prevent making my kids a bunch of orphans! LOL

The human body's electrical resistance is a bit difficult to nail down as it depends on numerous factors such as your size, weight, fat and water content, skin moisture, etc but generally accepted to be ~1000-2000 ohms. So even a lower voltage tube amp with a 300V power supply is capable of 150-300mA into your body which is obviously serious depending on how the engagement takes place and you ability to break the circuit.

PLEASE STAY SAFE MY FRIENDS!!

WopOnTour
 

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I had a GN once, very fun car. I sold it about 5 years ago with 14K on the clock, the mats still in factory plastic bags, and the plastic peel off crap still on the radio/dash stuff.:yes: The guy who bought it couldn't get the check written fast enough.:D The worst I ever got shocked was two fingers of the same hand across a 900v TX secondary. Made a little burn hole in, and out. I couldn't really use that hand for a day or two because the muscles were so sore from clenching up. I still swear I felt the lams vibrating in my teeth.:thmbsp:
 
FalconEddy said:
30mA to 40mA accidentally running through your hand feels SO much better than 400V+ that slams into your arm like someone wound up and hit it with a creosote-soaked telephone pole, eh?
Click to expand...

That happened to me only once so far. Crabbed over a Heathkit W-4M and managed to put my elbow onto something hot while I was holding the chassis. I went straight back into my chair and onto the floor in a nanosecond. Since that, I sit on one hand when HV is present.
 
Bosch ignition coils often have a warning label on them about the risk of a fatal shock.
 
Thanks for that Wop...I know AK has plenty of disclaimers about this stuff, I wonder if we should have an electrical safety sticky with stuff like this. Both the risks and good practices to prevent shock.
 
In a previous life I used to work in the semiconductor industry. One of the pieces of equipment we manufactured was a wafer saw that had a 3phase spindle motor. The oddity was that the machine didn't need a 3phase drop. There was a section of the power supply which converted 240V 1ph to 3ph.

I would wear thick gloves to tweak this stuff. I remember one guy wasn't wearing gloves and went into one of these. He got zapped and tossed his @ss about 4 feet. They had to surgically remove his wedding band because it has fused to his finger.
 
KGBMAN said:
They had to surgically remove his wedding band because it has fused to his finger.
Click to expand...

I met someone at a car show that managed to short across a car battery with a wrench through his wedding band. It was several weeks on from when he did it but it was the worst looking wound. He said he nearly lost the finger the burn was so deep.
 
soundmotor said:
I met someone at a car show that managed to short across a car battery with a wrench through his wedding band. It was several weeks on from when he did it but it was the worst looking wound. He said he nearly lost the finger the burn was so deep.
Click to expand...

They managed to save this guy's finger but he never regained full motion in it.
 
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