Tube Amps / Music Electronics
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|9/24/2000 1:05 AM|
||Blowing up DVOM|
Hey Guys, Tell me am I doing something wrong I have ruined 3 DVOMs in the last 4 days. It only happens on one amp that I try to check voltage on the OT leads from the tube. I did get an arc on one so I figured turn it to standby and then turn it on when i turned it off and pulled the lead it arced again all I wanted to do is get B+ voltage so I could set bias.also the amp does'nt bother my simpson 260-4 meter. What gives. thanks DonJ
|9/24/2000 2:17 AM|
are you sure that:
a) you're not using the "ammeter" or "A" or "mA" jacks on the meter? those are for current measurement and are low impedance.
b) the meter is rated to the voltage you're asking it to measure?
you shouldn't be getting arcs when you apply the electrodes of a voltmeter. a little spark, maybe, but SMALL.
|9/24/2000 3:48 AM|
I've blown up three DVMs in the last two and a half years and every time it was while measuring the AC output of a 50 watt OT, running into a dummy load at near full power.
All three times I tried to measure the AC, I did not shut the amp down first and connect the leads before measuring anything, I mearly turned the amp up and touched the probes to the plate lugs of the power tubes...Poof!
And yes, all three times the arc was very obvious.
I never do that anymore!
The first was a medium priced Radio Shack bench DVM meter I've had for years and the other two times, they were new 1000v 27XT Waveteks.
Wavtek replaced them free both times by the way.
Radio Shack told me to suck eggs and pound sand.
|9/24/2000 4:57 AM|
I think we have the same test bench! I've had one of those little rat shack meter over here for years. Never had a problem with it. I was sure I killed it when I got careless once. After I got all the glass from the fuse exploding picked out of the innards it worked fine.
The trick when you buy stuff from the shack is to make sure you get the 5 yr warantee. Something goes wrong and they'll replace it no questions asked. I can't tell you the number of those litte metal shelf speakers I've gone thru!
|9/24/2000 7:36 AM|
The trick when you buy stuff from the shack is to make sure you get the 5 yr warranty.
I picked up a set of earbud headphones from RS a few years ago to use with an ultrasonic leak detector. They were on sale for $14.95 and the salesman tried to sell me an extended warranty on them for $19.95. When I asked him why I would want to pay more for the warranty than the item himself he explained that I could throw them down on the floor and step on them, and they would give me a new pair for free...
But for something like a DMM it probably would be a good idea... if you remember where you put the sales receipt and warranty! Best Buy has a replacement plan based on the sales price of the item so it's like $10 for an item under $100 (this is for the 2 year replacement plan). Definitely recommended for something like a CD player which can wear out...
P.S. Measuring the DC volts on the output tube plates can be very hard on meters (possibly because of the AC component in the B+???) My RS meter will go into fits, automatically switching from a dc voltage range to an ohms range (and this is a model with a manual range switch!) I don't think that they test these DMM's on tube amps at all, but just on ss circuits with a well-filtered power supply and much lower voltages...
|9/24/2000 5:19 AM|
I blew up a couple of Waveteks also, then I bought a VTVM , less than $10 on ebay. The analog needle is more convenient for some measurements too.
I also upgraded to a Wavetek HD160, which I have been unable to blow up. I even inadvertently probed -3500V on a scope I was fixing and it lived through that. Only problem, cost like $200 I seem to remember.
|9/25/2000 2:15 AM|
I also have a HD160 and it's a very nice meter!!
I heard Wavetek was bought out by Fluke and the HD100 series is their new low cost commercial stuff now.
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