Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => AmpTools/Tech Tips => Topic started by: New2ampsman on February 21, 2011, 01:04:14 pm
-
I'd like to test the resistors and caps in an old Princeton Reverb Amp. I already own a fairly good auto-ranging DMM but I'm thinking of buying a Digital Auto-ranging Capacitance Test Meter and a Digital Auto-ranging clamp type DMM for checking live AC / DC currents and voltages. The B&K Precision units look good and the cost is not a concern for a hobby I enjoy but want to be safe in.
Does any one know if these instruments give realistic and meaningful readings for working on and testing tube amp circuits especially with the amp using"live voltages" ? Will they allow small currents to be accurately read i.e in the mA) range, without disconeecting components or is this wishful thinking
Thanks
-
You just need Volts and Ohms.
Caps are mostly right, or else they leak SO much you see it in the DC Volts readings. (I have never owned a cap-meter.)
Manual-ranging is often better.
Needle-meters are often better for our work than digytal, but that's a whole other topic.
WHY would you read current? Even when I had current meters, I rarely used them. Small stages, the current is obvious from the resistance and voltage. BIG power tubes are an issue; modern fad is to put 1 ohm resistors under the cathode (assumes a DMM is handy). Wall-cord current.... bah, if it isn't smoking or blowing fuse, who cares?; if smoking/blowing, don't measure, fix.
> an old Princeton
It was developed, built, and serviced with much "cruder" meters than a DMM. Often older test-point voltages are given for a "20K/V" _passive_ volt-meter. Only later did a 11Meg VTVM became common enough that most servicemen had one. Caps were "tested" by observing DC voltages, and by having known-good 0.1uFd and 22uFd 400V caps with clip-leads for speedy substitution.
Get a lamp-limiter, a second meter ($3.99 is fine), and a dummy load.
-
Hey PRR. Thanks for the confirmation. I've been doing it right for years.
A quick reading voltmeter is essential.
-
Thanks fellas ( I'm assuming you're fellas) but I think I showed my ignorance again - oops!
Ohm's Law is simple yet it's easy to forget its significance in so many situations. I'll keep on a-readin' the books. Hopefully the penny will drop BEFORE I electrocute myself!