Discussion in '"Boat Anchor" & Classic Equipment' started by WA6MHZ, May 23, 2014.
We are on the same page price wise.
HAW! That's a good one!
A word about used transformers that have spent a LONG time stored in damp conditions:
Before you start making a chassis and all that, get the transformer, clean it up, and then test it. Make up a test jig with fuses and no load on the secondary, and just run supply voltage to it (since there is no load, big transformers can be tested on 120 with the primaries in parallel). Let it run for a couple of days (with all necessary precautions to protect people and pets).
The idea is to get it powered up and warm and make sure it's good BEFORE you spend a lot of time and work designing around it.
Most old transformers that have not been abused are probably still OK. But it pays to be sure!
I would still use the high voltage meter to measure high voltage. Your plate voltage would be right in the upper third.
Just add a toggle switch to switch between grid current and high voltage.
Even though you don't need to calculate input power in order to sanctify rules it is still nice to have.
Oh and I am thinking $125.00 is reasonable.
for $125 will U clean off the rust and test to verify it is good?
I can test it or we can do it together, I'll take off the surface rust.
OK all U Math wizards out there!! Lets see if I calculated this right.
I put a power supply in series with the a variable resistor box and adjusted the resistance for full scale on the meter which is 500 uA. My power supply was 127mV and I measured 49mV across the meter. I measured 78 mV across the variable resistor which turned out to be 157 OHMS, so using OHMS LAW, the meter resistance came out to 98 Ohms.
Something doesn't seem right as the heathkit meter is 0-200uA and it is rated at 1400 Ohms meter resistance 98 seems awfully low, but can't see what I did wrong!
is this correct????
There is a quick and dirty way to do this and it is accurate.
Take your meter with some series resistor.
Put the series resistor / meter combination across a power supply and carefully adjust the supply for a full scale reading.
Place a resistance substitution box across the meter and adjust the R-Box for exactly half scale.
The value on the box is equal to your meter resistance.
Well, I'm not a math wizard but I'll have a go....
Let's see.....127 millivolts = 49 millivolts plus 78 millivolts...ok so far
78 millivolts divided by 157 ohms = about 500 uA
500 uA times 98 ohms = about 49 millivolts.
So the 0-500 uA meter is about 100 ohms internal resistance.
Of course this ASSUMES the meter you used to measure the resistances has very very high internal resistance so it doesn't load down the circuit. And it ASSUMES the resistance box is accurate.
You're not measuring a 0-200 uA meter. You're measuring a 0-500 uA meter. Different meter.
1400 ohms internal resistance sounds high for a 0-200 uA meter, too.
W8JI has a good piece on his website about metering grounded-grid amplifiers:
Note that W8JI recommends directly grounding the grid of a grounded-grid amplifier, and ungrounding the negative high voltage for metering purposes. If done as he recommends, this is the optimum way to meter a GG amp - but you have to do EVERYTHING he says to do.