Download Measuring PT capabil..

Wednesday, February 20, 2002 9:35:55 AM
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Ross M. ([email protected])
2/19/2002 6:03 AM
Measuring Transformer Specs - please help
I was planning to use a salvaged transformer to power my AX84 November project, but I don't know if i
could do that anymore. I still would like to measure what I can get from it though so that I can use it in a
lower powered project - however I'm having some difficulty.
I hooked it up to a 5Y3GT Rectifier tube to see what kind of DC voltage and current it would give me.
According to all of the schematics I've seen, you pull the DC off of pin 8 - one of the filament pins. I've
checked and double checked that everything is connected right, but when I go from pin 8 to the CT of the
Transformer all I get is about 130VDC and 300VAC.. I'm not sure what is coming off of there. The
transformer is said to be 260/0/260 in the schematic, but I pull about 270VAC off of each leg because of
higher wall voltages. What am I doing wrong? Once I get it right, how do I measure the current (I've
already blown a pocket meter trying to do this wrong). Would I have enough power for the November with
this transformer?
I'm sorry to crowd the board with so many newbie questions over the past few months, I just really want to
know how this stuff works. I'm going to borrow some old tube books from my uncle who is a retired
Electrical Engineer and he thinks I'm crazy trying to mess with tubes now that there are these "nice simple
transistors and microprocessors" for so cheap. Nice for building missles (his former occupation) but not
really guitar amps I told him.
Thanks,
Ross M.
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Tristan Money
2/19/2002 7:16 AM
Re: Measuring Transformer Specs - please help
Hey Ross, I'm 17 too, and about half way through a November, using a salvaged OT and PT. I took them
from a similar 2xEl84 amp. I'm using a 6CA4 for my rect., and getting about 340vdc from a 260-0-260 PT.
(after the first filter cap and a 47 ohm resistor)
It sounds to me like there is a mistake in connections, or maybe the rect. is bad.
Once you get it working, you could measure the current by taking a resistor selected for about 150 mA (in
my case it would be 340V/.15A = 2266.6 Ohms), and placing it from B+ to ground. See how much the
voltage drops when you do this. If it drops too much (beyond 100 volts or so) then I wouldn't use it. I might
note that this resistor will be taking 51 watts, but a 10 watt will take it for a few seconds. I'm sure there is a
better way, but this works.
You really can't use a meter to determine how much current your PT can handle. If you tried to measure
the current directly between B+ and ground then you caused a direct short.
People think I am crazy to work on tubed stuff too, but it's great since alot of people give me NOS tubes
and old tube radio's because they think they're worthless.
Good luck,
Tristan
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SpeedRacer
2/19/2002 8:08 PM
Wednesday, February 20, 2002 9:35:55 AM
Subject:
Re: Measuring Transformer Specs - please help
you're on the right track IMHO Tristan. The "right" way IMHO is to gradually swap resistors and then let
the PT adjust to the load and check the temp after say an hour or so. If it gets too hot to touch, you need
to draw less current.. see how much current you can draw w/o dropping the B+ a whole ton and/or
cooking the PT.
If you put the time in up front, you can save yourself a headache down the road.
Have fun!
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Enzo ([email protected])
2/19/2002 8:31 AM
Re: Measuring Transformer Specs - please help
Did you make a complete circuit for the DC supply? Or did you just rectify it and stop. Unfiltered DC reads
very low on a meter. For any meaningful readings you need to filter it at least.
You have it wired right. Well, you do if the two hot leads are going to the two plates in the 5Y3. The
filament is lit with the 5volts? HV CT to ground. That ought to yield good unfiltered DC. Unfiltered DC is
DC pulsing off and on. Your meter will give you an average which will be half - or thereabouts - what it
would be if filtered. If you measure the pulsing DC on the AC scale of your meter, it will measure the AC
component. In this case that will be substantial. It is common practice to put a meter on AC to measure a
DC supply for ripple.
PUt a cap on it and see what you get.
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Ross M. ([email protected])
2/19/2002 8:37 PM
Re: Measuring Transformer Specs - please help
The rectifier is good, I've checked it with another. I bet that it is because it is unfiltered, besides that I was
measuring HV to CT, and the CT wasn't grounded. I can't filter it properly because I don't have a capacitor
to do it with right now, I was just taking some readings with it. The filament was lit with 5V 2A, and all of
the connections were proper. I suppose I'll have to get a proper DC filter circuit and then mess with
measuring current and DC voltage. I'll try and reform a capacitor tonight to smooth off those ripples and
see if I can get a proper DC reading.
Thanks,
Ross M.
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tommy ([email protected])
2/20/2002 4:20 PM
Re: Measuring Transformer Specs - please help
Even a 0.1uF will act as a filter so your meter can read the peak value. 270*1.414 = 381VDC.
Put the 400V or larger rated cap from rectifier output to tranny Center tap. Put your meter across the cap.
You'll get much more than what you got with no cap at all.
Chris
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