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dscottguitars 11th June 2012 01:07 AM

Output transformer and shock from the speaker jack
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I have a couple of questions:

1.) I got a little shock from the speaker jack when I pulled it out to switch speakers in my cabinet. (I know you're not supposed to run the transformer without it being plugged in but I'm fast enough that I can do it.) Is this possible? I then plugged an 8 ohm resistor (dummy load) and tested the voltage at ~18.5v which is what I expected. Then I switched the meter to current, and got 4.2 amps. That is not right according to the math.

2.) The transformer is 4 and 8 ohm for a push pull setup, the power section is close to a Bassman clone. I am having trouble determining which lead is which. The resistance between the two secondaries is .7 ohms as is the resistance between the green secondary and ground. This tells me that would be the center tap and such would be the 8 ohm lead. The resistance between the yellow and ground is .9 ohms which tells me it is longer and should be the 4 ohm tap. It definitely sounds better in my 8 ohm speaker with this configuration, but with the little electric shock mentioned above I'm really not sure. Or sure that it's related.

Any help would be great,


dscottguitars 11th June 2012 01:13 AM

ignore first schematic
1 Attachment(s)
there is no speaker polarity switch anymore...

kevinkr 11th June 2012 03:05 AM

Good way to fry your output transformer, when unloaded abruptly the way you are doing it the primary will go into flyback mode as the field collapses and the voltage across the primary will go way up - this happens in milliseconds. The end result could be to destroy your output transformer. Think about what an inductor with a large amount of current flowing through it wants to do when you suddenly remove the current. Turn the volume/master gain down first!

Measure current by inserting the meter in series (in current measurement mode) with a load resistance or just calculate it based on 18.5V/8ohms which is a little over 2.25A.. Switching your meter to current is essentially placing a short across the output of your amplifier - hardly benign..

Might want to bone up a little on basic electronics before going too much further - imho you've been lucky so far. :D

dscottguitars 11th June 2012 01:44 PM

Thanks Kevin, I won't do that anymore. But what about the questions...

chrispenycate 11th June 2012 07:30 PM

Eighteen volts is enough to give you a tingle, but it's more likely you cut in the middle of a cycle, and the collapsing field in the transformer gave you fifty or sixty volts. Just think of what was stepped up and reflected back to the tubes, and be gentle with your output stages.

That diagram is wrong. The eight ohm tap should have higher voltage than the four, thus more turns. If you've got an oscillator (doesn't have to be a great one) you can run it into the output of the tranny (with the amp turned off; the anodes aren't going to give any reading while cold. Great, tubes, aren't they?) and you can measure the voltages at the various taps. Cold to eight ohm is the highest voltage, cold to four about two thirds of that (yes, I know one over square root two, but I'm not calibrating to a tenth of a dB for broadcast authorities here.)
But speaker impedances are rarely all that precise (or regular), and amps put up with this. Make it sound right.

Osvaldo de Banfield 11th June 2012 07:38 PM

To staying secure that the amp will not be run unloaded, replace the output jack with some that short the amp output. It is preferable a short for pentode output than an open circuit.

DF96 11th June 2012 08:41 PM


Originally Posted by dscottguitars
I'm fast enough that I can do it

You can switch output plugs in less time than the transformer insulation takes to break down? I'm not sure if that is ms or us, but you must be very fast!

If you get a small shock from the secondary, think how much bigger the primary voltage will be.

dscottguitars 11th June 2012 08:45 PM

Thanks everyone...

I now understand how I can get a little more than a tingle of shock with it being unplugged and in the middle of the cycle. Sounds like that would be normal as I shouldn't pull the speaker plug with it on, for more reasons than OT frying.

Chris: about the diagram, are you referring to the drawing of the OT and the way I have the 4 & 8 ohm taps drawn in? I didn't really think about correctness, only to show that it has two taps. And can one tell the impedance of the tap being used by measuring the voltage coming out if I use an 8 ohm resistor and select the four ohm tap on the switch and then the 8 ohm tap? I don't have an oscillator to use. So I am thinking that the voltage coming out will be different if I have it switched in 4 ohm mode with a 4 ohm resistor, than in 8 ohm mode with an 8 ohm resistor.

Printer2 11th June 2012 10:09 PM

I wire a small 100 - 200 ohm resistor across the output jack. The voltage will climb of disconnected but not as bad as an open circuit.

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