Help with matching speaker impedance to old tube amp

[idebb]

Hi!


I have an old tube amp that I´d like to connect to an 8 ohm guitar speaker. The amp has four speaker output jacks. Two of them are labeled 8 ohms and the other two are 4 and 16 ohms.

It´s a Dynacord MV17 from the sixties, and doesn´t have a user manual, obviously.

In the past I have connected the 8ohm speaker to one of the 8ohm outputs, leaving the second 8ohm output alone. But I´m beginning to worry that this will cause an impedance mismatch if I don´t use two 8ohm speakers connected at the same time to each of the outputs.

When opening the amp up it seems that the two outputs positive terminals are connected and so are the grounds. This means that they are connected in parallel, right? If so does this mean that they are in fact are 16ohms together?

If anyone could shed some light on this it would be much appreciated!

Thanks alot!

PS I´ve attached the schematic if it helps

[rknize]

Boy...I see your confusion. Ignoring the wiring and just comparing the test voltage levels on the schematic at each of the OPT taps, what they label as an 8 ohm tap does indeed appear to be an 8 ohm tap. Why they have two of them? Good question. Maybe so you can hook up two 16 ohm speakers in parallel? Maybe they got a good deal on quad terminal boards? Who knows.

[bob91343]

It appears that it's just two wires going to the same place. Eight Ohms, either one, but maybe there is some flexibility desired.

[idebb]

Good thoughts! It seems a bit clearer to me now! Thanks!

rknize...I´m curious how you see the impedances by comparing the test voltage levels...(you mean the 16000mV 11000mV and 8000mV readings right?) How did you translate that into Ohms?

[Eli Duttman]

Originally Posted by idebb

Good thoughts! It seems a bit clearer to me now! Thanks!

rknize...I´m curious how you see the impedances by comparing the test voltage levels...(you mean the 16000mV 11000mV and 8000mV readings right?) How did you translate that into Ohms?

THE Law, specifically Ohm's, "connects the dots". V = IR

[idebb]

Ok but there are two unknown factors, impedance and current. How should I get impedance knowing only voltage.

I´m quite new to electronics so sorry if I´ve missed something obvious...

[Hi-Q]

I think the circuit is a bad copy and the outputs are 4, 6, 8 and 16 Ohms. They have connected the 6 and 8 ohms to the same tapping as this can drive both impedances.
So you are safe just to connect to the 8 Ohm tap with your 8 Ohm speaker.
Les

[DigitalJunkie]

Originally Posted by Hi-Q

I think the circuit is a bad copy and the outputs are 4, 6, 8 and 16 Ohms. They have connected the 6 and 8 ohms to the same tapping as this can drive both impedances.
So you are safe just to connect to the 8 Ohm tap with your 8 Ohm speaker.
Les

I'm inclined to agree.. looks like a blurry '6' to me.

[rknize]

Originally Posted by idebb

Good thoughts! It seems a bit clearer to me now! Thanks!

rknize...I´m curious how you see the impedances by comparing the test voltage levels...(you mean the 16000mV 11000mV and 8000mV readings right?) How did you translate that into Ohms?

Impedance ratio is the square of the turns ratio and the turns ratio of each secondary tap relative to each other is given by those voltages. So the 16 ohm tap has 4x the impedance of the 4 ohm tap but only double the voltage (turns ratio).