Impedance switch

[mmaatt25]

Hi,

Not strictly stompbox territory I know, but I trust whats said here over anywhere else on the internet. I made 5F1 amp and wired the output for 8ohms. I want to use the amp with a 16ohm cab.
It's easy enough to wire the amp for 16ohms by just switching the tap on the transformer. But I want to make it switchable between 8ohms and 16ohms.
I've wire a 10 amp SPDT on-on switch L1 & L2 to the 8ohm & 16ohm taps and then wired the COM of the switch the tip of the output jack, making it switchable between 8 and 16ohms at the output.
I've checked the wiring with my DDM set to continuity and no matter what the switch position is I get continuity a both taps. I'm sure this is because of the transformer and that the taps are just different parts of the same winding, so I will get continuity.
The things is I'm not sure enough to turn it?? Have I got this right?

Many thanks

Matt

[defaced]

Yes, you can do this, and yes you've got a the correct approach.  If you post wiring diagram of the OT and pics of what you've got someone here can proof it for you.  Or if you're pretty sure you're right....

To test your setup, plug everything in except your guitar.  Turn the amp volume all the way down, then power up the amp.  Very slowly turn up the amp volume until you hear the noise of an unplugged guitar out of your speaker.  If you don't hear it after the volume is up where you know the amp should be giving you *something*, turn the amp volume back down, turn off the amp, and check your wiring.  No you're not supposed to do this, but by keeping the amp volume down you significantly reduce the risk of flyback voltage killing your OT.  For built in protection, a 125 ohm or so 5w resistor across the 16ohm winding will provide enough load in an emergency to keep your OT from dying.  

[mmaatt25]

Wired as stated, connected amp to my 16ohm, switch to 16ohms and all seems OK, played for 20-30 mins with the amp flat out.

Is there any way I can check that the switch is actually switching between the taps??  I know that its possible to mismatch with no immediate ill affects, but it's the fly back voltage that does the damage.

Thanks

Matt

[defaced]

Yes, but it's not super easy.  The way I would do it is to run a low voltage AC signal through the secondary and measure the corresponding voltage on the primary.  Switching taps will change the primary voltage.  You would need to do this with the tubes removed, power removed, basically have the OT isolated from the rest of the circuit. 

[mmaatt25]

OK,

I think I'm going to rely on the fact that the 16ohm tap goes to L1 of the switch, the 8ohm goes to L2 of the switch and the COM of the switch goes to the jack tip, I'm not religious but I'm going to use faith this is right!  

Thanks for your help Mike

Matt

[defaced]

You're a wise amp builder.  And you're welcome, enjoy your amp!

[PRR]

> check that the switch is actually switching between the taps??

1) Replace the transformer with two resistors, 8 ohms and 16 ohms. Put meter where speaker will go. Switch, and confirm that you get the right reading in the right setting.

2) Transfer the wiring to the transformer, being careful what-goes-where.

This assumes the transformer is marked correctly.

"Continuity" is a hasty-check. Good for lamp-wiring, maybe.

Learn to use the Ohms function and have a clue what to expect.

Yes, in this case the transformer's DC resistance (what you read on the ohm-meter) is much-much less than speaker impedance, therefore around 1 ohm, and many meters do not read such low resistances well.

If you can tell a difference 8 or 16 tap, such as 0.9 ohms and 1.3 ohms, then you can test for this at your switch and speaker-jack. However it is very hard to make a reliable sub-Ohm contact with standard probe tips. And some meters round-down anything under 2 ohms.

As Mike says, you can also feed low AC voltage to one tap, check voltage at other tap(s), then check voltage at switch. (Tip: from 8 to 16 ohms is 1.414 times voltage, not 2X voltage.)