Multi-Tap Output Transformer Question

[harry]

Hi there,
I'm working on rebuilding a PA head. When I bought it, the previous owner said it required a 4 ohm load.

Looking at the output transformer, there are 10 connections.

on one side:
1/2 are from the power tubes
3 is B+
4/5 are from the power tubes


on the other side:
6 and 7 are not connected.
8 is connected to the speaker jacks and ground.
9 is connected to the speaker jacks
10 is not connected, but looks like it once was.

(In addition there is a 1.8k resistor across 8 and 9 - is this to protect the amp in case there's no speaker connected? It doesn't look like it could handle much power - I'd guess it's a 1 watt. I have run the amp twice with no speaker connected by accident, and it wasn't damaged, so it seems to work.)

Anyway, my main question is - could connection 10 be an 8 ohm tap? Is there any way I can tell?

Harry

[Kramusha]

You could try to measure the DC resistance of 10 and 8. If the resistance is higher than the resistance of 9 and 8, the possibility is given.

Stefan

[harry]

Well, according to my (bad) multimeter:

the dc resistance of 9 and 8 was 0.9 ohms
the dc resistance of 10 and 8 was 1.2 ohms

I bit the bullet and tried it (obviously it had been connected and someone had cut the wire). Using an 8 ohm load the amp seems slightly louder but breaks up slightly sooner on the volume control (having said that, this could be due to my speaker distorting)

[harry]

Oh, also, there was feedback to the preamp coming from the 4 ohm tap. If I change the speaker connection to the 8 ohm tap, is it alright to leave the feedback on the 4 ohm tap?

[Kramusha]

Yes, that is ok.

To measure the transmission ration of the OT, you can put the 6,3V from the heater and connect it to the secondary. Be careful of the high voltages at the primary side.

Stefan

[harry]

Excellent, thanks

Does anybody know what kind of rating an impedance selector should have? Would I be ok with a 5A 250Vac switch?

Harry

[Kramusha]

I think yes, because we say, you have a 100W Amp and a 4 ohm load, the current is exact 5A. But, for a lifelong decision, I would take a stronger switch.

lg Stefan