AC30 speaker impedance question

[maynut]

Thanks for the insight PRR.
Yes I have no knowledge of the transformer, the whole chassis was sold as a project, which it looks like it really will be! 
It could have easily been a wrong part - the only hope I have that it is genuine is that it looks in similar condition and appearance to the mains transformer suggesting the same age/wear... which means nothing haha!

I'm going to double check my findings and measurements before I decide how to proceed with it I think...

[maynut]

Guys, I'm going to remeasure the output from the transformer again, but I fear my DMM is just not accurate at low voltages.
The highest voltage PSU I have is the 20V one I used previously.
I'm wondering if I can pump UK mains voltage through it to get a higher output which would be more accurately read by my DMM?

Tubegeek, when you described the process you said:

Connect a modest AC voltage to the primary. 12V, 24V, all ok. No need to go all the way up to line voltage. Any AC wall wart is fine.

Do you think I can use 240V 50Hz straight from the main supply?  Would it do any harm?

Thanks again everyone, really appreciate the help I'm getting here!

[tubegeek]

Could you? Yes. Would I advise a forum member whose workbench setup and safety knowledge is unknown to me, to do so?

No.

[maynut]

Touche!

Ok let me rephrase that -

If I disconnect the transformer completely from the amplifier circuit and remove it from the chassis, placing it on an insulated surface such as a wooden bench, and then connect 240V to the primary of the transformer in order to measure the output voltage at the secondary side, will the transformer be damaged (assuming it is in working order and to the spec of a typical AC30 output stage)?

I will disclaim that I have worked with mains voltages before and I shall be looking after my own safety without any blame to anyone on this or any other forum.

Thanks!

[maynut]

Actually I realised this is a silly question isn't it - we've already determined the data for an AC30 OT is based on 346V from the output stage of the amp.

I guess my question may now be, do I need to put a load on the secondary side of the OT before connecting up and measuring?

Thanks again

[tubegeek]

I just try to err on the side of caution because a) 240 is a hell of a jolt, and b) this forum attracts participants with a VERY wide range of experience. We get folks who know how to lay out circuit boards but don't know how to solder, running the gamut in a single individual!

You worked out for yourself how I knew it'd be OK.

LEFT HAND IN YOUR POCKET - you do not want to buzz your heart.

I'd put a 1K resistor across the 2ndary, sure, given the concern expressed by others for dangerous flyback voltages. If you have a 5W 100R, even better. Loading the primary with 10K can't hurt either.

Do you have alligator clip test leads? In the US, you can walk into Harbor Freight Tools and get a pack of 10 absurdly cheap - while you're there, get a $3 flush cutter - you'll always need one, theirs is not junk, and it's the best deal I've found anywhere for that item.

Not in US? The clip leads may be available at an auto repair shop.

[maynut]

Yep, completely understand, I know what you were getting at, and again I do really appreciate all the help - that's what the forum is here for right?

I think I will try with loading both sides if I can get suitable resistors, I too am cautious of that 240 jolt as you say...

It may take a little while but I will report back.

Thanks again

[tubegeek]

Load won't do.much about the jolt. Just prevent possible damage to the transformer. Good luck!

[maynut]

Ah I meant jolt to the transformer as well as me 
I shall make efforts to prevent both.
Thanks for advice again,
R

[maynut]

By the way I am still alive, have not been fried by a high voltage!

Just having trouble getting round to it what with Christmas approaching etc.

Should get to this over the coming week and will report my findings looking for advice.

Thanks!

[maynut]

Ok, I tested the OT again with mains voltage, here are my results.

Across the primary I had a 10k load rated 5W
Across the secondary I had a 100R rated 5W

Measurements:
Across the primary I get 243V
Across the secondary I get 10.7V

I'll go do some calculations and if you guys would like to calculate also to verify would be much appreciated!

Thanks again,
R

[maynut]

4k primary, 8ohm secondary? Well 7.75 but close enough...

Is this right?
Thanks

[PRR]

> Is this right?

My finger-math agrees.

4k is an about-right value for four EL84.

[maynut]

Hi PRR,

Thanks for that - yes I'm definitely working from the 4k as the primary value because that's what is listed in all the replacement OTs I have found for an AC30 or 4xEL84.  Everything including the input from yourself and the good folks here suggests that is correct

So just to confirm, trying to calculate the secondary is as follows:
voltages measured are ratio 243v:10.7v
which is also the turn ratio which simplifies to 22.7:1

then squared (22.7*22.7) is 515.76:1 equals the impedance ratio

so working from 4000 ohm primary divided by 515.76 equals secondary impedance of 7.756 or close enough 8 for a speaker.

Luckily I have an 8 ohm speaker I can test things with!  I am sure I will be back here to diagnose more problems haha!

As ever I am very grateful for your help with this!
Thanks again.