# How to determine correct load impedance for an amp



## Gizmo (Aug 7, 2008)

I have a prototype amp made by master amp designer Peter Belov.

It uses a prototype torroidal output transformer with no markings and the single speaker jack has no label.

I use both 8 and 4 ohm cabs, no 16 ohm, so I should be ok but I would like to se if there is a way to determine the correct load impedance...anyone got any ideas??


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## nonreverb (Sep 19, 2006)

Gizmo said:


> I have a prototype amp made by master amp designer Peter Belov.
> 
> It uses a prototype torroidal output transformer with no markings and the single speaker jack has no label.
> 
> I use both 8 and 4 ohm cabs, no 16 ohm, so I should be ok but I would like to se if there is a way to determine the correct load impedance...anyone got any ideas??


Here's a good site outlining the proceedure. You'll need a voltmeter and a Variable AC supply such as a Variac.


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## greco (Jul 15, 2007)

I think *nonreverb *forgot to put in a link.

Here is a thread from this forum that explains this topic in detail:

http://www.guitarscanada.com/amp-building/31359-so-ummm-more-impedance-questions.html

You will ned a low voltage AC transformer (or an AC wall wart) for this method.

Cheers

Dave


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## Gizmo (Aug 7, 2008)

Hey guys, thanks...it looks pretty straightforward. Wonder why I didn't find it.


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## greco (Jul 15, 2007)

* PLEASE BE CAREFUL *...putting only a few volts of AC into the secondary of the OT and measuring the primary can show a VERY significant increase in voltage !!

Good luck with it.

BTW..I can put your numbers into the formula if you want to send me a PM.


Cheers

Dave


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## Gizmo (Aug 7, 2008)

Oh yeah...been there done that. I still have the twitch as a result of 2kV knocking me across three benches in the first year of my Elec Eng degree. 

I still remember the days as a young apprentice being thrown a large cap that had been charged up by some twit...I guess I was the bigger twit 'cos I caught it...ouch.

Warnings always appreciated and thanks for the offer of the spreadsheet Dave.




greco said:


> * PLEASE BE CAREFUL *...putting only a few volts of AC into the secondary of the OT and measuring the primary can show a VERY significant increase in voltage !!
> 
> Good luck with it.
> 
> ...


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## greco (Jul 15, 2007)

So Sorry...I wouldn't have written this If I had known you are an EE.

Cheers

Dave


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## nonreverb (Sep 19, 2006)

greco said:


> I think *nonreverb *forgot to put in a link.
> 
> Here is a thread from this forum that explains this topic in detail:
> 
> ...


Oooops....my bad. Thanks Dave


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## Gizmo (Aug 7, 2008)

Hey Dave, no..I meant it...warning reminders are always welcome....often its the most experienced people that get complacent...please don't stop.
Over my drill press in the basement I have a big sign that says "SAFETY GLASSES!!!"...sometimes we don't stop to think.

Gizmo


greco said:


> So Sorry...I wouldn't have written this If I had known you are an EE.
> 
> Cheers
> 
> Dave


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## nonreverb (Sep 19, 2006)

Gizmo said:


> Hey Dave, no..I meant it...warning reminders are always welcome....often its the most experienced people that get complacent...please don't stop.
> Over my drill press in the basement I have a big sign that says "SAFETY GLASSES!!!"...sometimes we don't stop to think.
> 
> Gizmo


lol...got one of those "reminders" today with a Bassman I was servicing.


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