# Checking an OT for Usefulness



## bcmatt (Aug 25, 2007)

Are there some tests I can do with a DMM to find out about an OT that I have? I would like to put it to use in a future build, because I am poor, cheap, and hate waste, but I seem to be addicted to building amps.

It was sent to me by accident by Ceriatone at one point and I was told to just keep it.

It is the same dimensions and weight as the OT from my DC30, so I imagine it is meant to handle up to 30 watts easy.

So, if I am able to find the primary impedance, will that be all the information I need? Is there a method that I can use with a DMM to find this out?


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## greco (Jul 15, 2007)

Thanks for starting this thread....I'm looking forward to learning from the responses.

Dave


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## Wild Bill (May 3, 2006)

bcmatt said:


> Are there some tests I can do with a DMM to find out about an OT that I have?
> 
> It is the same dimensions and weight as the OT from my DC30, so I imagine it is meant to handle up to 30 watts easy.
> 
> So, if I am able to find the primary impedance, will that be all the information I need? Is there a method that I can use with a DMM to find this out?


First off, the weight does indeed provide a useful guess as to power capacity. More power needs heavier iron. Also, bass OTs need to be about 30% heavier than an OT for a regular guitar. This works both ways. A PA or bass guitar OT can be safely pushed for about 30% more power when used in a regular guitar amp.

As far as using your DMM, go to this link:

http://www.aikenamps.com/

On the left, click on "tech info", then "Advanced", then "Output transformers explained".

The article will explain how an output transformer is like a voltage transformer. It will step an impedance up or down, IN A FIXED RATIO!.


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## Wild Bill (May 3, 2006)

bcmatt said:


> Are there some tests I can do with a DMM to find out about an OT that I have?
> 
> It is the same dimensions and weight as the OT from my DC30, so I imagine it is meant to handle up to 30 watts easy.
> 
> So, if I am able to find the primary impedance, will that be all the information I need? Is there a method that I can use with a DMM to find this out?


First off, the weight does indeed provide a useful guess as to power capacity. More power needs heavier iron. Also, bass OTs need to be about 30% heavier than an OT for a regular guitar. This works both ways. A PA or bass guitar OT can be safely pushed for about 30% more power when used in a regular guitar amp.

As far as using your DMM, go to this link:

http://www.aikenamps.com/

On the left, click on "tech info", then "Advanced", then "Output transformers explained".

The article will explain how an output transformer is like a voltage transformer. It will step an impedance up or down, IN A FIXED RATIO!

You need a small wall -wart power suppply, one of those cubes that plugs directly into the wall socket. You need one that delivers AC volts and NOT DC! DC will fry your OT. Anything from 6 to 12 volts AC is fine.

You feed the low voltage AC into the speaker wires on the OT. If there are more than one wires for 4,8, 16 ohms or whatever just pick any one. You don't even have to know which is which, although it would save you time.

You then measure the stepped up voltage appearing on the plate leads of the output trannie. If it's a pushpull with a centretap just ignore the centretap lead. Use insulated 'gator leads or something 'cuz those plate wires will be showing several hundred volts and that can hurt!

If 12 vac gets stepped up to 240 vac then you have a voltage ratio of 20:1. Once you know the voltage ratio you can plug this into the equation in Mr. Aiken's article and figure out the impedance ratio.

Zp/Zs = (Vp/Vs)^2

Zp = trannie primary impedance

Zs = trannie secondary impedance

Vp = primary voltage

Vs = secondary voltage

Vp/Vs is your voltage ratio. In the above example it was 20. You square it, making it 400. Zp/Zs = 400. If you assume 8 ohms or whatever you plug in as Zs then you will have your value for the primary impedance.

I can never remember how to figure square roots and stuff so I just made an Excel spreadsheet titled "Unknown OTs". I set it up so that I can plug in the voltages and then plug in an arbitrary speaker value. For example, I might plug in "4" for the ohms and the spreadsheet will show that the primary impedance would then be maybe something like 6600 ohms. This is in the range for a pair of 6L6s. If I get something way too high or too low to be likely then I try subbing "8" or "16" to see if I get a primary value that makes sense and is useful. This way I can quickly determine which speaker tap wire is which and what the primary impedance will be. If the values all like to come out at an impedance more like 3400 ohms then the tranny was likely designed to be used with a pair of EL34's.

6V6s and EL84s like to have a load in the range of 6000-8000 ohms.

Don't forget, if you are using 4 output tubes you need a primary of only half as much. That 3400 ohm tranny could also be used for 4 6V6s, running 2 pair in parallel on each side. A 100 watt EL34 Marshall OT would likely be rated at 1700 ohms for its primary.

Remember also to go with what ever primary values are close. Don't get all anal about being a few hundred ohms off. The tubes don't care so why should you?

Hope this is clear enough!

:food-smiley-004:


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## bcmatt (Aug 25, 2007)

Yes! Thank You Wild Bill! I'd say this is a legendary post you just made!

I'll hopefully get to give it a try today and put this into practice. I'll post my results step by step so others can see your system in action.


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## bcmatt (Aug 25, 2007)

Well, now that 2 and a half years have passed, I finally did this and using a 12VAC wall wart (13VAC when I measured), I got 426V on the 4 ohm tap, 308V on the 8, and 214V on the 16.
So, that gives me a primary impedance of 4k3 on the 4 and 16 ohm taps, and 4k5 on the 8 ohm tap.
Do, you suppose that this was made to be for a quad of 6V6s or El84s?

So, now that I am very poor, but want to build a Herzog, you'd think I should be able to use this somehow to power a single single-ended 6V6. I guess it would just require the appropriate size/value resisters on the appropriate taps...


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## WCGill (Mar 27, 2009)

This is a common value for 4 6V6's or EL84's. You *might* be able to use this transformer for your Herzog but single-ended transformers are designed with an air gap which prevents core saturation at idle because the full DC current flows through the transformer under no-signal state. Push-pull amplifiers operate differently and consequently use transformers without the air gap and tend to be smaller in size. I'm betting your transformer would saturate, resulting in considerable distortion, and not of the kind you would like to hear.


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## bcmatt (Aug 25, 2007)

WCGill said:


> This is a common value for 4 6V6's or EL84's. You *might* be able to use this transformer for your Herzog but single-ended transformers are designed with an air gap which prevents core saturation at idle because the full DC current flows through the transformer under no-signal state. Push-pull amplifiers operate differently and consequently use transformers without the air gap and tend to be smaller in size. I'm betting your transformer would saturate, resulting in considerable distortion, and not of the kind you would like to hear.


Thanks for the insights.
Ya, After some reading (and sleeping on it), I think it is much easier to spend my money on specific trannies for the Herzog and save this OT for something down the road that comes up. It's not worth the trouble and potential hazard to save the 30 bucks I'd pay for an appropriate Herzog OT. 
I suppose with multiple secondaries, it could be adapted for 2x EL84 or 6V6 amps as well (18 watt, Tweed Deluxe) and limit(change) the speaker impedance choices.
I am pretty sure this is likely what Ceriatone uses for the DC30, which I believe is what also is used for the 2x EL34 Hot Cat as well...

I notice that the Tweed Bassman uses primaries about this value too, and am thinking this might be just on the edge of being big enough for that...


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