Anyone know what's going on with my OT

[Guitarman18]

A number of years ago I was given a 10W, 8 ohm Schumacher OT as a wedding present (?) which I promptly used in a Champ clone.

This transformer has now found it's way into one of my latest projects which is a SE dual channel amp.

The amp was sounding pretty thin and tight until I plugged it into a 4 ohm cab which gave it great bass extension and focus.

Could it be that this OT was always supposed to be 4 ohm and I was misinformed at the time? If I continually use it in 4 ohm operation(if it is indeed supposed to be 8 ohm), will I kill it?

Any thoughts would be greatly appreciated.

Many thanks, Paul.

[jjman]

Google the part # on it yet?

[Structo]

Is the transformer strictly a single ended output transformer?
Does it have a center tap on the primary winding?

What is the power tube(s) you are using?

The power tube has a impedance that should be matched (somewhat) to the primaries of the OT.

6V6 have around 8K output impedance.

6L6 have around 4K output impedance.

So when you tried the 4 ohm cab, you probably simply matched the impedance better so it became more efficient and sounded better.

[Guitarman18]

I did google it (EIA 606 -- 130 024038) and got as far as finding out that it was Woodward/Schumacher but have had no success finding out what the other numbers tell me.

Can anyone point me in the right direction for Schumacher codes?

I was told that the impedance was 5K and so would work with either 6V6 or 6L6.

Cheers, Paul.

[Andy Le Blanc]

A transformer is an impedance matching device, between the tube and speaker.
If you have access to a signal generator you can measure the thing
and find out just what the conditions are that you have created. SE trannies
also have a max rating for current, it might have something to with the
operating conditions you've set up for the tube as well.

[Guitarman18]

Thanks for the responses.

You got me thinking (=danger) so I've been looking over my books and see that the impedence ratio of a 6L6 with @ 320v on the plate gives 537:1 (4300/eight) which is pretty close to the tranny ratio (5000/eight=625)

However the 6v6 I'm using would have an impedence ratio 1062:1 (8500/eight). Does this tell me that the tranny wants to see a 16ohms speaker when using a 6V6? (8500/16= 531.25)

Have I got this right?

Cheers, Paul.

[Sonny ReVerb]

I believe that's correct. 6V6's like to see around twice the impedance of the 6L6's. So, say you have a 6L6 amp with 4 and 8 ohm speaker taps. If you sub in 6V6's, you should consider the taps as 8 and 16 ohm.

[Guitarman18]

Sonny ReVerb wrote:

I believe that's correct. 6V6's like to see around twice the impedance of the 6L6's. So, say you have a 6L6 amp with 4 and 8 ohm speaker taps. If you sub in 6V6's, you should consider the taps as 8 and 16 ohm.

Thanks for that Sonny. That now leaves me with a dilemma as it sounds infinitely better running it at the wrong impedence in this amp.

Would you:

a) set the correct impedence and redesign the amp to suit?

b) change the OT? ie. start again

c) operate it at the wrong impedance and wait for it to melt?

or d) any other solutions?



BTW I haven't tried putting in a 6L6 yet. I will try that when I get the chance.


Cheers, Paul.

[Alexo]

I would measure the actual impedance.

Hook a load resistor on to the secondary and plug a wall wart AC adaptor across the primary. Let's just say you end up with 18 volts across the primary and .75 on the secondary, 18/.75 = an impedance ratio of 24 to 1. 24 squared = 576. So each single ohm you would put across the secondary would give you 576 ohms impedance on the primary, an 8 ohm speaker would reflect 4k6, 16 ohms would reflect 9K2, etc...

Then you can see just how far "off" you actually are.

[Guitarman18]

Thanks Alexo, thats a great idea, I'll give it a go and report back.


Cheers,

Paul.

[Sonny ReVerb]

Here's some more reading to get you thinking:

http://www.diyguitarist.com/GuitarAmps/ ... edLoad.htm

http://www.harpamps.com/transformers.html

http://www.radioremembered.org/outimp.htm

The last one is from a radio perspective, but the basic principles apply to our amps as well.

[Guitarman18]

Sonny ReVerb wrote:

Here's some more reading to get you thinking:

http://www.diyguitarist.com/GuitarAmps/ ... edLoad.htm

http://www.harpamps.com/transformers.html

http://www.radioremembered.org/outimp.htm

The last one is from a radio perspective, but the basic principles apply to our amps as well.

Great links . Thanks for that.

I just need to find myself an AC producing wall wart.

Meanwhile, anyone have any particular preferences of cathode bias resistor values for a SE 6L6 @300v?


Cheers, Paul.