Output Impedance

[jaykick]

Forgive me if this has been address many times before, but I'm using a circa 1980 internet connection and dont have enough time to search (cargo ship in the ocean).

Am I correct in understanding that if I am using a 3500ohm out put transformer, with 4 ohm output, into a 2 ohm load, the 300B is actually going to see a 1750 ohm load? And if so, this is still fine for a 300B? Or should I go with a 5k output transformer?

thanks,

Jay

[Giaime]

Hello Jay!

You're correct, the 300B is going to see 1750ohm load. Bad thing, too low with strong distortion.

I suggest you find a proper output transformer, with a 2ohm secondary winding, or you could use a 6-8k one with the 4ohm secondary.

May I ask why are you trying to drive a 2ohm load with a 300B?

[kevinkr]

No this isn't fine for a 300B, using a 5K transformer would help, OTOH getting a transformer wound that has a 2 ohm secondary tap might be better still due to the dcr of a nominally 4 ohm secondary.

I'm sure electraprint would do this, and ASL in Hong Kong did it for some amplifiers I designed and had them build oem..

[tubemaster]

Hi Jay,

Am I correct in understanding that if I am using a 3500ohm out put transformer, with 4 ohm output, into a 2 ohm load, the 300B is actually going to see a 1750 ohm load?

Yes, this is correct. But bear in mind that a loudspeaker is not a pure resistive load but a reactive one too which makes matters a little more complex.

And if so, this is still fine for a 300B? Or should I go with a 5k output transformer?

This is a not an easy question! The 300B will not be damaged but the sound will be effected. Increasing the load will raise the distortion but also the available output power. Attached is a diagram showing the relations between load, output power and distortion in the 300B as published by Western Electric.

On the other hand a transformer with a 5K primary has some draw backs as compared to a transformer with 3.5K impedance in that its winding capacity will be larger which can effect high frequency response possibly resulting in a sound less clear and bright.

Besides it really also depends a lot on the individual output transformer, the type of 300B used and most of all on the characteristics of the speaker.

You might consider getting a Sowter SA08 output transformer which is very good indeed and can be configured to a 1 ohm secondary connection which will put you on the really safe side in halfing the load to 7K giving a little less power and a lot less distortion!

Regards
Michael

[Giaime]

Originally posted by tubemaster
Increasing the load will raise the distortion but also the available output power.

Hi,

I'm sure you meant "decreasing the load".

[Christer]

Originally posted by Giaime


Hi,

I'm sure you meant "decreasing the load".

I am no valve expert, but I think he referred to increasing the current demand, while you refer to decreasing the load impedance, which are the same thing.

[tubemaster]

Originally posted by Giaime
I'm sure you meant "decreasing the load".

Hello,

I meant increasing the load by decreasing the load resistance!

Michael

[Giaime]

Oh yes Michael,

in fact you're right. It's a bad practice to confuse "load" with "load resistance".

My fault

[Yvesm]

Originally posted by tubemaster

. . .
Attached is a diagram showing the relations between load, output power and distortion in the 300B as published by Western Electric.
. . .

Hey, where did you find THAT

Obviously a very poor tube for audio output use

Yves.

[tubemaster]

Originally posted by Yvesm


Hey, where did you find THAT

Obviously a very poor tube for audio output use

Yves.

Hello Yves,

why do you think that?

The diagram shows a distortion of 6% with 7.5 Watt at a load of 3.5K. That is very good! There aren't so many tubes that achieve this when operated in single ended triode mode without negative feedback!

Regards
Michael