Amplifier Fun.

[Paula A]

I always wonder why people stick with a large OP transformer with a single ended valve amp.. just run the anode into a fixed resistance load and take the speaker between cathode and chassis.. play about with the load resistance while monitoring by ear or with a scope for the flattest cleanest output

[Refugee]

That puts DC into the voice coil and biases the cone.
I tried it with a transistor when I was a teenager and got sound that was quite good.
The main problem was that the speaker coil got hot to the point that the magnet also got quite warm.
I learned why car radios of the time ran the speaker from an auto transformer arrangement with a PNP transistor and the speaker from the tap to the negative earth. The transformer was in the collector load.

[pwdrive]

P = Isq R, not very good for an ordinary valve as cathode drive for a loudspeaker, plus polarizing and biasing issues.
350ma through 8 ohm will just about give you 1 watt.
Also if a pentode or tetrode is used the speaker would be passing the standing screen current as well.
Also there will be a chicken and egg feedback/frequency response issue regarding any cathode ac decoupling.

Lawrence.

[Old Sparky]

I've never tried this, but could not the speaker be connected via a coupling capacitor of, say 47uf. This would obviously prevent DC from flowing through the speech coil, though it may do little about other unwanted side effects.

[tiv48]

It can be done see here http://www.r-type.org/articles/art-153.htm
Rob T

[pwdrive]

Yes it can be done but it gets more involved, there are many designs around but they all seem to use lot's of power valves in parallel push pull, I don't think you would get much output from a low impedance speaker connected in series with the cathode of a bog standard single ended output stage.

Lawrence.

[Refugee]

In there day valves were the most complex part of the amplifier and as such was the first item to be shown on the drawing board at the start of a design.
As the valves were also expensive in both manufacture and tax per valve so they were used sparingly.
Transformers were cheap in comparison so that was how it was done.
Once transistors came along it was another thing altogether and some sets had as many as six of them just for overload protection alone.