Might speaker filters be in the wrong place?

[ErikdeBest]

So

The output tube of the x-over could be powerfull enough to drive an output tube or phase splitter for a Pushpull output.

for the above case, as an example, you are proposing to include Xover functions between stages within an power amplifier? Such as our country fella Mathijs, from Machmat?
http://www.machmat.com/mysys/electri...es/kap2amp.gif

[beamnet]

errr... no...


We were talking active crossover. So get any tubed active crossover schematic and instead of an output cinch fantasize a power tube instead. Maybe substitute the last tube in the x-over with a beefier tube to drive the power stage...

[Brett]

I see some issues with it.
- filters will need to be symmetrical
- a hefty driver will be needed
- all the output tubes will need to be the same; eg a 300B typically needs >60V for full output whereas an EL84 needs about 10V
- what happens when the grids go positive and current drawn

Better to biamp fully IMO. Select your designs and OPT's based upon the frequency range required and optimise. Seems to me like less compromise, and as you're 2/3 of the way to a whole second amp, do the job right.

[Sheldon]

Originally posted by Brett
I see some issues with it.
- filters will need to be symmetrical
- a hefty driver will be needed
- all the output tubes will need to be the same; eg a 300B typically needs >60V for full output whereas an EL84 needs about 10V
- what happens when the grids go positive and current drawn

Better to biamp fully IMO. Select your designs and OPT's based upon the frequency range required and optimise. Seems to me like less compromise, and as you're 2/3 of the way to a whole second amp, do the job right.

I tend to agree that going all the way is the more practical approach, but hey, if we were practical we'd not use many tubes in the first place.

I would also think that filtering between output tube and OPT would be dicey. However, a single input and driver stage and multiple outputs is certainly doable.

To the issues cited:

Why would the filters would have to be symmetrical?

Yes hefty driver, or MOSFET follower(s).

Yes, easiest if output tubes are the same, but just need similar grid swing and plate voltage requirements - doable. Maybe single to multiple parallel tubes. As for grid current, either stay out of A2 or refer to previous comment.

Not surprised that George has wheels turning.

Sheldon

[Brett]

Originally posted by Sheldon
Why would the filters would have to be symmetrical?

I assumed a PP O/P stage

Originally posted by Sheldon
Yes hefty driver, or MOSFET follower(s).

No MOSFET followers here thanks.

Originally posted by Sheldon
Yes, easiest if output tubes are the same, but just need similar grid swing and plate voltage requirements - doable. Maybe single to multiple parallel tubes. As for grid current, either stay out of A2 or refer to previous comment.

Unless you have way more power than you need, it's not uncommon to find grid current on peaks in output stages.