“Trans” amplifier circuit

[CrowDaddy]

Have the gang at Bottlehead thought about the circuits Menno Vanderveen has published in audioxpress?
Seems that a reduction in effective plate resistance, and a reduction in magnetic distortion in the OT, would be worthwhile.  Have you had a chance to listen to his SE10, for example?

His recent designs use solid-state drivers to produce the current input for the output stage, but tube drivers should be able to do this as well.

Peripheral question:  he points out that this technique was first published in 1940.  Do you know of any vintage amps which used it?

[Paul Birkeland]

We had a kit called the Seductor that used plate-to-plate feedback between the output tube and the driver tube in the same manner as the SE-10 (without the cathode follower).

I have used this in a dozen or so prototype transmitting tube amplifiers, and we are chipping away at a design to scale this up to offer quite a bit more power. 

[CrowDaddy]

How about Menno’s statement that a SE pentode gives twice the power of a triode, with an (arguably) preferable distortion spectrum?
Perhaps it takes his OTs to get that spectrum…

[Paul Joppa]

Sort of true. In practice, it's usually a bit less that twice - pentodes are more efficient. You can get the same efficiency from a triode if you push enough grid current. In both cases, the source impedance is too high to be useful unless you use plenty of feedback. Vandeveen uses plate-to-grid feedback around the output stage, making it behave as a current-controlled voltage source - called a transresistance.

The driver acts as a voltage-controlled current source - called a transconductance. Vanderveen adds a local feedback to make that more linear. His driver cathode follower also is a feedback stage.

Lots of feedback means you need lots of open-loop gain, and therefor more tubes - the Vanderveen amp has four elements compared to two in most SETs. That eats up the cost savings from increased efficiency. No free lunch, no magic bullets.

[Paul Birkeland]

How about Menno’s statement that a SE pentode gives twice the power of a triode, with an (arguably) preferable distortion spectrum?
Perhaps it takes his OTs to get that spectrum…

The distortion behavior pushes more toward a solid state amp, where you get a sharper uptick in THD as you approach hard clipping, but not a lot before.  The actual composition of the THD in such an amp is different from a typical SET which makes these circuits sound different.   If your system and your listening tastes can make use of the extra power, it's a great way to go.  If you're listening only requires 5W or so of power, a 300B is still a better choice IMO.