“Trans” amplifier circuit

CrowDaddy · 2562

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Offline CrowDaddy

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on: June 12, 2023, 08:39:52 PM
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?



Offline Paul Birkeland

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Reply #1 on: June 13, 2023, 04:37:23 AM
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. 

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline CrowDaddy

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Reply #2 on: June 15, 2023, 08:26:23 PM
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…



Offline Paul Joppa

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Reply #3 on: June 15, 2023, 09:54:24 PM
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 Joppa


Offline Paul Birkeland

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Reply #4 on: June 16, 2023, 05:28:13 AM
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.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man