Interstage: Cap or Transformer Coupling

[Cogito]

Which one is preferred and why?
Me thinks transformer coupling yields better transient response.

[ratbagp]

You could also consider the Tubelab Powerdrive CF circuit.

http://tubelab.com/articles/circuits/power-drive/

ray

[David McGown]

Transformer coupling is generally regarded as better of the two, since it is not subject to blockage when entering Class A2 operation (i.e. when the driven signal at the grid swings positive with respect to the cathode and the grid starts pulling current). Therefore, you have more headroom with a transformer. On a full bandwidth amplifier, the quality of the interstage transformer is critical, and you want an interstage transformer that has enough inductance to support the driver tube at very low frequencies, and very good high frequency performance. This makes for a challenging design. It also needs to be carefully selected to match the driver tube plate impedance and operating current. However, you can use an interstage transformer to deliver more current (using a step down ratio) for output tubes needing more control, or increased voltage (step-up ratio) when using a driver that does not have enough voltage swing. Generally, it is preferred to either use a 1:1 or a step-down ratio for maximum bandwidth and driver current.

Hammond makes some very good bifilar wound 1:1 interstage transformers for around $80 each (126 series). I actually have used a pair that I used in my GM70 amp before I upgraded to Dave Slagle's (Intact Audio) custom wound interstages. These were in the amps shown at CAF three years ago. Actually, the price is not too bad in comparison with the crazy price that top quality signal capacitors can get (like copper foil teflon or Dueland caps), and I would expect the sound to be better.

You setup up a SE gapped interstage in a similar way as an SE output transformer, the plate of the driver tube is connected directly to the transformer primary, and the plate voltage is supplied through the primary winding. The secondary winding is connected to the output tube grid and ground. This does mean that you need to have a substantially lower voltage for the driver tube than the output tube, since you will not have the voltage drop of a plate resistor.

David

[David McGown]

I have used the Powerdrive circuit as well, and can recommend it, but it does somewhat increase the complexity, and you need to worry about a heatsink on the Mosfet. You can do the same by using a cathode follower driver with a beefy tube such as a ECC99, 6N6, 5687, or similar. Capacitor or transformer coupling is easier to setup (though you need to have real estate on the chassis for the transformer, which can be sensitive to location, i.e., keep away from power transformer to avoid inductive coupling).

[Roscoe Primrose]

I prefer neither... http://www.n4rp.com/images/5842-45.pdf

Roscoe

[David McGown]

Roscoe,

Classic Loftin-White direct coupled. Actually, this is simple to adjust and balance than the the DRD design discussed in another thread, and therefore better suited for a first time SE amp build. A just little more conceptually complex than a cap coupled circuit (relating to elevated cathode voltage of the driver in order to direct couple the driver plate to the grid of output tube), but you can adjust the output tube and driver tube operation independently. (Shashi, this is a good candidate).

David

[Cogito]

Transformer coupling is generally regarded as better of the two, since it is not subject to blockage when entering Class A2 operation (i.e. when the driven signal at the grid swings positive with respect to the cathode and the grid starts pulling current). Therefore, you have more headroom with a transformer. On a full bandwidth amplifier, the quality of the interstage transformer is critical, and you want an interstage transformer that has enough inductance to support the driver tube at very low frequencies, and very good high frequency performance. This makes for a challenging design. It also needs to be carefully selected to match the driver tube plate impedance and operating current. However, you can use an interstage transformer to deliver more current (using a step down ratio) for output tubes needing more control, or increased voltage (step-up ratio) when using a driver that does not have enough voltage swing. Generally, it is preferred to either use a 1:1 or a step-down ratio for maximum bandwidth and driver current.

David

Very Informative
It makes the case for separately driving the low bass frequencies and hi-passing (at line stage) rest of the frequencies, doesnt it?

I am going to try both PowerDrive and Transformer coupling topologies. PowerDrive seems to be the simplest and cheapest as TubeLab is selling the PCBs and standard products from DigiKey/Mouser are used.