VHT cathodyne PI

[pdf64]

Pete Millett sells these buffer boards on his ebay store. I haven't used this but have used some of his other stuff - well designed...

http://www.pmillett.com/A2_buffer.html

What’s the point of these for our application, I don’t see how they can be beneficial? Unless wishing to experiment with power triodes that are intended for grid current usage.

[roberto]

Hi pdf64,
in my previous post I was talking about a dedicated driver, dc coupled to the output tubes' grids, capable of driving grids into AB2.
The previous concertina can swing more than 100 Vpp needed reach the limit between AB1 and AB2. Isn't it?

[pdf64]

Hi pdf64,
in my previous post I was talking about a dedicated driver, dc coupled to the output tubes' grids, capable of driving grids into AB2.
The previous concertina can swing more than 100 Vpp needed reach the limit between AB1 and AB2. Isn't it?

Sorry roberto, yes you kinda set the scene, but I just thought that the conditionality, of driver for AB2, was worth clarifying.
I know next to nothing about the specifics of this amp, so dunno whether with a DC coupled buffer, AB2 would be possible And if it was, whether it would be a good idea.
The decision to operate in the grid current area really needs to be made at the very beginning of the power amp design process. Once the power supply and OT are done, it’s probably too late, the die has been cast. Because grid current operation will have such a significant affect on power valve dissipation, and on the requirements for preceding stages, yes, including the signal voltage swing available.
I don’t think it’s something that should be considered to be retrofittable to an existing design, certainly not without careful consideration.
That said, it’s an arcane topic that’s somewhat outside my competence
Cheers Pete

[roberto]

Thanks for your reply Pete!

I have an amp with CCS loaded nMosfet drivers dc coupled to the power tubes.
It is intended to go into class AB2 not with the purpose to get more power (loadline is optimized for AB1) but to avoid blocking distortion and "round" the behaviour approaching g1=0 and beyond.
This help to run the amp louder than with a standard phase inverter AC coupled, without being harsh.

No extra request from the PSU and the OT in this case.
But I agree with you that if the purpose of AB2ing is squeeze more power, a steeper loadline is needed to target curves at g1>k and so more current from the PSU as well.

Cheers
Roberto