Re: PP KT88 Amps
Posted: October 10th, 2021, 11:23 am
I have experience with Williamson and Ultra-Linear amplifiers not to mention my current incumbent -- my Cary 845 based SETs.
My findings were that yes the output transformers will ring or resonant with other reactances in the circuit. What I have done successfully is damp the primary with a clamping (damping) resistor.
For the most part a high-voltage rated 5-Mohm resistor strapped to the primary (plate-to-plate) gets the job done and the result is better transparency like cleaning fog off of a window.
https://www.ebay.com/itm/161128273433?h ... xysstSXSOf
The damper provides straight DC resistance rather than what some manufacturers use such as an R/C network (Zobel?) and prevents other issues that would occur due to a capacitor interacting with the transformer inductance.
And no -- the damper does not influence plate load and the relationship between the primary and secondary. That said -- I also have found that allowing the the un-used secondary taps to "float" has its problems. I've used 4-Kohm non-unductive power resistors on the un-used taps and 800-ohms on the tap feeding the speakers.
This scheme really tightens things up -- enables excellent transient behavior and suppresses reactive artifacts that do not serve the music. I've added the feature to five amplifiers to this date with the same positive result.
For the higher-voltage (1,200-volts) SET output I used 20-Mohm.
My findings were that yes the output transformers will ring or resonant with other reactances in the circuit. What I have done successfully is damp the primary with a clamping (damping) resistor.
For the most part a high-voltage rated 5-Mohm resistor strapped to the primary (plate-to-plate) gets the job done and the result is better transparency like cleaning fog off of a window.
https://www.ebay.com/itm/161128273433?h ... xysstSXSOf
The damper provides straight DC resistance rather than what some manufacturers use such as an R/C network (Zobel?) and prevents other issues that would occur due to a capacitor interacting with the transformer inductance.
And no -- the damper does not influence plate load and the relationship between the primary and secondary. That said -- I also have found that allowing the the un-used secondary taps to "float" has its problems. I've used 4-Kohm non-unductive power resistors on the un-used taps and 800-ohms on the tap feeding the speakers.
This scheme really tightens things up -- enables excellent transient behavior and suppresses reactive artifacts that do not serve the music. I've added the feature to five amplifiers to this date with the same positive result.
For the higher-voltage (1,200-volts) SET output I used 20-Mohm.