G'day from Oz,
I had a couple of Hammond 1650T (120W UL Taps 1900 Raa) Output Trannies on my shelf and so commenced a couple of Class AB1 monoblocks to replace my 6BQ5 10W a Channel power amp.
I have conected the driver valve anode resistor to the output valve anodes thus shifting it from an anode resitor to an output valve feedback resitor c.f. the resistor from an op amp output to its -ve input. This turns the output valve into a current to voltage converter. The main effect is to lower the valves rp which reflects across the output tranny as a much lower Output Impedance and hence improved damping factor.
To allow enough voltage for the driver stage, 12AT7, (which now has to act as a current source) I am running 525V rails which stress the screen connections of the output valves , 4 x EL34s, (UL connections to screens). I am considering deleting the screen connections from the output tranny UL taps and regulating a lower voltage supply for them.
The prototype single mono block I've got running sounds sensational.
Has anyone any experience with this circuit topology - sometimes called "Partial Feedback" particularly in the following areas:
1) Use of the UL taps for the O/P Valve Screens vs separate Regulated Supply
2) Perhaps using 6550s in place of the EL34s
Happy to correspond with anyone working on a similar design.
Cheers
Ginger
contact:
Ian Miller
Senior Electronic Engineer
Laser Airborne Depth Sounder
Tenix LADS Corporation
ian.miller@tenix.com
