#1 sv572 re born
Posted: Thu May 30, 2019 7:35 am
Following the temporary re-birth of the sv572 for Owston I had to do a bit of revision regarding the sv572 and valves in general. I brushed off the cobwebs, read my old notes and re-biased it, and it sounds great(to me).
I did some measurements with the scope and found that between 0v and 1.2v RMS input it has a consistent gain of 3.8 at 1k but as the input increases above 1.2v there is a very slight flattening of the waveform at the bottom and the gain drops to 3.75. I guess the clip point with this set-up is at just under 3 watts then, with an output of 4.55v into 8 ohms.
With the bias at 90v and anode at 500v it idles at 50ma. I drew out the expected loadline for 5k and this figure(50ma) is way below expected. I'm not sure why this is at the mo but it'll be exciting to find out. I'm sure I was aware of this way back in the day but there wasn't any mention in the build notes.
So, not content to just listen to it, I decided to model the circuit in spice, something I'd never done with this amp. Using Duncan's model for the sv572 I got some very curious results. The model shows a gain of about 1. The front end(aikido) models perfectly for voltage gain but the back end seems very strange. I'm very curious about this bit and will pursue it.
All in all a pleasant diversion form the normal time-wasting endeavours. I'm hoping I can persuade Steve to let me hook up to his baffles to hear the best from this.
I did some measurements with the scope and found that between 0v and 1.2v RMS input it has a consistent gain of 3.8 at 1k but as the input increases above 1.2v there is a very slight flattening of the waveform at the bottom and the gain drops to 3.75. I guess the clip point with this set-up is at just under 3 watts then, with an output of 4.55v into 8 ohms.
With the bias at 90v and anode at 500v it idles at 50ma. I drew out the expected loadline for 5k and this figure(50ma) is way below expected. I'm not sure why this is at the mo but it'll be exciting to find out. I'm sure I was aware of this way back in the day but there wasn't any mention in the build notes.
So, not content to just listen to it, I decided to model the circuit in spice, something I'd never done with this amp. Using Duncan's model for the sv572 I got some very curious results. The model shows a gain of about 1. The front end(aikido) models perfectly for voltage gain but the back end seems very strange. I'm very curious about this bit and will pursue it.
All in all a pleasant diversion form the normal time-wasting endeavours. I'm hoping I can persuade Steve to let me hook up to his baffles to hear the best from this.