16-09-2021, 12:33 PM
OK the problems have now been resolved. I did scope the first stage but determined that was not the issue, in the end I changed the transformer for another out of the three I bought and it made a major difference.
I had to increase the feedback capacitance from 390 pf to be 3n3 otherwise I had more gain at higher frequencies than lower, no more snubber across the primary.
I did have to fit a 39pF from the triode grid to ground, I found it stable when I had a scope probe connected but remove the probe and it went off at around 30KHz soon as the scope lead was connected again it stopped. This would not be an issue when I have an volume control and coax to connect it to the IF sound stage as there will be the capacitance of the lead, but it does for now.
The power output at 5% distortion is 1 Watt and the difference from 500 Hz and 20KHz is it drops 1.2dB at 20KHz, 10 KHz it is 0.3 dB down.
As to the other transformer, no idea what the issue could be, shorted turns or a mains transformer maybe?
Adrian
I had to increase the feedback capacitance from 390 pf to be 3n3 otherwise I had more gain at higher frequencies than lower, no more snubber across the primary.
I did have to fit a 39pF from the triode grid to ground, I found it stable when I had a scope probe connected but remove the probe and it went off at around 30KHz soon as the scope lead was connected again it stopped. This would not be an issue when I have an volume control and coax to connect it to the IF sound stage as there will be the capacitance of the lead, but it does for now.
The power output at 5% distortion is 1 Watt and the difference from 500 Hz and 20KHz is it drops 1.2dB at 20KHz, 10 KHz it is 0.3 dB down.
As to the other transformer, no idea what the issue could be, shorted turns or a mains transformer maybe?
Adrian
Learning as I go!
Youtube EF91 Valve
Youtube EF91 Valve
