22-09-2019, 07:26 AM
Thanks for the comments everyone.
So, leaving the power supply, (and the topic), for the moment, I've cobbled together a reasonable copy of the Grundig set's audio stage, but using a PCL80 and the mains transformer, (6W 0-115 0-115. 0-6 0-6. From RS.) for output.
Boy, did it oscillate well!
Got the NFB wrong way, also had to play with the pentode biasing. Grundig arranged for negative grid bias, I had to provide it separately.
Without anything on the other winding it did work, OK, but the problem was poor frequency response, and clipping at low power, couldn't do much to improve that.
Putting a load on the other winding to draw simulated "balance" current improved things remarkably, what surprised me was how little it took, about 4mA....as against the 16mA standing pentode anode current in the circuit.... You would think optimum would be Ia=Ib....
I don't know what tradeoffs are with the effect....
I was doing this with a resistive sink load on the second winding.
What happened with a 47uF capacitor from transformer balance winding to chassis?
It made it worse.... It does seem to be loading the output of the valve, with increased Ia, and increasing the low frequency "roll off" frequency, by killing the inductance no doubt.
....A well deserved "told you so" from Kalee 20.
I haven't tested for hum cancelation yet, its running on my stabilised supply at the moment....
Seems like a good way to utilise a cheap mains transformer in an SE amplifier though, as Mr Watterson said....
It was giving 600 to 700mW clean output from the rats nest lash up.
Soon have to start the presentable version
Thanks,
Amie.
So, leaving the power supply, (and the topic), for the moment, I've cobbled together a reasonable copy of the Grundig set's audio stage, but using a PCL80 and the mains transformer, (6W 0-115 0-115. 0-6 0-6. From RS.) for output.
Boy, did it oscillate well!
Got the NFB wrong way, also had to play with the pentode biasing. Grundig arranged for negative grid bias, I had to provide it separately.
Without anything on the other winding it did work, OK, but the problem was poor frequency response, and clipping at low power, couldn't do much to improve that.
Putting a load on the other winding to draw simulated "balance" current improved things remarkably, what surprised me was how little it took, about 4mA....as against the 16mA standing pentode anode current in the circuit.... You would think optimum would be Ia=Ib....
I don't know what tradeoffs are with the effect....
I was doing this with a resistive sink load on the second winding.
What happened with a 47uF capacitor from transformer balance winding to chassis?
It made it worse.... It does seem to be loading the output of the valve, with increased Ia, and increasing the low frequency "roll off" frequency, by killing the inductance no doubt.
....A well deserved "told you so" from Kalee 20.
I haven't tested for hum cancelation yet, its running on my stabilised supply at the moment....
Seems like a good way to utilise a cheap mains transformer in an SE amplifier though, as Mr Watterson said....
It was giving 600 to 700mW clean output from the rats nest lash up.
Soon have to start the presentable version
Thanks,
Amie.
