03-11-2016, 08:22 AM
Hi.
In the recent overhaul and rebuild of the World designs amplifier It didn't have any form of volume control it relied on the 1 meg grid resistor and 4.7k grid stopper.
During the time I did the work I didn't have the customers preamp.
When he got it back I got a call from him he told me the Bass was brilliant. A few weeks later he came back with the amp, speakers and preamp for an intermittency on the left hand channel (turned out to be his £200 phono leads) but I had a look in the preamp (solid state) and I was surprised to see the output coupling caps were measly 0.047uf. This of course was one of the reasons when the amp had been butchered on the few occasions it worked produced poor Bass.............. a 47k across the inputs. Now don't ask me the make of the preamp all I can say it was well over a £1000 and was designed for use with valve monoblocks.
I accept that a high value volume pot "can" lead to hum pick up especially in designs that may have a far from ideal layout but my point is that in high impedance inputs a 50k would be the wrong choice.
Yesterday I quickly knocked up an amp using the values the OP has used and yes it does work but I found that using line level there is no headroom at all and there was some instability, Raising the 12k to just 47k and increasing the cathode a couple of hundred ohms cured the instability and improved the drive no end, it still is bang on in the middle of the curve and on my distortion meter the overall output at 3 watts was under 10%, later on I reverted to an ECC83 and higher anode load with some NFB and the distortion was less than 5% the values were selected for best linearity.
The HT was 280v in this lash up.
In the recent overhaul and rebuild of the World designs amplifier It didn't have any form of volume control it relied on the 1 meg grid resistor and 4.7k grid stopper.
During the time I did the work I didn't have the customers preamp.
When he got it back I got a call from him he told me the Bass was brilliant. A few weeks later he came back with the amp, speakers and preamp for an intermittency on the left hand channel (turned out to be his £200 phono leads) but I had a look in the preamp (solid state) and I was surprised to see the output coupling caps were measly 0.047uf. This of course was one of the reasons when the amp had been butchered on the few occasions it worked produced poor Bass.............. a 47k across the inputs. Now don't ask me the make of the preamp all I can say it was well over a £1000 and was designed for use with valve monoblocks.
I accept that a high value volume pot "can" lead to hum pick up especially in designs that may have a far from ideal layout but my point is that in high impedance inputs a 50k would be the wrong choice.
Yesterday I quickly knocked up an amp using the values the OP has used and yes it does work but I found that using line level there is no headroom at all and there was some instability, Raising the 12k to just 47k and increasing the cathode a couple of hundred ohms cured the instability and improved the drive no end, it still is bang on in the middle of the curve and on my distortion meter the overall output at 3 watts was under 10%, later on I reverted to an ECC83 and higher anode load with some NFB and the distortion was less than 5% the values were selected for best linearity.
The HT was 280v in this lash up.
