More power
Our previous west meets east circuit can be improve further. In fact, a compromise made with the filament bias design is that coupling between driver (FET follower) and the output stage wasn’t DC. We want DC coupling to get best performance, to ensure we can drive well the output stage and provide sufficient grid current even when not operating in A2. This can be done with filament bias, however, since we are already introducing a negative supply, I’d prefer removing the filament bias and go for proper grid bias to get best performance of output stage in terms of maximum power and linearity.
The below circuit can be easily implemented with just few modifications from previous version:
What has changed here? Not much, the coupling cap C2 is now between the gyrator and the FET follower. The gate bias resistor R6 provides high impedance to the gyrator load to ensure maximum performance of the 01a driver (minimum distortion given size of load). Not as good as previous version, but good enough. The R6 is connected to a potentiometer which sets the bias voltage. The bias voltage is derived from V2, the -50V negative supply. You can see that this circuit will put more stress into the M1 FET as now there is an additional 25V of drop across it so power burned on this device increases.
The output of the follower is directly coupled (DC) to the output stage. The filament bias resistors are removed and we use the Coleman regulators directly on the filaments of the 4P1L.
This amplifier responds better to the grid current of the output stage once the output power goes over 3.5W. At 4.5W the distortion is just above 3% (3.2%) with a 3Vpp input signal. A tad more and you can get to the 5W and a bit more into A2 operation.
Hi again , for this amp which secondary connection alternative is used for the transformer LL1623 ? Hoping you can help . Thank you .
Hi Mike,
Check the data sheet:
The connection “B” is for 3K:8 assuming you have 8 ohm speakers.
cheers
Ale
Hi Ale, hope you are well. I am preparing to build V4 of this west/east amp.
Could you suggest the correct way to set the bias voltage with the pot ?
Thank you.
Hi Mike
I’d definitely add a 100uF (electrolytic) and good stacked film MKT (470nF) right at the +B/-B connections of the Source Follower. You don’t want to turn it into an oscillator.
the -50V in this case needs to be stable supply unless you are using a Rod Coleman Fixed Bias regulator or similar. That would be a better choice.
If you’re using a simple potentiometer, I’d add an output 470nF film cap to the wiper pot.
To start will bias it down to -50V and then move the pot up gradually until you bias the 4P1L. I’d add some current meter as with fixed bias you don’t have another way of looking at the DC current. Hassle of fixed bias, though! The HT supply capacitors should be able to withstand the maximum DC level without a load to prevent damaging them.
Cheers
Ale