46 driver breadboarded. The mu-follower gyrator, the filament bias resistor array and the nice teflon UX5 socket from Jakeband. The filament bias resistor array is formed by 4 10Ω 20W dale wirewound resistors. These get very hot so probably need to think an alternative layout or further resistors in parallel:
The performance is very good. I just picked up a random 46 from my stock and biased it at 204V (which is the operating point in my design) achieving less than 0.05% at 10Vrms. Need to re-run this test to see how will perform at 70Vrms:
Using my spice model created from a good 46 valve, THD should be around 0.15% at 200Vpp with a 100K load and performance is great to loads down to 100 ohms. Clearly the load in A2 will change from high impedance to some kΩ so this driver should maintain outstanding linearity all the way through:
really nice ! happy 2013
Thanks, happy new year too!
Ale, good morning, I read from your other article that LND150 helps to set V(a) for U2. So I suppose Rmu1 helps to set I(a)? what is the trick to determine Rmu1’s value?
In the view of the U2, does it only see Rmu1 as the plate load resistor/resistance? Will it be good to choose as small value as possible for Rmu1?
Hello, the anode current is set by the voltage defined by LND150 and the vgk. In this case given it’s filament bias, then the negative feedback of the Rf will also help determining this. Rmu is roughly 1/gm. Can’t find the equations now but it’s basically the mu-follower topology between U2 and M2. Varying Rmu will produce different distortion profiles as the output impedance will change. Downside is that output to grid voltage may not be close to U2 anode voltage and you need to bear this in mind when designing your stage as Vgrid=Vanode+Rmu*Ia.
Cheers,
Ale