My friend Rob was right. The UX120 sound is unique. Perhaps is due to its cylindrical anode, thoriated-tungsten filament. Anyhow, he was also right that they pick hum and are extremely microphonic!
Well, not a candidate for a preamp indeed. However, with a low mu of over 3, it just added an extra gain kick into my 300B system:
The modification of the Mule was quick an easy. Fitted back in the UX4 sockets, adjusted the SiC array to 6V (5.8V drop) on filament bias. The hybrid mu-follower boards were adjusted down to 85V and 7-8mA. They are DC coupled into a pair of Source Follower boards biased at 15mA
Also I fitted the copper screen board on the back as I had in my Ba/Aa preamps. This works perfectly and when grounded there is no hum picked by the valve. No need to cover the valve at all.
The sound is amazingly good. Level of clarity and detail which is unique of an DHT. Very similar to the 01a sound in my view. Just great. I had to put the volume pot at the output as the microphonic noise is terrible. Yet, level is set to be usable (very usable) in my system. Love this sound indeed.
The 300B still is driven by the D3a. I will experiment with them in the future as have few things to do before changing them.
