The VT-25 DHT Preamp is now part of my system as I do like it a lot. Therefore the Mule was vacant for a new experiment. It was the time for the 3B7 valve. This was introduced to me by my friend Paul, who has implemented it using the gyrator and is very happy with it. The 3B7 has a pair of triodes in the same bottle. I wired them in parallel as well as the filaments, which have same current specifications as the 01a.
Most of the mid-mu DHTs tend to be microphonic. This one is an exception, although some singing comes out of them, its very minor and not impacting the stage.
Here is my implementation:
The build was very quick given the minor modification needed on my mule setup. I reused the 01a Rod Coleman regulators and the filament bias is a pair of 27Ω Russian 2% NOS wire wound filaments. Was aiming for 15Ω, but this should do.
I’m running the gyrators at 20mA and anode is about 206-208V. The filament LT supply is reused from the 01a but can clearly have 10V or less of raw supply.
I’m settled with the IXT08N100D and the BSH111BK. I have some daugther boards for the BSH111BK now. I also purchased the BSN20BK which has higher gm, haven’t tried them yet.
Here is an interested picture of the frequency response:
ON both channels there is a slight dip coming closer to 10kHz. Less noticeable on one channel. I have some military JAN Sylvania boxed 3B7. Haven’t tested them all, but perhaps it’s my batch, I don’t know
Build
Well, a nice beer is needed to help out the build stage. What best than trying this cask aged Imperial Stout from Brewdog:
The build was fast as I said, that is the beauty of the Mule. I created a board to mount the Loctal sockets into the main wood chassis, that will allow me to exchange different sockets in the future:
The sound?
It’s rounded and softer than the other DHTs. Perhaps this is due to its higher H2. Overall has great bass and clear treble. Need to listen to this stage for some time. The 26dB are needed in my system when playing the discrete DAC (Starlight CD transport + discrete DAC) as I have a step-down transformer which delivers a mere 500mV signal.
Very nice result, a DHT to consider indeed. I like this 3B7.
Related
Most of the mid-mu DHTs tend to be microphonic. This one is an exception, although some singing comes out of them, its very minor and not impacting the stage. 
