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VT-25 in action
Now I’m back from our long trip, I found some time to play with the “Mule“. I wanted to revisit my old VT-25 preamplifier. Many years ago I had my first VT-25/10 preamplifier which was based on a gyrator load. Then it morphed to a transformer coupled (LL1660/40mA) version to drive my TVC before I settled into the 4P1L for some long time.
The circuit design
The VT-25 has always been on my list of favourite DHTs. It’s gone ridiculously expensive these days and is hard to get. I have a couple of pairs in very good shape luckily.
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The VT-25 isn’t easy to implement in filament bias due to the bias point and the high filament current. However, for a line-stage / pre-amplifier you can bias the valve at a lower point where it’s achievable without burning ridiculous amounts of heat (Like I did with the 46 driver in filament bias for the 814 SE Amplifier). The design of this circuit compromises available supplies. The HT is 230V so the operating point was chosen to be around 200V to ensure headroom for the top MOSFET (M3). The bias point operates the valve in about in a region where anode resistance is about 5kΩ. The high anode resistance is a challenge for a transformer couple stage, but with the gyrator this isn’t a problem at all, as the output impedance is determined by the BF862 and its operating point (Id). The gain is close to 8 (18dB). R4 needs to be 390kΩ to allow higher bias voltage than 180V. I reused a pair of available gyrator boards using the BF862. For a bias point of about 20mA/200V you need a filament resistor of 5Ω. I used a pair of 10Ω NOS Russian wire-wound resistors in parallel. They do get hot as need to dissipate about 7.2W. In my previous designed I starved the filaments even down to 1060mA. This time I dialled up the current to just 1.2A as had headroom on the supply. The bias point of 6V is healthy and provides good headroom for input signal. I’m using the latest version of Rod Coleman’s regulators and just changed the resistor to trim the current. The actual modification of the “Mule” took me a couple of hours.
Building and Testing
The filament regulators are using mounted TO-220 small heatsinks as well as the top MOSFET (M3). I have a multi-tapped secondary on my filament supply so I can accommodate multiple DHTs. That was a real bonus, so getting it to work was easy and fast job.
Here is a test of the distortion profile at 8Vrms output. The second harmonic is predominant as expected. Distortion stays below 0.1% when is driven to full 15Vrms output:
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The gyrator circuit provides a hybrid mu-follower with high bandwidth. Here is a snapshot of an early test biased at 15mA instead of 20mA:
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The actual response will vary depending the load. However, it’s very good! This is one of the unique things of this circuit topology.
How does it sound?
The most tricky question indeed. I love it, I always loved the VT-25. It has the clarity of the 01a or most of the thoriated-tungsten DHTs. I think the bass is strong and overall response is fantastic. I need to listen to it more to give further impressions. At this stage, this preamp can stage on my system for long. Wow, it does sound good!