Gyrator PCB Update – Rev07

The gyrator PCB has been updated to fit now a wider variety of lower enhancement MOSFETs with low capacitance and high transconductance. The best examples are the BSH111BK and BSN20BK which are great options for currents above 25mA:


The board offers now all the flexibility needed in terms of different TO-92 and SOT-23 package pin-outs to use whatever FET you want.


3B7 DHT Preamp

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. 

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VT-25 DHT Preamp Update

Well, it was just a question of time to make an update to the VT-25 DHT preamp. I wanted to try the BSH111BK as lower FET as shown below:

The result is visible on the frequency response. It does have an improved FR and the bandwidth gets up to 800kHz on same testing conditions. If you increase the HT to 280-300V you can increase the anode current to 30mA which should be good as well. 

I love the sound of this stage driving the 4P1L PSE output stage. It has a deep bass as well as clear treble. Dynamics on the system are fantastic. 

The bass on my system is fantastic now. Stronger than with previous setup. That is what you get when you marry low ra (4P1L pair) with a high-quality OPT.

I have a pair of pre-soldered BSH111BK boards (which are painful to solder) so I will give them away on a first come first serve basis

Weather is really nice today in London, so I’m going out right now…

cx-112a DHT preamp


This was one of my first DHT preamps. I found a quartet of NOS CX-112a Cunningham (globe) back in Buenos Aires many years ago and built one of my first DHT preamps. I loved it. I played with it before I moved into the 26 and then started the long exploratory journey with DHTs. 

The CX-112a can be easily fit in an existing 01a preamp. Take a look at what Thomas Mayer recently blogged about this valve, worth reading it. 

Well, you can get more current drive than 01a (nearly double) but no thoriated tungsten filaments. Anyhow, the gain is slightly higher but is very easy to adapt to my gyrator-based circuit, that I couldn’t resist to take the quartet out of my valve stash and make them sing again after so many years.

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VT-25 DHT Preamplifer

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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VT-25 / 10-Y triode curves

VT-25 triode curves 




Everyone loves this thoriated-tungsten DHT valve. I’ve only used it in a preamp and was hooked with its sound. Really warm and nice. Downside is, it’s very pricey these days and also is quite demanding from a filament perspective.  You can check the characteristics here.

For those who like testing their designs with LT SPICE, I produced a model which matches really well the traced curves. Would like anyone to use this one, to drop me a note with any feedback 🙂




RCA10Y VT25 spice model

Here is the VT-25 spice model. Let me know how it works for you!