An IHT preamp, oh yes!

I always loved the 27 valve. It was one of the first line stages I built many years ago before adventuring in the DHT world. I still have a large collection of them and I was very fond of the mesh anode ones. Please check Thomas’ blog in which he wrote a very nice note about it.

With the hybrid mu-follower (a.ka. gyrator) configuration, we can build a minimalistic and great preamp stage. The 27 has a mu of 9, so in some scenarios this may be a bit too much gain, but for many cases, it’s just what we need to drive the valve amps. Someone recently asked me for help on this, so here it goes my version:

The circuit is dead simple. The 27 is biased with a battery via a grid leak resistor (R1). C1 blocks DC from input and contributes to LF response by forming a pole with R1. 150nF is good enough but if you don’t have any, use 220nF. The operating point is 6mA looking at my old notebook. The supply doesn’t need any funky regulation, and 180-200V should do. The top FET should be either DN2540 or any other depletion of your choice. The lower JFET should be either a 2SK170GR or 2SK170BL (preferably). You can use a J310 here as well (or SMD BF862).

The sound is beautiful and THD is very low driven by H2 only, as you would expect from this triode.

If you don’t want battery bias, you can add a 1K5 resistor in the cathode with its decoupling cap and remove the battery and C1. R1 should be changed to 47k then.

Hope you enjoy this!

Ale