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
Hi Ale, your circuit looks very appealing. Thank you for such a great site! Your work is really appreciated. I would like to start off building a linestage using an IHDT, so am attracted to this tube and circuit. However, the gain is a bit more than I really want, could I use an OT to reduce the gain? Would that be compatible with the hybrid mu-follower (gyrator) or would I be better off using the SSHV2 similar to your 26 DHT Gen 1 design? Thanks for any help.
Hi Fred, You can use an OT if you want and will get very good results.
If you want to use a hybrid mu follower with lower gain, I’d recommend using other valve with low mu. I’d recommend 71a but is a DHT. It’s worth the additional filament supply complexity as it sounds much better than many IHTs.
Thank you, Ale. If I am following you correctly, if I use an OT, I do not use the hybrid mu follower. With the hybrid mu follower, in order to get a lower gain I just need a tube with a lower mu. The 70a looks tempting, but I am a little intimidated by the IHT. I will take another look at my project and see if I can convince myself to give the IHT a try. Thanks again, I appreciate your help.
I meant DHT!
Correct. The DHT isn’t as complicated as you think so. The challenge is on the filament supply which adds extra complexity. Also you need to pay close attention to layout.
In my opinion you don’t only need to look into gain only, but also on the overall sound performance of the stage. Each valve as it’s own unique solid contribution, however the overall sound depends on the end to end design, not just the valve.
There are a few IHT’s which have low gain, but again not necessarily means they sound as good
Thanks, Ale. I was typing late last night and, in addition to a few typos (e.g., IHT instead of DHT and 70a tube instead of 71a) my comment about “just” needing a tube with a lower mu was poorly worded at best. I did not mean to imply that any tube with a low mu would suffice! Your implementation of the 71a looks very interesting. It seems like either the 71a with hybrid mu follower or 27 with OT will be the way for me to go. I just have to decide which! Thanks again.
Hi Ale.
This blog is such a wonderful resource for new comer to VTA like me. I built 4 preamplifiers. But I don’t like transformer pre very much and I found this 27 pre design with a joy, as I have a couple of 27 tubes. The schematic looks like LTspice. I also use LTspice and wonder if you can share gyratorPCB2.sub file. Thanks!