It was pure lust and love at first sight. I found the RL12P35 german transmitting pentodes and couldn’t resist in buying them. A very nice set of NOS Telefunken and Valvo valves with also NOS sockets. These pentodes look like a de-rated LS-50/GU-50 and very interesting candidates for a nice PP amp:
Note the Nazi printing!
These have been used by Lorenz to build the classic and lovely PP amp LVA/BA30 RL12P35 Amp with anode to grid feedback (i.e. a la Schade) like the below amplifier:
Recently I was asked about whether I could write on my blog about how to design a filament bias stage. My immediate answer was:
I don’t have much time these days am afraid to write extensive articles (and sometimes to even write-up at all)
Thomas Mayer has written about it (see here). Of course, I completely forgot that Thomas never completed his intended series of posts around filament bias, so I decided to attempt explaining the practical aspects of its design in this blog.
Before you continue reading this post, I suggest you read first Thomas’ article above and get yourself acquainted with DHTs and triode amplification. I’m not going to cover any of that theory which I will give it for granted that the reader is experienced with valve circuits and in particular with the hybrid mu-follower amplification stage with gyrator load.
It’s always great to come back and revisit a great design. The 4P1L preamp performs flawlessly so I tweaked the gyrator board to see how it worked with the BF862 FET. The result is great, it sounds as good as it measures:
The 4P1L is biased to 150V/25mA which is the maximum current that the BF862 can do (IDSS max). You can see that the frequency response is flat up to 1.5MHz. The LF response of my test mule is affected by the AC coupling of the measuring gear. However it should be around 5-10Hz.
The distortion of low-level signals is really good:
THD @ 4Vrms
Predominantly H2, it’s very nice to see THD<0.015% for a 4Vrms output. The load is 100KΩ which is the typical input impedance of an amplifier (with exception of solid state gear)
This low distortion manifests across the entire audio band (ignore the THD below 20Hz which is a byproduct of my testing gear):
THD version frequency @ 4Vrms
The nice thing to see also, it’s how well the 4P1L can drive larger voltage swings:
4P1L THD @ 10Vrms
We can see H4 popping up, however odd harmonics are lower (H5 in fact is higher than H3). THD at 10Vrms is still below 0.03%!
It’s been a long time since I haven’t tweaked my speakers. After more than 9 years I decided to change the drivers after falling in love with the Alpair 10M from Mark Audio. I listened to my friend Andy’s system (4P1L PSE driving the Alpairs) and decided to get hold of them.
A simple upgrade
As I don’t have much time left for DIY audio these days, I needed a simple solution. I couldn’t build a new set of speakers despite the love I have for some horn-type designs. Bringing new speakers was out of the question, so I had to modify my existing boxes to replace the FE167E. Sadly they didn’t fit straight on, so my friend Tony made me a pair of adapter boards to fit these. Made of MDF I painted them in grey:
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.
More than 4 years ago I ran a lovely 71a preamp which sounded amazing. I used it for some time and enjoy its sound up until I continued with my exploration around DHT preamps. Recently I was asked about how to implement this lovely valve again.
The CX371a / 71a valve is a great candidate for a line stage with its low mu and anode resistance. In my experience you have to run it above 20mA and over 100V to get the best out of this valve:
The implementation of this preamp is dead simple and a few components are needed on top of the gyrator PCB:
I haven’t starved the filaments as I found this valve not to be microphonic. If you have an 01a preamp you can modify it slightly. The interesting thing is that you can run it with just 180V. Even 150V should work and you need 25mA on each channel. A J310 or BF862 lower JFET device will work fine and you will need a heatsink for the top device (e.g. DN2540). Filament resistor is anything close to 50Ω. I used some 51Ω Russian NOS wire wound resistors, but any combination will be fine.
As part of improving my bench test gear to do sweep tests and impedance measurements, I ended up building a great preamp and buffer gig based on the SSM2019 device as described previously here.
Here is the main circuit for the preamp:
The circuit is same as described before. I added a rotary switch to select gain from 0dB to 60dB (ignore the diagram labels). The circuit has a DC input (differential floating) and an AC input for voltages less than 60V. There is a switch for AC/DC selection and also a switch to ground the negative input for DC mode when we don’t want it floating.
The preamp AC output is not shown but is a 1uF with a 220k resistor to ground.
The buffer circuit is similar to the above but without AC circuit and no gain selection.