I posted several times about my 4-65a SE Amplifier and also traced in the past the curves using my analogue curve tracer to get a view of the loadlines of this fantastic DHT in triode mode.
Now that I have the uTracer I traced again the curves including grid current and A2 anode curves which are very handy for this type of transmitter valve.
My tests were conducted with the following heating and bias setup:
DC heater using Rod Coleman regulator @ 6V and 3.5A
Cathode connection to the negative filament terminal of the regulator and valve.
It’s been a while since I played with preamps. Here is my third iteration of the 26 preamp. I love the sound of this valve as many of you out there. I found my previous build a bit inflexible to modify bias points and play around so decided to go back to the workshop and update my preamp, again. The design is very simple as you can see in the figure below. I’m reusing an existing supply but you can get away with a 200V HT and probably a 15V raw DC for the filament circuit. Key characteristics are:
Filament bias as I’m a fan of it, no output or input caps sir!
Added a Salas SSHV2 regulator to provide HT voltage flexibility and vary the operating point of the 26. The SSHV2 is extremely quiet so will give a clean HT supply to the 26. Yes, the lovely colour of the glow valves are lost in this version 🙁
The Kelvin capacitor C1 (ala Morgan Jones) will help keeping Salas noise input low, specially HF.
R1 is a simple resistor to measure the anode current. It is located in a handy place to allow easy measurements
T1 is the Lundahl LL2745 in Alt R mode for 5.6:1 ratio. This provides the lowest gain and the lowest output impedance. In my new system, this preamp is more a line stage as don’t need further gain.
SW1 is helpful in case ground loops are to be broken
The Rod Coleman regulators are set to 800mA to starve the DHT and reduce microphonics and distortion. I found 800mA to be better sounding than my previous 760mA.
P1 is my stepped attenuator and R2 will help providing grid bias when breaking before making action of the attenuator.
RF is 2 pairs of 10Ω/20W parallel/series wirewound.
The preamp is extremely quiet. The below measurement is with a noisy Sylvania ST valve which picks up significant 50Hz hum. Notice that 100Hz harmonic is very very low (can’t be seen at -100dB). Distortion at 5Vpp input (8Vpp output) is lower than 0.03% which is what you would expect from a 26. With a good selection of valves you can get this down to 0.02% for sure:
The most important part of the build work is grounding and avoiding ground loops. A combination of star grounding is recommended. I do the following star ground combination:
Input stage
Filament bias return
Output stage
You definitely want to avoid the filament current ground introducing hum in the output stage so you want to keep the three star ground points separate.
26 DHT Gen3 ready to be tested
Listening to the 26 is a fantastic experience. If you haven’t built a DHT preamplifier then do it. The detail and colour of this stage is unique. I found ST valves being more detailed on the top end, a richer treble compared to the Globe ones. Globe ones are sweeter and mellow. Definitely prefer globes for classical and ST for rock and Jazz.
26 Preamp connected to the 4-65a SE Amplifier
The preamp is extremely quiet. Best build so far, I can’t hear hum on my high-efficiency FE167E full-range speakers and that is a real challenge.
I will try this new combination of 26 preamp and 4-65a SE for a while and report further impressions at a later stage.
Just need to fix my Lenco turntable and will be a happy man again 🙂
Hope you enjoy this post and encourage you to build one of this.
“One chord is fine,” he once said, alluding to his bare-bones guitar style. “Two chords are pushing it. Three chords and you’re into jazz.”
Thanks Lou for so many years of education, joy and great music that definitely inspired many music generations and myself. Hope you are taking a walk on the wild side now…
This is a great new arm board made by Carl to fit my Lenco plinth. I’m planning to add a second plater so needed to raise the tonearm and therefore a new board was required. Carl can make this arm board to fit any requirements. Just get in touch with Carl if you need a special arm board for your plinth.
I normally will write about the sound of Coltrane’s music in a sweet class A amplifier, but not this time. Of course I was listening to trane’s music in my 4-65a SE amplifier when I got tele-transported back in time. The 4-65a SE output valves are really setled in and the amp is playing at its glorious sonority capability. I’m afraid of touching it as it sounds good.
Anyway, if you haven’t listened “A love Supreme” by Coltrane, then you’re definitely missing a master piece of the jazz music. In fact, I recall playing this record back in the early 90s to a friend who was very fond of 60s rock and some of the 90s electronic music. The monocorde progression of trane’s scales with Elvin Jones and the rest of the Quartet creating the most incredible mood is magnetic, or hypnotic. I’m not looking to review one of the greatest records of all. Simply, Psalm (Part 4) is one of those tunes that gives me goose bumps. Every time I play it, is unique and that is the lovely thing about music. When I played music before, I was really into bebop but then discovered how beautiful was to express with fewer notes and slowly. Is like when I draw cartoons. You can express a lot with fewer traces and just plain indian ink. But a very small number of people have mastered the art of reflecting the most incredible things and experiences through fewer notes.
That Quartet was unique and I’m sure they were in trance when they played Psalm. They were taken to another dimension. We can enjoy their music still, but only God knows what they felt as part of that collective experience.
Anyhow, I’m still sitting here with my Single Malt in one hand and just decided that these lines were enough (hopefully) to reflect what I experience every time I listen to this record.
I made the following changes to the RIAA stage to improve its accuracy:
Measured all caps
Adjusted 75uS network to compensate for the capacitor variance and the cascode output impedance
Adjusted 318uS and 3180uS networks to compensate for the capacitor variance
All resistors were adjusted with a pair of 0.1% SMD resistors to avoid series connection to reduce noise level.
As shown below, the response is really good matching at less than 1% for frequencies above 50Hz. the error at low frequency is due to the rumble low pass filter, the hum and the soundcard LF response which introduce a slight distortion at very low frequency in the MLS measurement:
Last week I started with my JFET RIAA stage. This stage is designed for my current MM cartridge (Ortofon 3e) which has a relatively high output at about 4mV. The stage is a variation of the Werner Ogiers’ circuit, inspired by the John Curl Vendetta. Rod Coleman helped me out here to get this design optimised, so thanks Rod again for the contribution.
A two stage RIAA stage with only about 42dB gain is not ideal. I’m looking at replacing my cartridge soon and hopefully buying an DL-103 MC cartridge. I will then easily tweak the gain and RIAA network to use the Denon one instead.
I built a breadboard of one channel and tested it:
I posted several times about my 4-65a SE Amplifier and also traced in the past the curves using my analogue curve tracer to get a view of the loadlines of this fantastic DHT in triode mode.
