EF86 – 300B XLS SE Amplifier build

Here is a very nice build from Dan Kercher on his contraption of the 300B amplifier with auto bias. The driver is an EF86 with SiC diode biasing, the topology is a hybrid mu-follower and a source follower drives the output stage. Filaments are DC with Rod Coleman’s board:

Dan Kercher’s EF86 – 300B XLS SE Amplifier build

More details can be found below:

Hi Ale, 

I wanted to thank you for all of your PCBs.  I just finished a project that uses your Gyrators, SiC bias boards, and SiC Source Followers.  I'm so happy with the results.  Details posted on Audio Asylum.  

https://www.audioasylum.com/forums/tubediy/messages/27/276814.html


Cheers, 

Dan 

FT-3 Coupling Cap PCB

As several DIYers out there, I’m a big fan of the Russian teflon FT-3 capacitors in my preamps. I normally use the 220nF/600V ones as have plenty of them in stock. These are superb in my view, however mounting them is a royal pain in the back. I used turrets for some time, but I ended up building a simple PCB which has a cutout to fit the FT3, solder it firmly on big copper pads and then solder the connections from the PCB. Also added a bias resistor (either for charging/discharging the cap or for fixed bias) which is connected to a GND terminal. This GND terminal could be used for the negative fixed bias if required. The PCB can be stacked very easily as has 4 M3 hex standoffs:

Populated PCB (Without resistor) and empty PCB
lateral view with the FT-3 cap soldered on PCB

 

LL1943 / LL1933 SUT PCB V02 – Balanced option

After a long time, decided to update the PCB of the LL1943 SUT to provide extra flexibility on the grounding connection. This is in essence to ensure no ground loops and noise when either using balanced or un-balanced cabling from the cartridge into the PCB:

LL1943: note the additional jumpers added

The additional jumpers are in line with the recommendations from the Datasheet. Here is the diagram:

For a fixed connection (like in most of the cases once preamp has been tuned/optimised) you can replace the jumpers by solid core wire jumpers soldered to the pad for best connection and performance.

This PCB is also for the LL1933. Basically is the same SUT but with 1+1:8+8 instead of 1+1:16+16 windings.

I made a small batch (I won’t need more than 2 or 4! myself) so the remaining will go on first-come-first-serve basis.

845/813 SE Amp: And the iron is here

I’ve received several emails from people asking me why I’ve gone quite lately. Nothing to be alarmed, I’m just simply buried on the daily grind like many out there are on these Pandemic times. My work-life balance has changed dramatically. This left me with very little time on my hands to work on audio stuff. I won’t complain as many others are actually struggling to survive, that is tough.

Continue reading “845/813 SE Amp: And the iron is here”

801a preamp build

I wanted to share a nice review provided by Lionel Borcelot on his journey in building the VT-25/10Y/801a preamp as shared previously. I think this is a good example on what to expect and how to adapt/reuse existing components at hand to get the most out of this design.  Bravo Lionel, excellent job.

Continue reading “801a preamp build”

The low gain DHT pre-amp library

For more than 10 years I’ve been experimenting, designing and enjoying DHT pre-amps. Without over-dwelling on this subject, to me DHT preamps bring the unique sound transparency and detail we all seek for. They can make night or day difference in any system. And is not about gain, hence I’m focusing on sharing this blog entry.

Continue reading “The low gain DHT pre-amp library”

01a Low Gain DHT preamp (finished)

It was a half-hour job to mod my Mule preamp by swapping out the UX-120 and introducing the 01a (CX-301a in fact), add the resistor divider to deliver low gain on this preamp:

01a mule

I set the valves at 115V/4mA. The Source Followers are running at 15mA and the resistor divider is actually 100KΩ/33KΩ (Kiwame) which is what I had at hand. It worked like a charm.

The 01a is driving my Slagle AVC into the 300B amplifier. I can say I can hear now the subtlety of the thoriated-tungsten filaments of the 01a with the extra level of gain I was hoping for. The amp sounds very dynamic at high level and with the clarity of the 01a.

01a mule

This is indeed a clear winner. I will leave this preamp on my system for a while to confirm further impressions. For now, I just simply love it.

300B/845 Amp: Driver Options

For more than 6 months I’ve been playing with a lot of satisfaction my 300B amplifier with the 47 driver. Since then, I’ve made 2 modifications:

  1. Major one was the introduction of my Open-Baffle (OB) speakers. These are the “Liionidas” based on the LiiAudio C-15 and W-15 drivers. I need to do a proper write-up of these, but have to admit their sound has taken my system to another level. They are simply superb. Detail and dynamics are beyond expectations. Nothing less to be expected from the designer (Leon Huijgen) :
Initial test of the Liionidas OB speakers
  1. Second modification was minor, I removed the Zobel network from the input LL7903 to minimise loading to the source. Also, after several tests I found that the impact in the HF resonant peak is minor as the source has some resistance (below 100Ω) which is beneficial. So the 300B amplifier looks like this now:

Continue reading “300B/845 Amp: Driver Options”

01a Low Gain DHT preamp

How many times I’ve been asked “how do I lower the gain of the DHT preamp?” many builders love the DHT sound and yet don’t really need that gain. Of course when you use a transformer load, step down is the obvious choice. And it works really well when you invest in a very good quality OPT.

However, when you use DHT with high rp, generally most of the low current ones, you face a tough challenge with regard to frequency response.

With the hybrid mu-follower topology (aka gyrator) we have the problem that the gain is fixed to nearly mu.

I’m in love with the DHT sound and in a preamp a 01a and the 801a/VT-25 DHT Preamp Update preamp cannot be beaten in my view. They all deliver a gain of 8 with a hybrid mu-follower topology.

This can be overcome in the following way: Continue reading “01a Low Gain DHT preamp”