Source Follower – Bartola® Valves

01a Preamp by Nash

It’s always satisfying to see someone else enjoying the end result of their build work. Here’s a great example of Nash building his take on the 01a low gain DHT pre-amp:

Hi Ale,

Its now over two months since I have been listening to your low gain version of the 01A preamp in concert with a Slagle AVC after it.

Over a span of a few years I have built several preamps and I can honestly say that this one is substantially better than any of the others. As a classical music fan I value attributes like clarity, timbre, body, weight and speed and your design delivers! I would like to use the analogy of a superb cake with icing- the cake is the 01A pre and the icing is the AVC. Thank you for a great design!

I have recently replaced RMu and the divider resistors with the TT Electronics PFC series SMD resistors mounted on a 1206 adapter. These are tantalum nitride resistors and they sound wonderful in this application. 

Further, I am getting interested in your 300B SE builds. Are you using a preamp before these? Please send me a link to your latest work in this area which you feel I should consider.

Regards,
Nash Bapasola  in NJ

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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:

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

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:

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.

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.

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”

300B SE Amplifier Finished!

When everything was going to plan…

This build became one of the quickest and eventually the most painful from all, perhaps not really. However, it was very challenging in the end. I will tell you why in more detail. Yet, it has been a fantastic learning experience.

Continue reading “300B SE Amplifier Finished!”

300B SE Amp: build part VIII (Cap Boards)

I made a set of useful PCBs. They are intended to mount large (big really big) film capacitors: WIMA DC Link ones!

WIMA DC Link mounting boards + PIO cap board

I use the cost-effective 45μF/600V (MF Part No. DCP4I054507ID2KYSD) in many of my boards as the last capacitor in the filtering network. This is a 2 pin device, however when you go larger like the 80μF/900V (MF Part No. DCP4N058009JD4KYSD), this one has 4 pins and bigger size. The PCB for the later can also accommodate the smaller DC Link of 45μF/600V. The boards have turret or 2mm banana plug connections and an INS-1 Nixie indicator with its associated resistor. Finally a bleeder 3-5W resistor can be added.

The smaller board has the size of the Source Follower PCB. It can be mounted below it or can be used independently. Can fit a variety of PIO/Film capacitors for decoupling or for AC interstage coupling.

Speaking about the Source Follower PCB, I made also a new batch of PCBs as run out of the original ones. I made a minor modification and improvement by adding an LED indicator before the top MOSFET drain. This works in the same way as the gyrator Rev08 PCB. Can be used for normal operation or for A2 current source indicator. Also added an extra PIO/Film 100nF decoupling cap to be mounted under the PCB to decouple the high impedance node to the power supply:

SF PCB Rev03: decoupling cap under board

300B SE Amp: build part V (Fixed Bias)

Last year I developed a voltage reference using an HV LED. Unfortunately these devices from OSRAM seem to be discontinued. I managed to buy enough parts for my own use though. What is interesting from these LEDs is that the dynamic resistance is low. About 150Ω with good current, or between 300Ω to 500Ω. Tempco is very low and with such a low dynamic resistance, they are great for creating a voltage reference with a stable CCS:

The LEDs are extremely bright and found that with a 1mA current are dim enough whilst retaining the stability needed. I have a cap multiplier arrangement and the LED array is fed by a stable CCS. Jumpers on the board allow bypassing LEDs and there is also the option to use a trimmer for variable voltage adjustment. Very handy for screen grid supply and phono stages. The reference voltage is extremely quiet with more than 100dB PSR.

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Tuning the system for ETF

This year I entered the shootout competition and will bring my DHT system to the European Triode Festival in France. It’s comprised of the ER801a stage plus the 01a (if extra gain is needed) and the 4P1L PSE output stage

I will have to swap out the amorphous OPT for the Monolith Magnetic ones as the speaker load is 5R.

It’s going to be interesting!

300B Amps: an early breadboard

Browsing my schematic archive I found this early implementation from some years ago on the 300B:

The amp design is straight forward. Let’s start from the output stage. The 300B is run hot at 33W (376V/90mA) with a fixed bias of about -78V. I used a pair of LL1623/90mA OPTs which I had wired on 3K:8 mode.

Continue reading “300B Amps: an early breadboard”