D3a Spice Model (Pentode & Triode)

My latest 300B amplifier brought me again closer to the D3a. I have to say it’s an exceptional driver for this amp as well as it can perform in a phono stage at same excellent level.

I took the opportunity to trace again one Siemens D3a NOS boxed as “POS 1.176 Q 31-X 601”. This one was handpicked as it measured at 102% (31mA) in triode mode with a Gm of 39mS.

I wanted to develop a pentode model for phono experiments so put this lovely valve back in the eTracer and used the Extract Model tool from Derk Reefman to develop the model below.

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Modular approach (Part I)

As time is very limited these days, I’m focused in continuing my modular building approach in LEGO style. I have developed several PCB modules which are flexible to be used in multiple amplifier and pre-amplifier designs. Now, I used the power of PCBs to build some additional supporting modules to speed up my breadboarding over the IKEA boards. Not the most elegant approach, but building becomes a very fast process this way.

You will see what I’m saying when you see a few of the following additions:

Turret and 2mm female connectors in a strip

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Teflon Sockets

I’ve got a set of teflon sockets from Jakeband. These are fantastically made to order. Luciano from Jakeband sent me in addition some samples which I will use in the eTracer which I will bring along to ETF.19.

Honestly, these are fine pieces of craftwork. For example, you can measure and let Luciano know the diameter of your 845 (or any other transmitting valves) when ordering these valves so they fit perfectly.

I’ve used these sockets on my projects for years and am very pleased have to say. Of course you pay a premium price as these are hand-made with fine materials. In my opinion, these are worth every cent.

Also I had a pair of RCA sockets, look at them:

They are made on a single piece of tellurium copper. First coating in pure silver 99.99% thickness 18 microns and the second coating Gold 24 K 3 micron thick. Sterling job!

You can contact Jakeband directly to order your sockets. Just use the form I posted years ago here.

Cap Multiplier test

I managed to find the time this weekend and do a quick test on the Cap Multiplier PCB. It turned out that I missed a thermal pad on the PCB so had to add a short jumper. Nothing major, but the PCB needs adjustment.

Below is the diagram of one of the ways the PCB can be used. This is the most complex circuit, a basic one can be wired instead. The CCS (M1 and M3) provides better PSRR as well as regulation. A stable current is fed to R4 and P1 to set the voltage. C2 is the cap multiplier and the M5 used can be any suitable MOSFET. T4 provides current surge protection to the MOSFET as well as short circuit. R7 sets the current limit.

The boards fits various film caps, I have some WIMA DC Link which are great and fit perfectly:

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SiC MOSFET Follower Driver

How many more times

Led Zeppelin wrote a fantastic song on their first album: how many more times. You may not be a rock fan, but hey: what a great song. How many more times do I want to get back to this “slew rate” theme? I don’t know, as much as I have to. Plenty of comments out there of bad designs with wimpy drivers attempting to take the 300B/2A3 or even 45 valves to full tilt with disappointing results. Either way, they always blame the valves.

I came back to revisit the driving of capacitive loads effectively as I’m working on a new 4P1L PSE amplifier. Slowly, but getting there. Previously I looked at adding a buffer to the 01a preamp as a result of slew rate limitations found in Tony’s implementation of this preamp.

The circuit design

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CCS: not everything that glitters is gold (Part I)

Introduction

This is the first instalment of a series of blog posts around CCS for valve circuits. Hope you enjoy it as much as I did with the experiments conducted as a result of my interest in CCS-driven circuits.

The depletion cascoded CCS

It’s been long time since I’ve done some circuit analysis and algebra, hopefully I’ve got this right. Seems to get to the expected result, so hey: I’ve done it ok.

The analysis of this circuit starts by using the T-model of the MOSFET. I’ve omitted the parasitic capacitances to simplify the analysis. I leave you the challenge to add them in though. If we look at the typical self-biased depletion FET CCS we can find the output impedance by doing the following formulae crunching:

In summary, the output impedance looking from the source side is:

$Zout\approx Rs+\left ( 1+Rs\cdot G_{m} \right )\cdot r_{o} \approx Rs\cdot G_{m}\cdot r_{o} = Rs\cdot\mu$

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Situbes digital panel meter review

I’m a heavy user of fixed-bias output stages. Yes, I do prefer them despite the additional complexity. However, I’m not looking to open a can of worms around this subject. On the contrary, I wanted to report a fantastic product developed by Situbes.

SiDPM Digital “Panel” Meter

Here is the brief description from the website. I suggest you take a look at the datasheet as well:

The SiTubes DPM is a digital “panel” meter packaged in a standard octal tube envelope.

It measures a DC input voltage from 0 to 2V full-scale. Several selectable legends (V, mA, A, etc.) can be selected by an external programming resistor. It is particularly useful to measure supply voltages (such as plate voltage) and tube bias levels (normally plate current) in tube amplifiers.

The DPM can be powered from an AC or DC voltage. The power input is isolated from the measurement input, so floating measurements can be made up to 1500V above or below the power input. This allows high-side current measurements – for example, sensing plate current directly at the tube plate – or the ability to be powered from a supply that is not referred to ground.

My review

Looking at their construction you will realise instantly the high-quality of this product. Impressive finish and presentation. The tube is made of glass and fits very tightly to the 8-pin plastic base.

I did a simple and basic test on my work bench to test this device and its accuracy. In 5 minutes I wired it on my curve tracer to access the pins easily without soldering a test rig. With a 1Ω 1% resistor I configured the device to current mode and placed my 5½ digit bench meter in series for reference. The refresh cycle is very good, more than what you’d need in normal operation. Accuracy with the reference resistor was great. It’s calibrated as provided by the seller and error was below 1mA up about 200mA which is the planned use case for me. The OLED display has the right brightness for day operation. It’s just great.

You can use them easily to measure anode/cathode current, grid bias or supply voltages in multiple configurations.

They are pricey, but worth every penny. A top quality product which I’m keen to use shortly in one of my next builds.

6P15P triode strapped models

I’ve been playing lately with the 6P15P. Also I have developed several spice models of these Russian clones which I will be publishing shortly when I get the time to do the proper write up.

I use Dmitry’s Paint_kit tool a lot. It’s very good and accurate once you learn how to use it. It’s not easy to match triodes first time as many variables are in play. After working with Derk Reefman on the pentode models, I noticed a slight divergence on his triode models. Derk suggested that the best was to use the triode-strapped pentode for this. Here is a simple comparison between Dmitry’s model and Derk’s:

6P15P triode model comparsion — Derk and Dmitry’s models under test

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C3D02060F Spice Model

As the model for the C3D02060F SiC diode is not available, I decided to venture myself and attempt to build a decent model for Spice. I researched a bit around in the web and found this interesting article about creating your own Spice models for diodes.

Luckily I have the Locky curve tracer and can get the most out of tracing sand devices. I did a quick trace on one of the SiC diodes to capture a good sample set of Vf and If points.

How did I go about to develop the model? Quite simple. I started with ploting the ideal diode response with the Schokley current model. Then I played around with N and Is to fit the curve. Note that this is an iterative manual process, which takes a bit of time but can be done without much difficulty. Finally I created the model in Spice and adjusted Rs by testing manually a pair of Vf/If points. Not perfect, but good enough for what I need:

The graph above has the actual tracer plot (“I”), and both the ideal model (“Schokley model”) as well as the output of LTSpice simulation (“Spice”).

This is a great diode for cathode bias when using drivers which need +15mA at least. If you are interested in the model it can be downloaded from here: C3D02060F model. Hope you find this useful.

Diode cathode bias

Playing with the semiconductor curve tracer I did a quick test of potential candidates for diode cathode bias:

The popular HLMP-6000 is a superb LED with its low impedance. The SiC diodes have proved to be a great match with an impedance lower than 2Ω. Bias voltages will be around 0.7-0.8V for low currents. The classic Schottky SB540 has a very low impedance, but its forward voltage is so low that is not practical for diode cathode bias. What surprised me was to see the 1N4007 to be a good match. The impedance is higher than the LEDs or SiCs, but good enough. The green LED on the opposite extent has a significant dynamic resistance over 10Ω.

Interesting to see that a minimum of 2mA should be run through with small signals to ensure the diode operates in the linear region. The higher the better. An arrangement with an extra source of diode current (e.g. LND150 or DN2540 CCS shunting current to the diode) can be used when dealing with lower cathode currents due to the valve being used.

Further tests are required….