Alex Gregory came up with this fantastic cartoon. It doesn’t represent may of us DIY’ers but surely does on some of the folks out there who burn crazy money on hi-fi
A monster DHT amp
Lately I haven’t had any time for audio work unfortunately. Changing nappies to a 4 week old baby whilst working long hours is tough. I can get the odd 30 minute here and there and every time I try to get upstairs to the workshop something pops up. Never mind, hopefully things will get easier in the near future.
I’ve been asked about the 4P1L pentode driver. It’s been a long time since I did those tests and never got around to listen to the driver sound. Tests were promising but never managed to include this driver on my amp.
Driving transmitting valves is a challenging task. Especially if we want to take them to A2-land (unless they operate in A2 whilst in zero grid bias). Driving big transmitting valves like 211, 805, 845, 813 or GM-70 require a large swing of volts for the driver which should do this linearly. The load is quite demanding in particular when we approach the grid to 0V (or biased positively) and using a triode as driver also puts a daunting task to the previous stage due to the Miller effect. It’s not easy to find triodes that can swing 300Vpp with very low distortion.
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:
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.
Playing this afternoon with the SiC C3D02060F, which can happily run +20mA with very low dynamic resistance. Ideal for the 6e5p/6e6p driver I had in mind for the 300B amp:
The 6e5P/6e6P will run comfortably around 30-40mA and bias tends to be around 3.5-3.7V to swing nice volts as needed. Therefore, we will need 4 SiC in series. 4Ω resistance is good enough and not adding much when reflected to the anode…
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….
On Sunday 19th our lovely baby girl Sofia arrived in St George’s Hospital in London. Both mum and baby girl are doing fine and we are trying to adjust to the new lifestyle.
She’s a musical baby. In fact, back in the operations theatre room I sung to her “Rocky Racoon” to sooth her once the medical team gave it to me for the first time. We used to play the Beatles’ white album frequently during her pregnancy and am sure she recognised the tune.
She loves “Ella Fitzgerald” and it’s one of my favourite records to play on at 2 or 3 in the morning to get her back to sleep
I’m still enjoying playing the valve amps whilst it last. It will be probably for another 4 months so best to get the most out of them
I’m a happy dad…