All about electronic valves and hi-fi
I think a gain of about 130-140 should be ok. Perhaps if I get around in adding the 6SF5 stage then it may be good enough.
So this got me thinking. Of course I have on my list 2 driver tests:
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.
I really love the Ba DHT preamp, if you need the gain in your system, is likely to be one of the best sounding DHT preamps in my experience. As received many requests for the SPICE model for the Ba DHT, here it is:
**** Ba TRIODE Composite DHT *****************************************
* Created on 10/13/2017 18:33 using paint_kit.jar 2.9
* www.dmitrynizh.com/tubeparams_image.htm
*
* Traced and model by Ale Moglia [email protected]
* (c) 2017 Ale Moglia and Bartola Ltd. UK
* www.bartola.co.uk/valves
*———————————————————————————-
.SUBCKT DHT_Ba 1 2 3 4 ; P G K1 K2
+ PARAMS: CCG=1P CGP=3.8P CCP=1P RFIL=7
+ MU=14 KG1=8940 KP=84 KVB=5232 VCT=-3.5 EX=1.47 RGI=2000
* Vp_MAX=350 Ip_MAX=10 Vg_step=1 Vg_start=0 Vg_count=11
* Rp=4000 Vg_ac=55 P_max=1.5 Vg_qui=-48 Vp_qui=300
* X_MIN=75 Y_MIN=51 X_SIZE=492 Y_SIZE=530 FSZ_X=1192 FSZ_Y=679 XYGrid=false
* showLoadLine=n showIp=y isDHT=y isPP=n isAsymPP=n showDissipLimit=y
* showIg1=n gridLevel2=n isInputSnapped=n
* XYProjections=n harmonicPlot=n harmonics=y
*———————————————————————————-
RFIL_LEFT 3 31 {RFIL/4}
RFIL_RIGHT 4 41 {RFIL/4}
RFIL_MIDDLE1 31 34 {RFIL/4}
RFIL_MIDDLE2 34 41 {RFIL/4}
E11 32 0 VALUE={V(1,31)/KP*LOG(1+EXP(KP*(1/MU+V(2,31)/SQRT(KVB+V(1,31)*V(1,31)))))}
E12 42 0 VALUE={V(1,41)/KP*LOG(1+EXP(KP*(1/MU+V(2,41)/SQRT(KVB+V(1,41)*V(1,41)))))}
RE11 32 0 1G
RE12 42 0 1G
G11 1 31 VALUE={(PWR(V(32),EX)+PWRS(V(32),EX))/(2*KG1)}
G12 1 41 VALUE={(PWR(V(42),EX)+PWRS(V(42),EX))/(2*KG1)}
RCP1 1 34 1G
C1 2 34 {CCG} ; CATHODE-GRID
C2 2 1 {CGP} ; GRID=PLATE
C3 1 34 {CCP} ; CATHODE-PLATE
D3 5 3 DX ; FOR GRID CURRENT
D4 6 4 DX ; FOR GRID CURRENT
RG1 2 5 {2*RGI} ; FOR GRID CURRENT
RG2 2 6 {2*RGI} ; FOR GRID CURRENT
.MODEL DX D(IS=1N RS=1 CJO=10PF TT=1N)
.ENDS
*$
You can download the file here: Ba spice triode model
Feedback was that more pictures were preferred. So here they are. I have little time, but slowly I will make progress I hope. The main chopping board is what is left now. Layout is tricky as have not enough space given the size of the OPTs 😀
I made a set of useful PCBs. They are intended to mount large (big really big) film capacitors: WIMA DC Link ones!
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:
It didn’t take long (or at least as long as I thought it would) to finish the driver board. It has a D3a hybrid mu-follower with SiC cathode bias arrangement:
The board is mounted on top of this previous board.
And the fixed bias PCB is completed. All individual PCBs mounted over a ground plane PCB. It will be a stacked build. On top of this PCB, another one will hold the driver. Firstly the D3a in a hybrid mu-follower configuration:
Tested and bandwidth of these source followers is nearly 10MHz with plenty of current drive at 20mA idle.
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.
Wiring job is done. The fixed bias supply delivers from +50V to -300V. It has more voltage capability as am planning to use this same supply for future builds (e.g. 845 SE). The filament supply set to minimise power dissipation on the filament regulators for the 300B. All working fine, so am happy with this board. Filament noise is 0.3mV.
Playing with the layout a bit, here is a view of potential layout of the amplifier board. The PCBs for mounting sockets, turrets and switches are very handy. It accelerates the build process and provides full flexibility.
You may not like the open look and feel, but who cares! I love the aesthetics and those OPTs are enormous!
