All about electronic valves and hi-fi
All about DIY audio and hi-fi using the sublime vacuum valves (or tubes)
Thanks to the great feedback from 45, we found out that I made a mistake in modelling the LL1682 OT in my previous post. In a nutshell, I was getting half of the power, doh!
I should have started from scratch, looking at the push-pull curves and estimating at least the A1 power from a pair of 6C4C in push-pull. So following the B.J. Thomson method plotted the curves in push-pull:
I love the sweetness of my 45 SE amplifier, but you know what? A great push-pull (PP) amp has a fantastic presence, bass and dynamic response. Whenever I listen to a good PP amp, I get to the conclusion that I need to have different amps ready to be played depending to the music I want to listen to! My last two years have been devoted to what Morgan Jones calls in his book “single-ended madness”. And yes, my 4-65a SE in class A2 is slowly coming to life and when ready so then I will be properly mad.
6C4C amps I listened so far made a great impression both in SE and in PP. Owning all components required, I embarked on refining a full DHT push-pull design and again, cap-less (excluding the power supplies of course).
I always found my FE167E full-range driver bass reflex speakers to be lacking of the fullest treble. I clearly knew it was a compromise of this full-range driver, but after 3 years of listening to them, I decided to do an upgrade to the system. The FT96H horn tweeters are a nice match for the price range I was looking for:
After playing for some time with Dmitry’s great DHT composite triode models, I looked at refining the model by matching my own set of curves of the 4P1L in triode-mode. Here is my take on it:
The Rod Coleman Filament Regulators were already tested in the New Year. It seems a long time indeed. Here is a selection of pictures after fitting the regulators in their heat-sinks and doing a final test before assembly:
Thought it was going to be an easy task as I’ve done it before many times and building a Shunt regulator seems to be not the challenging part of this amplifier build. We all know that life brings surprises and specially when we are not expecting them. My 4-65a SE amplifier requires a very stable DC as part of the DC-coupling design. The Salas Shunt Regulator version 2 (a.k.a. SSHV2) is a good choice for this task.
After building it very quickly I struggled to get it to work. To cut a long story short which involved some IRF840, PNP and JFET replacements, I discovered that the stabilising RC wasn’t connected as the 330nF MKP capacitor was not properly soldered to the right holes. The PCB has multiple holes to accomodate capacitor sizes, however only the top two correspond to one capacitor pin and the remaining bottom ones are for the other. My logic of placing the capacitor in the centre clearly didn’t work and the capacitor was disconnected in the end. Finally, when hooking the regulator to the raw supply and switching it on, the whole thing produced the unwanted smoke particular of sand devices getting blasted. What happened? The maximum input voltage to the regulator evidently exceeded the CCS voltage and the top FET (M1) blowed away and therefore the regulator cascode CCS (J1) and the pass FET (M3) as well. My PCB was already suffering from multiple solder work and was reaching to its usable life. I looked at using HV parts as hand to increase the robustness of the regulator. The pass-FET was replaced by a 1kV part (STE5NK100Z) and the Mosfet CCS DN2540 pair for an IXTP01N100D which is also 1kV part:
All worked well until I realised that the differences between DN2540 and 01N100D’s VGS(th) and gm made the CCS maximum to be limited to about 40mA given the test point resistor value. As M2 can be a simple DN2540, I replaced it back and all worked well to get 60mA and deliver about 280V @ 40mA rock-solid!
It was suggested to me recently by Piotr to explore the 2e24 after looking at the 2e22. This small directly heated pentode have about 10-12W of anode dissipation depending how it’s wired. I suspected this DHT in triode mode will have a high anode resistance so as soon as I managed to get hold of a sample, I submitted it to the mercy of my curve tracer:
Thomas Mayer presented in his blog a great article about this DHT. A very interesting valve for preamps given its unusual high-mu (circa 30). It could be used in filament bias despite its high filament current needs (see the curves). Ideally choke loaded or IT transformer coupled, I suspect it could perform well with a mu-follower gyrator to provide a low output impedance given its high Ra (for those who are not uncomfortable with some sand assistance 🙂 )
Here are the curves:
I found Ra to be around 20-25KΩ, instead of the higher value highlighted by Thomas.
Here is the SPICE model:Triode 841-Composite
Which produces a nice set of curves extending to the positive grid current region suggesting an interesting use in A2:
This is one of the other great DHTs I received from Vegard Winge for tracing.
Here are the curves and the SPICE model: 864 DHT Composite
The SPICE composite model based on Dmitry’s:
**** 864 VT-24 DHT Composite ******************************************
* Created on 03/16/2013 19:30 using paint_kit.jar
* www.bartola.co.uk/valves
* Curves image file: 864 SMALL.jpg
* Data source link: 864 SMALL.jpg
* Created by Ale Moglia valves@bartola.co.uk
*----------------------------------------------------------------------------------
.SUBCKT TRIODE_864-Composite 1 2 3 4 ; Plate Grid K1 K2
+ PARAMS: CCG=2.3P CGP=5.3P CCP=1.3P
+ MU=7.14 KG1=13560 KP=98
+ KVB=1.88 VCT=-0.07 EX=1.41 RGI=2000
* Vp_MAX=200 Ip_MAX=0.008
* Vg_step=2 Vg_start=0 Vg_count=11
* END PARAMS -----------------------------------------------------------------------
* cathode resistor is 4.4 ohm, the pins K1 and K2 are 1.1 ohms from the ends of it
RFIL_LEFT 3 31 1.1
RFIL_RIGHT 4 41 1.1
RFIL_MIDDLE 31 41 2.2
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 3 1G
RCP2 1 4 1G
C1 2 3 {CCG} ; CATHODE-GRID
C2 2 1 {CGP} ; GRID=PLATE
C3 1 3 {CCP} ; CATHODE-PLATE
D3 5 3 DX ; FOR GRID CURRENT
D4 6 4 DX ; FOR GRID CURRENT
RG1 2 5 {RGI} ; FOR GRID CURRENT
RG2 2 6 {RGI} ; FOR GRID CURRENT
.MODEL DX D(IS=1N RS=1 CJO=10PF TT=1N)
.ENDS
*$
Hope this is useful…
Ale
Vegard Winge kindly sent me some great DHTs for tracing including the 307a directly heated pentode. The sample traced is not an original Western Electric but a lovely Raytheon RK 75 307a NOS. There is limited information of this valve in triode mode and the folks at DIYaudio are looking at potentially using it for a DHT headphone amp.
This valve has a filament of 5.5V and 1A and an anode dissipation of 21W in class A (including screen dissipation) when triode-connected.
Let’s see how this valve performs in triode-mode:
How well can we match a triode model for this valve?
