Bank holiday weekends give us time to think in a fast world. A time where analogue beauty is almost eclipsed by the digital norm. In fact I like both or at least the right blend of them. At the end of the day we can create (as our predecessors did before) with what we have. Some may do good things and some may still produce a pile of rubbish.
Author: Ale Moglia
"A mistake is always forgivable, rarely excusable and always unacceptable. " (Robert Fripp)
4-65a SE Amplifier: testing filament regulators
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:
4-65a
Testing the 4-65a filament regulators
4-65a filaments are lovely
46 filament bias and the regulator
46 Driver regulator
4-65a SE Amp: Shunt regulator
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.
New pass FET
Fitting new pass MOSFET and CCS
Salas SSHV2 shunt regulator
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!
2e24 DHT
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:
A high-mu DHT (841/VT-51)
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:
864 / VT-24 DHT curves
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 [email protected] *---------------------------------------------------------------------------------- .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
307a DHT in triode and Schade feedback
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?
The Shunt Cascode Driver
A heavy-weight driver
Rod Coleman came up with a brilliant design recently which baptised as “shunt cascode” driver. For those who cannot stand a pinch of sand in their circuits, I suggest you skip this post now. This hybrid circuit is actually a folded cascode if we consider the book terminology. What makes attractive of this design is its outstanding performance against the classic multistage designs aimed at achieving a large drive signal for output stages such as 300B, 6C4C/2A3, etc. I personally haven’t build it yet but according to Rod the sound is superb.
Before building a stage which will replace my current 45 SE driver, I thought it made sense to analyse the circuit and understand why is claimed to be such a great alternative for today’s designs.
Russian pentodes in triode mode
After some proper time tracing the curves, here are a set of Russian high-frequency pentodes in triode-mode for comparison. I was looking at closer ones to D3a and these are the ones I had at hand and wanted to include in the tests:
| 6Z5P | 6Z9P | 6Z11P | 6Z49P-D | 6Z51P | 6P15P | 6E5P | 6E6P-E | D3a | |
| Vf [V] | 6.3 | 6.3 | 6.3 | 6.3 | 6.3 | 6.3 | 6.3 | 6.3 | 6.3 |
| If [ma] | 450 | 300 | 440 | 300 | 300 | 760 | 600 | 610 | 315 |
| Pa [W] | 4 | 3.75 | 6 | 3.4 | 3.5 | 13.5 | 10.6 | 8.8 | 4.5 |
| Gm [ma/V] | 7 | 35 | 33 | 17.75 | 29 | 16 | 32 | 25 | 34 |
| μ | 44 | 36 | 40 | 44.3 | 79.5 | 22 | 30 | 34 | 68 |
| Ra [Ω] | 6K2 | 1k | 1K2 | 2K5 | 2K7 | 1K6 | 960 | 1K3 | 2K |
| Ia [ma] | 16 | 29 | 40 | 15.7 | 23 | 52 | 50 | 30 | 22 |
| Va [V] | 252 | 125 | 150 | 200 | 150 | 250 | 200 | 200 | 200 |
Well, as you can see in the table above, the 6e5p and 6e6p-e (both tetrodes) were included in the list. Some interesting points to highlight:
26 DHT Preamp Gen2 by Rui Lourenço
Here is a great post by Rui Lourenço who finished a sublime incarnation of the 26 DHT preamp Gen2. I hope you find this post as inspiring as delightful it is for me to see someone taking my 26 DHT preamp version to the next level of perfection. I think Rui’s pictures will speak for themselves showing the great craftsman skills and amount of effort and dedication put into this art work.
My Type 26 Tube Preamplifier adventure – by Rui Lourenço (Portugal)
I’ve started my interest in diy audio about 2 decades ago, basically at that time because I had no means of buying some of the fantastic equipment’s I saw in several Audio Shows, also because the bug was there, and some of my friends were doing it. Today, fortunately I have the means, but the bug was kept and personally I believe that good implemented diy projects can most of the times be several notches above commercial products, or otherwise you would have to spend some important thousands of Euros to acquire them at a similar performance level. You all agree that the personal filling of achievement vs your friends drop jaws when looking to some of the wonderful projects we find everywhere, is also very good.