Valve Leakage Test

Years ago when I built my analogue curve tracer I added a small, yet very effective valve leakage test circuit. Due to my laziness, I failed to test a transmitting tetrode which I bought on-line and despite being NOS it damaged my uTracer. I followed the repair and re-calibration process and got the tester back again running, however, I regretted not having used this simple one step test I normally did before. Lesson learned, now I do use it back again!

Here is the circuit in case you don’t have a proper tester and you want to build something similar yourself:

You can test for leakage current using a simple amplifier made out of a NPN transistor and an indicator. In this case I used a Russian Neon (80V/0.5mA) and the existing supply on my tester (+/-80V). You can replace all this with a simple LED and the supply you have at hand. The circuit is designed to turn on the bulb when 5 μA leakage current is provided on the base of Q1 thanks to grounding the valve element next to the one under test. So for example if we want to measure cathode to grid leakage, we simply ground the cathode and we connect the tester to the grid. Same process is repeated with the other valve elements.

When I asked for some help in the DIYAudio forum, someoone gently recommended this text. Unfortunately I don’t read German, but what I got out of this adivce was:

valves with poor vacuum (i.e. failed the test described on the procedure in 18A)
1. preamp valves with less than 4 μA are usable
2. output valves whit less than 10 μA are usable
Valves that are good and show little Gas on the gas test:
1. should have less than 0,6-1μA for preamp valves
2. And should have less than 1.5-2μA for output valves

So the 5μA threshold was good enough in my view. It does work well and the beauty is that when neon light is very dim is an indication that it may be a workable valve despite the tiny leakage in particular with output valves.

Hope this helps

Ale

01a Preamp Gen2

The return of the 01a stage

I remember my first 01a pre-amplifier to be one of the best sounding ones I ever had. The uniqueness of its tone, detail and clarity was astonishing. Perhaps it is due to the warm tone it provides and I guess this is the reason why Thomas Mayer branded his design as the “sound processor”. I fell in love with the sound of a CX-301a and the joy of listening to this stage was so great that I found a fantastic excuse now to re-build this stage. My Starlight Discrete DAC has a very low output due to the step down transformer it has. I can only get 500mV as maximum output level. Not enough to drive my system to full level. This was a perfect argument for me to look at building a simple amplifier stage that could add the sonority of the 01a in my system.

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HY1269 curves

I’m a big fan of the thoriated-tungsten filaments and I value them not just because they do look very cool, but fundamentally due to how they sound in a single-ended stage. I hope to build a nice push-pull amp with these type of valves soon.

I found a pair of HY1269 valves recently. This valve is not well-known amongst the used ones out there but they do have some interesting characteristics as a directly heated tetrode that would be interesting to consider it for a plate-to-grid (Schade) feedback configuration. With its 30W of anode dissipation capability, it’s a good candidate for an output stage. However, it’s quite likely that you will have to drive it in A2 to get the most out of this valve. Like designs using 811a and similar transmitting valves, the HY1269 can be operated in class A2 even with no signal on the grid.

As per my “Robustiano” design using the 6P21S, it would be nice to see extracting 6W or more out of this transmitting valve. I’m sure that the thoriated-tungsten touch will provide a lovely sound if properly implemented.

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6P21S tetrode curves and model

Time ago I generated the tetrode curves for this great directly heated tetrode using my analog curve tracer. I originally used this tetrode in triode-mode. Although it’s a good candidate for a SET amplifier with its 21W in triode-mode, I always wanted to find out how it will perform with Schade-type anode to grid feedback. Building an accurate beam-type tetrode model was key. Luckily now, Derk Reefman has developed an accurate model for these type of valves.

I also worked with Ronald Dekker and insisted him to build in the “Schade” feedback capability in the uTracer using software rather than hardware:

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4P1L with screen as anode

A friend in DIYAudio came up with a great idea of using the 4P1L in a different way for a pre-amp/line stage. Given availability of IT and its gapped current, he suggested connecting the 4P1L differently. Instead of using the anode as the anode element, the screen is used as anode. The 4P1L screen has a maximum current of 10mA and dissipation should be within the 1.5W.

The 4P1L curves in this mode

Let’s look at an initial transfer curve with Uak=100V:

I wired the 4P1L in the following way to allow tracing with the uTracer:

The connection is slightly different as the one suggested by Indra. If you are looking to implement filament bias, you will have to rearrange the anode and suppressor grid connection and expect a slight shift on the curves given the change in bias. Filaments are in series here, however is preferred to wire them in parallel when using this valve in filament bias as a smaller filament resistor will be required given there is twice the filament current when filaments are connected in parallel. This will help to keep the output resistance lower as the size of this resistor is smaller (remember it is reflected multiplied by μ+1 times.

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6e5p beam tetrode SPICE model

Thanks to the great work that Derk put into his great Extract Model tool, I helped him to refine and debug the application by tracing the challenging 6e5p beam tetrode. After 4 versions we managed to optimise the model:

The model fits really well including the kink section but given the saturation of the 6e5p tetrode at currents above 50mA there is a slight divergence as the valve cannot reach the same anode current at higher voltages.

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4П1Л / 4P1L Pentode Curves

A versatile CCS load

I’m a heavy user of CCS loads. I generally use them to test my valves regardless of using my curve tracer or not. I tried multiple CCS types in the past with good results until I ended up burning one FET or protection zener or whatever due to the abuse of it.

Testing high current loads is not easy at high voltages. The DN2540 is rated at 400V. Not enough. You can use an expensive 01N100D which is another depletion 1KV MOSFET that has a lower Ciss (54pF against 200pF) or you can look at the cheaper enhancement FETs which require a different bias arrangement. If we are looking at modifying the classic cascode self-bias pair, it is a convenient opportunity to improve the VDS bias of the lower FET to improve the frequency response by lowering the Ciss. Remember that in a FET the Ciss is proportional to the VDS. The classic cascode pair has a disadvantage as the lower FET is biased with VDS lower than 1-2V to ensure the upper FET is biased correctly.

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6ж52п / 6Z52P in triode mode

I have only one 6ж52п which measures within specs:

Operating point in pentode:

Ia=42.1mA @ Va=150V/Vs=150V
gm=4.47mA/V
Is=4.4mA

Triode curves:

Triode SPICE model:

And resulting SPICE model:

**** 6Z52P TRIODE ******************************************
* Created on 11/17/2013 19:25 using paint_kit.jar Version 2.4 Beta. May 2013 
* Curve traced and model by Ale Moglia 2013 valves@bartola.co.uk
* Curves image file: 6Z52P TRIODE
* Data source link: www.bartola.co.uk/valves 
*----------------------------------------------------------------------------------
.SUBCKT TRIODE_6Z52P 1 2 3 ; Plate Grid Cathode
+ PARAMS: CCG=13.5P CGP=0.5P CCP=2P RGI=2000
+ MU=76.5 KG1=34.7 KP=252 KVB=1272 VCT=-0.11 EX=1.39 
* Vp_MAX=200 Ip_MAX=200 Vg_step=0.5 Vg_start=0 Vg_count=8
* Rp=4000 Vg_ac=55 P_max=11.2 Vg_qui=-48
* X_MIN=64 Y_MIN=48 X_SIZE=421 Y_SIZE=530 FSZ_X=995 FSZ_Y=675 XYGrid=false
*----------------------------------------------------------------------------------
E1 7 0 VALUE={V(1,3)/KP*LOG(1+EXP(KP*(1/MU+(VCT+V(2,3))/SQRT(KVB+V(1,3)*V(1,3)))))} 
RE1 7 0 1G ; TO AVOID FLOATING NODES
G1 1 3 VALUE={(PWR(V(7),EX)+PWRS(V(7),EX))/KG1} 
RCP 1 3 1G ; TO AVOID FLOATING NODES
C1 2 3 {CCG} ; CATHODE-GRID 
C2 2 1 {CGP} ; GRID=PLATE 
C3 1 3 {CCP} ; CATHODE-PLATE 
D3 5 3 DX ; POSITIVE GRID CURRENT 
R1 2 5 {RGI} ; POSITIVE GRID CURRENT 
.MODEL DX D(IS=1N RS=1 CJO=10PF TT=1N) 
.ENDS 
*$

4-65a Triode Curves

Tracing the transmitter valve curves

I posted several times about my 4-65a SE Amplifier and also traced in the past the curves using my analogue curve tracer to get a view of the loadlines of this fantastic DHT in triode mode.

Now that I have the uTracer I traced again the curves including grid current and A2 anode curves which are very handy for this type of transmitter valve.

My tests were conducted with the following heating and bias setup:

DC heater using Rod Coleman regulator @ 6V and 3.5A
Cathode connection to the negative filament terminal of the regulator and valve.

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