Tag: valve curve tracer

6CB5a output valve in triode mode

6CB5a RCA NOS under test

6CB5a, as many TV power pentodes, is a great valve for a single ended amplifier. Specially now that popular valves of similar anode dissipation are getting really expensive.

Thomas Mayer came up with a great design for this superb TV pentode that wired in triode mode is very linear and can sustain at least 25W of anode dissipation. If you want to check Thomas’ design, have a look at his website which is very interesting.

If you want to look at the specifications, you can download the datasheet here.

When testing this valve on the curve tracer it was evident the fantastic features and linearity at every point.

As a power valve, testing a 6CB5a for harmonic distortion showed THD=0.07% @ Vo=+22.22dBu (10Vrms) at the following operating point:

  • Ia=40mA
  • Vg=-50V
  • Va=253V
6CB5a RCA NOS THD

Transconductance in triode is high: 6.4 mA/V @ Ia=40mA, Vg=14.6V, Va=100V. In summary:

  1. Gm = 6.4 mA/V
  2. μ = 4.5
  3. ra = 700Ω

Here is a set of triode-strapped curves that you may find very useful if you’re planning to use this valve:

6CB5a RCA triode curves

 

Also you may want to try the following spice model. It can be improved, but I think is an accurate representation of the curves shown above. Please try it and let me know what you think about it…

6CB5a SPICE model

12P17L curves and Spice model

A great russian pentode valve similar to 4P1L, but with indirectly heated cathode. You can check the valve specifications here.

I tested transconductance in left-handed triode mode: Gm=9.6mA/V @ Va=150V, Ia=50mA , Vg=-5.6V.

Here are the curves for this valve:

12P17L triode curves

For the ones who want to test the SPICE model here is my take on it:

You can try the model and please let me know your results! You can always email me

Curve tracer finalised

After a long process, here it is. The curve tracer is finalised. It includes the following features:

Valve curve tracer

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  • Sockets: 4 pin, 5 pin, 7 pin, octal, loctal, 9 and compactron (10 and 12 pins)
  • Anode sweep: 0-330V
  • Anode current: 100mA (max)
  • Grid step generator: 8 steps, 0 to -80V and 0 to -5V steps
  • Grid output for calibration
  • Oscilloscope: X (x10 attenuation) and Y
  • Y amplifier:
    • x1/x10 differential amplifier
    • 1Ω / 10Ω sense resistor
    • Polarity inversion optional
    • Image sync adjust (coarse and fine)

Valve transconductance tester & THD meter

  • Anode current CCS 0-100mA (0.1mA resolution)
  • Anode voltage 0-600V (1V resolution)
  • Grid bias: 0 to -80V (0.1V resolution)
  • Transconductance meter:
    • 0-2,000 μmho scale (1 μmho resolution)
    • 2,000 – 40,000 μmho scale (100 μmho resolution)
    • Input test signal: 100mVrms @1kHz
  •  THD meter:
    • Soundcard I/O BNC connectors
    • CCS load or external load

img_3000

 

Finishing the curve tracer

 

Today I did a bit of extra work on the curve tracer with a view of finishing it. It has been a long and painful journey, but I’m reaching the end of it.

Tracing curves with the oscilloscope4P1L under testCurve tracer and 10Y10Y under test

The transconductance tester is working perfect. I need to use the following ranges in my true RMS AC voltmeter:

  1. 0-2,000 μmho: 100mVrms scale
  2. 2,000μmho-50,0000μmho: 1Vrms scale

It’s probably the DC bias which affects the low scale. As an example when testing a 46 in triode mode (see datasheet for details), I tried the following operating point: Vg=-33V, Ia=22mA and the measure should be around 2.35 mVrms over 220mVdc. But in my bench voltmeter, above 17mA in the 46 doesn’t like it and cannot measure it, so need to change scale. I tested low transconductance valves in the lower AC scale such as CX301a, 26, 4P1L, 71a and then using the high AC scale, used 6e5P, 6C45, 6N6P amongst others.

The tracer now has a common-mode mains filter. This was required as at certain times during the day, specially in the evenings when the mains is really noise or my wife is using the microwave oven!, when tracing curves with the 1Ω sensing resistor and low anode currents (e.g. CX301a) then the noise level was sufficient to impact and distort the traced image. With the common-mode mains filter it works brilliantly.

Now need to place bottom plate and standing feet. Job done then and will move to some proper audio work!

Testing the circuit today, I measured 29 46 valves.  Ended up discarding two which measured low and then when tested with the tracer found that curves weren’t good at all. Probably electrode misalignment as they weren’t just with low transconductance. Will upload some examples as it’s very interesting to see the difference

 

 

Updated 6e5P SPICE model

6e5p under test

Having fixed the bias offset problem in my tester (actually my oscilloscope). I took again the curves this evening to get a better model….

If you want to read how did I manage to get here, please read this post

 

6e5p triode strapped curves (offset fixed so grid voltages are fine now!)

6e5p triode-strapped

6e5p valve

The 6e5p is high-frequency indirectly-heated tetrode from our friends in Russia. The specifications can be found here.  Anode can easily dissipate 8W and screen can take up to 2W and has a high transconductance of around 30 mA/V

Wired as triode this chap becomes very attractive. The anode resistance drops to around 900Ω – 1KΩ and effective mu is about 30-35. This turns this valve into a low anode, medium mu and high transconductance fellow which is highly regarded as a driver in SE amplifiers. Check out there in the jungle and you will find many good examples of how this valve is being used effectively.

When testing this valve on my curve tracer I found that it probes to be a challenging device. You need to leave this guy running on its own for a while (Lars recommended 30 min to 1 hour). I found that indeed after 20-30 min it stabilise.

First run on my tracer

Dmitry came up with a very good model. When I created a model based on my curves found a mismatch between my notes and simulation. Checking my notes I think I set up the tester to start plotting curves at 0V with a step of -0.5V, however looking at the model produced by Dmitry’s tool, I got this:

SPICE model to fit 6e5p in triode mode

 

It looks like the curves starts at -2V. Need to re-check and probably trace this valve again. Either way it does match very well and not far off from Dmitry’s model from above.

Here is my model.

 

I’m planning to use this valve in my OTL (cap-less) headphone amp. Stay tuned…

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