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Old 1st November 2012, 01:26 AM   #1
TheGimp is offline TheGimp  United States
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Default 6A3 test, what am I doing wrong?

I bought a 6A3 and I am testing it to insure it is good.

Since my Mercury 1000 does not support this tube, I am testing it in the manufacturer's data sheet configuration.

I have a 6.3V DC supply on the filaments.

Each filament terminal has a 1.5K resistor to ground, for a parallel equivalent of 750R. This should produce 60mA of current at -45V bias.

I wired two 5.1K resistors in parallel for a 2550R load. Spec is 2500R.

Using a variable bench supply for B+ I adjusted it until I measured Vgk=-45V and took my measurements.

Vak=220.5V

I am feeding in a 1Vrms signal.

I am measuring a 2.0Vrms signal at the plate.

Datasheet specifies amplification factor of 4.2 at 250Vak, -45Vgk.

Is this indicative of a bad tube or am I measuring it wrong?
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Old 1st November 2012, 01:31 AM   #2
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If your tester suports , a 2A3 test it like that with a 6.3 volt fillament or test it like a 6B4G with an adapter socket .
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Old 1st November 2012, 01:37 AM   #3
Bigun is offline Bigun  Canada
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You will only get the full gain if you bypass the cathode resistor with a suitable capacitor.
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Old 1st November 2012, 01:48 AM   #4
TheGimp is offline TheGimp  United States
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Tester supports 6B4, but I only have one 4pin socket so I'll have to tear my test set apart.

I bypassed the cathode resistors and now have 3Vrms.

tks Bigun

I tested It while you were posting.

What caps should I use? I used two sprague 50V 1000uf caps, one across each cathode resistor.

Adding a 40uF motor run cap (low esr) made no difference I could measure.

Last edited by TheGimp; 1st November 2012 at 02:01 AM. Reason: comments on caps
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Old 1st November 2012, 01:02 PM   #5
Bigun is offline Bigun  Canada
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I would have thought that those caps were plenty - 1000uF is a big cap for cathode bypass providing the test signal is not really low frequency (i.e. at the test frequency you want this cap to have a low impedance compared to the load on the anode).

So now you have a gain of around 3.

What should you have ? Well this is a handy link: Vacuum Tube Amplifier Circuits and Equations

The gain of the tube will be equal to mu only with an infinite load impedance, otherwise you will see that the internal resistance of the tube comes into play, you can't bypass this internal resistance with a capacitor like you did with the cathode.

If you plug in the numbers, gain = mu x Rp / (Rp + ra) = 4.2 x 2.5k / (2.5k + 0.8k) = 3.18

So you'd expect a gain of 3.18 and you are measuring roughly 3. Seems OK to me
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Old 1st November 2012, 02:45 PM   #6
TheGimp is offline TheGimp  United States
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Hmmm.

I was thinking the specs in the data sheet took into account all of that since it specified a gain of 4.2, Rp = 2500 and ra=800.

I realize mu is defined with Ia=constant but just miss-interpreted the data sheet.

I should have used a current source for Rp instead of the 2500R load.

As an additional test I dropped the filament voltage from 6.29V to 5.01V, gain did not drop and Vak only dropped 1V out of 219. So the tube is truly NOS as it is operating in Space Charge Limit mode and not in Temperature Limit mode.

Last edited by TheGimp; 1st November 2012 at 02:48 PM.
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Old 1st November 2012, 04:24 PM   #7
kevinkr is offline kevinkr  United States
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CCS would be ideal for measuring mu.

You can also calculate the transconductance by measuring the plate current while keeping the plate voltage fixed and varying the grid voltage over a sufficiently wide range, while also recording the values at a number of points. You can also fix the grid voltage, vary the plate voltage and measure the plate current, do this for several grid voltages, collect enough data points and you can construct passable plate curves.. I've done this and found I needed a fair number of points in the cut-off region to get a good graph, the linear portion of the curve not being a big deal It is not as bad as it sounds particularly if you have someone to record the values as you do the measurements.
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Old 1st November 2012, 08:11 PM   #8
Bigun is offline Bigun  Canada
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Quote:
Originally Posted by TheGimp View Post
Hmmm.

I was thinking the specs in the data sheet took into account all of that since it specified a gain of 4.2, Rp = 2500 and ra=800.
Perhaps it's a case of being careful not to confuse data sheet specifications with typical operating point ? Many data sheets provide both. The specifications in tube data sheets are, by convention (it seems) based on certain conditions, for example, mu is not the gain of the tube, it is a ratio of the ability of the grid voltage to influence the current flow vs the plate voltage ability to vary the current flow. You can use it to calculate the actual voltage gain in a particular circuit configuration. The mu depends on the tube internal design, the gain on the external circuit. I think your tube is fine. Time to get it into an amp
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Old 2nd November 2012, 04:29 PM   #9
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Old 2nd November 2012, 05:08 PM   #10
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Don't forget that the internal plate resistance also affects gain, so load resistor is the external 2550 ohms in parallel to the ra of the triode, so gain measured this way always be lower than triode´s mu.
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