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First shot at a battery DHT preamp stage
First shot at a battery DHT preamp stage
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Old 19th November 2019, 01:40 PM   #71
piano3 is offline piano3  United Kingdom
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I'm suggesting the differential stage only as a way to easily characterise the valve (and match, if you like) since you don't have a set of triode curves for it. Careful, BC557 is PNP, I think.
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Old 19th November 2019, 04:41 PM   #72
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Oh I see, I missed that interpretation...

I guess you could match a pair in such a way, balancing both phase outputs for amplitude - a good pair would cancel a bit of 2nd harmonic, I guess.

Or rather a good pair match would cancel almost all signal, when measured on 2 ch of a scope, with one channel inverted and added to the other.

Using CCS anode load and diode string for Vgk bias volts, seems to work alright to adjust plate voltage and current.

I could try using bypassed cathode resistor to set plate volts, but it almost doesnt seem worth the effort, for the more continuously variable range it would afford.

And yes, either BC557 or 237 is PNP, I can never recall from P/N alone.
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Old 21st November 2019, 10:25 AM   #73
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Default Some magic happened.

I've been tinkering with the odd 30 mins here and there, and taking FFT for a looking at when I have time.

So I have plugged some numbers in Excel at last - eyeballing the figures and THD looks improved - but the calculated result is better than I had thought.

Shocked how much difference a cheap simple, far from ideal CCS anode load is working!

4-6mA quiescent current seems the sweet spot

Roughly speaking THD stays under 0.5% across almost all the operating range (literally, until I drive the valve into clipping the top half of the wave)

Harmonic spectrum is dominated by 3rd, at about -50dB across the span of output voltage.

All other harmonics are about -60dB down.

This is still open loop.

At 13V rms output (1.8V rms input) THD remains under 0.5%.

This surprised me a little, being as at this point, the grid bias voltage is being exceeded by the signal voltage by quite a long way.
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Old 21st November 2019, 03:44 PM   #74
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I read somewhere that these rod valves don't produce significant grid current until grid to cathode voltage is more than a volt positive.
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Old 21st November 2019, 04:49 PM   #75
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Well there is certainly some truth in that.

Driving about 5Vp-p into the grid, while Vgk is at 2.6V, means that I am in the region of >1V positive grid bias.

By the time I am clipping the top half cycle, I suspect the grid is 2 to 3V positive, and grid current clipping occurs.
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Old 21st November 2019, 08:11 PM   #76
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I assume you mean 5V RMS? 5V p-p isn't going to produce grid current.
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Old 22nd November 2019, 05:41 AM   #77
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No, I mean 5V p-p, 1.8V RMS.

Enough drive voltage to take the grid above 1V positive.

I don't know if this is enough to cause grid currents or indeed what grid current may or may not be produced- I'm not measuring it.
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Old 22nd November 2019, 02:08 PM   #78
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Today I set out to double check everything, and retest at the same operating points, to confirm all my measurements are repeatable.

Filament volts and B+ on.
Quiescent current 5mA & Vgk checked.
Then left for lunch to return after 20 minutes and check everything, take a FFT and recheck voltages.

On return I find that the quiescent current has remained solid 5mA, less than 1/20th mA change.

But anode volts have slowly drifted from the set point of 62V to about 70V.

I'm not sure if this is the valve stabilising, the diode string stabilising, the CCS adjust wirewound potentiometer or the gain of the BJT stabilising as everything comes up to a stable working temperature.

If I had to guess I'd say the BJT.

TIP47 isn't the most high bandwidth, high Hfe device, just what was immediately available.

For the currents and voltages I'm looking at, something like a Zetec E-line case PNP would work, 6mA looks to be just inside the SOAR curve for one device at reasonable voltages. Better Hfe and Vce too.

Worst news is that once warmed for 30 minutes like this, the maximum output of the CCS is reduced, where once I briefly got to 8mA I can now barely make 6mA. This is the reason I suspect the BJT.

Better news is that once warm, I can dial the bias current down, and the anode voltage responds, and remains stable.

So there is definitely something related to warm up stability and some hysteresis - I.e. x anode current, with y grid cathode voltage, does not give exactly the same result; less quiescent current, all else being as equal as I can make it, is required to drive the valve toward cutting off.
Clearly there is a gradient or curve to the DC loadline, close to horizontal but still wonky. That's the simple one BJT CCS I figure.
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Old 22nd November 2019, 06:37 PM   #79
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I have noticed I have been stating incorrect part no.s for the BJT.

It's not TIP47.

I'm not sure why I wrote that (its NPN).

To correct myself, the BJT is TIP42C (which is PNP)

I've just bought a handful of 2SA2142 and ZTX558 to experiment with.
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Old 26th November 2019, 10:26 AM   #80
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Well I figured out my wandering anode volts.

Filament volts.

Even a 50mV change is enough to move plate volts by about 5 Volts, if not more.

So my stabilised supply being used for filament, wound down near its minimum output, seems to be wandering around, ever so slightly.

So little that it's barely noticable, but enough to perturb the anode voltage, and fox me for a while!
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