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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 6th November 2019, 04:36 AM   #41
mondogenerator is offline mondogenerator  England
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Here's the thing.

I intend to use a battery for filament supply.
Also, I was hoping to use a battery for B+ (or a small PSU if impractical, I.e. for B+ of 90V or less)

But I have been using a lab PSU for these tests.
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Old 6th November 2019, 12:09 PM   #42
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Default Its Alive!!!

Using diodes worked a treat, one 1N4701 for the time being (first thing to hand).

Plate volts have risen a small but appreciable amount.

LED next, and again further improvement.

Resistance didn't work, but was easy enough to try (perhaps not enough R)

Now, I wonder.... maybe a neon?

If so that may set the grid at about half B+ and could work a treat.

Or things may go awry, as I'm not certain negative resistance will be a good thing....
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Old 6th November 2019, 12:10 PM   #43
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Many thanks for the brain jolt, particularly PRR and Piano3
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Old 6th November 2019, 12:12 PM   #44
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I did a battery linestage many years ago using CCS loaded STC 3A/109A triodes with a string of 80 x Poundland 800mA/H Nicads for HT and 4 x 2000mA/H NimH for the filaments . If I was to re-do this , I'd use 2P1P or 2P2P valves which are DHP with B7G base . 2P1P works especially well at low voltages , has low Ra , but has a very low current (60mA) filament so probably microphonic . I have briefly tried 2P2P , these are similar to 2P29L and sound pretty good . Food for thought , although it's a right P.I.T.A when you want to listen to music but the batteries are flat as the linestage was left running overnight...

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Old 6th November 2019, 03:43 PM   #45
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One thing to remember; although these valves use exotic technology, their transfer functions are similar to familiar ones. They are not designed to work at particularly low voltage (only a few valves are). The most negative grid voltage on the 1j29b (pentode) characteristics is 2.5 so I would think that much more than this will cut current flow off completely. It would probably be best to treat it as if it were a small EF95 of something similar.
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Old 6th November 2019, 03:52 PM   #46
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Hehe yes batteries draining because of absent mindedness would be annoying - quite often my mind is absent!

I have now got plate to bias nicely about 1/2 HT:

(47V-57V and HT of 90V, using Ra between 17k and 68k, anode current between 0.6mA and 2.2mA, gain between 5.5 and 6.5 V/V, and 100k load)

I still have quite a lot of DC anode current left to play with.

With even lower HT, I wouldnt be surprised if Ra becomes very small indeed, in order to maintain plate volts and around 1/2 HT, and keep current at the level I see now.
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Old 6th November 2019, 03:55 PM   #47
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Hi Piano3,

My little experiments show otherwise. (At least when triode wired)

(Vg = -5V, at 2.2mA quiescent current, plate at 57V, HT of 90)

How it sounds and performs WRT THD, is another matter, and something I havent tested yet.

Of course, higher HT is always going to allow greater headroom for large signal output, at lower THD, than a lower HT will.
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Old 6th November 2019, 05:55 PM   #48
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Yes, that's not surprising; it was your stated intention in post 42 to try to set the GRID at half B+ that I was questioning. OK, so a typo!

Glad that you are making progress.
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Old 6th November 2019, 06:38 PM   #49
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Oh I see... I missed that point of reference.

I agree, a neon isnt a good idea.

I dont want the grid at half HT, upon engaging my brain; I forgot to step back from that comment and state my error of judgement

Looking at the circuit operation at that stage, felt quite alien, in comparison to IDHT circuits. I believed that I may have to elevate the grid very much more than a DHT circuit.

On that count, I was very very wrong! So not a typo, just me not getting the gist if what you were saying

Initially, using a single diode, the impact on plate voltage was small - perhaps setting plate volts at around 30V.

Now I have things working better but far from optimised, I understand the mechanics of this type of circuit a little better.

It's not actually as far removed from working with IDHTs.

Which pleases me no end!

There are still many question marks about how well these tubes will work at low voltages - I.e. just because a skilled designer can and has made a very nice radio receiver operating on just 9V plate supply, doesnt automatically mean an audio circuit using low voltages, can be made to operate at low THD.
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Old 6th November 2019, 06:46 PM   #50
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Still, a neon in the right place, might make for a OK but crude oscillator.

And I have kinda always wanted to make a Theremin, or glissade.
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