• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

First shot at a battery DHT preamp stage

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I have build some small buffer amps for the TDA 15xx d/a converters to add a little gain. The others i use for little power amps a la Zotl. At this moment i try to create a 10w amp with 6 tubes per channel. I do not know if this will actually gives me 10w, but i have a good feeling about it. The smaller version is very promising and can actually drive my 4ohm speakers without any audible distortion up to 2w.


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I've been doing similarly with 6P30B-R, with a single tube, then 2, then 3; per channel.
This has been mentioned in Lampie's thread on this amplifier, so I dont want to repeat myself :)

However, I have come to the conclusion that 1W/tube is reasonable, at 1% or less THD. (for 6P30B-R), less than this for 6P30B (single anode versus twin anodes).
My triple tube stage gives about 3W at 1% THD.

I expect that 6 x 1P24B will be similar, maybe slightly less, due to the slightly lower dissipation (4.5W versus 5W) so I'd expect 5W at low distortion.

Matching tubes for idle current helped a lot in reducing THD, and the 6P30B-R were difficult to find enough good matches.

Maybe this is easier with 1P24B, I have no idea!
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Sadly the Gòogleplex hasn't offered me any curves, either pentode or triode wired for the 1J29B....

You can always sketch on the pentode curves.

When Vp=Vg2, the triode and pentode curves align. Ip(tri) is really Ip+Ig2, but in the tube you cite Ig2 is <10% of Ip so is lost in uncertainty.

At Vp=0 all I stops (very-very small). And usually small Vp makes small current (I'm unsure why this plot seems to stop at Vp=5V).

Now sketch Childs Law curves through zero and the Vg2 points. No great accuracy is needed, not worth finding your French Curve.

We see that Mu is about 9 and rp runs near 10k at high current but 20k at medium current, over 40k near 1mA.


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Thanks! Of course, I am showing my lack of basic principles here. I'll have a go at that, given some time.

I had a chance to try out the basic circuit for 15mins during coffee break today.

Rather disappointing, I may have an error in my connections somewhere. More than likely, IMO.

Using 68k anode load, 1M grid leak resistor, and g2 tied to anode, g3 to ground.

Powering up with B+ of 100V, the anode sits at 22V, grid is 0V. :mad:

I must have something wrong in the filament connections, and Cathode (is this a cathode, or a screen?)

Increasing grid leak to 5M then 15M has no effect.

Reducing B+ to 80V still leaves the anode at 20V, almost no change at all.


It does amplify, at least into an open circuit 0.142 Vrms in, for around ~1 Vrms out

Until I connect the output load resistor, 100k. This kills the output stone dead.

When I get more time, I'll break it all down again, and check all connections, and use another tube.

Probably also more like 1M load (seems very high though)


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I didn't expect changing the filament wiring would make any significant difference. There is nothing in post 25 to suggest that the tube is necessarily behaving abnormally; why not raise the high tension to 250v or, if you can't do that, then replace the 68K load resistor with a single transistor CCS. A single MJE 350 or MPSA 92 would have a dynamic impedance of a significant fraction of a megohm at a few milliamps.
I'm sorry

Firing the test up again, and accidentally raising the filament volts above 2V momentarily....

Now plate volts sit even lower.

I've cooked the filament :idea:

This particular tube, I had demoed to my 7 year old using an AA battery, so I had probably messed up
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The datasheet maximum is the anode to cathode voltage; a couple of milliamps will drop most of this on the 68k load resistor.

Edit: for instance, the data sheet has an operating point (pentode) of 5.3 mA with an anode voltage of 60.
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You are quite right of course.

I did briefly run the B+ up to 250V, and noted that plate volts rose to about 30V (up roughly 1/3rd from B+ of 100V)

Anode current:

(100-22)/68k =1.15mA
(250-30)/68k = 3.25mA

So then I guess I could increase Ra to 180k-200k to stay in the same ballpark for anode current, as the first pass.

Being completely unfamiliar with DHTs, it takes me a while to work it through mentally; but I figure then that would bring up the anode voltage, allowing more voltage swing.

But also, I imagine load driving ability is reduced as a consequence. (I may have had RL of 1k rather than 100k in the first instance - :headbash: So that is something, along with other bits, I should check!)

Looking at the CCSource you suggested. I have a feeling I dont have any MJE350, but I do have lots of T03 BUS14 switchers. Complete overkill mind you.

I am trying a fresh tube, when the chance presents itself, just in case!
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I thought this too, but I need to go and read some more, and figure out how to go about it properly.

At a guess I'd hang a cathode resistor off the earthy end of the filament, sized to pass the filament current, and then use a higher voltage filament supply to elevate the whole lot up a volt or two, but still pass 1.2V across the filament.

I have thousands of 1N4148 so I could do that also, in the same way.

I think a cog has started turning, and I've been doing this all wrong!

My dropper resistance of 180R is the Rtop of a potential divider.
It should be the bottom half!

Like Heater Elevation in a IDHT Something I've also never *needed* to do, so have zero experience of.
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