• 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.

My first creation

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Ok, here it is, a small, 0.1W headphones amp using a sub-miniature 1P24B tube per channel.
Don't be naughty, it took me quite a bit of learning.
Considering line out level as 2V, it should be ok ( the voltage swing on the grid is -4/+4V), but a simple 4x amplifier with (cough cough) sand should be easy enough to make, and be small at the same time.
The cathod is connected like this as the tube is DH and eagle only has IH models.

Now, continuing with the transformer question... :p
 

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The g2 resistor says 90k. That's a misprint? Or must the g2 voltage
be much lower than plate on this type? Did you intend triode or
pentode mode? If former, the resistor must have a cap in parallel
for AC. If latter, you need a cap from g2 to cathode.
Sorry, I'm not familiar with this tube, but I think my comment applies
anyway.:)
(Pentode mode without NFB may be problematic even with HPs.)

Regards, Morgan
 
Pentode connected.
In the datasheet the grid is usually 25 volts more negative than the plate, and using the plate curves, G2 is set to 1mA at the targeted 90V, so 90V/0.001mA = 90K

You're quite right, the cap to ground is missing. Maybe i was thinking i could get away with it at the time (aham :angel: )

I must correct myself, the estimated power is 140mW, not 100mW (boy, those 40mW can't be discarded!).
 
If I may continue to nitpick...:angel:
g3 should go to cathode, not ground (but probably doesn't make
much difference).
And still troubled by the pentode mode high Zout (which gets
even worse due to the lack of cathode bypass cap.)... Do you
get any (distinct) bass??
If sensitivity is higher than necessary, you could try some plate
feedback (I seem to be recommending that all over the place,
but it does work well except in low-level places like phono pre:s).
This will lower sensitivity, Zout and distortion, and fix the freq.
response. Thinking 1M via 0.1 uF from plate to g1, 47k-100k from
input jack to g1. That's all!
Or connect g2 to plate via small resistor, increase bias to get
the same plate current as before (oh, and bypass the cathode
resistor) - and live happy in triode-land.:D
(The tube will probably survive even if 90V screen is not "allowed"
in the data sheet.)

Morgan
 

PRR

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Joined 2003
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Your volume pot is drawn wrong.

As shown, at full-down, it shorts the source. That's not how it is usually done, and will make some sources unhappy.

When turned up enough to not-short the source (about 1%-10%), you will get FULL output (the high-side of the pot is "small" resistance compared to naked grid).

I too am dubious about the 90K in G2. It probably "works". It should work better (in some way) with G2 bypassed to cathode (but then way take the 90K from plate?) for real-pentode mode, or with a very small-value (<10K) resistor to plate for semi-triode mode.

data
 
hi

Just out of interest, how much heater current does that valve need?

I just curious, with a 16 volt bias and a 2.6v bias battery I am assuming that the heater is a 13.4 volt one? so is the heater about 150ma? if so, is there that much current available?

just interested. look forward to hearing from you.

kind regards

bill
 
IIRC this is a directly heated tube.
Normally, you would connect the cathode resistor and G3 together to the heater center tap and power the heater with AC. If you use DC heating, you will have less of a variance in current on the cathode surface if you connect the heater in parallel mode (1.3V).
This may not be a big problem, depending on bias. If bias voltage is on the same order as the ehater voltage, DC heating a DHT is not a godo idea because you will get current hogging on the parts of the filament where effective bias voltage is lower due to flament voltage drop, as current there will be higher. This sort of thing also changes the effective tube curves as well.
 
If bias voltage is on the same order as the ehater voltage, DC heating a DHT is not a godo idea because you will get current hogging on the parts of the filament where effective bias voltage is lower due to flament voltage drop, as current there will be higher. This sort of thing also changes the effective tube curves as well.

Some DHT tubes were designed with this in mind. Such tubes will have the filament pins specified as "f+" and the other "f-". The grid would be constructed to allow an even current distribution from the filament when DC in the right polarity is used to run the filament.
 
'Happy?'

Moderately. Are you able to decipher the text on the data sheet?
It would be useful to know what the max ratings are, as well as
transconductance and mu g1g2 if possible. Looks like max plate is
150V, max screen 125. Max plate dissipation??

I've been trying to guess the triode-connected curves from the
diagram in the data-sheet. This has the screen as 125V. Imagine
a line from origo to 125Vplate/0V grid (just above the top of the
diagram!). This gives you a rough idea of the triode-ct 0V grid-
line. Your op point needs to be well to the right of this line.
Looks like you need a bit more than 90V plate to get any
swing at all (remember the extra volts "wasted" across the
cathode resistor too)...
In triode-mode, it should be OK to exceed max Vg2 slightly, so I
suggest you up your B+ voltage to 150V for starters. This will
give you about 125V effective plate to cathode (subtracting
bias voltage and drop across output transformer). Increase
cathode bias resistor to about 1800 ohms (guessing here). You
get maybe 10 mA idle current and a good deal more voltage swing
available. Progress is made. Oh, and you seem to need more than
a 5k transformer - more like 10k:30 ohm. Or else the load line
will be very steep...
If we go back to pentode mode, your 90V plate looks better, but
the screen will have to be at about the same voltage to get any
useful output. So, minimal dropping resistor from B+ direct to
screen, and large cap from screen to cathode. Pentode mode
would benefit from more B+ too, though. Maybe go for 125V plate
and screen, with a bias of about -17/-18V or so? (12-14 mA plate
current)

Triode mode will not have enough sensitivity for direct connection
to a source, so will need an input stage. Pentode mode might
make it, but not if shunt feedback is applied. (Guessing more here.)

'BTW, what value should i choose if using a cathode bypass cap?'

Hard to say without knowing more about the tube data. But probably not less than 100uF or so.

Let's hope PRR chimes in again and corrects these ramblings...

Good luck, though.

BTW, good advice on filaments above. I guess feeding the outer
ends of the filament with AC and attaching the cathode resistor
and bypass cap to the filament centre-tap would work. But if it's
too noisy, you will need DC on paralleled filaments and maybe
adjust the bias resistor slightly.

Morgan
 
Simpleton wrote:
"I'll make a prototype to see how it goes (i've already bought the
components) and check your thoughts later"

Yes, go ahead fire it up! You will no doubt get sound out and also
get an idea how the tube behaves at different currents, etc. You
can always go back and optimize later if it's too weak, distorted or
noisy.
BTW, if you already have a transformer that only gives 90V you
could easily make a voltage doubler, which should bring you close
to the double voltage. In the end, considering various drops in the
PSU, etc, probably pretty close to the more optimal 150V.
Anyway, it's better to have too much voltage than not enough.
Since the current consumption will be modest, I guess your
transformer won't be over-stressed with a doubler, unless it's very
small.

BTW, that 5k transformer looks pretty reasonable in pentode mode,
since pentodes don't mind a steeper load line. The emphasis will be
on second harmonic, whereas a higher load will lessen second, but
increase higher harmonics. Only disadvantage is that gain will be
less with the smaller load.

Morgan
 
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