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Old 31st August 2005, 12:28 PM   #1
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Join Date: Jan 2005
Location: Lisboa
Talking My first creation

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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Old 31st August 2005, 03:16 PM   #2
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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
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Old 31st August 2005, 03:25 PM   #3
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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 )

I must correct myself, the estimated power is 140mW, not 100mW (boy, those 40mW can't be discarded!).
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Old 31st August 2005, 04:45 PM   #4
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If I may continue to nitpick...
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.
(The tube will probably survive even if 90V screen is not "allowed"
in the data sheet.)

Morgan
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Old 31st August 2005, 04:53 PM   #5
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Sorry, I missed the fact that you have a volume pot. So: 47k to 100k
from *vol pot wiper* to g1.
You could in fact both triode-connect and use shunt feedback, but
maybe sensitivity would be too low to be usable.

Interesting project, BTW!

Morgan
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Old 31st August 2005, 05:42 PM   #6
PRR is offline PRR  United States
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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
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Old 31st August 2005, 10:00 PM   #7
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So i realizaed PRR... that's what you get by drawing the schematic in a hurry i'll post it with all corrections as soon as time allows.
I'll correct the pot, g3 to cathode, and i suppose i can directly connect g2 to plate without any harm.
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Old 1st September 2005, 09:16 AM   #8
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Happy?
BTW, what value should i choose if using a cathode bypass cap?
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Old 1st September 2005, 10:12 AM   #9
billr is offline billr  New Zealand
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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
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Old 1st September 2005, 10:48 AM   #10
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Actually, the filament voltage is 1.2V or 2.4V at 230/115mA
The bias is set by the resistor, not by the battery.
These valves are very cute to look at
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