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Old 10th January 2010, 03:41 AM   #1
Bob_345 is offline Bob_345  United States
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Default space charge 12fx8 amp or preamp schematic?

Does anyone know of any 12fx8 amp or preamp Schematics? this is my first tube project and i have been having a hard time on finding any schematics.
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Old 10th January 2010, 04:25 AM   #2
Bob_345 is offline Bob_345  United States
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or guitar amp schematics?
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Old 10th January 2010, 09:01 AM   #3
Arnulf is offline Arnulf  Europe
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This is a receiver tube (heptode + triode) so you're mostlikely to find it used in some sort of radio receiver, serving in RF portion of the circuit, which is irrelevant for your project.

However that doesn't mean we cannot come up with a schematic for you to try We do need some details first:

1: What is your B+ voltage ?

2: What kind of load (= input impedance of the amplifier) do you intend to drive with this preamplifier ?

3: Which audio source is going to end up feeding this preamplifier ?

4: What voltage amplification factor are you looking for ?
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Old 10th January 2010, 09:51 AM   #4
Bob_345 is offline Bob_345  United States
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The b+ voltge is going t be 12 volts and the load is going to be 4 to 8 ohms. The input is most likely a guitar as I do not want do destroy any iPods or anything by accident.
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Old 10th January 2010, 10:30 AM   #5
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Looks very difficult to me; the triode curves are a disaster, (but maybe they sound good for a guitar-effect), it has an amplification of only 10x, and the plate resistance is .5 MegaOhm. Maybe you can do something cool with the heptode...

If you want to drive 4 or 8 ohms you'll need a poweramplifier; this tube will never be able to drive a speaker.(or i'm missing something)
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Old 10th January 2010, 11:01 AM   #6
Arnulf is offline Arnulf  Europe
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Quote:
Originally Posted by Bob_345 View Post
The b+ voltge is going t be 12 volts and the load is going to be 4 to 8 ohms. The input is most likely a guitar as I do not want do destroy any iPods or anything by accident.
Ugh, let's clear up your misconceptions first:

First of all, as pauldune has already told you, there is no way you will drive 4R/8R load and get anything useful out of it, unless you are into microwatts You can cobble together a preamplifier though.

Second, why on earth would you destroy your iPod by using it as the signal source ? It would take something really stupid (like deliberately shorting its output) in order to destroy it.

Third, is B+ set in stone or do you have some leeway in designing your power supply ? I would opt for 17-18V DC if I were you because that would allow you to (A) run heater off of CCS (using a simple LM317 regulator wired as a current source) and (B) use active anode load (using JFETs) in order to get the maximum swing out of your tube. This would give you more flexibility and allow you to use more feedback (if desired) to compensate somewhat for inherent nonlinearity of low voltage operation, plus you could use cathode bias with correspondingly lower value of grid resistor (= lower input sensitivity to hum).

Ultimately, preamp is expected to provide different things in different roles - with guitar you might appreciate any distortion it can generate, with audio (music) you will mostlikely want faithful reproduction instead.


@pauldune: could you please point me to the datasheet showing the characteristic curves ? All I can find are limiting values
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Old 10th January 2010, 03:25 PM   #7
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Quote:
Originally Posted by Arnulf View Post
Ugh, let's clear up your misconceptions first:



@pauldune: could you please point me to the datasheet showing the characteristic curves ? All I can find are limiting values
http://www.classiccmp.org/rtellason/tubedata/12FX8.pdf
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Old 10th January 2010, 05:29 PM   #8
Bob_345 is offline Bob_345  United States
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Quote:
Originally Posted by pauldune View Post
Looks very difficult to me; the triode curves are a disaster, (but maybe they sound good for a guitar-effect), it has an amplification of only 10x, and the plate resistance is .5 MegaOhm. Maybe you can do something cool with the heptode...

If you want to drive 4 or 8 ohms you'll need a poweramplifier; this tube will never be able to drive a speaker.(or i'm missing something)
ya actualy a distortion pedal or boost would soun great on my current guitaramp. i am very new to tubes so thanks for the help. i also have a 6av5ga power tube laying around so i might be able to make a poweramp with that.
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Old 10th January 2010, 09:26 PM   #9
Bob_345 is offline Bob_345  United States
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would a 12k5 work as a sort of powertube? i know sopht amps uses one in their 12 volt amp
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Old 10th January 2010, 10:24 PM   #10
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I know little to nothing about low voltage tubes, but here a site dedicated to it.
A lot of schedules, but unfortunately mostly little radio receivers. The 12k5 is also mentioned, it is capable of providing 50mW. Dont think that is enough..
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