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space charge tube preamp
space charge tube preamp
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Old 19th December 2016, 03:10 AM   #1
mashaffer is offline mashaffer  United States
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Default space charge tube preamp

Has anyone actually experimented with 12V space charge (car radio) tubes as audio pre-amplifiers? Decent fidelity possible? Strictly low gain or are mic/phono type gains possible? Crossovers?
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Old 19th December 2016, 04:21 AM   #2
BinaryMike is offline BinaryMike  United States
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space charge tube preamp
There's an excellent article on this topic by Quan Ronald in Linear Audio volume 10.
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Old 19th December 2016, 06:51 AM   #3
jazbo8 is offline jazbo8
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space charge tube preamp
And there's Pete Millett's Low-Voltage Tube Hybrid Headphone/Line Amp
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Old 20th December 2016, 01:30 PM   #4
kylej1050 is offline kylej1050  United States
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They can be used and actually preamp is the only 'real' application for them since none of them are capable of big output numbers. You can get some output with the power tetrode units but it won't be very loud unless you use high voltage finals or SS buffer(which is the best option TBH).

I'm working on a schematic for a 12AJ6 stereo phono preamp but without my shop and some experimentation I'm not sure how linear I can get them.

The nice thing about these low-voltage tubes is they're cheap and you can safely experiment with them using standard silicon parts like an LM317 without any worry.
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Old 21st December 2016, 12:15 AM   #5
stocktrader200 is offline stocktrader200  Canada
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found quite by accident that a 6922 produces good sound with as little as 6v on the plate
gain wise, there is very little difference in output level from 41v B+ to 135v B+ 10k plate resistor

Last edited by stocktrader200; 21st December 2016 at 12:17 AM.
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Old 21st December 2016, 01:58 AM   #6
kylej1050 is offline kylej1050  United States
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Tubes with higher heater demands are usually better suited for low-voltage duty. The 5687 is a good example as is the 5814A. The 12U7 low-voltage dual triode is just a specially tested 12AU7.

I can confirm the 6922 as I have a hybrid headphone amp that uses a 6922 with CCS plate load that runs on only 24V and was my daily driver for years. Haven't tried the 6N23P in that guy yet...
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Old 21st December 2016, 05:59 PM   #7
mashaffer is offline mashaffer  United States
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Quote:
Originally Posted by kylej1050 View Post
They can be used and actually preamp is the only 'real' application for them since none of them are capable of big output numbers. You can get some output with the power tetrode units but it won't be very loud unless you use high voltage finals or SS buffer(which is the best option TBH).

I'm working on a schematic for a 12AJ6 stereo phono preamp but without my shop and some experimentation I'm not sure how linear I can get them.

The nice thing about these low-voltage tubes is they're cheap and you can safely experiment with them using standard silicon parts like an LM317 without any worry.
Thank you I look forward to seeing what you come up with on the phono preamp so please post.
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Old 21st December 2016, 05:59 PM   #8
mashaffer is offline mashaffer  United States
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Thank you for all the info guys. Much appreciated.
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Old 2nd March 2018, 08:24 AM   #9
engels is offline engels  Israel
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Default 12AJ6

I've experimented with spacecharge tubes for quite a while. 12AJ6 is NOT a hi-fi tube, it has a significant midrange bump and low frequency rolloff. Out of all preamp tubes I'vetried I'd say 12DV7 is the most hi-fi, while is's till far from being as hi-fi as 12AX7 and such.

Here's a list of the tubes with my impressions about their sound in a neutral circuit:
12AJ6 - grainy, not much lows, midrange bump, fuzzy, good for guitar.
12FM6 - less gain, lezz fuzz, still not much low end.
12DU7 - 9-pin, way more lows than the rest of the pack. less midrange bump.
ECH83 6V triode-heptode, both are suitable for audio use, no lows, lots of middle. lo-fi sound.
12BF6 - similar to 12AJ6 with even more midrange bump. good for guitar.
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Old 2nd March 2018, 08:47 AM   #10
DF96 is offline DF96  England
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Quote:
Originally Posted by engels
12AJ6 is NOT a hi-fi tube, it has a significant midrange bump and low frequency rolloff.
Are you saying that the 12AJ6 has significant internal resonances in the audio range? If not, it cannot have a bump or a rolloff - circuits have bumps and rolloffs, valves work from DC to VHF. Of course, the distortion produced by a valve+circuit may give the impression of frequency response problems, or the circuit may be inappropriate for that valve.
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