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Old 23rd December 2003, 06:37 PM   #1
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Default 12 volt tube preamp help

Hi everyone,

Go easy on me, i'm a newbie and know really little about electronics.

The hardest schematic i ever had to read until i starting browsing this forum a couple of months back is a 4th order passive crossover.

As yet, i have yet to create anything with regards to audio, but i have DIYed my entire car audio system(with regards to its installation that is heh) and i do have a passion for audio.

I do hope to build myself a tube amp one day, but thats another story.

In the hope of buliding up my working knowledge to that aim, i have set a slightly less ambitious project for myself, a PC audio system!

I already have a schematic (from tubecad i think) for a neat little 4w class A SS amp(attached) which some people have tried and seems to work ok. I like it cos its simple and most importantly, runs straight off my PC power supply(12 volt) without a transformer(not sure if thats the right term)

I do love that magic tube sound though, and as such, i want to somehow get a tube in the signal chain. The best thing that would be if i could DIY a really simple tube preamp to this effect that will run off a 12V supply.

I've looked absolutely everywhere(this forum, and all over the web) for a schematic like this but havent been able to come up with anything.

The closest thing i found that suits my needs is this:
http://www1.jaycar.com.au/productVie...Max=&SUBCATID=

Actually, its perfect for me in the sense that it has everything i'm looking for. Problem is, i think the actual cost would be far lower than the kit price given and i really want to build everything from scratch with a schematic(since i'm dealing with really simple stuff here already).

I'm not sure what tube to use. My car audio amp(commercially bought) uses 12ax7s. Recently one of the channels started to crackle so i'm swapping the Electro Harmonics 12ax7 tubes for svetlana ones which i just bought. I expect only 1 tube to be faulty or problematic(at most) so that would leave me with 3 spare EH 12ax7s for the preamp.

Some threads have mentioned that 12ax7s have too much gain, and since preamps dont exactly cost a bomb anyway i'm open to suggestions as to what tube to use.

The main thing i need is the schematic though.

Thanks a lot for your time(and reading my long post)
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Old 23rd December 2003, 09:39 PM   #2
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How does one edit a post? Anyway just to be a little more clear, i'm looking for a tube preamp which uses 12V as its B+. If i'm not wrong, this would eliminate the need for any kind of transformer?

I've also been looking a full tube amp schematic with a 12v B+. I think its not something thats possible though(not even for a simple 3w amplifier?). But i think a preamp would be a little easier to come across.

Appreciate the help fellows!
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Old 23rd December 2003, 10:40 PM   #3
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Hi, luvly -

I don't know of any such commercial product but here's a list of 'space charge' tubes that were meant to work off of low voltages like 12VDC:

12AC6 rem cutoff pentode
12AD6 pentagrid conv
12AE6 duo diode medium mu triode
12AE7 dual triode
12AF6 rem cutoff pentode
12AJ6 duo diode hi mu triode
12AL8 med mu triode power tetrode
12BL6 rem cutoff pentode
12CN5 rem cutoff pentode
12CX6 rem cutoff pentode
12DE8 diode rem cutoff pentode
12DK7 duo diode power tetrode
12DL8 duo diode power tetrode (diodes have own cathode)
12DS7 duo diode power tetrode
12DU7 duo diode power tetrode
12DV8 duo diode power tetrode (diodes have own cathode)
12DY8 med mu triode rem cutoff tetrode
12DZ6 rem cutoff pentode (curves are in 1963 RCA tube handbook)
12EA6 rem cutoff pentode
12EC8 med mu triode semiremote cutoff pentode (semiremote?)
12EG6 pentagrid amplifier (no idea what this is)
12EK6 rem cutoff pentode
12EL6 duo diode hi mu triode
12EM6 diode power tetrode
12F8 duo diode rem cutoff pentode
12FK6 duo diode lo mu triode (triode curves are in 1963 RCA tube handbook)
12FM6 duo diode med mu triode
12FX8 med mu triode pentagrid conv
12GA6 pentagrid conv
12J8 duo diode power tetrode
12K5 power tetrode
12U7 twin med mu triode
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Old 23rd December 2003, 10:50 PM   #4
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Wow...thanks for the info there.

Alright....i'm not sure if i'm on the right track but in order to narrow down the list i was thinking of going triode...only reason for that is because i see it mentioned a lot and from what little i've read about tubes they seem the simplest(it appears simple to me cos triodes only have a cathode,anode and the heater as compared to the rest LOL)

Of the remaining list, i'd be happy with the one that provides the most 'tubey' sound, since tube sound is really what i'm after.

If something has a 12V B+, does it mean 12 strictly? I was thinking of perhaps implementing the same preamp design in a DIY hybrid amp for car audio(an environment where voltages swing from 12V to 14.4V constantly). The inspiration came from this schematic. Hoping i could use the same preamp design for it in future.

I'm not sure if it matters, but it would be best if the preamp can accept and provide gain to high voltage inputs(anywhere from 100mV to 8V). I'm not sure if this would affect the schematic. At this point it seems to me that the only thing this would affect is the value of the pot on the inputs.

Oh yeah, i won't need a switch on the inputs or anything, since in any case i only plan on using it on a single input.

I don't know if i'm being to picky. I hope not, and i sincerely apologise if i do. If i'm asking too much...i'll still be very happy with a simple 12v single tube preamp with no adjustable gain with as few components in the signal path as possible.

Thanks again

Kevin
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Old 23rd December 2003, 10:53 PM   #5
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Sorry, this is the schematic i plan on using for a car audio hybrid tube amp.
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File Type: gif car class a amplifier.gif (4.4 KB, 633 views)
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Old 23rd December 2003, 10:56 PM   #6
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And this is the one i was planning to use in my PC.

Man, solid state everywhere...someone help me get a tube into these things

I'll only need about 3W per channel for the PC amp. I was hoping to get a full tube amp schematic using 12V as its B+ but it seems hopeless so the preamp will have to do!
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File Type: jpg schematic ss 4w class a pc amp.jpg (12.3 KB, 574 views)
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