Author Topic: Tube preamp for the truck  (Read 5029 times)

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Offline MadScientist267

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Tube preamp for the truck
« on: July 26, 2015, 01:41:16 am »
A little musing/brainfarting earlier in IRC left me with a lingering desire to build a tube preamp for the audio system in the truck...

Obviously it's not efficient. In fact the lack of efficiency is nearly being completely ignored. The only real efficiency consideration is using just a single 12AX7.

A zillion ohm plate resistance means the raw signal coming from the tube would be effectively useless to try and pump straight into the master pot... Will need some emitter follower trickery to get impedances back in check...

It's not entirely only about the majesty of tubes... there is actually a practical need. My primary audio source is one of my old phones, and between what seems to be anemic drive in the final in said phone, and the fact that the signal gets played with, isolated, split, and otherwise passively screwed with... By the time it gets to the Alpines, it's rather lacking.

What better way than for those times when an extra bump is needed, to use a good old fashioned valve? ;D

Supplies are easy, methinks... For HV, the device that came to mind is an old cold cathode laptop backlight inverter... Should be plenty. For filament, a buck converter (pronounced "tweaked car cell phone charger") should be able to handle that with little issue.

Now, I know there's gonna be issues... Supply noise, impedance matching, and so on... but I don't think any of them are insurmountable... Heavy filtering and clever use of transistors following the plates, should come together without too much issue... The signal would legitimately be amplified by the tube, and add magic to the music for those "special occasions". The efficiency hit would be fairly well hidden by the gnarly suction occurring inside the power amps  8)

I'm probably just nuts... but then again I'm building a solar oriented uhaul RV conversion... to live in full time... I can't have but so many marbles lined up... Shouldn't the tunes be able to at least reach full capacity?  :o

Steve
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Offline bj

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Re: Tube preamp for the truck
« Reply #1 on: July 26, 2015, 07:35:03 am »
     I understand the need for the tunes completely.   When lightning took out the shop amp a few years ago,
(along with a lot of other stuff) I was a bit lost.   Always had a love for tube stuff, but went the other way
and built a solid state kit amp/preamp.  Lots of tweaking to get it to sound good to my poor hearing, but worth
every second spent.
    Good luck with the project.  It will be worth it.
"Even a blind squirrel will find an acorn once in a while"
bj

Offline MadScientist267

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Re: Tube preamp for the truck
« Reply #2 on: July 26, 2015, 09:20:42 am »
It would probably be a little while before it got built, as there are still things that haven't seen round 1 yet, but no doubt I think it would be fun to put together and dial in as well.

In a world where transistors are only made up of a few atoms and come by the bazillion in all the kooky toys we've got at our fingertips today, it's all too easy to forget where it all began. I have had the desire for some time to make a complete stereo tube amp, something of the flavor of 100W/channel clean... but of course that's quite an undertaking to really do right, and the twists and turns of life have other ideas for me... LOL

This would allow me to at least sneak the essence in somewhere and make the whole project that much more unique ;)

Steve
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Offline oztules

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Re: Tube preamp for the truck
« Reply #3 on: July 29, 2015, 06:00:35 am »
I can't put my hand on it straight away... but I have still got my pre amp using a pair of   12AX7 as the pre amp and tone control triodes.... think the circuit came straight from the RCA receiving tube manual of about 1968 or thereabouts... it even had a rudimentary printed circuit board instead of point to point..... think I used texta and nitric acid back then....

Plate voltage can be as low as 90v, and heaters only 150ma@12v... so not a big ask at all now days... easy booster and use the 12v almost direct for the heaters...

As for the power amp..... not sure I would do that... too hard to find big audio transformers etc, and real power needed then... probably 500v at near an amp for the plates for near 400w. of plate current for the stereo 100 watts.... unless you can pull off B class and get to better efficiency, and lower duty cycles.... but usually AB1 or 2.

So I vote for a valve pre amp to get the smooth 2nd harmonics, and then slum it and do a decent solid state power amp for the output.

.... just sayin...


.........oztules
Flinders Island...... Australia

Offline MadScientist267

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Re: Tube preamp for the truck
« Reply #4 on: July 30, 2015, 10:56:35 pm »
Oz -

Would be cool to see the schematic if you do happen to run across it... No out of the way digging needed tho, it'll be a while before I could do it.

The power amp has always been a bit of its own thing... That came about back in my married days, a project of scale one might say. It'll probably never get built now, or at least isn't anywhere in the crosshairs due to everything else more or less "taking out the scent trail" so to speak :-\

The power amps in play are both Alpines, a 100x100 and a 40x40, biamped, "split" subs (main and auxiliary for focusing). Sounds really good as it sits, just falls short from time to time on certain files, or even a well mixed album etc sometimes gets me slowly opening the pots, only to bury them way earlier than expecting. So a little boost that has some aesthetic appeal would be perfect.

We shall see hehe
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Offline oztules

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Re: Tube preamp for the truck
« Reply #5 on: July 31, 2015, 02:04:29 pm »
This looks similar from memory

...........oztules


[MOD EDIT - Cleared invalid attachments - Steve]
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Offline MadScientist267

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Re: Tube preamp for the truck
« Reply #6 on: August 01, 2015, 02:24:59 am »
I like the first one... clever use of followers to get complete control of impedance... Probably a bit more than I am looking at for this, really just because it will be redundant (more on this in a sec). Looks like it could be made into a rather nice device however.

The second one is closer I think to what I had in mind, only with a single tube doing stereo (so only a single stage). The output impedance for this one is closer to what I was expecting, which would indeed need a follower of some sort to make it remotely useful with the existing system. I'm willing to cheat on that and just use transistors, as a fair portion of this is really just ambiance that the tube visually provides (a little glow, some mystique, conversation piece, etc)... Of course tho I don't want to just light the filament up... If it's not part of the circuit, it would be a "lie" LOL ;)

Luckily I don't need insane gain, +9db could be considered the most extreme I'd need it to pull off... if it isn't stable or introduces too much noise and/or distortion at that level, less gain is acceptable. Big thing is drive that volume pot (10k) and everything behind it with a decent blow.

The schematics give good starting points with useful values... have to tinker around with the pencil and paper for a bit and see what might come of it. As long as it doesn't degrade the signal, provides a little umph, and the tube is legitimately inline, I'm good with whatever comes of it.

Steve
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Offline rossw

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Re: Tube preamp for the truck
« Reply #7 on: August 01, 2015, 05:13:15 am »
this is really just ambiance that the tube visually provides (a little glow, some mystique, conversation piece, etc)... Of course tho I don't want to just light the filament up... If it's not part of the circuit, it would be a "lie"

So, a simple driver, driving a small (say, 2mW) laser diode, hitting something that'll light up and look pretty... PWM so the average intensity will change and be visually obvious, AC-coupling the output via a photoreceiver of some sort...

Offline oztules

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Re: Tube preamp for the truck
« Reply #8 on: August 01, 2015, 10:15:01 am »
I used this one back in the day as well
4245-0

No problems driving Philips motional feedback speakers from a Nackamichi 1000 tape deck. The 1v output will match most amp inputs.... but not drive a current stage.

It was driving into this:
4247-1

4249-2

If you look at the circuits in B in the last picture, you will see an interesting way to split the spectrum into three frequencies by sum and differences of frequencies.... very interesting back then....


Blew a lot of money back in the day for this sort of stuff......young and silly.

Then designed my own feedback speakers using impedance rather than accelerometers built into the cones... sigh....

Ross..... musicolor springs to mind there.....


...................oztules
Flinders Island...... Australia

Offline oztules

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Re: Tube preamp for the truck
« Reply #9 on: August 07, 2015, 06:15:03 pm »
Dug it up out of the rubbish I keep... near 40 years old... still works too :o

4256-0


It would appear that in 40 years or more, my work has not improved much at all.... if any..... I made a mess then and still do.
My wife ( who I have kept as long as the preamp)  thinks I am still a messy mongrel.


.............oztules
Flinders Island...... Australia