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Old 22nd December 2009, 12:04 AM   #1
Luke352 is offline Luke352  Australia
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Default Tube pre-amp for a vehicle?

Hey guys, I'm just posting this here from the Analogue Line Level section as it was suggested I may get better assistance/better knowledge in this section.

My original post:


As the title says, I'm interested in building a Tube Pre-amp/ gain stage for my car, I'm just interested to see what results I'll get and also for a bit of fun to keep myself occupied.

I have my current HU that has up to 5v pre outs, and then off it goes down my RCA's to my amps, which have there own pre-amp section with variable gain controls, so I don't need this Pre-amp to have volume as a part of it's design or to have much actual gain, infact straight through up to maybe a 2x gain would be perfect as a little boost to the signal voltage wouldn't hurt. Basically I just want to experiment and see what sort of results I get by adding a bit of tubey goodness to the signal.

So can anyone suggest some designs or kits that would be able to work off a 12v power supply and that are fairly simple in design and low gain.

Thanks Guys and Gals.

Luke



I got this reply:

Tubes and vibration - not a good combination. If you want the tube sound in a car I would look at a FET based tube simulation.

I guess it depends on how reliable you need it to be and how much you are prepared to spend on replacement tubes. I don't really know how well tubes will survive in this environment, I know they were used in early car radios. That might be a good place to look for inspiration for designs. Unfortunately I have no car tube circuit diagrams.

Regards,
Andy


My next post:

Yeah vibration will be an issue I'll admit, but there are several amps on the market that use Tube Pre-amp sections, so I figured the vibration issue can't be too bad, aslong as I can mount them in a shock absorbing setup.

I guess for initial trial it won't have to be too reliable but if I like what I hear I'll invest more time into making it more resilient to vibration and shock.

I figured something like this 12AX7 Tube (valve) Preamplifier Kit would be ideal if modified to use a 12V-14V DC supply rather then the 17V power pack it is designed for. But then the issue becomes how resilient will the design be to a DC supply voltage that will swing from around 12.5-13.8 volts.

The other issue with that design is it has a 4x gain, I only want 2x - 2.5x at an absolute max, I also can't find the kit on the SC online shop. I'm sure it could be modified for lower gain , but that is beyond my abilities, I can solder and understand what symbols and values mean on a circuit diagram but as to understanding how a design works that is still beyond me I'm afraid.


Anyway as it turns out that kit out of production which I missed in in the link, it was pointed out to me.

So guys, does anyone know of any designs or kits that may be useable for a purpose like mine?

Any help is greatly appreciated!

Luke

Last edited by Luke352; 22nd December 2009 at 12:11 AM.
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Old 22nd December 2009, 12:12 AM   #2
Luke352 is offline Luke352  Australia
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Update looks like I found a source for the Circuit Boards, now I just need to figure out how to lower the gain.
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Old 22nd December 2009, 12:18 AM   #3
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go with a cathode follower design - zero gain, plenty of drive to supply your power amps. Look for metal cased valves - less microphonic and more able to withstand the vibration in a car. You'll need something that runs right down low in voltage unless you want to build a special high voltage supply. Like most things, its all been done before - google is your friend!
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Old 22nd December 2009, 12:19 AM   #4
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Quote:
Originally Posted by Luke352 View Post
Update looks like I found a source for the Circuit Boards, now I just need to figure out how to lower the gain.
voltage divider at the front end is the easiest way.
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Old 22nd December 2009, 12:26 AM   #5
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here you go - Space Charge and Other Low-Voltage Tubes
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Old 22nd December 2009, 01:26 AM   #6
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Locktal based tubes were intended for use in the mechanically stressful automotive environment. The triodes in the 14N7 are electrically identical to those in the revered 6SN7. So, the type makes excellent cathode followers and its heater will run directly off of the "12" VDC of the vehicle's electrical system. The "gotcha" is getting a B+ rail that's "tall" enough, say about 180 V. More would definitely be better.
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Old 22nd December 2009, 03:52 AM   #7
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Quote:
The "gotcha" is getting a B+ rail that's "tall" enough, say about 180 V. More would definitely be better.
There are two ways to do this.

One way is to get one of those cheap 100 Watt 120 volt power inverters ($20 or less), take apart and tap into its internal DC supply. It is usually in the 160 volt range. These "inverters" use a high frequency step up DC to DC converter to create 160 volts DC and them use a mosfet chopper circuit to make 60 Hz AC. This is the most elegant and efficient way to get the DC. It is possible in some cases (if the inverters are isolated) to stack two or more IDENTICAL units to generate a higher voltage.

There is also the lazy way. I have been told that it won't work, or you will get hum or your power transformer will buzz. Yes, simply plug your 120 volt tube amp (preamp or whatever) into the power inverter. NOTE, the output of these cheap inverters is NOT a sine wave (regardless of what it says). Some transformers will indeed buzz (the Hammond 200 series) and the actual B+ voltage will usually be a little lower than it is when the amp is plugged into the wall outlet. A poorly filtered power supply may result in some hum, and a choke input type supply may not work right. Obviously a big tube amp will need more than a 100 watt inverter.

In spite of these gotchas I have run a few tube amps in the car by simply plugging them into a big 800 watt inverter that I got on sale for $50. I ran the tube amp directly from the line outputs of the factory CD player in my 99 Mustang (Mach 460 system). Even at this level microphonic effects were a problem on our pot hole infested roads.

The Simple SE works well plugged into an inverter except for a mild buzz emitted from the Hammond made Allied power transformer. The amp (and the inverter) was in the trunk of the Mustang so the buzz was not audible inside the car. The twang of vibrating grid wires could be heard on some road surfaces and was obvious when driving over speed bumps with the volume turned down. Tube rolling was needed to find a quiet set. Tripple mica 6201's were the quietest 12AT7's and I used some military surplus 6L6 type output tubes (I don't remember the number right now).

In a moment of insanity (actually about 10 days) I put my 845SE in the trunk. The car is a convertible so the trunk is small and the 2 chassis amp plus the inverter filled it up. This amp worked too, but the microphonics were much worse. The game ended when a pot hole resulted in a broken filament in an 845 tube. Fortunately it was a cheap Chinese tube. I never tried a Tubelab SE since it uses expensive DHT's, and DHT's had already proven troublesome.

I usually drove that car with the top down and I eventually decided that the tube amps sound was wasted in that environment. I wound up hooking the factory 250 watt amp back up in the car after I fixed it. I have since sold that car, and the city has repaved the roads, so I may try it again some time.

My first car was a 1949 Plymouth that came with a tube radio. It had a pair of 6V6GT output tubes and was loud! I found an identical radio in a junkyard, and mounted both radios in the trunk and hooked them up to some home made speaker cabinets in the back seat. I fed them with the output from a Panasonic portable cassette player. This was the loudest car stereo in my high school parking lot since boosters, amplifiers and powered speakers hadn't been invented yet! (1970)
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Old 22nd December 2009, 04:02 AM   #8
Luke352 is offline Luke352  Australia
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The kit I've mentioned actually has DC-DC power supply diagram complete with winding instructions for the step up to 250V required for the 12AX7. The more I read into it the simpler the whole conversion seems, I am concerned about the fact that the voltage supply in car can vary quite a bit so my heater voltage and step up voltage will be constantly changing as I'm unsure what effect this will have.

Also as has been pointed out vibration etc.. is going to be an issue but I'd like to try it and just see what happens, I can always figure out a way to try and shock/vibration isolate things as much as possible.

Oh I also had a read into using a voltage divider in the front end, seems simple enough and probably preferable to using a pot since it's more accurate and it's less stuff to have to try and set correctly.

These other tubes mentioned look interesting especially the low voltage ones, but I have no idea how to integrate these into a design.

Thanks for the help so far.

Luke
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Old 22nd December 2009, 07:27 AM   #9
m6tt is offline m6tt  United States
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ECC86 or one of the other space-charge tubes were designed to be used with 12v on the plate, there were also some 30s tubes (I forget!) designed to run off a 20-30v farm supply (?). The ECC86 are frame grid, reducing microphony. They are apparently very similar in construction to ECC88, but with different sound and operating points. Low Z too, can load it around 1.5k it looks like. Perfect for 10k inputs.

There were some pentodes and heptodes that could be used as is or triode strapped.
Googling "space charge tubes" should give you more info.
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Old 22nd December 2009, 11:32 AM   #10
Brit01 is offline Brit01  United Kingdom
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I've found the perfect solution.

Just needs a bit of elbow grease

antigua radio de autos,a valvula - $ 200.00 - MercadoLibre
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