Tube amp for my car
 

This is going to be an odd request.. But I love the sound of tubes.. Even the headphone amp on my Mac is tube powered, and in the livingroom when watching movies and stuff on the Apple TV, we use an amp that is tube powered.. Love it, LOVE it. Especially when people think tubes aren't that good, and they hear a gun shot or explosions in some action movie with regular 8 ohms speakers, and they are like "wow, and there's no subwoofer?!"

Now I want it for my car. Someone I work with, has a Fisher 500c receiver, that is pretty banged up. Electrically it works and had the caps replaced, but it has no casing, and is just a bare chassis.. I was thinking, if I replaced the speakers in my car with a higher impedance other than 4 ohms, perhaps 6 or 8 ohms, I could use this amp, and even use an inverter to provide it the AC power it needs.

Now, that should be easy to do. My car is already wired up for an amp, which has bit the dust, and it needs to be replaced, so this is why I am thinking of doing this.

Can that wonderful old amp be retrofitted into a car? It would be mounted in the trunk of the car, but on my Caprice, the trunk actually doesn't get hot at all, even in hot weather, it's rather nice in there, so heat won't be as much of a concern.. Though I would install fans anyway just to play it safe.

Is it a crazy idea, or an interesting idea? Should I do it, or is there a better solution for tube sound in the car?

My commute to work is an hour, so good tunes is very important for me.

It takes all kinds, and if doing this pleases you, by all means go ahead. You can build or buy an inverter. Just the heaters in the tubes draw so much current that it might be wise to take them off the filament winding of the power transformer and run them directly from the battery instead. That would improve the efficiency of the setup, which will be pretty grim in any case.

So you like to wait for tubes to warm up, and you like the distortion many tube amplifiers provide. I prefer clean amplifiers, something solid state is much better at providing than even the best tube amplifiers.

I don't mind warmup time. I'm not in a rush.. And even when tubes provide distortion, it's much more graceful to the ears than a transistor distorting. I don't hear that distortion. Not even in the livingroom amp.

I suggest trying it with an inverter first, then see if you want a permanent mod. You might want to come up with some kind of shock-absorbing mount too. I am not sure if tubes would like all that road vibration.

You should get a Panasonic CQ-TX5500D - Japanese-market car stereo with tube drivers! There's one on the 'bay for $645 BIN right now.

If you do go with the Fisher, like CoogarXR said, you should put in some kind of isolation to preserve the tubes (and cut down on microphonics when going over bumps).

The idea of tube-type amplifiers in car radios is not new. Years ago, car radios used miniature tubes and they worked quite well; no microphonics unless the output tube had loose elements, and the radios themselves were built to withstand road shakes, bumps, and other things that would shake a standard radio's tubes apart. However, the old automobile radio reverberation systems with actual metal spring delay lines were prone to noise problems if the car in which they were installed went over a bump in the road. The springs would bounce against each other or against the unit's housing, resulting in a loud "BONG!" sound from the radio's speakers (the main one in the dashboard and the second one in the back of the car, near the rear seat). My dad had one of these reverb units in his 1963 Chevy BelAir; the reverb worked very well, no problems with the springs causing the noises I mentioned -- at least none I can remember.

BTW, Cadillac car radios probably had reverb as standard equipment, as these were luxury cars with all the bells and whistles (and a price to match). My mother had a '57 Caddy with an AM radio that had a signal-seeking tuner and preset buttons for favorite stations. I wouldn't have been surprised if that car had a reverb system as well.

Problem with this one, is that the 94 caprice's radio is not a double DIN spot.. It's close, but not quite. I already have a custom radio in there anyway, that's hybrid digital on FM and AM (I refuse to call it HD radio as it's not high definition), and has a USB port so I can just plug in my flash drives for music.


As far as microphonic, i'll definitely find a way to make it shock absorbed so the tubes fare well.

Ran a modified Dynaco Stereo 70 in a '71 Plymouth Duster many years ago.

Rewired the heater circuitry for direct operation from 12VDC. Pulled out the stock power transformer and the 5AR4, and obtained B+ from an old surplus dynamotor from a 50s era mobile 2-way radio setup. Negative bias supply came from a little DC-DC converter circuit I cobbled together.

Soon discovered that EL34s and 7199s don't like vibration very much. Tubes developed loose internals and microphonics very quickly, even with the entire amp shock mounted on rubber isolators. If I were to try something like this again, I would strongly consider the use of highly ruggedized mil-spec tubes. Bendix Red Bank 6384s would be great for the output tubes, but they are getting harder to find and go for $$$ if you can find them.

You should definitely do it with an inverter first, to be sure you're gonna be happy with the results (IMHO). Sort of a 'proof of concept' run.

Should be an interesting project. As long as you have fun doing it... I actually put 8-track players in two of my previous cars just for the heck of it. (Link)

If you don't like iBiquity's marketing term "HD Radio" you can always call it "IBOC" (the engineering name, in-band on-channel)... :music:

Cadillacs DID NOT come with reverb units. They had the standard AM signal seeking radio. AM/FM mono was optional by '63 and did not have the signal seeking feature. They were tube with solid state outputs(hybrid). As far as that Fisher is concerned, you will most likely need to replace the electrolytics. They dry up and short. More than likely will have very dirty tube sockets. I've worked on tons of Fishers and they always have some issues. I suggest that you install brand new 7591's.

I can just hear the buzzing from the crappy stepped signwave out of the inverter. The power transformer in the receiver would not like the stepped sign wave either and would be very inefficient.

Also speakers used with tube equipment are usually very efficient, over 100db 1 watt 1 meter. Automotive speaker are usually horrible, around 85db.

Also, if the amp uses 200 watts thats 20 amps at 12v, then factor in all the inefficiency of the inverter it would be 30+amps continuous on the cars electrical system and the alternator wont support that. If you factor in fuel pump, headlights, blower fan, compressor clutch, engine management, etc it wont do it. Maybe if the engine is always at 2000rpm but not through a regular driving range.

When guys a thousand watts of solid state power its very low duty cycle, Tubes dont do that, they draw a fairly consistent current from a whisper to a scream.

I always wanted to convert one of those old tube motorola police radios to a ham rig for my truck but its just way to impractical

If you are keeping it in the trunk and using your cars existent solid state tuner then removing the tubes associated with the radio would be a wise move to save power. If you wire the tubes to take the car's 12V mains directly then why not go a step further and add a DC to DC converter for the B+(you can get them pre-made or design and build your own).

I have a home made car headphone amp designed with sub-miniture hearing aid tubes that has been partially dismantled by me for tubes for a project that I got sidetracked on. Even when I do part it out I intend to draw a schematic so I can duplicate it someday if I desire.

I prefer tube sound to SS as well. If you like vinyl and 78's like me you will find tubes to be substantially more forgiving of record scratch. I can really demonstrate this by taking a worn 78 and playing it through a tube amp followed by a solid state one(my 65 Zenith SS console sounds GREAT on a good LP, but put a 78 on and the record noise overpowers the music).
Also I find the highs on my tube gear to be sharper and more melodic than those of mt better SS gear which makes them feel a bit jagged.

As mentioned above, another approach for getting some tube sound in the car would be to use a tube buffer stage ahead of a SS amp. And the tube buffer built using say subminiature tubes intended for fighter jets or guided missile service. Say a pair of triodes in cathode follower connections. And you may be able to take apart an inverter to get at the high voltage DC before it's chopped into square wave AC, to get the B+ for the tubes.

I was thinking of that but with a 12AX7 type tube. I had not though of opening the inverter and removing the chopper fets. Brilliant!
All you need to do is ad the 2nd and 4th order harmonics to give it the tube sound.
I saw an ipod docking station with a 12AX7 preamp with the same intention.

There's the 6112 twin triode submini tube, somewhat similar to a 12AX7. With a reasonable mu around 70 IIRC. It's a 6.3V heater tube, but use a pair and build an SRPP circuit, and the heaters in series to run them off the 12V car power. With wire leads you could just solder into the circuit, so no tube getting loose issues, and it's designed for mobile work.

thanks.That's the ticket. :thmbsp:

As how much power is required, I have a 140 amp alternator, and my engine idles roughly at 900rpm. It idles like that because it's a 350 chevy small block with a 400 crank in it, making it a 383, along with it's cam profile.

So even at idle, im sure I can get some power in the electrical system that should be useful for something.

But using these simpler designs are interesting too.

Are you thinking of a switchmode AC-DC power supply? Every 12V-120VAC inverter I have seen does the primary side switching at 12V, and the AC voltage is just stepped up using a transformer. No high voltage DC available.

More modern, and smaller ones, look to be switchmode DC to DC converters running somewhere around 30KHz or higher, with subsequent choppers to make it look like 60Hz AC squarewave. Assuming that the DC-DC conversion has no DC path between its primary and secondary, or if the negative end of the secondary side is tied to the input 12VDC, you can use it for B+ ahead of the 60Hz chopper ckts. You may need additional filtering to get rid of switching crud off this B+.

If you have an inverter that runs at 60Hz 12V to a 60Hz transformer, and the secondary is isolated, just feed it into a bridge rectifier and filter caps to get the B+.

Ok so I tried a bit of what was recommended, especially feeding the tubes that can, off of 12v directly.

I can get sound, and it's feeding the speakers sound that can get fairly loud. Only issue is, while it's seemingly not causing the volt meter on my car to go low at all, and voltages are ok and normal so far, but my check engine light oddly comes on, on super bassy music (like 90's alternative rock), and it's always the same trouble code from the cars computer, right O2 sensor read error.. Very weird, but when I disconnect the amp, the code goes away. It doesn't always do this with the amp attached though.

Anyway, here's an audio sample. I won't be able to keep this video up, it's being scrutinized for the music contents.

http://www.youtube.com/watch?v=_w-h263ur10

EDIT: And I did notice, at 1:28, when the songs bass starts playing, I have noticed some distortion in the bass guitar sounds. It's as if the tubes are possibly suffering from enough current, but are getting the voltage though.

For the other stuff that needs an inverter, I chose an inverter that produces the best sine wave, that I could find.

I did just this very thing in my '68 Chevelle in the late 70's with a Trio W-45 stereo integrated amp and a Tripp-Lite PV-150 150w power inverter. Don't remember what kind of under dash tape/radio thing I was using.

And, that's exactly what I heard, as well. So, out it went. :D

Probably not wise to run the filaments directly from the cars power due to the fact that it's at least 13.8 volts and higher when cold. Filaments wont be too happy. A regulated PS would be in order. B+ is 375V for the 7591's plus 200V+ and 160V for the others. The 12ax7's are already run in series in that receiver, However the rest are not. The current draw for that receiver is 3A max.. Removing the FM tuner and phono preamp tubes will raise the B+ voltages and is not advised without modification. Maybe just sell it on eBay. You'll get more than $200 for it easily and buy something better.

A series power resistor, or even the resistance of the supply wiring may be all you need to get it to 12.6V.

Something else to be very mindful of: When mounting the amp in the trunk or anywhere else, be very careful where you drill holes for screws or anything else. You really don't want to drill a hole into the gas tank, catalytic converter, exaust, drivetrain and such. And be sure that where you do place a nut and screw or pointy end of a sheet metal screw won't get hit by something as the car bounces along on a bumpy road. And be sure you can still get the spare tire and jack out if you ever need to. Or some access port the mechanic may need to get into (fuel pump switch and such).

If your proof of concept test works out, you may want to take the iron out of the amp, and custom build a chassis to create a new amp to fit. And use rugged tubes.

Drop by AudioKarma, our sister forum site. one is for car audio http://www.audiokarma.org/forums/forumdisplay.php?f=149
and tube audio http://www.audiokarma.org/forums/forumdisplay.php?f=14