Vacuum Tube Car Amp

[RicharD]

OK... time for another hair-brained idea from the buttery guy.  I'd like to put a tube amp in my pickup truck.... that I just wrecked.    Cell phone related fender bender.  I should be put up for public ridicule.   Me = dumbass  anyway...  Here's my plan:

Starting with what I have on hand is a matching pair of PP OT's harvested from an old HiFi that were driven by pairs of 6BQ5's.  Output power should be good enough for me.  No need to thump.  So counting on thumbs, I need about  200mA @ 300ish VDC = 60VA.  Here comes the uncharted territory for me.  I've never built a power inverter.  I gotta get from 12VDC to 300VDC.  I searched all of the internets  and found this circuit.

It's 500 watts which is way more than I need.  If I understand this circuit correctly, just like in a tube amp, the last 8 transistors are sharing the 500W load.  If I knock it down to a single pair  I should get up to 125VA.  My thumb count was 60VA so I think I'm golden.   Mouser sells a transformer rated at 135VA , 230:24VCT.  Now, if this inverter is 100% efficient, and I'm swinging +/- 12V, then I should get 230VAC out.  230VAC bridged gets me in the 300VDC neighborhood.  The million dollar question is how close to 230V will I get?  I'm half way expecting this to end up as a "roll your own" transformer.  Another question, besides will the dang thang work, is there any compelling reason why the multivibrator has to be set at 60Hz?  I'm not plugging in my TV, I'm going straight back to DC.  Wouldn't the supply be theoretically quieter if the AC frequency was say >20kHz, up and out of the range of hearing?

Looking back to the source, 135VA @ 12V is >11 amps.  That tells me this thang is gonna get hot and I'm gonna be using some big honkin' wire.  Probably #10 THHN.     I've attached my schematic.

[chocopower]

Just a question..

in the South Texas Amp i see you draw a fixed biased cathodyne P.I. like some Ampegs used too. Some people call it the "Paul C mod"... anyway..

I think you missed the resistor bettwen the V1B grid (pin 2) and the "C" power node....  
It uses to be a 2,2M o higher value resistor.
I expected to see, too, a coupling cap bettwen the anode and grid (pins 6---2)

I,m right, or this is just another diferent way to do it?

[kagliostro]

Hi Richard

Nice project

what about to use a commercial european car inverter (12v / 220v)

are that difficult to obtain in the Stetes ?

http://cgi.ebay.it/INVERTER-600W-TUTTE-LE-SOLUZIONI-12V-A-220V-240V-/280583760027?pt=Trasformatori_e_Raddrizzatori&hash=item415418689b

this one is too big, 600W but there are many other lower in price and W

like this that is a 200W

http://cgi.ebay.it/INVERTER-DIGITALE-PER-AUTO-200W-DC-12V-AC-220V-USB-5V-/180629642014?pt=Trasformatori_e_Raddrizzatori&hash=item2a0e5d9b1e

Kagliostro

[sluckey]

in the South Texas Amp i see you draw a fixed biased cathodyne P.I. like some Ampegs used too. Some people call it the "Paul C mod"... anyway..

I think you missed the resistor bettwen the V1B grid (pin 2) and the "C" power node.... 
It uses to be a 2,2M o higher value resistor.
I expected to see, too, a coupling cap bettwen the anode and grid (pins 6---2)

No, it's just a typical direct coupled self-biasing split load PI. No need for a voltage divider from the C node or a coupling cap because it's direct coupled from the previous stage and self biased because of the cathode resistor.

[Tiny_Daddy]

As I recall those radios used a mechanical vibrator. AES sells a selection of them. Now all you need is a power transformer with the proper windings.

[Jack_Hester]

Why would an amp designed around 'space charge' tubes not provide enough power to build your car stereo around?  It would eliminate the need for an inverter.  Not sure how much power you want to achieve.  

Jack

[RicharD]

in the South Texas Amp i see you draw a fixed biased cathodyne P.I. like some Ampegs used too. Some people call it the "Paul C mod"... anyway..

I think you missed the resistor bettwen the V1B grid (pin 2) and the "C" power node.... 
It uses to be a 2,2M o higher value resistor.
I expected to see, too, a coupling cap bettwen the anode and grid (pins 6---2)

No, it's just a typical direct coupled self-biasing split load PI. No need for a voltage divider from the C node or a coupling cap because it's direct coupled from the previous stage and self biased because of the cathode resistor.

Exactly.  This is a plagiarized circuit.  I believe the direct coupled stages are supposed to be better because there is no RC network between them.   Error's I've just noticed:
No dot where C5 connects to ground
R8 & R9 should be .1 ohm

I've never messed with a shunt coil vibrator.  I have some reading to do.

http://dev.emcelettronica.com/vibrator-power-supply-design-mallory-standard-vibrators

[kagliostro]

Mechanical Vibrator ......... or a ..................... Dynamotor

like here http://www.montagar.com/~patj/tubeart.htm

No I'm kidding that's too crazy

I think the best way is an inverter, but if possible I would like to use a commercial model, don't build it

Kagliostro

(EDIT) p.s.: give a look to the bias sypply in the link I posted, they use a 400Hz transformer, as from my experience you can find it cheap at Ham Fest or Surplus sales - the advantage is that those transformers are more little than those at 60Hz

[Tiny_Daddy]

Just remembered: 40 years ago I rewound a 400Hz 3-phase core in delta-y with rectifiers to supply 120VDC from the jeep's alternator. The alternator was modified to bring out the 3 phases. Shouldn't be a problem making a higher-voltage supply this way. The regulation will be very sloppy though, relying on the alternator's regulator.

[DummyLoad]

now this looks like something that might contain the smoke...

http://w5jgv.com/inverter/index.htm

[PRR]

> I've never built a power inverter.

So don't. You are an electrician, I think? Transistors are outside your job; transFORMers are right up your alley.

As Daddy says: "120VDC from the jeep's alternator."

There's AC inside the Alternator. Open it, look for the diodes, tap between the brushes and diodes. It's 3-phase, may be Y or delta. Just grab one phase for now.

It's about 12V.

Frequency: Assume engine idle is 600RPM or 10 rev per second. Alternator is belted-up about 1:3, 30 revs per second. See if it looks like 30Hz. It may be that heavy alternators are wound 6-phase, which might work out to 60Hz. That's at idle, 5 to 10 times higher speeding over the mountain with heavy load in low gear.

If it is really 30Hz, you ought to use "24V 60Hz" windings with this 12V 30Hz.

Then figure your transformer ratio, use 2 or 3 cores to keep loading somewhat balanced.

> I should get 230VAC out.  230VAC bridged gets me in the 300VDC neighborhood.

Iinverter is square-wave, not sine. The peak of a square is equal to the average-AC and RMS. No 1.414.

Not sure why you'd want to run 6GK6 with 300V on G2. A mere 160V G2 will pass 1,400mA.

> 60VA

Four 6GK6 could suck more. But will the OTs enjoy it?

Before you get heroic.... blow $20 on a cigar-lighter inverter. Voltage-double the "120VAC" and see if it comes out near 260V DC. That ought to be enuff for ex-6BQ5 OTs. And if not, you can use it to re-charge the cell-phone and laptop.

> quieter if the AC frequency was say >20kHz

Iron is smaller (and not stock power-iron), transistor and rectifier losses are larger.

DynaMotors were awful yet very effective.

> radios used a mechanical vibrator

They fail a lot, and rarely fed more than 8-Watt amplifiers (p-p 6AQ5).

> Why would an amp designed around 'space charge' tubes not provide enough power to build your car stereo around?

The largest "Space Charge" tubes do maybe 40mW, 0.040 Watts. Since a 2-Watt car radio is futile, any non-crap car radio does 8 Watts, 0.040W may be insufficient. Unless you sport a hundred of them, but then the heater plus charge-grid current demand is near the max output of a pickup's alternator. What the Space Charge tubes could do is tickle the Base of a 2N301 power transistor to make 2 to 4 Watts audio while presenting a high impedance to the detector. I'll post plans to anyone who shows a 20 ohm S-E OT.

[RicharD]

>You are an electrician...

Given the amount of money and attention I've been giving to the United States Treasury, I think I'm an electrical contractor.... for now anyway.     I'm also a tech school drop out, flunkies of 83, so I can probably keep these transistors cool.

>There's AC inside the Alternator.

True enough but this means the car has to be running + I gotta get all greasy hacking on the alternator.  That might just void my warranty.  I really don't wanna go this route.

>Not sure why you'd want to run 6GK6 with 300V on G2.
Plagiarized amp schematic. The dropping resistor between nodes A & B will more than likely end up a choke.  Normally one makes the power supply fit the amp but in this case, I'll probably make the amp fit the power supply.  It's more a question of logistics + building something I haven't built before.

>Four 6GK6 could suck more.

True.  The 200mA @ 300ish volts is idle current and again hijacked from elsewhere.  I didn't take into consideration full power but the "off the shelf" transformer is 135VA so I think I'm still in the ball park.

>blow $20 on a cigar-lighter inverter

This did cross my mind and I probably will do this simply to dissect it and see what makes it tick.  Most of these are 150W inverters.  If I hang a doubler off it's output, that gives me 75VA and I could be in trouble.

>Iron is smaller (and not stock power-iron), transistor and rectifier losses are larger.

Those are compelling reasons.  I'm not opposed to rolling my own toroid transformer, but for the sake of experimentation, I'd like to start with a common stock part.

I'm guessing since there is little to no talk about the inverter circuit itself, it's not a good circuit.  I've seen some FET based circuits that are more complex and probably better.

Thanks for the feedback.

-Richard

[sluckey]

I know you're doing this more for the challenge of the project, but... Have you considered buying a good quality 'off the shelf' inverter and just plugging any ole amp into it? Maybe a tv, microwave, small beer fridge, etc?   

[PRR]

> 150W inverters.  If I hang a doubler off it's output, that gives me 75VA and I could be in trouble.

No, essentially the full 120 Watts should be available. (150VA? Cigar is often a 10A fuse on 12V-14V supply, you can't get 150 Watts for long and they know it.)

> little to no talk about the inverter circuit itself, it's not a good circuit

No, I think most of us are just clueless.

I built an inverter decades ago and it mostly didn't work. Base drive requirements were one problem. Testing was another problem.... you need a BIG 12V bench supply or you have to do development balanced on a fender. (And the van had no fender-tops to set stuff on.)

[RicharD]

>No, essentially the full 120 Watts should be available.

I got myself confused.  The trade off with a voltage doubler is current, not wattage.


>I know you're doing this more for the challenge of the project.....

Pretty much.  It's all brain candy for me.  I remember Tubenit posting that he liked to do layouts and they were the equivalent of a crossword puzzle for him.  Some days I think I'll go insane if I have to twist another wirenut.  Television makes me nervous.  Electronics keeps me sane. 

>you need a BIG 12V bench supply

My truck is 20 feet from my bench and my scrap wire comes on 500' rolls.  I need to do a few more hours of research but i think I'm gonna press on with this experiment.

[Jack_Hester]

The largest "Space Charge" tubes do maybe 40mW, 0.040 Watts. Since a 2-Watt car radio is futile, any non-crap car radio does 8 Watts, 0.040W may be insufficient. Unless you sport a hundred of them, but then the heater plus charge-grid current demand is near the max output of a pickup's alternator. What the Space Charge tubes could do is tickle the Base of a 2N301 power transistor to make 2 to 4 Watts audio while presenting a high impedance to the detector. I'll post plans to anyone who shows a 20 ohm S-E OT.

'In the day', what was the typical car radio power out?  I remember it being a challenge to hear the radio in our '50, and later '53 Ford, when Mama would have the windows down (just too hot).  But, we could hear it loud and fine, with the windows up.  Then again, those may have not been the 'space charge' tubes that I'm refering to.

Jack

[billcreller]

 Some of the old GM car radios had a pair of Delco 6V6s, and had a fair amount of output.
For an automotive type vibrator power supply, I thing AES has solid-state "vibrators" for use in old car radios.
 Also, marine supply places have various sized inverters for 120 VAC if you want to go that way.  My boat had a 175 watt in it, but now has a 2000 watt in it.

[PRR]

Portable battery radio: 0.1W

Domestic (living-room later kithen) radio: 1 Watt

Cars have had everything. Early (1920s) was stack-of-batteries, radio was a novelty and perhaps not used much while motoring, maybe a Watt.

A very minimal practical car radio (such as 1950 Ford) might be 2W. Buicks and up would have two 6V6 giving 8W.

I have recently seen two early "transistor" car radios. Experimental in 1956, push-pull transformer output at around 2 Watts. Soon after Delco made zillions of 2N301-types, cooking 6W dissipation and 2 Watts output, but driven with a string of tubes selected/adapted for 12V operation. RF/IF were fairly ordinary tubes. But power gain of a one-transistor power stage is under 1000:1, so 2W out needs much more than 0.002W in, yet conventional tubes at 12V can't do much over 0.002W out. Hence the Space-Charge wacko tubes and 0.04W output, enuff to flog 2N301 to 2W. Not a lot more because 6W dissipation is a lot, space is tight, cars can get hot, Germanium can't take high temps. And '301 was both expensive and in short supply so rarely used push-pull.

Early Lear car tape players had 2W to 4W per channel and *two* (wow!) channels. That's totem-pole on 12V into 8 or 4 ohms. Such radios were sold (cheap) into the 1980s. I thought I could use one with a 3V speaker for mono AM, but in a retired mailtruck 2W just did not cut it, I rigged a bridge to get 6+Watts.