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heater 28th August 2004 01:58 AM

Tube ghetto blaster ?
I did not get my quota of beer for a Friday night so I was sitting home in the dark listening to some cool reggae. This caused me to entertain some strange ideas. One of which is why not a tube ghetto blaster ?

Now I don't have much experience with tubes, having only just recently built a tube/MOSFET hybrid, but I'm old enough to remember portable tube radios with 90v batteries.

This would pose some nice technical challenges:

Small size
Low(ish) weight
Low power heaters
The HT supply, from a voltage doubler perhaps ?
Probably push-pull
High efficiency speakers.
The box is the speaker cabinet.
Perhaps a hybrid.
Probably not stereo.

So what tubes ? What transformers ? What speakers? What schematic ? ....

I want this to sound good, not just a boom box, even if it's not so loud.

Any ideas anyone ?

rcavictim 28th August 2004 06:24 AM

The main problem with vacuum tube ghetto blasters is that the gasoline generator adds too much weight and noise. :xeye:

Sch3mat1c 28th August 2004 07:23 AM

Install a new set of batteries, heat it up and have a night on the town...

I'd say 12V of D cells heating 2 or 4 6V6's (as class B as you can get, with as much NFB as it needs), wire the heaters in series pairs. As for B supply, you can get about 40 million AAA cells.... or run an SMPS, say +300 or 400V.

I'd expect about 5 hours per set of batteries, depending on how loud you play it.


heater 28th August 2004 10:18 AM

Well I should have known it. Steve Bench has already done this. His couple watt amplifier is here:

and the power supply here:

He claims "15 hours of operation from 8 alkaline". I have a bunch of 12v 2.2 amp hour lead sealed lead acid batteries I'd like to use. So we're in bussiness.

Any one with ideas for a box and especially high efficiency speaker ?

Perhaps a mini guitar amp style box. Or something of the wall ?

rcavictim 28th August 2004 05:29 PM


Since it appears that you are serious and sch3mat1c suggested 6V6's may I suggest you look at the 1619. That is a metal 6L6 beam power tube with a 2.5 volt directly heated filament and G3 is not internally connected. DH filaments get you more emission per watt than their sleeved indirect heated cathode counterparts.

With lots of power but sacrifice in quality you can use 6N7's tranformer coupled class B P-P. One tube will give you 10 watts! There is a schematic called "Class B amp for portable use" in the RC-14 RCA vintage tube manual on page 208. I could scan and post it if you wish.

The gell cell rechargeable idea is good. I have similar thoughts for a home based hi-fi tube amp that can be separated from the mains during use and charged when not in use. I have about 240 volts available at 5.5 AH. Far too many projects on the burner though so this one is just a remote dream right now.

heater 29th August 2004 09:55 AM

Yes rcavictim please post that RCA battery schematic. At this stage I will have a look at any and all options I can find. As this will be mono and used outdoors without mains power the sound quality can be traded off a little against efficiency.

My experience so far is that the thing you want to build always looks really simple, I get frightened away from complicated schematics, but the big headache is the power supplies. Transformers that can't be had immediately etc etc. So for this project I'm going to start with Steves power supply. Might also buid one for the front end of my hybrid amp.

Now what about the box ? I woke up this morning with a vision of building this is the style of an old 78 gramaphone with a nice horn on top. Any one got any pictures of such a machine for inspiration ? Would look really cool on the picnic table in the summer. Maybe not practical as a portable device though.

Any one have thoughts on a speaker ?

rcavictim 30th August 2004 08:11 PM


After dinking around with scanning this image for a half hour and finding that in my only mode available, grey scale, I cannot post anything legible here in the 101 kB file size limit. Therefore please PM me your email address and I'll send you a readable copy. Thanx.

heater 30th August 2004 09:59 PM

2 Attachment(s)
rcavictim: thank you again for the RCA battery amp schematic. I cropped it a little and am posting it here. The GIMP under Linux is a wonderfull thing.

Nice simple looking circuit. I worry about the transformer coupling, transformers always seem to be big, heavy and expensive. Is there somthing small and cheap from Hammond I could use here.

I also worry about the heater current, 800ma is going to empty my gel cell in 2 hours.

The 6ak6 tubes of Steve Benches circuit are 150ma. But then we are looking at a lot less power output. Decisions, decisions...

Geek 9th September 2004 07:17 AM

*cough*vibrator supply*cough* ;)

rcavictim 9th September 2004 06:10 PM


Originally posted by Geek
*cough*vibrator supply*cough* ;)
No THAT is a classy idea! :D Then you could use an 0Z4 recrifier toob too, no filament current to worry about.

I'm sure I'd be going with a SS power inverter to make the B+ on such a rig if I were so inspired. Hammond made toroidal transformer sets for DC-DC upconverting tube plate supplies. You can also look at aircraft war surplus tube gear with 400Hz transformers for size and weight savings here. Much, much smaller core and less windings for the same job.

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