Guitar Tube-amp From Junk Hi-fi

[Tristan Chambers] picked up an old speaker box some friends acquired at a yard sale. It didn’t have any inputs, and there weren’t any tuning knobs like a radio would have, so it’s a mystery what this was originally used for. [Tristan] traced out the circuit and figured out where he could input audio signals which allowed him to hook up an iPod, but it was mono and not very loud. He ended building his own vacuum tube preamp from a schematic he found on the Internet so that he could use it with an electric guitar. As the video after the break shows, the box not only puts out some pretty good sound but it’s nice and loud too.


21 thoughts on “Guitar Tube-amp From Junk Hi-fi

  1. @Focusrally: Yes, there are a few “Carrier Current” speaker systems of the era when 12AT7 and 6AL5 tubes were commonplace. IIRC some Rauland produced models were used in commercial installs for executive suite remotes etc.

    But, there’s another type of radio with no user accessible tuning settings. FM “SCA” subcarrier channels were used in some cities for services like book reading to the blind, and a doctor’s office network:

    The “reason” for such a history lesson is to introduce the Hacking utility of a control system used in SCA service. Sub to Barely audible after demod at least- tones activated relays that could “do things” all across a city.

    Traffic Light time range settings/reversible lanes etc or in background music services-raise/lower volume so one store’s commercials got muted in a competing store or raised in volume on speakers of the advertised stores:> Hacking traffic lights was and is a CRIMINALLY bad idea. But? Hacking the audio boost/mute of SCA services was a test of RF Hacker chops indeed! Made a bit easier of course by looking at the Motorola PL reeds more than one such system used.

    Sadly- such gear now is almost all history and winding up as excellent re-assignment materials.

    But knowing the past can explain foibles in re-assignment surgery. shortest explanation= Audio suffered horribly by shoehorning into a SCA subcarrier or carrier current path and oftentimes the amplifier sections tried to expand/equalize that audio in compensation..

    If Tristan has any identifier data at all- I’d love to see it posted. Oh- This is a neat hack of course..

  2. You should be able to cut out the IF circuit and wire in the preamp, it’s the one with the shield on the socket tween the two IF cans. Already shielded, it and short leads with built in ground will kill hum but not from guitar. This thing must have been a dedicated radio or carrier current campus radio, or big Muzak setup.

    It is transformer operated, I see, thankful design. There may be caps feeding the RF signal in from the power-line, these can be death caps. Find them and kill. It could be transformer coupled though, but even that don’t have a modern safety rating so clip it out. This coupling could be causing hum also.
    What causes aspect ratio fubars on video? I can’t stand to watch such. Please check settings and post a test, adjust if necessary then push good vids. Widescrean sucks, westerns made it, we could have done with out that. 3to4 works best on a subject, either a hot model or a hacked model. Some vids on HaD have been clipped off on the right because of this whole aspect ratio mess. I have a test on echodelta9 channel Youtube, it looks great. Stay tuned for a guitar hack soon to be posted that will set standards.

  3. I recognize this. This was a “wireless” speaker. It would recieve audio over a radio frequency from a device that plugged into the receiver. Then you would put this in another part of your house and plug it in and have a “wireless” system…

  4. @echodelta the 60hz hum is from two things: the single coil pickup on the guitar and improper grounding in the guitar circuitry, and more importantly the unsheilded audio cable (read: automotive jumper cables) connecting the guitar to the amplifier. There was an IF stage. I removed the tubes early on in the hacking process, and yes it did improve noise.

    @mjrippe you are a sweet heart. I was totally bumbed when I realized that I didn’t have a socket for the 12ax7. I was hoping to use one from the radio but they had the wrong number of pins. Next time… I might rebuild the circuit because I was getting some internal feedback when I buttoned everything up.

  5. that overdrive is almost perfect for most types of music. Not choppy enough for most metal but would be perfect for alot of early heavy metal or rock.
    Nice work on that sounds great.
    Now time to learn to shred.

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