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An Interesting Challenge - Tube Console Radio into a Harmonica Amp


kbp810

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A local blues player up this way came across an old Zenith Console Radio and snagged it with the hopes of bringing it back to life to use as a harmonica amp. It was non-working condition, as in turning it on instantly blows the fuse. He'd received some advice about maybe just being some bad tubes (and some did actually test bad), so he tried swapping them all out for new modern tubes; unfortunately it would still instantly blow the fuse. 

That's just about the time I came into the picture - been chatting for a little bit, and met up today to snag the chassis from him to see what the problem might be. Didn't take long to pin point some initial culprits. Just upon a quick visual inspection, there's at least 3 weeping capacitors and at least a couple of toasted resistors. 

The original plan was to see if it could be saved; but the more I inspect, the more I think this is going to be a gut and start from scratch. There's a lot of the circuit that can be eliminated for the intended purpose anyways. Would keep it point to point construction; and plan to reuse the tube sockets, terminals, transformers, pots (hopefully), and should be able to reuse some of the wirewound resistors within... but most everything else is pretty much at or near it's end of life. At the end, it would essentially be a "new" amp using original and NOS parts (where I can); hiding in an old console cabinet (no pictures of the cabinet at the moment). 

Phew... just look at that birds nest... 

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Hmmm... I think I see the problem.   There's one bad solder joint on the 5th pin of the third tube socket, and a conductive spiders web.   Should clear up any problems right there.

 

I remember the first time I looked at the guts of my Valco amp from the 50's.   That's pretty much the style.

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This was destined for a total tear down anyways; the more I snipped, the worse it got. At the end I was hoping to at least salvage the transformers; unfortunately, I accidentally yanked a lead out from the OT in the process (it slipped out with barely any effort; it's life expectancy was also apparently up). 

Evidence of some little mini fires inside... (found a few of these like this)

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A wire that fried and partially brazed itself to the chassis in the process (black/formerly red "wire" near the bottom of the picture - see the last picture for the imprint it left)

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Wholesale clean out - some work yet to be done; things were soldered/brazed in and/or riveted on and will take some additional persuasion to remove. 

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  • 2 weeks later...

I know, I've been slacking with the updates. Just about done at this point... took a lot of pre planning to try to utilize as many existing holes, mounting points, and terminal strips as I could. Not wholly point to point, as it just wasn't always practical, so let's just say "pointish to pointish" 

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15 hours ago, bolero said:

Nice!

What's the circuit based on?

A slightly modified 5E3; channel 1 will be for a guitar, and is voiced a little a hotter; channel 2 is for a harmonica (via microphone) with an XLR input and impedance matching so that guitar and mic will be at similar levels. And addition of a master volume, adjustable negative feedback, and 4/8/16 ohm outputs round it out. 

Since it's built using an old Zenith chassis, I'm calling it the "5EZ" 

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Well, mission accomplished and handed back off to it's owner today. 

The XLR/mic input took a lot of back and forth triage to get it right; excessive hum and uncontrollable feedback was the bane of my existence this morning, before I realized I had to decouple and isolate the mini-impedance transformer from the chassis; else, the entire chassis became microphonic. After that, was clean and quiet as could be. Also added a switch so it could be "turned off" (shorted to ground) when not in use so it wasn't picking up noise when nothing was connected to it. 

On guitar channel, sounded just like a slightly hotter 5E3; on mic, clean, with just a little bite of distortion blowing through the harmonica. 

No cab for this one, but didn't want to hand over an open chassis, especially with largely open/p2p construction, so found a little scrap plank of walnut, cut to size, rounded off, and ran some screws through some rubber feet directly to the chassis mounting points. 

 

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