Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: passaloutre on March 07, 2025, 10:27:21 am
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I picked up this 1958 Magnavox console (no radio) for cheap on marketplace in rural Louisiana. The turntable was a junk 80s replacement, and I originally planned to return it to its former glory with a proper contemporary changer, but I already have a hifi system that I love, and some recent discussion about stereo vibrato got my wheels turning...
This thing came with 2 5Y3GT rectifiers, 4 6V6 output tubes in stereo, 2 12AX7 phase inverters, and a preamp with 2 more 12AX7s, not to mention the speakers: a 15", a 12", and a couple tweeters. A veritable jackpot at under $100.
I decided to try my hand at a harmonic tremolo, but rather than mix the two signals back together, what if they each fed a separate power amp and separate speakers?
After considering several preamp designs, I settled on the 5E11 Vibrolux: the era was correct for the hardware, it would give me some nice chewy mids to pan left and right, and, most importantly, it would only need a single triode. The preamp feeds a simple crossover centered around 400-600 Hz, and then the two signals each feed their own Vibrolux-style power amps: cathodyne PI, fixed-bias push-pull 6V6s, the lows going into the 15" and the highs into the 12". My spice sims showed the crossover to be a little lossy, so I added a modest gain stage to push a little more signal through it.
I used my other 12AX7 to construct an oscillator feeding its own cathodyne PI, with the resulting antiphase signals modulating bias on the two power amps. I didn't know if I would need to buffer the two modulation signals, but my spice sims suggested it didn't make much of a difference.
It went together mostly on Sunday, with some additional tweaks throughout the week. The large power amp chassis provided ample room, and the dual-chassis configuration helped keep the sensitive parts away from the noisy parts. This was my first time using LEDs to bias the preamp stages (with the added bonus that the LFO LED can go on the front panel to provide some visual interest), and my first time using LTSpice as a design aid (which substantially reduced my usual post-build tweaking).
Even with the small speaker separation, the stereo effect is palpable. The watery flutter sloshes between your ears in a delightful way. My final tweak will be to install additional speaker jacks so I can hook up external cabinets and really throw the sound around the room. I still need to button it all back up into the cabinet and make it pretty, and this one might actually be living room approved!
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Here are some photos. I hope to have some sound clips recorded this weekend.
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Here’s a quick clip with my phone camera. I hope to make some more with close mics on each speaker.
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Nice, but needs verb for depth.
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I’m probably maxed out on heater current, but I could maybe throw some mosfets together for reverb…
Meanwhile, I’ve got a reverb pedal
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A verb pedal will work fine.
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That is so cool! I haven't seen a variation on Fender's harmonic tremolo before and yours sounds fantastic. After I finish my next build, I'd like to try out your idea, but make it dual SE and wiggle the "U3 and U4" in your circuit.
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This is not really Fender's harmonic tremolo. It's a two phase dual bias vary trem such as found in a Princeton Reverb. Probably get some nice effects if the speakers are separated.
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This is not really Fender's harmonic tremolo. It's a two phase dual bias vary trem such as found in a Princeton Reverb. Probably get some nice effects if the speakers are separated.
I forgot that fender didn't invent that circuit, but borrowed it from Wurlizter/ Dorf, so it really isn't a "Fender" variation like I wrote, but passaloutre's circuit does look like it splits into a high pass low pass and then gets wiggled oppositely.
passaloutre, where'd you come up with such an efficient solution?
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passaloutre's circuit does look like it splits into a high pass low pass and then gets wiggled oppositely.
Ah, yes. I overlooked that. C5/R13, and C6/R12.
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I borrowed the crossover directly from the early 60s Fender circuits with the harmonic trem. The difference is what happens after the crossover. In the Fender, the highs and lows go to separate gain stages but get remixed to mono before the phase inverter. In mine the highs and lows go to separate phase inverters and power amps entirely for the stereo effect.
I think your dual single-ended idea would work great. At the risk of cutting into my own supply, I'd advise you to look on marketplace/craigslist for old stereo consoles. For every one of these dual push-pull amps, there are dozens out there with dual single-ended EL84 or 6V6 stereo power amps. I've picked up several in the $30-50 range. People just want to get rid of them.
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That console reminds me of Guitar Bob’s TV cab amp
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Haha, unfortunately those legs aren’t part of the console, just the bench that it happens to be sitting on.
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Sounds pretty good, I have the parts for stereo SE and eventually might build it.
What do U and V stand for in the schematic?
It looks like a lot of capacitors and resistors. Were the chassis's stripped and then rebuilt?
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U and V are LTSpice's nomenclature: U denotes a vacuum tube in this schematic, and V are voltage sources. The rectifier and bias supply are not shown in the schematic, because I didn't have any reason to model them. The rectifier is two 5Y3GTs in parallel, and the bias supply is straight from Merlin's page. The "V"s on the left side of the schematic are just sine wave signal sources I was using for transient analysis; that's where your input jack would go.
Yes I stripped out all the original console circuit except for the transformers, rectifiers, and heater wiring, and built my circuit with new resistors and capacitors.
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At the risk of cutting into my own supply, I'd advise you to look on marketplace/craigslist for old stereo consoles.
You are a true gent! I scour my local spots but have taken in too many orphans and not finished enough of them. Luckily it sounds like you are on the other side of the US, so I think there will be plenty enough scores for us both!