Okay, so I spent some time bringing the stock unit up to full voltage on a variac and let it set overnight – no overheating, weird smells or capacitor barf. That is the good news. The bad news – the RIAA equalization and kludgy balance control make the amp sound not necessarily bad, but just a bit ‘off’ with line inputs.
Going back to my original instincts, I ripped out the entire tone forming front end and balance circuits. Inputs now have a simple 100k grid bias resistor, and the feedback is thru 10k fixed resistors. While inside, I installed an IEC power receptacle, installed a power switch, replaced the coupling and local feedback capacitors, and installed a set of true binding posts to allow easier speaker connections. Here is the ‘after’ picture that you can compare with the ‘before’ shot in the previous post:
I also installed a dropping resistor on the pilot light circuit and made a simple red LED power indicator.
Sound is much better now, and in line with what one would expect from vintage Magnavox – not too mellow, not too punchy, but just right.
Frequency response and harmonics follow. The low end has a bit of a bump to it – maybe to match with the smallish speakers originally fitted to the console. Frequency response is plotted at ~2.5 watts output into an 8 ohm resistive load. The first pair of scope shots are at 1.5 watts (~2.5% THD @ 1 Khz). The second pair are at just north of 2 watts RMS, where you can start to see the odd order distortion harmonics kick up.
Listening with high efficiency (>90dB) speakers at around 1.5 watts seems to be the sweet spot for this amp. It is plenty loud, with lots of headroom available for transients. I have it running in my basement with open baffle Corals and it sounds fantastic.
Available now for $150 + actual shipping. Plug-and-play, includes all the original glass and a heavy gauge IEC power cord. Just what you need to perk up your Thanksgiving.
Foxtrot 9 Audio 