Magnavox 8601-20
Many moons ago, I had a collection of what I believed were the finest push-pull EL84 amps designed during the golden age of hifi, namely the Acro 20/20, Dynakit stereo 35, Leak 20 and Pilot SA232. When this gutted Magnavox 8601-20 console amp showed up as an unfinished project on FB marketplace for $25 sans tubes, it was an opportunity to hear the EL84 tube in a single ended amplifier. Given the amount of parts I've collected through the years, I knew I could get it up and running with minimum outlay.
!!!WARNING!!!
The voltages in this project are potentially lethal!
Proceed at your own risk!
ADDENDUM
In the ensuing days after this article was uploaded, the pair of PSVANE EL84s I used for bench testing manifested signs of instability.π
After replacing them with a pair of GE 6BQ5s or Hitachi EL84s, the amp in triode mode easily managed over 1000 mWPC into a 4Ξ© load as pictured above. Since there was no significant change in square wave performance, I'm leaving the scope pics below.
To my ears the revisions in this updated schematic further refined the sound. π πΆ π
I couldn't restore the amp to its original state because I didn't have a 6EU7 tube in my stash. Instead, I chose the harmonically rich-sounding 6CG7 (9-pin equivalent of a 6SN7) as a resistance coupled input/driver stage amongst other miniature med-mu dual triodes in my parts bin, like the 12AU7 and 5670/2C51. Each input/driver section has its own B+ supply rail. Each EL84 output tube has been configured to have its own cathode bias resistor + bypass cap. The tuner section's 6.3V filament winding is used to heat up the EZ81/6CA4. Since I didn't plan to use negative feedback, I added a 2H/100mA choke in the power supply to keep hum/noise at 2mV or less.
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| Resto-mod Magnavox 8601 square waves @ 100 Hz, 10 kHz and 1 kHz in pentode |
I initially tried running the EL84 as a pentode and measured 1.35W driving a 4Ξ© resistive load. However, as depicted by the 100Hz square wave, bass performance was dismal and the upper midrange was a bit harsh. It's been my experience, after having been in this hobby for over three decades, test bench performance improves in a pentode amp after injecting a bit of negative feedback but at the expense of a lucid midrange. I didn't want that compromise...
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| Resto-mod Magnavox 8601 square waves @ 100 Hz, 10 kHz and 1 kHz in triode |
So I connected the EL84 in triode (screen grid to the plate via a 100Ξ© resistor), power dropped to about 500mW per channel, but the square waves started looking more like a hi-fi amp. Using music as input signal, the amp sounded more powerful with more bottom end definition due to the triode's lower output impedance. The upper midrange glare was gone supplemented by a shimmering top end.
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| Silvertone RTR OPTs (top) Original Magnavox OPTs (bottom) |
In the midst of testing and listening, I remembered that I had a pair of OPTs pulled out of a Silvertone tube RTR tape deck. As pictured above, the Silvertone OPTs have finer laminations + a slightly thicker stack. I measured and determined that both OPTs have similar primary to secondary impedance ratios ~ 5K:4-6Ξ©. Let's see how these Silvertones perform compared to the stock units...
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| Resto-mod Magnavox 8601 square waves @ 100 Hz, 10 kHz and 1 kHz in triode + Silvertone OPTs |
Technical performance was further improved! With 0.4 Vrms 1kHz sine wave input, power is up to 700mW per channel, the square wave tilt at 100 Hz and 1 kHz is a smidgen less, while the rise time at 10 kHz is significantly faster. All of these improvements were also notable sonically across the audio band! Nevertheless, I'd like to remind readers that I've tested amps and output transformers that projected beautiful waveforms but couldn't make music and vice versa. The scope pics I upload merely serve as a guide in my DIY pursuits and enhance the optics of this blog.π
Top view of the Resto-mod Magnavox 8601-20 with Silvertone OPTs, tubes, power supply choke and capacitor, updated input and output jacks + PIO coupling caps and more underneath, which brings the street value of this unit above $100. That said, it's still a significantly cheaper project than a genuine SE-DHT amp.
Let's take this amp for a spin in the following setup: 99dB/1W/1M sensitive Altec 753C speakers, Intact Audio nickel AVC, triode'd 6F6/Hashimoto HL20K6 line stage, RCA derived 5693 phono stage, Tamura TKS83, spherical tipped EntrΓ© EC15x on a Garrard 401 + Bokrand AB309.
I'm not a stranger to milliwatt powered amps, having built my first SE71A amp decades ago and still enjoying that amp in its current guise. Therefore, the lack of headroom from a 700 mWPC ~ 1300 mWPC amp isn't a major concern as long as I'm driving 90dB+ sensitive speakers with benign impedance curves. I turn down the volume control a few notches and revel in the milliwatt triode amp's uncanny ability to retrieve low level detail and dynamic nuances. Besides, lower SPLs are kinder to our ears and most welcomed by neighbors who may have different tastes in music.π
Just like my Simple triode 6V6 amp, this triode-strapped single ended EL84 amp offer an enticing glimpse into the organic sound of directly heated triodes + it's a fun DIY project!π»
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