Before 2018 drew to a close there was a buzz on various audio forums about the auction of a vast collection of audio gear in the northeast. My attention was caught by lot number 4, a seemingly unmolested Dynakit Stereo 35. I bookmarked the site and registered for bidding.
Fast forward to mid December, my wife and I drove through the new Tappan Zee Bridge aka Gov. Mario M. Cuomo Bridge
and scenic Merritt Parkway (Route 15) to collect my loot, which included the stereo 35.
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| Feuillantine - layers of dark chocolate mousse, hazelnut buttercream, chocolate cake with hazelnut wafer crunch |
On our way home, we let the rush hour traffic subside by exiting in Pleasantville, NY. As a reward for my wife's enduring patience, we had dinner at Jean-Jacques and took home this decadent Feuillantine.
As mentioned in this JEL Arkiv entry, my passion for vacuum tube amplification was sparked by a Dynakit Stereo 35 I found in what was left of Radio Row in downtown NYC in the early 80s. Unfortunately, I had to unload it in the 90s to finance Tango and Tamura SE output transformers for my SE-DHT amp projects. It was a sentimental journey to reacquire this amp.
Removing the cage reminded me of the days when I serviced classic tube gear at Angela Instruments in the 90s. I quickly recalled the routine of checking for shorts in the multi-section capacitor and identifying continuity in the transformer windings. After those static tests were established, the amp was plugged into my trusty GenRad Variac and I slowly applied AC voltage while carefully monitoring the current draw. My hunch was right, this was a healthy amp that just needed a lot of cleaning and a little updating.
!!!WARNING!!!
The voltages in this circuit are potentially lethal! Proceed at your own risk!
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| Dyna Stereo 35 Schematic |
The secret to the ST35's success was its elegantly simple circuit design. It uses a 7247 twin triode which is essentially half of a hi-mu 12AX7 + half of a med-mu 12AU7. The hi-mu section functions as the input voltage amplifier, while the med-mu is configured as a cathodyne-phase inverter driving a push-pull pair of cathode biased 6BQ5/EL84 pentodes connected to wide bandwidth Dynaco Z565 ultra-linear output transformers. The judicious use of both positive and negative feedback made this a very stable amplifier.
Aside from replacing the original mylar coupling caps with polypropylene Wonder Caps in typical 80s fashion, the only mod I ever did to this amp was bypass the "green ceramic disc cap" (C1) and filter network (R1 & C2) in the input. This idea came from page 67 of Bascom H. King's review of the Mark Levinson ML10A preamp + ML9 amp in the August 1985 issue of Audio.
Since I wanted to keep this amp as original as possible, I avoided removing parts and cutting traces like I did as a Dynaco hacker in the 80s.
This is the stock wiring of the RCA input jack. Audio signal + is connected to solder tag No. 1.
I unsoldered the wire from solder tag No. 1 and soldered it directly to the input grid, pin 7 of the 7247 input/driver tube. From above it looks stock but now the audio signal goes directly into the input grid (shunted to ground by R2) of the hi-mu voltage amp section. This mod effectively bypasses the filter network (C1, R1 & C2) which protects the amplifier from a preamp that produces DC offset at the output. If your preamp passes DC (or you aren't sure) DON'T DO this modification.
Since the 90s I've been convinced that paper in oil (PIO) caps sound more like live music. Although I'm not 100% sure whether the surplus mil-spec West-Cap/Astron cap combo I installed are PIO, the amp doesn't sizzle like it did with the original mylars.
Dynaco skimped on the power supply of the ST35 by using solid state rectifiers and a wire wound resistor in the 𝜫-filter. The least I could do was replace the original 60Ω, 5W resistor with a proper C-354 choke from Dynakit.
Minimal phase shift @ 100Hz = excellent low frequency response
Fast rise time @ 1kHz with just a hint of overshoot
Again a tiny overshoot @ 10kHz with minimal and well damped ringing
9.89 Vrms squared = 97.8121/8Ω load = 12.23 watts per channel
before clipping, both channels driven
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stereo 35 sans cage |
Speakers I've used in the past with the Dyna Stereo 35 include DIY mini-monitors using Radio Shack components, BBC LS3/5As and original Quad ESLs. It had a hard time driving a pair of Magnepan SMGas I briefly owned in the mid-80s, so I unloaded them. 😉
I'm listening to jazz tunes with the stereo 35 driving Altec 755Cs in 618 cabs as I type. It doesn't have the mid-range magic nor does it present micro-dynamics like my Simple 46, but this fine PP EL84 comes pretty close.
Just like Feuillantine, the dynakit stereo 35 is yummy!
