The story on this one goes all the way back to 1976 when Victor Belenko defected to Japan in a MIG-25. Our analysts all had a good laugh at how backwards the plane was – a welded stainless steel frame, unshaved rivets, and archaic vacuum tube based radar and avionics. What no one mentioned was the fact that we had no way of welding the particular stainless alloy the Soviets were using on the airframe, the exposed rivet heads were inside the boundary layer, and had absolutely no effect on aerodynamics, and that the simplistic vacuum tube based radar system had a range exceeding the solid state units our planes had in operation at the time. This Smerch (Tornado) Radar System was designed to perform during and after a Nuclear Doomsday, and may well have been one of the most rugged, indestructible set of electronics ever built! Thanks to its vacuum tube amplifier, this Radar was immune to Radiation and any Electronic Countermeasures the US could throw at it. It is rumored that on the ground, this Radar System would stun or kill rabbits around the airfields out to 300 yards. Mounted in the supersonic MIG-25, this Radar system completed a highly capable Interceptor Platform designed to duke it out with the US and NATO for air-superiority in WWIII. Fast forward about 15 years; The Soviet Union Implodes, the Iron Curtain falls, and the west suddenly has access to markets of the Sovie t Hegemony. Almost immediately audiophiles start sampling the once forbidden fruit of Soviet vacuum tubes. It doesn’t take long for the 6S45P-E out of the Smerch Radar to become a hot topic. Capable of just under two watts of output with a line level audio input, and considered to be very similar to the vaunted Western Electric 437 triode, experimenter’s embrace this little firecracker with both arms. However, taming the little beast proves to be a chore. Not easy to domesticate, the temperamental little tube is prone to outbursts ranging from brutal oscillations to spectacular outright meltdowns. Despite the challenges, I’ve built several amps using this tube, and the sound is always absolutely gorgeous. I’ve come up with several methods to balance current draw and block parasitic oscillations, all with varying degrees of success. Since early 2004 I have been searching for any information on the original implementation of this tube in the Smerch radar. I did know that it was used as part of the pulse scan preamp, but that was about it. I did have one lead in the Ukraine that claimed to have the original schematics, but the asking price of $10,000 was about $9,990 more than what I was willing to pay. The trail went cold for a few years. But just after Thanskgiving of last year, while on a business trip to San Diego, I was making small talk over lunch, when one of the meeting attendees revealed to me that his Father-in-law was restoring a Mig-23 at a nearby hangar. The business at hand was completely forgotten, and I was able to arrange a look-see that afternoon. Once at the hanger, and when approached from the rear, the plane itself didn’t look to be in bad shape, but the wind went out of my sails when I walked around the front to see an empty cavity in the nosecone. I explained that I was really interested in the radar electronics, and my hopes were restored when the Father-in-law said that both the radar and IFF assemlby were shipped with the plane and were in some boxes in the hangar, but had been seriously “busted-up” to comply with import restrictions. A little searc hing and box shuffling revealed a metal canister and radar dish that looked like it belonged in an NTSB crash investigation lab. The entire unit was dented and crumpled without a straight edge anywhere. Looking at the dents and dimples, you could make out the shape and size of the sledgehammer head that had been used to ‘de-commission’ the unit. The dish and gimbal looked to have taken the worst of the beating, which revived my hopes that maybe something inside the chassis might have survived. The access panel was relatively easy to pry open, revealing a nearly untouched bank of 16 vacuum tubes. Granted, this was an older version of the Smerch, and the power amplifier tubes had been smashed when the access door was crumpled in with the sledgehammer, but the preamp section and 6S45P-E tubes were intact. I doubt even Bouchard was as ecstatic as I when he discovered the Rosetta Stone. My new benefactor allowed me to take as many photos as I wanted, and he even let me take four of the floating socket assemblies, complete with tubes (two of which are now inside this amp). He probably would have let me have the entire assembly, but my excitement probably convinced him that he was sitting on some sort of electronics gold mine. At the airport, I had another dilemma: I had no checked luggage, and trying to carry on something that looked like glass vials with lots of wires coming out of it was only asking for a full body cavity search and confiscation of the offending device. Luckily for me, the car rental agency had mail service, and I was able to mail the parts to my home address with no issues .
Over the Christmas break I had plenty of time to study the original Soviet implementation and experiment with some new ideas of my own. The amplifier shown here is the fruit of those efforts. The tubes, sockets and wiring are the originals from the MIG Radar. Both channels are operated fixed bias ( it appears even the original designers could not get a cathode bias to function or balance properly across channels). The prominent front panel meters read quiescent tube current directly, which can be adjusted using the two locking potentiometers on the right of the faceplate. I think the best sound is delivered at around 40 mA, but you can run them richer or leaner as you see fit. Inputs are via the two RCA jacks directly below the Ammeters, and there is also a stepped attenuator utilizing Vishay Dale precision resistors for volume control. Output transformers are vintage Telefunken. Precision air-gapped and interleaved, they give a very natural musical tone combined with a low end punch that is completely disproportionate to their physical size. The power supply is hash free, direct choke loaded, and utilizes all German Oil Capacitors. Tube Filaments are DC heated for minimum noise as well. All electrical power has circuit breakers; upstream of the power transformer there is a 15 amp breaker for main power and the switched AC outlet in the rear, and there are individual 5 amp breakers for both the B+ power and 6.3 Volt filaments on the output side of the power transformer. The monstrous speaker terminals are actually the original gold plated vintage antenna connectors from the military radio chassis used for the amp. The entire wiring harness layout and component placement was done by ear – I assembled everything loosely in the chassis, and with the amp under power and connected to a set of headphones, I maneuvered parts and wires for minimum noise and best sound. Although the wiring and placement was very labor intensive, the end result is an amp that is absolutely silent. No background noise, no hum, no nothing. I measure 0.00013 volts across the speaker terminals at idle open circuit. With an 8 ohm load resistor connected I read exactly 0 - as in the big goose egg, zilch, nothing, nada. With a maximum output of 1.37 watts (3.32 volts across an 8 ohm load), and even conservatively using the open circuit noise floor levels, the amp delivers a SNR better than 88 dB. A number this good is hard to achieve with even high powered amps, and is absolutely unheard of for any amp putting out less than 10 watts per channel. Frequency response is flat from about 32 Hz all the way out to 22Khz. The -3dB point is right at 20 Hz for the low end, and an astronomical 32 Khz on the top. The wide bandwidth can partially be attributed to the fact that there are no coupling capacitors in the circuit – the input goes straight to the grid to directly regulate the output. Finishing touches include a green LED power indicator and a textured black epoxy finish on the ventilated chassis.
How does it sound you ask? Just like my earlier versions using this tube, sound from this amp is absolutely magical. Power it up, and the first thing you will notice is that it is dead quiet with absolutely no hum or micro phonics - partially due to good internal shielding and placement,, but mostly a tribute to Soviet vacuum tube technology that was still advancing the state of the art 20 years after we had completely abandoned it in here in the west. The 6S45P-E is the Soviet analogue to the Western Electric WE-437 and Telefunken EC-8010, both of which have reached almost cult status (and prices). The sound can only be described as clean, crisp, and dead accurate. Try to imagine a very clean solid state amp, but without the trademark harshness (some would say hatefulness) and edginess. Even though the amp is single ended with the associated second order harmonic coloration (or imaging, or whatever you want to call it), you can listen to this amp for days and days without your ears getting tired - music comes through fast and accurate without any buttery soupiness, but still open and airy without a hard edge. More so than any of my previous builds, this amp truly has a live, in-your-face feel that I have never experienced before. That’s it and all that I have to say for now. Please feel free to email with any questions or to request additional photos. IFor you locals, I do plan on taking the amp over to Rocky Mountain Vintage sometime this week to listen to some pairing with their horns – if you would like to join in on the fun, email me and we can set a time.