Repair

Dr. Hill working in the lab.

Download the Service Manual: Plasmatronic Service Manual

Keeping a pair of 35 year old Plasmatronic 1A system running is clearly not the easiest task in the world.  It involves learning about gas distribution, vacuum tube power amplifiers, control systems, and some strong lifting abilities.

Yet when you break it all down, it does make sense and will work.  The key is patience.

As an aside- I was stuck on an ignition problem for months.  After actually reading the manual I was able to fix the problem in thirty seconds.

Tools recommended are a strong back, a good table with proper lighting near your speakers, a pair of thick oven mitts (really handy for carrying the plasma units), some Caig cleaning products, a gaggle of Q-Tips for cleaning, and a tolerant support partner.

We recently resurrected our set (30 L and 30R) after a twenty year hiatus.  I removed both the original cone drivers and replaced them with a Dynaudio midbass and Audio Concepts AC-12 subwoofer at the recommendation of a good friend and industry colleague.

You’ll need a good soldering station, pipe tape, Allen wrench set, adjustable wrench for the gas connection, access to a tube tester, a roll of trimmable velcro, and a couple of circuits of AC power in the room.  The WT3 woofer impedance program ($99) made short work of the cone related issues.

As a non-general rule with the disclaimer (it’s your house, don’t burn it down)- you can operate the system on a single 20A 120V circuit.  You will need at least two plug strips- one for each speaker- as two on a strip will trip the internal breaker on the strip.

I would also recommend using a triamplifier configuration and bypassing the internal Passover between the woofer and midbass unit.

We are currently a Yamaha SP2060 is upstream of the Hill Interface Unit in our system feeding a tri-amplified configuration in our system and bypassing the internal passive crossover.    We selected the Emotiva 5 channel 200 watt amplifier; only using four of the channels.  It has balanced inputs, good protection against faults, plenty of current (never approaching clipping) and drives the mid bass and subwoofers nicely since it is only used up to 700 Hz.  Speaker wire is some Mogami sourced 10 AWG for the subwoofer and 12 AWG for the mid bass, primarily so as not to confuse which was which.

Currently the Yamaha crossover has the low end driver is configured with a Thru High Pass Filter at 20 Hz, and a Low Pass Filter at 100 Hz,  -12 dB Butterworth.

The midbass driver is High Pass Filter at 100 Hz, -12 dB Butterworth.  It is High Pass Filtered at 710 Hz, – 6 dB.   I have electronically inverted the polarity, but frankly the difference is minimal.   It is delayed 2 mS relative to the bass driver

The high frequency driver is set at 710 Hz, – 6 dB per octave.  It is delayed 1.5 mS relative to the midbass driver.

A good room fan is important.  I chose a Vornado model which is relatively quiet and discretely placed it out of sight.

My son chose Pink Floyd’s Wish You Were Here for first listening, and frankly we were stunned.  I’ve listened to that disc since it first came out, went to the LA concert performance of it in my youth (yes, the infamous Darryl Gates debacle), and still heard things on it I’ve never heard before today.  I have written over 350 audio articles in my career, and frankly do not have the words to describe the difference between listening on the Hills and any other system.  I only wish I had the listening acuity of my youth back.

We have five audio systems in the house- an all Quad electrostatic system, two Gale GS401 based systems, and one Sonus Faber system and they are well loved and appreciated.  I regularly go to CES and the local Hi Fi show and have compared with the best Wilson and Magico have to offer.  All marvelous kit- but…

But as Lynn Olson once wrote, they are all 737’s parked near a Lockheed SR-71.

Probably like watching the 1927 Yankees play the Red Sox.   Hey, someone had to go 51-103 for the Yankees to win all those games.  The Quads do some things better than the Gales and vice versa.  The Sonus Fabers also have their strengths.   But the audio quality of the Hills is superior in every aspect.

 

One of the Plasma Amplifiers Not Working

If you were to ask me the most common failure issue with these speakers, of all things it boils down to a silly little resistor.   Symptoms include one of the five electrodes will not light, the front panel meter gives you silly readings, and the indicator lamp remains lit constantly.

Each speaker houses five tube amplifiers within based on the 6MJ6 (also known as a 6JE6-C) Beam Pentode vacuum tube.  This tube was generally used for TV horizontal deflection amplifier applications back in the day, but it is a seriously robust tube with thicker glass.

The anode of each tube feeds a large vertically oriented green ceramic 25 kΩ 100 Watt wirewound resistor.  Directly underneath that big green resistor on the opposite side the printed circuit board is a 1.75 kΩ 5 Watt resistor which connects directly to one of the five tubes feeding the plasma chamber.   That’s the troublemaker.  You generally have to unscrew the P.C. board to access them.

I have had THREE of these 1.75 kΩ resistors fail (there are ten total, five per speaker).  Originally made by Clarostat (Model VPR5), they end up overheating and cracking down the middle.   You can get NOS replacements online, but I’d suggest an alternative to these little devils.  They overhead, the internal wiring blows, and they crack in the middle.  Here’s a photo of two examples…

 

My suggestion is two-fold.  Replace them with a higher rated wattage resistor (10 watt ceramics are easy to find) and keep a few extras in stock.  Schematically, the little devils are highlighted in rounded red rectangles below:

 

 

Plasma Chirping

The right plasma started acting up a little bit.   There was an intermittent (once every ten seconds) “chirping” sound which would make the bias meter oscillate wildly on one the five internal amplifiers.

The interesting part was the top electrode metal cap came off easily, and underneath the top of the tube was badly corroded.

Turned out to be a bad tube.  Replacement sorted the problem out instantly.   Always handy to keep a stock set in hand.   They’re up to about $30 apiece for NOS on eBay now.

 

Interface LED Display (July 2014)

The lower segment of the left channel LED display wasn’t working.   Turned out one of the ground traces on the display printed circuit board wasn’t passing ground.   Soldered in a jumper to bypass the trace, and worked perfectly once again.

I would suggest replacing the Interface power cord as well.  The original rubber jacketed version deteriorates with time.

 

The Most Valuable Drawing

I want to give credit where credit is due (one of our members forwarded this to me) but frankly can’t find the original email.   Click on the image to enlarge, save it to your hard drive, and print it out.

 

 

 

 

 

 

Speaker Schematic Diagram (click to enlarge)