Tuesday, December 3, 2013

BIG IRON - the 200W monoblock monsters!

Not long ago I was contacted by David Counsell of dc10audio to create an amplifier for his company to sell along with his speakers. We collaborated and came up with a few ideas, the first of which was a variation of my 66-100 amplifier using a higher plate voltage and KT-120 output tubes, higher grade capacitors and wiring and a different color scheme. A higher VA rating required an all new power transformer, and a driver stage with more current capacity for those big tubes. This was successfully completed last spring and sold to one of his clients, who is very happy with the amplifier.

At 120 watts / channel this amp took control of the big Martin Logan CLX's at my buddies shop Reference Media in Everett, WA. The sound was big, open, tight and nuanced.

Fast forward a couple of months. David and I agreed to do the Rocky Mountain Audio Fest together and he wanted a super amp, a statement product to go with his speakers- a pair of 200 watt monoblock amplifiers. With the show only a few months into the future and knowing the lead time for such things as custom wound transformers and all new casework, I was  just a bit nervous at this proposition. After some consideration I determined that I could do the project if the 66-100 casework could be used. This would require a modification of the design since the stereo amp has 3 transformers and the monoblocks would have only 2 per unit. The solution was to create a custom aluminum top plate to cover the mounting holes for the stereo version, while at the same time dressing up the appearance of the amplifier and stiffening the chassis to accommodate the larger transformers.  

The first thing I needed to do was to order the custom output transformers so that I could build up a prototype and prove the design. Since my usual supplier doesn't make a power transformer above 350VA or an output transformer above 100W I had to find another vendor. After some internet searching and sending quote inquiries, I found a company that would wind the output tranny's. The power transformer was covered since I had already found a local commercial supplier that could build what I needed for the 68-120 project. I just had to specify a larger core size and higher VA rating to match the new output transformers.

So it all came together, the BIG IRON arrived and I was able to build one mono amplifier prototype that easily made 200W into 4 ohms with 4 6550's. (It would make 240W with KT-120's but 200 watts is enough power for most people and the 6550's sound better anyway, at least in this design.) The next step was to create a parts list based on that design; specifying the best parts for their respective applications while trying to insure they won't be discontinued can be a challenge. Over the years of doing this I've had to scramble to find replacement parts for those which had been discontinued more than once.

David wanted to incorporate some high-end wire and capacitors for this build so I used silver / teflon for input wiring, silver plated copper / teflon wires for the output transformer secondary leads. All other circuit wiring is point to point on the component level and signal carrying interconnect wires are silver plate copper / FEP. The capacitors ended up being the Jupiter beeswax / paper line and took a few weeks to arrive. These I ended up bypassing with some small value polypropylene film and foil caps, which improved the focus and tightened up the bass somewhat. We also used some really nice rhodium plated binding posts that cost a fortune. Here's a shot of the interior wiring during construction.

This design is merely a single channel of the 66-100 on steroids. Instead of 2 6550's producing 100 watts there are 4 6550's pounding out 200. The driver stage took some consideration- the drive requirements would be a challenge especially at high frequencies where the miller effect of the power tetrodes will cause distortion and attenuation. At first I explored a differential pair with cathode followers but thought the layout would be too complicated. I like to keep things simple so I just paralleled the two triode halves in each driver tube and tied both composite tubes together at the cathode thus creating a 'super-tube diff pair' (I could use that in marketing..). So essentially each parallel triode pair drives two paralleled 6550's for each half of the output tranny's primary. Got it? It worked like a champ! Of course this amp retains all the features found in the 66-100 like cooling fan and individual bias pots for each output tube and a built in meter for setting bias, and design features like local feedback in the output stage and optimized concertina phase splitter. Look at the huge PSU reservoir caps and the smoothing choke, also found in the 66-100.

So as Murphy would have it, what works great in prototype has issues in the final build. I spent a considerable amount of time chasing down frequency response aberrations, ringing and such. I experimented with the feedback level (very little loop feedback), compensation caps in the driver stage, replacing those beeswax caps with poly's, etc. Then I noticed that the response was nice and flat when the amp was open on the bench, but when the cover was grounded to the rest of the chassis the response had a rise in the high frequencies. Very strange. Turns out that the output transformer's core doesn't like to be grounded but there's no way around that! I ended up using a simple zobel network across the primary.

Here's the frequency response and THD plots at 200W into 4 ohms. The THD measurement shown is 1.6% @ 1kHz. Note the flat distortion spectrum above 40Hz. Below 40Hz the distortion rises due to transformer core saturation. This is pretty good for full output! But who listens to music at 200W?

 Here's the plot at 25W - .5% @ 1kHz.

Notice much less low frequency distortion in this plot. 25W is pretty much the most average power you need but having 200W in reserve gives you much more headroom and potential dynamic range.

Here's the 3W plot - .3% @ 1kHz. This is where most of your music lives. And this is why these amps perform so well even on high-sensitivity speakers.

So you're wondering how it all sounded? Very much like the 66-100 but more power, a little more liquidity and finesse. I attribute this to the upgraded wiring and caps, and of course the output iron is different as well. All of these things add up and contribute to the overall presentation that an amplifier will give. But the fact that it's so similar to the 66-100 is due to the circuit topology and layout itself being mostly the same. Hard to improve on an already proven design!

David was thrilled at the show and we had very good response to the sound in our room but that's another story..

These were built exclusively for dc10audio and carry their branding. They are available directly from their website: They make some very nice high-sensitivity speakers, which happen to mate very well with ESA products~

Here's a shot of our setup at RMAF:
The speakers are very interesting - the tweeter horn doubles as the reflex vent for the woofer, and they incorporate a tone-wood resonator panel that damps internal vibrations while creating a passive radiator effect. The sound is immediate and refined and the overall bass response is incredible. You really have to hear it to understand - they don't sound like your typical high-end speaker, they sound like music. Lovely.

If you look closely you will see the ESA 66-001p Vacuum Tube Reference preamp. A new preamp was planned for the show but we ran out of time to get it all built and dialed in so the ESA unit was used to very good effect. Stay tuned for another post on the development of the new preamp, which uses circuit boards instead of point to point in an effort to cut down on labor.

Happy Listening!

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