This particular circuit is perhaps one of the most complete designs we have done, however Single Ended circuits dont really sit well with either of our audio philophosies , as we are both mostly electrostatic speaker users and these type of speakers just dont work with Single Ended amplifiers.
Over a period of more than 20 years I've built many SE amps using all the classic output tubes- 300B,211,845,2A3,45,212E, and some with rarer tubes like the 10,71A, DA41, 813,811,805.
My favourites were the 211 and 212E and there's no denying that a well designed SE amp working into an ideal load, can be very nice sounding, however the high output impedance of all SE amps make them less than ideal in terms of accuracy.
Have a look at this frequency response measurement of a very expensive SE amp- the black line is the response into a simulated speaker load- hardly what you could call accurate!
Plate resistances are usually over 1000 ohms, some many times that.
One tube that has found a lot of favour with SE designers is the Russian 6C33C-B . This tube has a plate resistance around 100 ohms and with a good output transformer the performance , while still not ideal, can be much improved.
The tube does have some quirky features though that makes it a little difficult to work with- the characteristics vary quite a lot from tube to tube, making them difficult to bias consistently, they also change with operating temperature. Using a combination of cathode and fixed bias helps a lot (100R on the cathode and negative bias supply on the grid) but owners need to keep a close eye on the operating points to ensure consistent performance.
Kevin Covi came up with a circuit that gives excellent performance and maintains consistent bias , and operation regardless of the tube characteristics and temperature. The servo circuit in the amplifier has worked flawlessly for the 5 or 6 years I have had this amplifier. Once set to 250ma current , it has sat there consistently- this extends tube life, and ensures the amp is always working optimally.
The actual amplifier circuit design is nothing groundbreaking or revolutionary- just very good solid design engineering- notice the use of a 300B triode to drive the difficult 6C33C-B, the very solid power supply with shunt regulator. Many variations of this circuit were tried, choke loads, different driver tubes etc, but this circuit ended up being the most pleasing.
I believe this amplifier is the equal of any SE amp available now, including the ridiculously priced exotica marketed for those with too much money.
Kevin has come up with many fantastic designs over the years, and I this is one of his design high-points.
Like all SE amps it has to be very carefully matched with speakers, and no SE amp can be regarded as "accurate", however with the right choice, something like a Tannoy Gold Monitor , the combination can be very rewarding .
Keep an eye out for other circuit designs to be showcased here.
