ANOTHER ITERATION OF THE 6V6 STEREO AMPLIFIER

GENERAL CONTENTS Technical Documentation The Audio Circuit Basic Schematic The Power Supply Basic Schematic A Diagram of Typical Operating AC and DC Voltages A Dimensional Drawing of the Chassis Layout Construction Photos The Chassis Rim, First Mill The Chassis Rim, First Mill, Detail The Chassis Rim, Inletted and Assembled The Chassis With Drilled top plate The Assembled Chassis, Right-Front View The Assembled Chassis, Top-Front View A Large Image of the Amp on the Bench The Story This is a version of the 6V6 amp that I use myself; most of my virtual cronies are familiar with it.  My now-friend Will approached me with the (then) idea of restoring his Bogen monoblocks and converting them to my circuit.  I didn't like the layout of the Bogens much, they had the rectifier mounted in the middle of the audio circuit and other strangeness, and also I have had gear with a volume control for each channel, which I found to be a colossal pain.  Will proved amenable to my idea of doing what I usually do with old gear; saving and restoring the transformers and letting everything else go the way of all things. Most of the change in this amp from my own example is on the power supply side, of course; the audio circuit is essentially identical to mine, except for some upgrading of passive components.  Since Will had been so agreeable to my general implementation proposal, I went with his component specs; which proved to be fortunate, as he seems to have been correct in every instance. On the audio side, the only changes are in the source switch , which is a nice silver E-Switch toggle, the volume pot, an Alps Blue, and the coupling capacitors, Jensen Copper PIO's; all these components upgrades from what's in my amp.  The biggest circuit change is the 11K load presented by the Bogen output transformer to the 6V6's, rather than the 7.2K of my Kenwood receiver trans.  I was a bit worried about this lightish load, both from a power and a distortion standpoint - more about this in a minute - but they are nice beefy iron, compared to the peanuts in my amp. The power supply is, of course, a rather different critter.  B+ main is completely dual mono, with two transformers, 5Y3 rectifiers, and filters. Heater power is also dual mono, just out of convenience. The diff-amp and bias negative supplies are shared, both being taken from a 15VA Amveco toroid.  These supplies use ultrafast silicon diodes, well shunted on both the filter and transformer sides.  Will didn't want to go for boutique filter caps, so I used Panasonic TSHA and TSHB types, which I will be using in everything from now on.  Nice call, Will. I am disappointed to have to tell you that this amp sounds noticeably more transparent than my own.  There are some half dozen substantial differences between the two amps, but my most probable suspects boil down to two or three:       General beefiness of the power supply, especially the large filter chokes. I'm not a monster advocate of vacuum rectification, I don't think the 5Y3's could cause the difference heard; and the dual mono B+ supply could stabilize images a bit, but I'm not an image freak and tend not to hear such things.  This is, however, a very butch and heavily filtered supply for a 6V6 amplifier.       The Jensen Copper Foil caps.  Signal capacitors have been a headache for me since I started doing this stuff; any time I can remove one, I can improve the resolution of the circuit.  These juice-can sized things seem to be the least of capacitive evils so far, except for the not inconsiderable problem of jiggering space in the chassis for them.       The output transformers, especially their high (numerically; low, electrically) load to the output tubes.  They're also big and nicely built, another example of the good iron that was often used in even budget old time gear.  As a test of the loading, I tried hooking my 8Ω speakers up to the 16Ω tap; the sound became looser, tube-ier, not as see-through.  Given my own preference, I would now load the outputs of this amp at the higher common impedence seen for tranformers of this type, 10K. This little developmental info page was written to be referenced by Will's review of this amp.  I am indebted to him for good and patient collaboration on the D&D of the piece (always a difficult process), for his much-more experienced ears than mine (he's heard most of the famous gear, which I have not), and for the considerable work he's done in making up such a comprehensive and well constructed review. And also for the fact that he likes it so much, of course! Thanks, Will, Poinz © Eric Kingsbury; June 26, 2004 Email me at: poinxie@yahoo.com Best viewed at 1024 x 768