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^Y^ A 6Y6 Amplifier
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The primary goal of this design was to play these tubes in Triode mode. The 6Y6 is a strong, low voltage high current tube like the 50L6. I've always liked the 50L6 both in pushpull and triode SE modes. So the plan was to proceed using triode operation.
After some experimentation, I opted for grid bias instead of cathode self-bias. This was in part due to the limited voltage I had available from the power tranny and in part due to the sound. It was going to take better than 40V Pk-Pk to drive this tube to full ouptut in Triode mode. Since I had already eliminated the output stage cathode bypass cap, I decided to do the same in the input stage. I don't like to bypass if I don't have to as I think it alters the sound too much. The 5965 was not going to do the job as I had hoped. After many considerations, I chose the 12AT7. Not just any 12AT7. I prefer the ones with the larger plates when I can find them. Some of them have just rolls of metal for the plates like Sylvania's. The Telefunken are the best, with the largest plate area. I don't have any of those. Next is the RCA production run from which the tube used in this amp was taken. This tube features a plate rolled at one end, but supported at both ends with a square top getter. So with a mu of 60, and a pinout common to many tubes, this would allow easy "tube rolling" for tubes with a mu of 40 or greater. Sure, you can play 12AU7's in this amp (u=20), but they will be biased in the dirt and won't have enough gain.
 Since I opted for grid bias, I had to have a voltage source. Hence the second tranny. This 16VAC tranny feeds a full wave voltage doubler to a CRC filter with a resistor network so set the bias. The net negative voltage feeding the network is 40V. The center resistor is adjustable and wired so that in the event of wiper failure, the bias with go more negative thus preventing damage to the output section. There is a single bias control for both channels, so a fairly close matched pair of output tubes is recommended. A 10% match is plenty good and easy to attain. Bias can be set empirically by the grid voltage or can be measured by virtue of there being a 1.1 ohm resistor on the cathode of each output tube. This feature also allows you to check tube matching. (About the 1.1 ohm resistor: I have a large number of 1-ohm 5% resistors. I ran a large batch of them on my bridge and on my HP 4800. Virtually ALL of them checked at 1.1 ohms - or slightly higher. I picked two that were 1.1 exactly.) Full biasing instructions are with the amp.
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You can see in the pictures about everything you need to know. All resistors and capacitors were hand selected for balanced channels. I do this in all my amps, but don't often mention it. The power supply is conventional full wave feeding a CRC filter to the outputs. Both the screen (for pentode operation) and the driver voltages are further RC filtered thus reducing noise. The actual measured noise is about 60 db below 1 volt output. There are strategically placed film caps for "hash" noise.
 The wiring is straight forward with all the DC power and ground runs in solid copper, and the AC runs in stranded. Mil spec silver plated shielded input wiring with silver wire feeding the input grids. I am going to be rethinking this whole silver wire thing in the future. All the other signal wires belong to the devices they are attached to. A nice pair of PIO coupling caps are the only caps in the signal path. Of course, the power supply is also in the signal path and low loss good grade caps are used here as always. Resistor selection consists of vintage carbon comp along with a few metal and carbon film. The speaker connectors were taken from an old dead Marantz receiver. Input control is Alps. The unit is internally fused. |
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It was the triode sound on the breadboard during development that kept this amp moving forward. It's everything I've come to expect from a small triode amp. Excellent tight bass and smooth highs characterize this one. I'm not a big fan of over-warm sound and you won't find it here. I added the Pentode mode almost as an afterthought. While working with this as a pentode, I incorporated my special pentode mode network. This network reduces high order harmonic distortion giving the Pentode a more pleasant sound. My wife and I played our usual run of music and enjoyed the amp a lot. Personally, I hear little difference between this and a Darling. In fact, I like the way this one makes the image come forward. My Darling is WIDE and moderately deep. This amp has better FRONT to BACK and still has good width. It also has a tad better distortion character with less 2nd and far less 3rd. I'd charactize it as sturdier sounding especially in the bass to mid-bass where the detail is complete and unvague.  Showing GE 6Y6's
Thanks for looking. |