Dimitrios Tsifakis (dtsifakis AT gee-mail dot com)

The 807 has always been my favourite valve. I have used it on a number ofRF transmitters and modulators and I thought it was only fair if myhi-fi audio amplifier uses it too. There are plenty of good designs out thereshowing different audio amplifiers using 807. The Williamson designis a famous instance. My desire was to design something simple from scratch, so Iturned to the 807 datasheet looking for single ended designs. There are many datasheets out there but the onethat is by far the most complete is the STC one.

1. A 30-W tube amp using two 807 power tubes in push-pull arrangement. Schematic from David Sarser & Melvin C. Sprinkle (November 1949). This entry was posted in Amplifiers, Tubes and tagged Amplifiers by matthias.
2. DIYTube 6L6 Clementine Single Ended Tube Amp Shannon Parks, owner of DIYTube, has made a number of projects available since 2003 when the DIYTube ST35 clone was made available. DIYTube is both Shannon's small business located in central Illinois and a close-knit forum featuring a handful of helpful and prolific members.

With cathode follower grid drivers, the 807 can easily produce some 30W of audio output, though specced at 26.5W. If you're willing to go Class AB2, you can get 80W or more, which requires increasing the plate voltage to 600Vdc, and is not recommended with 6L6s.

Triode or Tetrode?

The majority of hi-fi valve ethusiasts will rave about the quality ofthe triode valve. The 807 is not a triode, but it could be connected intriode mode by connecting the screen grid directly to the anode. In this case,the maximum voltage that can be applied to the plate will limited by the maximum allowable screen voltage which is 300 V. That's a low value and that'sthe reason why the 807 wouldn't be an ideal valve to use in an ultralinear design. The datasheet shows that a triode strapped807 is not expected to produce much more than a watt of audio. The tetrodeconnected 807 produces over five watts. I have found that given the sensitivityof my speakers (89 dB) and my listening room and preferences, five watts is more than enough.

While the valveenthusiasts will argue that one triode watt is plenty and they would preferthat to anything else, I was of the opposite opinion. In the STC datasheet,there are tables with suggested configurations for tetrode operation. In the tables above, we see that the 807 will produce more power if used as a tetrode. So what's the difference? The difference appears to be in thedistortion of the final amplifier. The tetrode amplifier is expected to havemore harmonic distortion, as seen below. In general, even harmonics (2nd, 4th,etc) are said to be more 'pleasing' to the ear, while odd harmonics (3rd,5th, etc) are believed to be an unpalatable form of distortion. One probablyhas to experience this and form their own opinion. In any case, I decidedthe described amplifier will produce a healthy dose of even harmonics!

Driving the 807

Given that this is a class A1 amplifiers, the driving requirements are simple. In Class A1, there is no current flowing on the 807 grid. A resistance-coupled voltage amplifier seems to be all that is needed. Inthe 6J5 datasheet, we get some instructions on how to do this. Theschematic is as follows:

The proposed values for the components of the 6J5 amplifier are seen inthe schematic below. They result in a voltage gain of about 18.

Final schematic, as built

Design comments

• I would have preferred a valve rectifier but the first version was built with silicon diodes. This was dictated by the availability of the powertransformer that did not have a centre tap or 5 V winding. In the future,the amplifier power supply will be re-designed for a 5AR4 full waverectifier after a suitable power transformer is found.
• I had a lot of hum initially. After some investigation, the filaments werefound not to be the source of the hum, it turned out to be the screen. Tetrodesare very sensitive to the screen voltage variations, in fact you can producean amplitude modulation transmitter with the modulation applied to the screengrid. I added R8 and C5 to further regulate the screen and that solved the problem.R6 and C4 do the same thing for the 6J5.
• The output transformer, T1 (L1-L2), is made by Edcorand it is the single ended, 3750 ohm to 6 ohm model CXSE25-6-3.75k. It has anUltralinear tap which is not used. It was purchased for a different design, so it's notoptimal, but the mismatch is small and the result is acceptable. If I use 4 ohm speakers, thenthe ratio is closer to the one recommended by the datasheet. But then again, the onlyspeaker that I know that has a constant impedance on the audio frequency range isa resistor! In reality, speaker impedance varies wildly with frequency, and themanufacturer specified impedance is probably just a hint that most of the time, theimpedance is close to that value.
• The full wave rectifier sees a 100 uF capacitor which presents a very low impedance onstartup. The transformer is a modern, toroidal type one and doesn't seem to mind this.The resistance of the winding is such that the startup current is limited to a lowervalue to the peak current of the diodes. The diodes are 1N5408 which have a surge currentof 200 A. R9 and C6 provide more AC ripple filtering and R9 is adjusted to bring thevoltage close to the datasheet suggested value. This is a class-A amplifier which implies that the current requirement is almost constant and independent of theload. Most hifi monoblock designs would have a separate power supply for each channel,but in reality, that may be not required as the crosstalk introduced by the shared supply will be very very low.

  All filter capacitors have 330k bleeder resistors across their terminals.

• Cathode voltages for the 6J5 and the 807 are shown on the schematic. Small deviationsare acceptable, as this is a single ended amplifier. If you are happy with what you'rehearing, there is probably no need to find a closer matched valve pair. Speaking of valves,I am using two new old stock Westinghouse (Made in Canada) 807 valves which I purchased for$2 each from the local radio club trash and treasure. The 6J5s are made by RCA. I have acouple of 6J5G valves which look much better, having the 'shoulder type' shape. They arereserved for special occasions, as they are hard to find. I have also tried 6C5 valveswhich are also a medium miu single triode with similar performance to teh 6J5. Anotheralternative would be a single 6SN7 or 12AU7, both double-triodes, which could haveone triode per channel.
• The amplifier is connected directly to an Akai CD-A7 compact disk player. This is a vintage CDplayer which has an output volume control knob. This is an unusual feature but itis handy in this case, as I don't have to implement the volume control function onthe amplifier. In the future, this may change as the CD player may be replacedwith a newer one.
• C2 is a Sprague 'Orange drop' 715P polypropylene capacitor. It's a cheap and cheerfulcapacitor which seems to work pretty well. Many valve audio enthusiasts may recommendfancier, audio-grade, expensive capacitors, but I can't quite say I have heard anydifference in the various capacitors I have tried. Your miles may vary..
• All resistors are 1 W rated except R5 and R9. R9 is 10 W rated and R5 is two 5W 330Rresistors in parallel, as I didn't have the 170 ohm cathode resistor suggested by thedatasheet. I think 165 ohm is close enough!

Listening

Microsoft office 2013 for mac torrent file windows 7. One could write all sorts of subjective comments on the performance ofan audio amplifier, but I will resist that. In general, the amplifiersounds pleasing to my ears. Is it because of the even harmonicsor because I like the looks of it?Or is it because I built it? In any case, the result is positiveand that's what it's all about!

Photos

References

807 Tube Audio Amplifier Schematics

Audio Amplifier Schematic Diagram