Single Ended 808/811A Monoblocks.... drum roll...... tada!!! This is my latest build. It uses a 1930's transmitting triode 808. A switch is also provided to toggle from 808 to 811A.
Brief OverviewI wanted to gain knowledge and experience on Class A2 amplifiers as well as zero-bias type of tube. In the absence of an inter-stage transformer, I initially thought of using 6EM7 to achieve a positive grid for 808 or 811A tubes.
After going through discussions with Tony at elab(see link below) and readings from the internet, I finally came-up with using 717A as an Input tube, a triode connected 6V6 to provide a positive grid voltage and current to 808/811A tubes.
Full Discussion at:
http://www.elab.ph/forum/index.php?topic=34991.1740Output TransformersWho doesn't know Edrel's Irons(AT&S) and the quality it has? This build uses an SE 15 Watt OPT which has an M6/Z11 core of 1 1/4 X 1 1/2(inches). Primary Impedance of 3,500 Ohms with Ultra-Linear(which I didn't use in this circuit) and a Secondary Tap of 4, 8 and 16.
Power Transformers and ChokeI tried to bring down the cost at a reasonable level because I have 5 other builds to fund but I'm not selling a single one of them.
I'll wound my own power transformers and choke for the upcoming projects. (
committing something publicly will most of the time gets a job done). These Irons are M50 which runs hot if not properly compensated via specifications.
Power Transformer specifications:
Secondary:
320-CT-320V @ 200mA - I'm using it about 120mA only.
7.5V @ 6A - Just about what I need to supply the Heaters of 808 which is 7.5 @ 4A FW Bridge.
6.3 @ 2A - I'm using it at about 675mA for 6V6 and 717A heaters
Choke is 1H 200mA (Appproximate since this was measured at about 2H without a load)
These Transformers came bare so I had to put some flux band and paint the end bells. I used a 100W Soldering Iron so the solder was cold and messy. (It needs at least a 150Watt Soldering Gun to install a flux band).
The Power and Choke is wound by Mang JV. His shop, is located at inner part of Main street of Paco Manila. He's been supplying transformers to guitar amp DIYers at Philmusic.
Layout and enclosure workFront and Rear ViewGut ShotsBelow the sockets of of 717A Tube
Below the sockets of 6V6
717A Input TubeWhat's up with those transformer wires? Why not cut them short?Re-usability. The tubes I'm using are very rare. Ones they go or you run out of tubes, you can easily re-use your transformers into another amp.
In actionTesting and Final Configuration?I only have 2pcs of 808 tubes(the rest are 811A) that's why I was aiming for longer tube life. At initial startup, 808 tubes have plate voltage of 375, plate current of 60mA, grid voltage at around +20. Both tubes are dissipating at only about 22.5 watts. From Tony's advice, I could still push it to about 100mA or around 40Watt dissipation and still get a reasonable tube life.
On the 2nd day of testing. I tried to increase the Amps overall B+ by adding a parallel resistor to the existing B+ dropping resistor. Now I'm getting a B+ of 400VDC. I've tested back both 808 tubes at 1ohm cathode resistance. One 808 can dissipate at a maximum of 35 Watts (plate voltage at 390 and plate current of around 90mA, while the other tube is only at about 29watts. Not. sure if it's gassy or just have low plate dissipation. I've swap them too and got the same results.
Now, I have them both dissipating at about 29Watts per tube and just have to live with it for the meantime. When they retire, I'll replace them with 811A.
How does it sound?Wifey said: "I like that I'm hearing percussion in just the right amount. Cymbals are sprinkled across but doesn't dominate. There is so much clarity in the vocals section it feels like the singer is in front of you. Lastly, the bass feels so smooth that I would like to assume the recordings had fret-less bass on them."
I say, this amp is a very sweet sounding amp. The likes of 300B. This is very good when you're listening to bossa nova, smooth jazz and ballad music. Since I listen mostly to Rock music, I tend to like the dynamics that Tetrodes and the Pentodes have. I played Reggae music on this amp and the bass sounded like a fret-less bass with long sustain. It's supposed to be tight and for Rock a bit punchy. But the Mids and Highs are sparkling.
Well, at the end of the day, the Wifey is very happy with the sound. I wouldn't have to argue with that wouldn't I? Thanks also to
Paul Chin for this wonderful Anthony Audio Gold Titus BS. It really brings out the beauty of the amp
Schematic* All 22uf Capacitors were replaced with 47uF
* B+ dropping resistors have an effective resistance to 250ohm. In my build, I've installed 2 parallel 1K plus a 500ohm resistor. Installed the 500ohm resistor at the final tweaking.
* Bridge Rectifier right after the 7.5 secondary tap. Note that 4A is wrong and you need 6A.
AcknowledgementsThis amp could not be made without all the advises from Tony Tecson. Thanks for always being there.
Edrel and his irons. The amps sound would not be as good as now without AT&S OPTs.
Gerry Sta. Maria. Our regular tube pimp in the DIY community.
To my best ever sound critic, my wifey
Reference Circuitshttp://www.single-ended.com/direct811.htmhttp://park21.wakwak.com/~eyp/jk1eyp/amp/811A/811AS.htmWhat's next?A lot of builds
but we can't get to the bottom of this journey until we know how to wound our own transformers. So I'll stop building for a month or two and would my own transformer.