5.1 channel tube audio amplifier project

ryuji

2[H]4U
Joined
Jun 11, 2004
Messages
3,115
I just finished my project after a few months of planning/assembly but mostly waiting for parts to arrive in mail. Decided to share.

Design:
The amplifier has five 807 pentodes running in single-ended fixed bias with Edcor GXSE10-8-5k output transformers. the pentodes are driven by 12AT7's. in addition, i have made use of 12AU7's as pre-amplifiers for all 5 channels, plus a sub-woofer pre-amp for a external solid state amplifier. I am making use of an 5C8S rectifier tube for the 320ma of current required for powering so many channels. The power transformer is a Antek AN-4T400 which is rated for 400VA. The VR-105 tubes are wired in series and used to provide regulated power for grid 2 on the pentodes. Filaments are powered by a modified random pc power supply i tweaked to provide 12.6v

Pictures:
I have pictures i have taken as i built it. Can take more pictures/better pictures of certain parts if people are interested. more pictures can be found at:
http://ryujiwarui.com/photos/main.php?cmd=album&var1=tube/

close_up.JPG

tube_underside.JPG

psu.JPG

psu_closed.JPG

installed.JPG

night.JPG


Schematics:
I used multiple schematics to build the amplifier:
Power tube design:
7868SE_with_12at7_driver.png

Pre amp:
12au7_preamp.gif

Bias supply:
807-tube-schematic.png


Additional information about anything available by request, just post a question
 
So, whats the basic gain from running in pure pentode mode, verses UL, I see your OPT's have UL taps, or triode mode, with grid 2 tied to the anode?



UL mode is good for 1 or 2 more watt's, whats B+ and B+2 voltages? are you screen driven? pure class A or do you slip into AB2 at any point?



I see your sporting a hard to find schematic for a 7868 pentode, Got any plans for a 7868 based amp?
 
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Single ended Triode is good at mid-range but not so good at the full dynamic range.Triodes also lack in power output. Ultralinear mode is a balance struck between the two via a 40% tap on the output transformer.

my b+ voltage is presently 385v and my B+2 voltage is 310~315v. the b+2 only powers grid 2 on the pentodes. Im running pure class A

i have no present plans to use a 7868 pentode amp but that doesn't mean i wont in the future
 
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Makes sense, so you threw efficiency to the wind, and stay in pure class A all the time? hows that 10% efficiency doing for your rooms heat production?


Whats the Bias current also? are you anode + screen measuring current? or just cathode current biasing?



Russian rectifier, and Russian PIO's, How do they sound?




You might not be interested, but I have a set of matched, 1941 schuab-lorenz 4.5ohm 5x7's id let go for $12,000 per driver. I have full TS specs if you want them, the gravy of them is 116db sensitivity @ 1w
 
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The tubes as a whole put out a bit of heat, but its not so bad. my pc does worse(c2d at 4.5 ghz, ati 5870)

Bias is presently set at 32ma idle measured by the cathode resistor voltage drop

The russian PIO's sound great and the rectifier functions perfectly
 
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Nice job but why go to the trouble when your speakers are small and in weird positions? Does it sound any different than before?

Maybe build some speakers?
 
Nice job but why go to the trouble when your speakers are small and in weird positions? Does it sound any different than before?

Maybe build some speakers?

there is a HUGE difference with this new amp vs what i was running. night and day.. cant even really begin to accurately describe what good tube audio sounds like compared to silicon amplifiers

my speakers are equal distance from where my computer chair is except the center channel. which i believe is the spec for surround sound?.

speaker_placement.gif


but yes, i will be upgrading my speakers. amp was my first step into doing some serious audio
 
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