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Old 13th January 2004, 11:07 AM   #11
Zoran is offline Zoran  Serbia
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Maybe it is the best way
to determine Ri is to make real circuit
set the +Ug for the desired point
controling the Ia for the desired value
and put for R_Load a powerful resistor_pot.Changing the Vbattery and the value of RL until we get the same
Voltages at the RL and Ua with of course
desired Ug and Ia...
Than the value of Ri will be the same as the RL_pot...?
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Old 13th January 2004, 11:09 AM   #12
Zoran is offline Zoran  Serbia
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But that is little bit complicated
requires the 50-100W pot.
and the about 2000V controlable Pover suply...
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Old 4th May 2004, 12:24 PM   #13
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Default Need more detailed information, please.

Quote:
Originally posted by Fuling
Hereīs the basic topology of my amp:

Triode wired E81L input stage, EL86 cathode follower direct coupled to 811A grid. I can post a more detailed schematic within a couple of days if anyoneīs interested.
I am fairly interested in building up a SE by using 811a as power tube. Can you give me more details about the features of this tube and related schematics? Thanks a lot.
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Old 4th May 2004, 01:49 PM   #14
Fuling is offline Fuling  Sweden
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I finished my 811A monoblocks quite some time ago and used them for a while. Unfortunately I had some serious problems with mechanical noise from various parts, mostly from the rectifiers for the driver stage (PY82).

I decided to tear them apart and rebuild them on MDF breadboard "chassis" using different rectifiers.
This was some months ago and since then the project has more or less died, simply because of all the work involved.

Just a couple of days ago I made some new plans for this project.
This time it would be a simplified version using solid state rectifiers and a mosfet buffer instead of the cathode follower.
The biggest problem would be to fit the filament power supplys inside the chassis (no monoblocks this time).
I donīt think 811Aīs can be AC heated without severe hum problems.


If you want to use these tubes, Iīd say go ahead.
They donīt sound bad at all, but they require some negative feedback to get the output impedance down and theyīre a bit of a mess to drive because of the grid current.
Fortunately they donīt require much voltage swing, only 50V p^p or something. Nothing a grunty cathode or source follower canīt handle.

A better solution might be to use interstage transformers, but suitable ones ainīt cheap.

One word of warning: A bit of timing seems necessary in the startup process: if the grid voltage is applied when the filament is applied before the plate voltage (when the filament is warm) the grid current might reach dangerous levels.
This might be a problem when using slow warmup rectifiers for the
B+.
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Old 4th May 2004, 02:38 PM   #15
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Thank you for your thorough introduction. It seems directly heated tube rectifier will be good option for it can get heated quickly to add high voltage to the anode of 811a, or FU811, its soviet and Chinese equivalent. I have a schematic as following that uses 5z3p as rectifier. 5z3p is the Chinese version of western style 5U4G, directly heated double diode. Welcome your comment on this design. Note: 6N9P=6SL7.
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File Type: jpg fu811a.jpg (24.8 KB, 425 views)
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Old 4th May 2004, 04:04 PM   #16
Fuling is offline Fuling  Sweden
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That schematic (never seen that particular one before) looks prefectly sound to me. Maybe the operation point seems a bit "cold"to me, I ran my tubes at about 430-440V and 100mA into 5,6kohms load.
Canīt tell if one is better than the other, though.

That amp is fully DC-coupled, good for the sound but bad if some part goes south somewhere and takes the rest with it.
Maybe the risk is smaller in this kind of amp because of the positive bias. If the output tube looses its bias it will turn off instead of going wild.

5U4 or equivalents should be fine as long as they can handle the voltage.

For the cathode follower, use something with relatively high current capability and gm. 6V6 would not be my first choice, I picked EL86 and it worked fine.
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Old 5th May 2004, 01:34 AM   #17
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Thank you for your comments. I find another schematic on a Japanese site. It seems eerie that the cathode of 6L6, the driver, is suspended rather than being grounded through a resistor, as the other two schematics show. Is this design operational?
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Old 5th May 2004, 10:00 AM   #18
Fuling is offline Fuling  Sweden
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That schematic looks familiar.
If all the voltages and currents corresponds correctly there is no need for a cathode resistor on the driver stage, since the 811 grid will act as one.
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Old 5th May 2004, 12:34 PM   #19
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Generally speaking, how large would the grid current of 811a would be? I mean, would it be a little bit wasteful to use 6L6G, a tube with modest current handling capacity as its driver? How about EL84? It has high S value and sufficient plate current capacity?
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Old 5th May 2004, 01:35 PM   #20
jim is offline jim  Belgium
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Default Schematic with EL84

Kenneth,

Here's a schematic with a EL84, I built this one.
Due to the low plate voltage output power is 5W max.
Sounds pretty good (and looks good too).

811A & EL84

and here's a collection (zip file) of 811A schematics :

811A collection

Greetings,
Jim
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