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Old 21st December 2006, 07:59 PM   #1
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Default 807 vs 811A output: Sonic preference?

Never heard either of these tubes, but the price looks attractive for the output power I want to achieve. Besides I like the looks of big tubes in the dark :-) What are the thoughts of each sonically. If I am not mistaken these tubes should have a similar sound to 6L6's. Looks interesting. Any experience with these in a SE parallel setup?
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Old 21st December 2006, 08:52 PM   #2
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Default Tubes...

The 811A and its 'relation' the 572B isnt an easy tube to drive for audio....

The 807 is very basically a 6L6 with anode-top-cap connection intended for RF service, so you can follow a good/popular 6L6 scheme for that tube...

Ive built an 811A P-P amp running at 420V anodes/plates, but you need to run them in Class A2, meaning the Grid is always Positive with respect to Cathode. This means a driver stage capable of supplying many 10's of mA to achieve any real power in the Anode cct. A good strong tube like a KT66/88/etc will supply that if it ALL must be tube...

I found in SE they are rather 'flat' to my taste so I re-designed into P-P....

I used MOSFETS to drive them. Works well, and sounds pretty good. Use a transformer of around 4-5K P-P, and around 60mA Bias, for which you'll need around 20V Positive on the Grid at 400V Plate/Anode Expect around 30-50W, in P-P depending on how you drive them

811A are very good asthetically...The Thoriated Directly heated cathodes will light up the room, and form a great talking-point, unlike the 807, although the 807/6L6 is a better choice to make a an amp with!
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Old 21st December 2006, 08:58 PM   #3
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Default Re: 807 vs 811A output: Sonic preference?

Quote:
Originally posted by Curly Woods
Never heard either of these tubes, but the price looks attractive for the output power I want to achieve. Besides I like the looks of big tubes in the dark :-) What are the thoughts of each sonically. If I am not mistaken these tubes should have a similar sound to 6L6's. Looks interesting. Any experience with these in a SE parallel setup?
Here, you are asking for a comparison that really won't mean very much. The 807 is a beam tetrode, whereas the 811A is a high u, "zero bias" RF power triode. Vastly different VTs which will require vastly different circuit topologies. About the only point in common is that both types have a rather highish r(p).

I have experience with 807s, and I can tell you that, given the right Q-Point, ~7.0db(v) of local NFB, and a bit of gNFB that they sound fantastic. And, yes, it's just like a 6L6GB with a plate connect button so that it can operate at higher Vpp's than can a 6L6GB where all connections are made at the base. As far as "glowey bottle kewelness", 807s really aren't all that impressive, as they have rather thin cathodes, and the heater requirement is but 6.3V @ 0.9A. Not like the Th/W filaments of the RF power triodes at all.

As for 811As, were I going that route, I'd opt for 812A instead. Same Pd, both will still require grid-positive (i.e. enhancement mode bias) to work right, but the 812A is a bit more linear, and has a lower r(p) dur to its lower u factor. Lower r(p) means better bass performance.

As for these, Class A2 triodes won't sound the same. They seem to perform their best when working at high volumes, and are very dependent on the quality of the driver stage. The latter isn't a prob with the 807, since it's a very easy load even in Class A2, or more likely, AB2.
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Old 21st December 2006, 09:50 PM   #4
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Default 811A

Forgot to add, another dis-advantage of the 811A type is hum can be a problem from the directly heated cathode, unless DC is used or some cunning design is done...The Heater of one 811A needs 6.3V at 4A so if you decide to regulate this supply, you may need a BIG heatsink!

I have AC heating on my 811A P-P, but hum is audible if you listen closely at the speakers. At normal distance, it is just barely audible between tracks if you listen hard......I really should do something about it, but never seem to get the time! One thing in its favour, It has been very reliable, Bias good and stable, It just 'works'.......

As Myles pointed out, These two valves are totally different animals really. For ease of design and ongoing reliability/performance the 807 would be recommended over the 811A, but dont look half as 'cool'!

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Old 21st December 2006, 10:56 PM   #5
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I have built SE amps with both 807 and 811A.
Let me put it this way:
Unless yoy have great experience in designing and building tube stuff or are on a limited budget, stay away from 811A.
These babies differs a lot from the regular triodes we use in hifi amps, they require massive DC PSUs for the filaments and very low-impedant driver stages.
I gave them a try just for the challenge and there was some degree of success but I never got it quite right (PSU problems).
Will give A2 another chance soon, this time Ill use 808s instead just because they look cool.

807 on the other hand is an easy tube to work with, capable of producing great sonics too if you dont need a lot of power.
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Old 21st December 2006, 11:03 PM   #6
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Default Thanks for the input

I really appreciate the thoughts and suggestions.
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Old 22nd December 2006, 12:49 AM   #7
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I've done SE 811A, SE 845 and have listened to SE KT66. If no
more than 9W works with you, then any of these projects are attainable. The 811A sounds different than the 6L6 by a long leap.
811A: crystal clear, detailed. KT66: warm, lush, rounded out. The
difference in sonics is the distortion profile. The 811A is extremely linear. I liked driving my 811A with a 6Y6GT as a cathode follower.
You can drive it a 6V6 or EL84 just as well.

I'd not attempt parallel 811A. You need to add another 25W of
filament power to do the second 811A. This is just not worth the
cost of additional hum injection, finding a driver that can power
a 500 ohm load at 40mA idle. That is just madness.

You are far better off doing a 800V amp using 211/845 than PSE 811A.

Dont be scared of considering the 211 or 845. They are $40 ea.
The socket is another $13. They last forever (dont drop 'em).
And, they look incredible. I think a nice project for not alot of
money would be a Cathode biased 211 (permits ultrapath bypass
of Output Transformer), 6600 ohm SE OPT. Use the mu-follower
driver from the old Svetlana App Notes (EL84 + 6N1P).

-- Jim

-- Jim
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Old 22nd December 2006, 07:17 AM   #8
Giaime is offline Giaime  Italy
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Default Re: Tubes...

Quote:
Originally posted by Alastair E
Ive built an 811A P-P amp running at 420V anodes/plates,
Hello! Could you post a schematic?
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Old 22nd December 2006, 11:49 AM   #9
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Default 811A P-P scheme...

Its already here somewhere....

Have a search on all my Posts, It will be under a thread about the 811A Cant quite remember where, Ill try and find it or if not Ill re-post it for you.

In the scheme as I recall, I made a small silly error on the 'Mu-Stage' but its glaringly obvious! The driver/O/P etc is fine however...

Here you go....811-A amp

About halfway down the page....
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Old 22nd December 2006, 12:28 PM   #10
Giaime is offline Giaime  Italy
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