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Old 7th August 2006, 01:15 PM   #1
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Default 808 Se

Good day to everyone
I have 2 pcs 808 triodes and I think it's time to use them.
I do believe that I have to use it on A2 Class according to datasheet.
Does anyone has expiriance with A2 drivers?
Please note that I prefeer the use of triodes in all stages.
I have never build an A2 amplifier before can anyone describe how does this sound?

Thanks in advance
Chris
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Old 7th August 2006, 01:35 PM   #2
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A2 can sound very, very good. I'm currently using a Shishido style 6EM7/801A amp which runs the 801A in A2, with positive grid bias.

The disadvantage of A2 is the low grid impedance, so you need a very powerful driver to drive the ouput tube - almost a small power amp. The 801A amp I use uses a 6EM7 with a step-down transformer. In addition, the transition from A2 to A1 causes the grid to stop conducting suddenly and unless the the driver has very low output impedance this will cause non-linearity.

Nobu Shishido was the master of using transmitting triodes in A2 for audio. See the link for some of his designs. There is an 808 deisgn.

Cheers,
Pete

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Old 7th August 2006, 09:46 PM   #3
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Steve Bench article on an A2 amplifier.
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Old 8th August 2006, 01:38 AM   #4
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This is a good place to consider a power MOSFET for grid drive. It'll work better than any VT for slamming current into the 808 grids since the Rd(on) of these guys is so very low. Class A2 is another one of those "all or nothing" propositions: done right, it can sound just great. Done wrong: Click the image to open in full size.
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Old 9th August 2006, 02:18 PM   #5
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Thank you all for your answers.

The use of a Mosfet is very resonable but I wouldn't like to use any "solid state" on this project.

After a little googling I come up with this japanese schematic using a 6L6 as cathode follower to drive the 808. Notice that the cathode of the pentode is directly connected to the grid of the 808 and as far as I understand the current passing throught 6L6 act as grid current for the 808. I have never seen this again and I do believe it is worth to try it just, to see how this thing "plays music".

Unfortunately I have to use a pentode on this configuration (I hate pentodes!!).
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Old 9th August 2006, 03:10 PM   #6
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Well, the pentode in this case is strapped as a triode. ;-) Actually, pentodes generally make better cathode followers than triodes.

I note that the circuit uses NFB, presumably to get the output impedance down. The 801A amp that I'm using had global NFB and cathode feedback to. I ended up disconnecting the global NFB, because I preferred the sound witout, even though the bass was a bit "looser". I left the cathode feedback in though.

If you build it, let us know how it turns out....

Cheers,
Pete
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